GNU Awk

GAWK: Effective AWK Programming
A User’s Guide for GNU Awk
Edition 5.3
October, 2023

Arnold D. Robbins

“To boldly go where no man has gone before” is a Registered Trademark of Paramount
Pictures Corporation.

Published by:Free Software Foundation
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URL: https://www.gnu.org/
ISBN 1-882114-28-0

Copyright c 1989, 1991, 1992, 1993, 1996–2005, 2007, 2009–2023
Free Software Foundation, Inc.

This is Edition 5.3 of GAWK: Effective AWK Programming: A User’s Guide for GNU Awk,
for the 5.3.0 (or later) version of the GNU implementation of AWK.
Permission is granted to copy, distribute and/or modify this document under the termsof the GNU Free Documentation License, Version 1.3 or any later version published by
the Free Software Foundation; with the Invariant Sections being “GNU General Public
License”, with the Front-Cover Texts being “A GNU Manual”, and with the Back-CoverTexts as in (a) below. A copy of the license is included in the section entitled “GNU Free
Documentation License”.
a. The FSF’s Back-Cover Text is: “You have the freedom to copy and modify this GNU
manual.”To my parents, for their love, and for the wonderful example they set for me.
To my wife, Miriam, for making me complete.Thank you for building your life together with me.
To our children, Chana, Rivka, Nachum, and Malka, for enrichening our lives in innumerable ways.

iShort Contents
Foreword to the Third Edition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Foreword to the Fourth Edition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Part I: The awk Language
1

Getting Started with awk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2
3

Running awk and gawk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
Regular Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

4

Reading Input Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

5

Printing Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99

6
7

Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
Patterns, Actions, and Variables . . . . . . . . . . . . . . . . . . . . . . . . 147

8Arrays in awk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177

9Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193
Part II: Problem Solving with awk

10 A Library of awk Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241
11 Practical awk Programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279
Part III: Moving Beyond Standard awk with gawk
12 Advanced Features of gawk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335
13 Internationalization with gawk . . . . . . . . . . . . . . . . . . . . . . . . . . 357
14 Debugging awk Programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367
15 Namespaces in gawk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383
16 Arithmetic and Arbitrary-Precision Arithmetic with gawk . . . . 389
17 Writing Extensions for gawk . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405
Part IV: Appendices
A The Evolution of the awk Language . . . . . . . . . . . . . . . . . . . . . . 473
B Installing gawk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 491
C Implementation Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 515
D Basic Programming Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . 525
Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 529
GNU General Public License . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 541
GNU Free Documentation License . . . . . . . . . . . . . . . . . . . . . . . . . . 553

ii GAWK: Effective AWK Programming

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 561

iii

Table of Contents
Foreword to the Third Edition . . . . . . . . . . . . . . . . . . . . . 1
Foreword to the Fourth Edition . . . . . . . . . . . . . . . . . . . . 3
Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
History of awk and gawk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
A Rose by Any Other Name . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Using This Book . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Typographical Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Dark Corners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
The GNU Project and This Book . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
How to Contribute. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Part I: The awk Language
1 Getting Started with awk . . . . . . . . . . . . . . . . . . . . . . . 17
1.1 How to Run awk Programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
1.1.1 One-Shot Throwaway awk Programs . . . . . . . . . . . . . . . . . . . . . . . 17
1.1.2 Running awk Without Input Files . . . . . . . . . . . . . . . . . . . . . . . . . . 18
1.1.3 Running Long Programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
1.1.4 Executable awk Programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
1.1.5 Comments in awk Programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
1.1.6 Shell Quoting Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
1.1.6.1 Quoting in MS-Windows Batch Files . . . . . . . . . . . . . . . . . . 23
1.2 Data files for the Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
1.3 Some Simple Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
1.4 An Example with Two Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
1.5 A More Complex Example. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
1.6 awk Statements Versus Lines. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
1.7 Other Features of awk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
1.8 When to Use awk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
1.9 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

2 Running awk and gawk . . . . . . . . . . . . . . . . . . . . . . . . . . 33
2.1
2.2
2.3
2.4
2.5

Invoking awk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
Command-Line Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
Other Command-Line Arguments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
Naming Standard Input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
The Environment Variables gawk Uses . . . . . . . . . . . . . . . . . . . . . . . . . 42
2.5.1 The AWKPATH Environment Variable . . . . . . . . . . . . . . . . . . . . . . . . 42

iv GAWK: Effective AWK Programming

2.5.2 The AWKLIBPATH Environment Variable . . . . . . . . . . . . . . . . . . . . 43
2.5.3 Other Environment Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
2.6 gawk’s Exit Status. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
2.7 Including Other Files into Your Program . . . . . . . . . . . . . . . . . . . . . . 46
2.8 Loading Dynamic Extensions into Your Program. . . . . . . . . . . . . . . 47
2.9 Obsolete Options and/or Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
2.10 Undocumented Options and Features . . . . . . . . . . . . . . . . . . . . . . . . . . 48
2.11 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

3 Regular Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
3.1 How to Use Regular Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
3.2 Escape Sequences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
3.3 Regular Expression Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
3.3.1 Regexp Operators in awk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
3.3.2 Some Notes On Interval Expressions . . . . . . . . . . . . . . . . . . . . . . . 57
3.4 Using Bracket Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
3.5 How Much Text Matches? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
3.6 Using Dynamic Regexps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
3.7 gawk-Specific Regexp Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
3.8 Case Sensitivity in Matching. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
3.9 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

4 Reading Input Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
4.1

How Input Is Split into Records . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
4.1.1 Record Splitting with Standard awk . . . . . . . . . . . . . . . . . . . . . . . . 65
4.1.2 Record Splitting with gawk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
4.2 Examining Fields. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
4.3 Nonconstant Field Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
4.4 Changing the Contents of a Field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
4.5 Specifying How Fields Are Separated . . . . . . . . . . . . . . . . . . . . . . . . . . 73
4.5.1 Whitespace Normally Separates Fields . . . . . . . . . . . . . . . . . . . . . 74
4.5.2 Using Regular Expressions to Separate Fields . . . . . . . . . . . . . . 74
4.5.3 Making Each Character a Separate Field . . . . . . . . . . . . . . . . . . . 75
4.5.4 Working With Comma Separated Value Files . . . . . . . . . . . . . . 76
4.5.5 Setting FS from the Command Line . . . . . . . . . . . . . . . . . . . . . . . . 77
4.5.6 Making the Full Line Be a Single Field . . . . . . . . . . . . . . . . . . . . . 78
4.5.7 Field-Splitting Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
4.6 Reading Fixed-Width Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
4.6.1 Processing Fixed-Width Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
4.6.2 Skipping Intervening Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
4.6.3 Capturing Optional Trailing Data . . . . . . . . . . . . . . . . . . . . . . . . . . 82
4.6.4 Field Values With Fixed-Width Data . . . . . . . . . . . . . . . . . . . . . . 82
4.7 Defining Fields by Content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
4.7.1 More on CSV Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
4.7.2 FS Versus FPAT: A Subtle Difference . . . . . . . . . . . . . . . . . . . . . . . 85
4.8 Checking How gawk Is Splitting Records . . . . . . . . . . . . . . . . . . . . . . . 86

v

4.9 Multiple-Line Records . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
4.10 Explicit Input with getline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
4.10.1 Using getline with No Arguments . . . . . . . . . . . . . . . . . . . . . . 89
4.10.2 Using getline into a Variable . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
4.10.3 Using getline from a File. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
4.10.4 Using getline into a Variable from a File . . . . . . . . . . . . . . . 92
4.10.5 Using getline from a Pipe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
4.10.6 Using getline into a Variable from a Pipe . . . . . . . . . . . . . . 93
4.10.7 Using getline from a Coprocess . . . . . . . . . . . . . . . . . . . . . . . . 94
4.10.8 Using getline into a Variable from a Coprocess . . . . . . . . . 94
4.10.9 Points to Remember About getline. . . . . . . . . . . . . . . . . . . . . 94
4.10.10 Summary of getline Variants . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
4.11 Reading Input with a Timeout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
4.12 Retrying Reads After Certain Input Errors . . . . . . . . . . . . . . . . . . . . 97
4.13 Directories on the Command Line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
4.14 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
4.15 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

5 Printing Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
5.1
5.2
5.3
5.4
5.5

The print Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
print Statement Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
Output Separators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
Controlling Numeric Output with print . . . . . . . . . . . . . . . . . . . . . . 102
Using printf Statements for Fancier Printing . . . . . . . . . . . . . . . . 102
5.5.1 Introduction to the printf Statement . . . . . . . . . . . . . . . . . . . . 102
5.5.2 Format-Control Letters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
5.5.3 Modifiers for printf Formats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
5.5.4 Examples Using printf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
5.6 Redirecting Output of print and printf . . . . . . . . . . . . . . . . . . . . . 108
5.7 Special Files for Standard Preopened Data Streams . . . . . . . . . . . 111
5.8 Special File names in gawk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
5.8.1 Accessing Other Open Files with gawk . . . . . . . . . . . . . . . . . . . . 112
5.8.2 Special Files for Network Communications . . . . . . . . . . . . . . . . 112
5.8.3 Special File name Caveats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
5.9 Closing Input and Output Redirections . . . . . . . . . . . . . . . . . . . . . . . 113
5.9.1 Using close()’s Return Value . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
5.10 Speeding Up Pipe Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
5.11 Enabling Nonfatal Output. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
5.12 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
5.13 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

vi GAWK: Effective AWK Programming

6 Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
6.1 Constants, Variables, and Conversions . . . . . . . . . . . . . . . . . . . . . . . . . 119
6.1.1 Constant Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
6.1.1.1 Numeric and String Constants . . . . . . . . . . . . . . . . . . . . . . . 119
6.1.1.2 Octal and Hexadecimal Numbers . . . . . . . . . . . . . . . . . . . . 120
6.1.1.3 Regular Expression Constants . . . . . . . . . . . . . . . . . . . . . . . 121
6.1.2 Using Regular Expression Constants . . . . . . . . . . . . . . . . . . . . . . 121
6.1.2.1 Standard Regular Expression Constants . . . . . . . . . . . . . . 122
6.1.2.2 Strongly Typed Regexp Constants . . . . . . . . . . . . . . . . . . . 123
6.1.3 Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
6.1.3.1 Using Variables in a Program . . . . . . . . . . . . . . . . . . . . . . . . 124
6.1.3.2 Assigning Variables on the Command Line . . . . . . . . . . . 125
6.1.4 Conversion of Strings and Numbers . . . . . . . . . . . . . . . . . . . . . . . 126
6.1.4.1 How awk Converts Between Strings and Numbers . . . . 126
6.1.4.2 Locales Can Influence Conversion . . . . . . . . . . . . . . . . . . . . 127
6.2 Operators: Doing Something with Values . . . . . . . . . . . . . . . . . . . . . . 128
6.2.1 Arithmetic Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
6.2.2 String Concatenation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
6.2.3 Assignment Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
6.2.4 Increment and Decrement Operators . . . . . . . . . . . . . . . . . . . . . . 133
6.3 Truth Values and Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
6.3.1 True and False in awk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
6.3.2 Variable Typing and Comparison Expressions . . . . . . . . . . . . . 135
6.3.2.1 String Type versus Numeric Type . . . . . . . . . . . . . . . . . . . . 135
6.3.2.2 Comparison Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
6.3.2.3 String Comparison Based on Locale Collating Order . 139
6.3.3 Boolean Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
6.3.4 Conditional Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
6.4 Function Calls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
6.5 Operator Precedence (How Operators Nest) . . . . . . . . . . . . . . . . . . . 144
6.6 Where You Are Makes a Difference . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145
6.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145

7 Patterns, Actions, and Variables . . . . . . . . . . . . . 147
7.1 Pattern Elements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
7.1.1 Regular Expressions as Patterns . . . . . . . . . . . . . . . . . . . . . . . . . . 147
7.1.2 Expressions as Patterns. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
7.1.3 Specifying Record Ranges with Patterns . . . . . . . . . . . . . . . . . . 149
7.1.4 The BEGIN and END Special Patterns . . . . . . . . . . . . . . . . . . . . . . 150
7.1.4.1 Startup and Cleanup Actions . . . . . . . . . . . . . . . . . . . . . . . . 150
7.1.4.2 Input/Output from BEGIN and END Rules . . . . . . . . . . . . 151
7.1.5 The BEGINFILE and ENDFILE Special Patterns . . . . . . . . . . . . 151
7.1.6 The Empty Pattern . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
7.2 Using Shell Variables in Programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
7.3 Actions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
7.4 Control Statements in Actions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154

vii

7.4.1 The if-else Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
7.4.2 The while Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
7.4.3 The do-while Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
7.4.4 The for Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
7.4.5 The switch Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157
7.4.6 The break Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
7.4.7 The continue Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
7.4.8 The next Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
7.4.9 The nextfile Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161
7.4.10 The exit Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162
7.5 Predefined Variables. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163
7.5.1 Built-in Variables That Control awk . . . . . . . . . . . . . . . . . . . . . . 163
7.5.2 Built-in Variables That Convey Information. . . . . . . . . . . . . . . 165
7.5.3 Using ARGC and ARGV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173
7.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175

8 Arrays in awk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177
8.1 The Basics of Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177
8.1.1 Introduction to Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177
8.1.2 Referring to an Array Element . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
8.1.3 Assigning Array Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180
8.1.4 Basic Array Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180
8.1.5 Scanning All Elements of an Array . . . . . . . . . . . . . . . . . . . . . . . 181
8.1.6 Using Predefined Array Scanning Orders with gawk . . . . . . . 182
8.2 Using Numbers to Subscript Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . 185
8.3 Using Uninitialized Variables as Subscripts . . . . . . . . . . . . . . . . . . . . 186
8.4 The delete Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186
8.5 Multidimensional Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188
8.5.1 Scanning Multidimensional Arrays . . . . . . . . . . . . . . . . . . . . . . . . 189
8.6 Arrays of Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189
8.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191

9 Functions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193
9.1 Built-in Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193
9.1.1 Calling Built-in Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193
9.1.2 Generating Boolean Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194
9.1.3 Numeric Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194
9.1.4 String-Manipulation Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196
9.1.4.1 More about ‘\’ and ‘&’ with sub(), gsub(), andgensub() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205
9.1.5 Input/Output Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208
9.1.6 Time Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211
9.1.7 Bit-Manipulation Functions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216
9.1.8 Getting Type Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219
9.1.9 String-Translation Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220
9.2 User-Defined Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221

viii GAWK: Effective AWK Programming

9.2.1 Function Definition Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221
9.2.2 Function Definition Examples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223
9.2.3 Calling User-Defined Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . 224
9.2.3.1 Writing a Function Call. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224
9.2.3.2 Controlling Variable Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . 225
9.2.3.3 Passing Function Arguments by Value Or by Reference . 227
9.2.3.4 Other Points About Calling Functions . . . . . . . . . . . . . . . 228
9.2.4 The return Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229
9.2.5 Functions and Their Effects on Variable Typing . . . . . . . . . . . 230
9.3 Indirect Function Calls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231
9.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237

Part II: Problem Solving with awk
10 A Library of awk Functions . . . . . . . . . . . . . . . . . . 241
10.1 Naming Library Function Global Variables . . . . . . . . . . . . . . . . . . . 242
10.2 General Programming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243
10.2.1 Converting Strings to Numbers . . . . . . . . . . . . . . . . . . . . . . . . . 243
10.2.2 Assertions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244
10.2.3 Rounding Numbers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246
10.2.4 The Cliff Random Number Generator . . . . . . . . . . . . . . . . . . 247
10.2.5 Translating Between Characters and Numbers . . . . . . . . . . 247
10.2.6 Merging an Array into a String . . . . . . . . . . . . . . . . . . . . . . . . . 249
10.2.7 Managing the Time of Day . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249
10.2.8 Reading a Whole File at Once . . . . . . . . . . . . . . . . . . . . . . . . . . 251
10.2.9 Quoting Strings to Pass to the Shell . . . . . . . . . . . . . . . . . . . . 252
10.2.10 Checking Whether A Value Is Numeric . . . . . . . . . . . . . . . . . 253
10.2.11 Producing CSV Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253
10.3 Data file Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254
10.3.1 Noting Data file Boundaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254
10.3.2 Rereading the Current File . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256
10.3.3 Checking for Readable Data files . . . . . . . . . . . . . . . . . . . . . . . . 257
10.3.4 Checking for Zero-Length Files . . . . . . . . . . . . . . . . . . . . . . . . . . 258
10.3.5 Treating Assignments as File names . . . . . . . . . . . . . . . . . . . . . 259
10.4 Processing Command-Line Options . . . . . . . . . . . . . . . . . . . . . . . . . . . 259
10.5 Reading the User Database . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266
10.6 Reading the Group Database . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270
10.7 Traversing Arrays of Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274
10.8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276
10.9 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276

ix

11 Practical awk Programs . . . . . . . . . . . . . . . . . . . . . . 279
11.1 Running the Example Programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279
11.2 Reinventing Wheels for Fun and Profit . . . . . . . . . . . . . . . . . . . . . . . 279
11.2.1 Cutting Out Fields and Columns . . . . . . . . . . . . . . . . . . . . . . . . 279
11.2.2 Searching for Regular Expressions in Files . . . . . . . . . . . . . . . 284
11.2.3 Printing Out User Information . . . . . . . . . . . . . . . . . . . . . . . . . . 288
11.2.4 Splitting a Large File into Pieces . . . . . . . . . . . . . . . . . . . . . . . . 293
11.2.5 Duplicating Output into Multiple Files . . . . . . . . . . . . . . . . . . 298
11.2.6 Printing Nonduplicated Lines of Text . . . . . . . . . . . . . . . . . . . . 300
11.2.7 Counting Things . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304
11.2.7.1 Modern Character Sets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304
11.2.7.2 A Brief Introduction To Extensions . . . . . . . . . . . . . . . . . 304
11.2.7.3 Code for wc.awk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305
11.3 A Grab Bag of awk Programs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307
11.3.1 Finding Duplicated Words in a Document . . . . . . . . . . . . . . 307
11.3.2 An Alarm Clock Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 308
11.3.3 Transliterating Characters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311
11.3.4 Printing Mailing Labels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313
11.3.5 Generating Word-Usage Counts . . . . . . . . . . . . . . . . . . . . . . . . 314
11.3.6 Removing Duplicates from Unsorted Text . . . . . . . . . . . . . . 316
11.3.7 Extracting Programs from Texinfo Source Files . . . . . . . . . 317
11.3.8 A Simple Stream Editor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321
11.3.9 An Easy Way to Use Library Functions . . . . . . . . . . . . . . . . . 322
11.3.10 Finding Anagrams from a Dictionary . . . . . . . . . . . . . . . . . . . 328
11.3.11 And Now for Something Completely Different . . . . . . . . . . 330
11.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 330
11.5 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 331

Part III: Moving Beyond Standard awk with gawk
12 Advanced Features of gawk . . . . . . . . . . . . . . . . . . 335
12.1 Allowing Nondecimal Input Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335
12.2 Boolean Typed Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 336
12.3 Controlling Array Traversal and Array Sorting . . . . . . . . . . . . . . . . 336
12.3.1 Controlling Array Traversal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 336
12.3.2 Sorting Array Values and Indices with gawk. . . . . . . . . . . . . . 341
12.4 Two-Way Communications with Another Process . . . . . . . . . . . . . 343
12.5 Using gawk for Network Programming . . . . . . . . . . . . . . . . . . . . . . . . 346
12.6 Profiling Your awk Programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 348
12.7 Preserving Data Between Runs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 352
12.8 Builtin Features versus Extensions. . . . . . . . . . . . . . . . . . . . . . . . . . . . 355
12.9 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355

x

GAWK: Effective AWK Programming

13 Internationalization with gawk . . . . . . . . . . . . . . . 357
13.1 Internationalization and Localization . . . . . . . . . . . . . . . . . . . . . . . . . 357
13.2 GNU gettext . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357
13.3 Internationalizing awk Programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 359
13.4 Translating awk Programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 361
13.4.1 Extracting Marked Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 361
13.4.2 Rearranging printf Arguments . . . . . . . . . . . . . . . . . . . . . . . . . 362
13.4.3 awk Portability Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363
13.5 A Simple Internationalization Example . . . . . . . . . . . . . . . . . . . . . . . 364
13.6 gawk Can Speak Your Language . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 366
13.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 366

14 Debugging awk Programs . . . . . . . . . . . . . . . . . . . . 367
14.1 Introduction to the gawk Debugger . . . . . . . . . . . . . . . . . . . . . . . . . . . 367
14.1.1 Debugging in General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367
14.1.2 Debugging Concepts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367
14.1.3 awk Debugging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 368
14.2 Sample gawk Debugging Session . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369
14.2.1 How to Start the Debugger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369
14.2.2 Finding the Bug . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369
14.3 Main Debugger Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 372
14.3.1 Control of Breakpoints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373
14.3.2 Control of Execution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374
14.3.3 Viewing and Changing Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375
14.3.4 Working with the Stack . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377
14.3.5 Obtaining Information About the Program and the Debugger
State . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377
14.3.6 Miscellaneous Commands. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 379
14.4 Readline Support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 381
14.5 Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 381
14.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382

15 Namespaces in gawk . . . . . . . . . . . . . . . . . . . . . . . . . . 383
15.1 Standard awk’s Single Namespace . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383
15.2 Qualified Names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383
15.3 The Default Namespace . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383
15.4 Changing The Namespace . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 384
15.5 Namespace and Component Naming Rules. . . . . . . . . . . . . . . . . . . . 384
15.6 Internal Name Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 385
15.7 Namespace Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 386
15.8 Namespaces and Other gawk Features . . . . . . . . . . . . . . . . . . . . . . . . 387
15.9 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 388

xi

16 Arithmetic and Arbitrary-Precision
Arithmetic with gawk . . . . . . . . . . . . . . . . . . . . . . . . 389
16.1 A General Description of Computer Arithmetic . . . . . . . . . . . . . . . 389
16.2 Other Stuff to Know . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 390
16.3 Arbitrary-Precision Arithmetic Features in gawk . . . . . . . . . . . . . . 392
16.3.1 Arbitrary Precision Arithmetic is On Parole! . . . . . . . . . . . . . 392
16.3.2 Arbitrary Precision Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 392
16.4 Floating-Point Arithmetic: Caveat Emptor!. . . . . . . . . . . . . . . . . . . 392
16.4.1 Floating-Point Arithmetic Is Not Exact . . . . . . . . . . . . . . . . . . 393
16.4.1.1 Many Numbers Cannot Be Represented Exactly . . . . 393
16.4.1.2 Be Careful Comparing Values . . . . . . . . . . . . . . . . . . . . . . . 393
16.4.1.3 Errors Accumulate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 394
16.4.1.4 Floating Point Values They Didn’t Talk About In
School. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 395
16.4.2 Getting the Accuracy You Need . . . . . . . . . . . . . . . . . . . . . . . . . 395
16.4.3 Try a Few Extra Bits of Precision and Rounding . . . . . . . . . 396
16.4.4 Setting the Precision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 397
16.4.5 Setting the Rounding Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 398
16.5 Arbitrary-Precision Integer Arithmetic with gawk . . . . . . . . . . . . . 399
16.6 How To Check If MPFR Is Available . . . . . . . . . . . . . . . . . . . . . . . . . 400
16.7 Standards Versus Existing Practice . . . . . . . . . . . . . . . . . . . . . . . . . . . 401
16.8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 403

17 Writing Extensions for gawk . . . . . . . . . . . . . . . . . 405
17.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405
17.2 Extension Licensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405
17.3 How It Works at a High Level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405
17.4 API Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 407
17.4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 407
17.4.2 General-Purpose Data Types . . . . . . . . . . . . . . . . . . . . . . . . . . . 410
17.4.3 Memory Allocation Functions and Convenience Macros . 413
17.4.4 Constructor Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 415
17.4.5 Managing MPFR and GMP Values . . . . . . . . . . . . . . . . . . . . . 416
17.4.6 Registration Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 417
17.4.6.1 Registering An Extension Function . . . . . . . . . . . . . . . . . 417
17.4.6.2 Registering An Exit Callback Function . . . . . . . . . . . . . . 419
17.4.6.3 Registering An Extension Version String . . . . . . . . . . . . 419
17.4.6.4 Customized Input Parsers . . . . . . . . . . . . . . . . . . . . . . . . . . 420
17.4.6.5 Customized Output Wrappers . . . . . . . . . . . . . . . . . . . . . . 424
17.4.6.6 Customized Two-way Processors . . . . . . . . . . . . . . . . . . . . 426
17.4.7 Printing Messages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 427
17.4.8 Updating ERRNO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 427
17.4.9 Requesting Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 428
17.4.10 Accessing and Updating Parameters . . . . . . . . . . . . . . . . . . . . 428
17.4.11 Symbol Table Access . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 429

xii GAWK: Effective AWK Programming

17.4.11.1 Variable Access and Update by Name. . . . . . . . . . . . . . 429
17.4.11.2 Variable Access and Update by Cookie . . . . . . . . . . . . 430
17.4.11.3 Creating and Using Cached Values. . . . . . . . . . . . . . . . . 432
17.4.12 Array Manipulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 433
17.4.12.1 Array Data Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 433
17.4.12.2 Array Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 434
17.4.12.3 Working With All The Elements of an Array. . . . . . . 436
17.4.12.4 How To Create and Populate Arrays . . . . . . . . . . . . . . . 439
17.4.13 Accessing and Manipulating Redirections . . . . . . . . . . . . . . . 441
17.4.14 API Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 442
17.4.14.1 API Version Constants and Variables . . . . . . . . . . . . . . 443
17.4.14.2 GMP and MPFR Version Information . . . . . . . . . . . . . 443
17.4.14.3 Informational Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . 444
17.4.15 Boilerplate Code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 444
17.4.16 Changes From Version 1 of the API . . . . . . . . . . . . . . . . . . . . 446
17.5 How gawk Finds Extensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 447
17.6 Example: Some File Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 447
17.6.1 Using chdir() and stat() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 447
17.6.2 C Code for chdir() and stat() . . . . . . . . . . . . . . . . . . . . . . . . . 449
17.6.3 Integrating the Extensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 455
17.7 The Sample Extensions in the gawk Distribution . . . . . . . . . . . . . 457
17.7.1 File-Related Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 457
17.7.2 Interface to fnmatch() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 460
17.7.3 Interface to fork(), wait(), and waitpid() . . . . . . . . . . . . 461
17.7.4 Enabling In-Place File Editing. . . . . . . . . . . . . . . . . . . . . . . . . . 461
17.7.5 Character and Numeric values: ord() and chr() . . . . . . . 463
17.7.6 Reading Directories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 463
17.7.7 Reversing Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 464
17.7.8 Two-Way I/O Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 464
17.7.9 Dumping and Restoring an Array . . . . . . . . . . . . . . . . . . . . . . 465
17.7.10 Reading an Entire File . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 466
17.7.11 Extension Time Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 466
17.7.12 API Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 467
17.8 The gawkextlib Project . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 467
17.9 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 468
17.10 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 469

xiiiPart IV: Appendices
Appendix A The Evolution of the awk
Language . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473
A.1 Major Changes Between V7 and SVR3.1 . . . . . . . . . . . . . . . . . . . . . 473
A.2 Changes Between SVR3.1 and SVR4 . . . . . . . . . . . . . . . . . . . . . . . . . 474
A.3 Changes Between SVR4 and POSIX awk . . . . . . . . . . . . . . . . . . . . . 474
A.4 Extensions in Brian Kernighan’s awk . . . . . . . . . . . . . . . . . . . . . . . . . 475
A.5 Extensions in gawk Not in POSIX awk . . . . . . . . . . . . . . . . . . . . . . . 475
A.6 History of gawk Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 478
A.7 Common Extensions Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 486
A.8 Regexp Ranges and Locales: A Long Sad Story . . . . . . . . . . . . . . 486
A.9 Major Contributors to gawk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 488
A.10 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 490

Appendix B

Installing gawk . . . . . . . . . . . . . . . . . . . . . 491

B.1 The gawk Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 491
B.1.1 Getting the gawk Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . 491
B.1.2 Extracting the Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 491
B.1.3 Contents of the gawk Distribution . . . . . . . . . . . . . . . . . . . . . . . . 492
B.2 Compiling and Installing gawk on Unix-Like Systems . . . . . . . . . . 495
B.2.1 Compiling gawk for Unix-Like Systems . . . . . . . . . . . . . . . . . . . 495
B.2.1.1 Building With MPFR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 496
B.2.2 Shell Startup Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 496
B.2.3 Additional Configuration Options . . . . . . . . . . . . . . . . . . . . . . . . 496
B.2.4 The Configuration Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 497
B.2.5 Compiling from Git . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 498
B.2.6 Building the Documentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 498
B.3 Installation on Other Operating Systems . . . . . . . . . . . . . . . . . . . . . . 499
B.3.1 Installation on MS-Windows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 499
B.3.1.1 Installing a Prepared Distribution for MS-Windows
Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 499
B.3.1.2 Compiling gawk for PC Operating Systems . . . . . . . . . . 499
B.3.1.3 Using gawk on PC Operating Systems . . . . . . . . . . . . . . . 499
B.3.1.4 Using gawk In The Cygwin Environment . . . . . . . . . . . . 501
B.3.1.5 Using gawk In The MSYS Environment . . . . . . . . . . . . . . 501
B.3.2 Compiling and Installing gawk on OpenVMS . . . . . . . . . . . . . 501
B.3.2.1 Compiling gawk on OpenVMS . . . . . . . . . . . . . . . . . . . . . . . 501
B.3.2.2 Compiling gawk Dynamic Extensions on OpenVMS . . 502
B.3.2.3 Installing gawk on OpenVMS . . . . . . . . . . . . . . . . . . . . . . . . 503
B.3.2.4 Running gawk on OpenVMS. . . . . . . . . . . . . . . . . . . . . . . . . 503
B.3.2.5 The OpenVMS GNV Project . . . . . . . . . . . . . . . . . . . . . . . . 505
B.4 Reporting Problems and Bugs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 505
B.4.1 Defining What Is and What Is Not A Bug . . . . . . . . . . . . . . . . 505
B.4.2 Submitting Bug Reports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 506

xiv GAWK: Effective AWK Programming

B.4.3 Please Don’t Post Bug Reports to USENET . . . . . . . . . . . . . . 507
B.4.4 What To Do If You Think There Is A Performance Issue . 508
B.4.5 Where To Send Non-bug Questions . . . . . . . . . . . . . . . . . . . . . . . 509
B.4.6 Reporting Problems with Non-Unix Ports . . . . . . . . . . . . . . . . 509
B.5 Other Freely Available awk Implementations . . . . . . . . . . . . . . . . . . 510
B.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 513

Appendix C

Implementation Notes . . . . . . . . . . . . 515

C.1 Downward Compatibility and Debugging . . . . . . . . . . . . . . . . . . . . . . 515
C.2 Making Additions to gawk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 515
C.2.1 Accessing The gawk Git Repository . . . . . . . . . . . . . . . . . . . . . . 515
C.2.2 Adding New Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 516
C.2.3 Porting gawk to a New Operating System . . . . . . . . . . . . . . . . 518
C.2.4 Why Generated Files Are Kept In Git . . . . . . . . . . . . . . . . . . . . 519
C.3 Probable Future Extensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 521
C.4 Some Limitations of the Implementation . . . . . . . . . . . . . . . . . . . . . . 521
C.5 Extension API Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 522
C.5.1 Problems With The Old Mechanism . . . . . . . . . . . . . . . . . . . . . . 522
C.5.2 Goals For A New Mechanism. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 522
C.5.3 Other Design Decisions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 523
C.5.4 Room For Future Growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 524
C.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 524

Appendix D Basic Programming Concepts . . . 525
D.1 What a Program Does . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 525
D.2 Data Values in a Computer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 526

Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 529
GNU General Public License . . . . . . . . . . . . . . . . . . . . 541
GNU Free Documentation License . . . . . . . . . . . . . . 553
ADDENDUM: How to use this License for your documents . . . . . . . . . 559

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 561

Foreword to the Third Edition 1

Foreword to the Third Edition
Arnold Robbins and I are good friends. We were introduced in 1990 by circumstances—and
our favorite programming language, AWK. The circumstances started a couple of years
earlier. I was working at a new job and noticed an unplugged Unix computer sitting in thecorner. No one knew how to use it, and neither did I. However, a couple of days later, it
was running, and I was root and the one-and-only user. That day, I began the transition
from statistician to Unix programmer.
On one of many trips to the library or bookstore in search of books on Unix, I foundthe gray AWK book, a.k.a. Alfred V. Aho, Brian W. Kernighan, and Peter J. Weinberger’s
The AWK Programming Language (Addison-Wesley, 1988). awk’s simple programming
paradigm—find a pattern in the input and then perform an action—often reduced complex
or tedious data manipulations to a few lines of code. I was excited to try my hand at
programming in AWK.
Alas, the awk on my computer was a limited version of the language described in thegray book. I discovered that my computer had “old awk” and the book described “newawk.” I learned that this was typical; the old version refused to step aside or relinquish its
name. If a system had a new awk, it was invariably called nawk, and few systems had it.
The best way to get a new awk was to ftp the source code for gawk from prep.ai.mit.edu.
gawk was a version of new awk written by David Trueman and Arnold, and available under
the GNU General Public License.
(Incidentally, it’s no longer difficult to find a new awk. gawk ships with GNU/Linux, andyou can download binaries or source code for almost any system; my wife uses gawk on her
VMS box.)
My Unix system started out unplugged from the wall; it certainly was not plugged intoa network. So, oblivious to the existence of gawk and the Unix community in general, anddesiring a new awk, I wrote my own, called mawk. Before I was finished, I knew about gawk,
but it was too late to stop, so I eventually posted to a comp.sources newsgroup.
A few days after my posting, I got a friendly email from Arnold introducing himself. He
suggested we share design and algorithms and attached a draft of the POSIX standard sothat I could update mawk to support language extensions added after publication of The
AWK Programming Language.
Frankly, if our roles had been reversed, I would not have been so open and we probably
would have never met. I’m glad we did meet. He is an AWK expert’s AWK expert and agenuinely nice person. Arnold contributes significant amounts of his expertise and time to
the Free Software Foundation.
This book is the gawk reference manual, but at its core it is a book about AWK programming that will appeal to a wide audience. It is a definitive reference to the AWK language
as defined by the 1987 Bell Laboratories release and codified in the 1992 POSIX Utilities
standard.
On the other hand, the novice AWK programmer can study a wealth of practical programs that emphasize the power of AWK’s basic idioms: data-driven control flow, patternmatching with regular expressions, and associative arrays. Those looking for something
new can try out gawk’s interface to network protocols via special /inet files.

2

GAWK: Effective AWK Programming

The programs in this book make clear that an AWK program is typically much smallerand faster to develop than a counterpart written in C. Consequently, there is often a payoff
to prototyping an algorithm or design in AWK to get it running quickly and expose problems
early. Often, the interpreted performance is adequate and the AWK prototype becomes the
product.
The new pgawk (profiling gawk), produces program execution counts. I recently experimented with an algorithm that for n lines of input, exhibited ∼ Cn2 performance, while
theory predicted ∼ Cn log n behavior. A few minutes poring over the awkprof.out profile pinpointed the problem to a single line of code. pgawk is a welcome addition to myprogrammer’s toolbox.
Arnold has distilled over a decade of experience writing and using AWK programs, anddeveloping gawk, into this book. If you use AWK or want to learn how, then read this book.
Michael Brennan
Author of mawk
March 2001

Foreword to the Fourth Edition

3

Foreword to the Fourth Edition
Some things don’t change. Thirteen years ago I wrote: “If you use AWK or want to learn
how, then read this book.” True then, and still true today.
Learning to use a programming language is about more than mastering the syntax.
One needs to acquire an understanding of how to use the features of the language to solve
practical programming problems. A focus of this book is many examples that show how to
use AWK.
Some things do change. Our computers are much faster and have more memory. Consequently, speed and storage inefficiencies of a high-level language matter less. Prototyping
in AWK and then rewriting in C for performance reasons happens less, because more often
the prototype is fast enough.
Of course, there are computing operations that are best done in C or C++. With gawk
4.1 and later, you do not have to choose between writing your program in AWK or in
C/C++. You can write most of your program in AWK and the aspects that require C/C++
capabilities can be written in C/C++, and then the pieces glued together when the gawk
module loads the C/C++ module as a dynamic plug-in. Chapter 17 [Writing Extensions forgawk], page 405, has all the details, and, as expected, many examples to help you learn theins and outs.
I enjoy programming in AWK and had fun (re)reading this book. I think you will too.
Michael Brennan
Author of mawk
October 2014

Preface 5Preface
Several kinds of tasks occur repeatedly when working with text files. You might want to
extract certain lines and discard the rest. Or you may need to make changes wherevercertain patterns appear, but leave the rest of the file alone. Such jobs are often easy withawk. The awk utility interprets a special-purpose programming language that makes it easy
to handle simple data-reformatting jobs.
The GNU implementation of awk is called gawk; if you invoke it with the proper options
or environment variables, it is fully compatible with the POSIX1 specification of the awk
language and with the Unix version of awk maintained by Brian Kernighan. This meansthat all properly written awk programs should work with gawk. So most of the time, we
don’t distinguish between gawk and other awk implementations.
Using awk you can:• Manage small, personal databases• Generate reports• Validate data
• Produce indexes and perform other document-preparation tasks• Experiment with algorithms that you can adapt later to other computer languages
In addition, gawk provides facilities that make it easy to:• Extract bits and pieces of data for processing• Sort data
• Perform simple network communications• Profile and debug awk programs• Extend the language with functions written in C or C++
This book teaches you about the awk language and how you can use it effectively. You
should already be familiar with basic system commands, such as cat and ls,2 as well asbasic shell facilities, such as input/output (I/O) redirection and pipes.
Implementations of the awk language are available for many different computing environments. This book, while describing the awk language in general, also describes the particular
implementation of awk called gawk (which stands for “GNU awk”). gawk runs on a broad
range of Unix systems, ranging from Intel-architecture PC-based computers up throughlarge-scale systems. gawk has also been ported to macOS, z/OS, Microsoft Windows (allversions), and OpenVMS.3
1

The 2018 POSIX standard is accessible online at https://pubs.opengroup.org/onlinepubs/9699919799/.
These utilities are available on POSIX-compliant systems, as well as on traditional Unix-based systems.
If you are using some other operating system, you still need to be familiar with the ideas of I/O redirection
and pipes.
3
Some other, obsolete systems to which gawk was once ported are no longer supported and the code forthose systems has been removed.
2

6

GAWK: Effective AWK Programming

History of awk and gawk

Recipe for a Programming Language
1 part egrep 1 part snobol
2 parts ed
3 parts C
Blend all parts well using lex and yacc. Document minimally and release.
After eight years, add another part egrep and two more parts C. Document very welland release.
After 35 more years, add Unicode and CSV support, sprinkle lightly with a few choicefeatures from gawk, document very well again, and release.
The name awk comes from the initials of its designers: Alfred V. Aho, Peter J. Weinberger, and Brian W. Kernighan. The original version of awk was written in 1977 at AT&T
Bell Laboratories. In 1985, a new version made the programming language more powerful,
introducing user-defined functions, multiple input streams, and computed regular expressions. This new version became widely available with Unix System V Release 3.1 (1987).
The version in System V Release 4 (1989) added some new features and cleaned up the
behavior in some of the “dark corners” of the language. The specification for awk in the
POSIX Command Language and Utilities standard further clarified the language. Both thegawk designers and the original awk designers at Bell Laboratories provided feedback for
the POSIX specification.
Paul Rubin wrote gawk in 1986. Jay Fenlason completed it, with advice from Richard
Stallman. John Woods contributed parts of the code as well. In 1988 and 1989, David
Trueman, with help from me, thoroughly reworked gawk for compatibility with the newer
awk. Circa 1994, I became the primary maintainer. Current development focuses on bug
fixes, performance improvements, standards compliance, and, occasionally, new features.
In May 1997, Jürgen Kahrs felt the need for network access from awk, and with a little
help from me, set about adding features to do this for gawk. At that time, he also wrote thebulk of TCP/IP Internetworking with gawk (a separate document, available as part of thegawk distribution). His code finally became part of the main gawk distribution with gawk
version 3.1.
John Haque rewrote the gawk internals, in the process providing an awk-level debugger.
This version became available as gawk version 4.0 in 2011.
See Section A.9 [Major Contributors to gawk], page 488, for a full list of those who havemade important contributions to gawk.

A Rose by Any Other Name
The awk language has evolved over the years. Full details are provided in Appendix A [TheEvolution of the awk Language], page 473. The language described in this book is often
referred to as “new awk.” By analogy, the original version of awk is referred to as “old awk.”

Preface 7

On most current systems, when you run the awk utility you get some version of new
awk.4 If your system’s standard awk is the old one, you will see something like this if you
try the following test program:$ awk 1 /dev/null
error awk: syntax error near line 1
error awk: bailing out near line 1
In this case, you should find a version of new awk, or just install gawk!
Throughout this book, whenever we refer to a language feature that should be available
in any complete implementation of POSIX awk, we simply use the term awk. When referring
to a feature that is specific to the GNU implementation, we use the term gawk.

Using This Book
The term awk refers to a particular program as well as to the language you use to tell thisprogram what to do. When we need to be careful, we call the language “the awk language,”and the program “the awk utility.” This book explains both how to write programs in the
awk language and how to run the awk utility. The term “awk program” refers to a program
written by you in the awk programming language.
Primarily, this book explains the features of awk as defined in the POSIX standard. It
does so in the context of the gawk implementation. While doing so, it also attempts todescribe important differences between gawk and other awk implementations.5 Finally, it
notes any gawk features that are not in the POSIX standard for awk.
This book has the difficult task of being both a tutorial and a reference. If you are anovice, feel free to skip over details that seem too complex. You should also ignore themany cross-references; they are for the expert user and for the Info and HTML versions of
the book.
There are sidebars scattered throughout the book. They add a more complete explanation of points that are relevant, but not likely to be of interest on first reading. All appear
in the index, under the heading “sidebar.”
Most of the time, the examples use complete awk programs. Some of the more advanced
sections show only the part of the awk program that illustrates the concept being described.
Although this book is aimed principally at people who have not been exposed to awk,
there is a lot of information here that even the awk expert should find useful. In particular,
the description of POSIX awk and the example programs in Chapter 10 [A Library of awk
Functions], page 241, and in Chapter 11 [Practical awk Programs], page 279, should be ofinterest.
This book is split into several parts, as follows:• Part I describes the awk language and the gawk program in detail. It starts withthe basics, and continues through all of the features of awk. It contains the following
chapters:− Chapter 1 [Getting Started with awk], page 17, provides the essentials you need toknow to begin using awk.
4

Only Solaris systems still use an old awk for the default awk utility. A more modern awk lives in/usr/xpg6/bin on these systems.
5
All such differences appear in the index under the entry “differences in awk and gawk.”

8

GAWK: Effective AWK Programming− Chapter 2 [Running awk and gawk], page 33, describes how to run gawk, themeaning of its command-line options, and how it finds awk program source files.
− Chapter 3 [Regular Expressions], page 51, introduces regular expressions in general,and in particular the flavors supported by POSIX awk and gawk.
− Chapter 4 [Reading Input Files], page 65, describes how awk reads your data. It
introduces the concepts of records and fields, as well as the getline command.
I/O redirection is first described here. Network I/O is also briefly introduced here.
− Chapter 5 [Printing Output], page 99, describes how awk programs can produce
output with print and printf.
− Chapter 6 [Expressions], page 119, describes expressions, which are the basic building blocks for getting most things done in a program.
− Chapter 7 [Patterns, Actions, and Variables], page 147, describes how to write patterns for matching records, actions for doing something when a record is matched,
and the predefined variables awk and gawk use.
− Chapter 8 [Arrays in awk], page 177, covers awk’s one-and-only data structure: theassociative array. Deleting array elements and whole arrays is described, as well
as sorting arrays in gawk. The chapter also describes how gawk provides arrays ofarrays.
− Chapter 9 [Functions], page 193, describes the built-in functions awk and gawk
provide, as well as how to define your own functions. It also discusses how gawk
lets you call functions indirectly.
• Part II shows how to use awk and gawk for problem solving. There is lots of code here
for you to read and learn from. This part contains the following chapters:− Chapter 10 [A Library of awk Functions], page 241, provides a number of functions
meant to be used from main awk programs.
− Chapter 11 [Practical awk Programs], page 279, provides many sample awk programs.
Reading these two chapters allows you to see awk solving real problems.
• Part III focuses on features specific to gawk. It contains the following chapters:− Chapter 12 [Advanced Features of gawk], page 335, describes a number of advanced
features. Of particular note are the abilities to control the order of array traversal,
have two-way communications with another process, perform TCP/IP networking,and profile your awk programs.
− Chapter 13 [Internationalization with gawk], page 357, describes special features
for translating program messages into different languages at runtime.
− Chapter 14 [Debugging awk Programs], page 367, describes the gawk debugger.
− Chapter 15 [Namespaces in gawk], page 383, describes how gawk allows variables
and/or functions of the same name to be in different namespaces.
− Chapter 16 [Arithmetic and Arbitrary-Precision Arithmetic with gawk], page 389,
describes advanced arithmetic facilities.
− Chapter 17 [Writing Extensions for gawk], page 405, describes how to add newvariables and functions to gawk by writing extensions in C or C++.

Preface 9• Part IV provides the appendices, the Glossary, and two licenses that cover the gawk
source code and this book, respectively. It contains the following appendices:− Appendix A [The Evolution of the awk Language], page 473, describes how the
awk language has evolved since its first release to the present. It also describes
how gawk has acquired features over time.
− Appendix B [Installing gawk], page 491, describes how to get gawk, how to compile
it on POSIX-compatible systems, and how to compile and use it on different nonPOSIX systems. It also describes how to report bugs in gawk and where to getother freely available awk implementations.
− Appendix C [Implementation Notes], page 515, describes how to disable gawk’s
extensions, as well as how to contribute new code to gawk, and some possible
future directions for gawk development.
− Appendix D [Basic Programming Concepts], page 525, provides some very cursory background material for those who are completely unfamiliar with computer
programming.
− The [Glossary], page 529, defines most, if not all, of the significant terms usedthroughout the book. If you find terms that you aren’t familiar with, try looking
them up here.
− [GNU General Public License], page 541, and [GNU Free Documentation License],
page 553, present the licenses that cover the gawk source code and this book,
respectively.

Typographical Conventions
This book is written in Texinfo, the GNU documentation formatting language. A single
Texinfo source file is used to produce both the printed and online versions of the documentation. Because of this, the typographical conventions are slightly different than in otherbooks you may have read.
Examples you would type at the command line are preceded by the common shell primaryand secondary prompts, ‘$’ and ‘>’, respectively. Input that you type is shown like this.
Output from the command is preceded by the glyph “ a ”. This typically represents thecommand’s standard output. Error messages and other output on the command’s standard
error are preceded by the glyph “ error ”. For example:$ echo hi on stdout
a hi on stdout$ echo hello on stderr 1>&2
error hello on stderr
In the text, almost anything related to programming, such as command names, variable
and function names, and string, numeric and regexp constants appear in this font. Codefragments appear in the same font and quoted, ‘like this’. Things that are replaced by theuser or programmer appear in this font. Options look like this: -f. File names are indicatedlike this: /path/to/ourfile. Some things are emphasized like this, and if a point needs
to be made strongly, it is done like this. The first occurrence of a new term is usually its
definition and appears in the same font as the previous occurrence of “definition” in thissentence.

10 GAWK: Effective AWK Programming

Characters that you type at the keyboard look like this. In particular, there are special
characters called “control characters.” These are characters that you type by holding down
both the CONTROL key and another key, at the same time. For example, a Ctrl-d is typed
by first pressing and holding the CONTROL key, next pressing the d key, and finally releasing
both keys.
For the sake of brevity, throughout this book, we refer to Brian Kernighan’s versionof awk as “BWK awk.” (See Section B.5 [Other Freely Available awk Implementations],
page 510, for information on his and other versions.)

Dark Corners
Dark corners are basically fractal—no matter how much you illuminate, there’s
always a smaller but darker one.
—Brian Kernighan
Until the POSIX standard (and GAWK: Effective AWK Programming), many features of
awk were either poorly documented or not documented at all. Descriptions of such features(often called “dark corners”) are noted in this book with the picture of a flashlight in themargin, as shown here. They also appear in the index under the heading “dark corner.”
But, as noted by the opening quote, any coverage of dark corners is by definition incomplete.
Extensions to the standard awk language that are supported by more than one awk
implementation are marked “(c.e.),” and listed in the index under “common extensions”
and “extensions, common.”

The GNU Project and This Book
The Free Software Foundation (FSF) is a nonprofit organization dedicated to the production
and distribution of freely distributable software. It was founded by Richard M. Stallman,
the author of the original Emacs editor. GNU Emacs is the most widely used version of
Emacs today.
The GNU6 Project is an ongoing effort on the part of the Free Software Foundation
to create a complete, freely distributable, POSIX-compliant computing environment. The
FSF uses the GNU General Public License (GPL) to ensure that its software’s source codeis always available to the end user. A copy of the GPL is included in this book for yourreference (see [GNU General Public License], page 541). The GPL applies to the C language
source code for gawk. To find out more about the FSF and the GNU Project online, see
the GNU Project’s home page. This book may also be read from GNU’s website.
A shell, an editor (Emacs), highly portable optimizing C, C++, and Objective-C compilers, a symbolic debugger and dozens of large and small utilities (such as gawk), have all
been completed and are freely available. The GNU operating system kernel (the HURD),
has been released but remains in an early stage of development.
Until the GNU operating system is more fully developed, you should consider using
GNU/Linux, a freely distributable, Unix-like operating system for Intel, Power Architecture,6

GNU stands for “GNU’s Not Unix.”Preface 11

Sun SPARC, IBM S/390, and other systems.7 Many GNU/Linux distributions are available
for download from the Internet.
The book you are reading is actually free—at least, the information in it is free to anyone.
The machine-readable source code for the book comes with gawk. (Take a moment to check
the Free Documentation License in [GNU Free Documentation License], page 553.)
The book itself has gone through multiple previous editions. Paul Rubin wrote the very
first draft of The GAWK Manual; it was around 40 pages long. Diane Close and Richard
Stallman improved it, yielding a version that was around 90 pages and barely described theoriginal, “old” version of awk.
I started working with that version in the fall of 1988. As work on it progressed, the FSF
published several preliminary versions (numbered 0.x). In 1996, edition 1.0 was released
with gawk 3.0.0. The FSF published the first two editions under the title The GNU Awk
User’s Guide.
This edition maintains the basic structure of the previous editions. For FSF edition 4.0,
the content was thoroughly reviewed and updated. All references to gawk versions prior
to 4.0 were removed. Of significant note for that edition was the addition of Chapter 14[Debugging awk Programs], page 367.
For FSF edition 5.0, the content has been reorganized into parts, and the major new additions are Chapter 16 [Arithmetic and Arbitrary-Precision Arithmetic with gawk], page 389,
and Chapter 17 [Writing Extensions for gawk], page 405.
This book will undoubtedly continue to evolve. If you find an error in the book, please
report it! See Section B.4 [Reporting Problems and Bugs], page 505, for information on
submitting problem reports electronically.

How to Contribute
As the maintainer of GNU awk, I once thought that I would be able to manage a collection of
publicly available awk programs and I even solicited contributions. Making things available
on the Internet helps keep the gawk distribution down to manageable size.
The initial collection of material, such as it is, is still available at ftp://ftp.
freefriends.org/arnold/Awkstuff.
In the hopes of doing something broader, I acquired the awklang.org domain. Late in2017, a volunteer took on the task of managing it.
If you have written an interesting awk program that you would like to share with therest of the world, please see http://www.awklang.org and use the “Contact” link.
If you have written a gawk extension, please see Section 17.8 [The gawkextlib Project],
page 467.

Acknowledgments
The initial draft of The GAWK Manual had the following acknowledgments:
Many people need to be thanked for their assistance in producing this manual.
Jay Fenlason contributed many ideas and sample programs. Richard Mlynarik
7

The terminology “GNU/Linux” is explained in the [Glossary], page 529.

12 GAWK: Effective AWK Programmingand Robert Chassell gave helpful comments on drafts of this manual. The
paper A Supplemental Document for AWK by John W. Pierce of the Chemistry
Department at UC San Diego, pinpointed several issues relevant both to awk
implementation and to this manual, that would otherwise have escaped us.
I would like to acknowledge Richard M. Stallman, for his vision of a better world andfor his courage in founding the FSF and starting the GNU Project.
Earlier editions of this book had the following acknowledgements:
The following people (in alphabetical order) provided helpful comments on various versions of this book: Rick Adams, Dr. Nelson H.F. Beebe, Karl Berry,Dr. Michael Brennan, Rich Burridge, Claire Cloutier, Diane Close, Scott Deifik, Christopher (“Topher”) Eliot, Jeffrey Friedl, Dr. Darrel Hankerson, Michal
Jaegermann, Dr. Richard J. LeBlanc, Michael Lijewski, Pat Rankin, Miriam
Robbins, Mary Sheehan, and Chuck Toporek.
Robert J. Chassell provided much valuable advice on the use of Texinfo. He
also deserves special thanks for convincing me not to title this book How to
Gawk Politely. Karl Berry helped significantly with the TEX part of Texinfo.
I would like to thank Marshall and Elaine Hartholz of Seattle and Dr. Bert
and Rita Schreiber of Detroit for large amounts of quiet vacation time in theirhomes, which allowed me to make significant progress on this book and on gawk
itself.
Phil Hughes of SSC contributed in a very important way by loaning me his
laptop GNU/Linux system, not once, but twice, which allowed me to do a lotof work while away from home.
David Trueman deserves special credit; he has done a yeoman job of evolving
gawk so that it performs well and without bugs. Although he is no longer
involved with gawk, working with him on this project was a significant pleasure.
The intrepid members of the GNITS mailing list, and most notably Ulrich
Drepper, provided invaluable help and feedback for the design of the internationalization features.
Chuck Toporek, Mary Sheehan, and Claire Cloutier of O’Reilly & Associates
contributed significant editorial help for this book for the 3.1 release of gawk.
Dr. Nelson Beebe, Andreas Buening, Dr. Manuel Collado, Antonio Colombo, Stephen
Davies, Scott Deifik, Akim Demaille, Daniel Richard G., Juan Manuel Guerrero, Darrel
Hankerson, Michal Jaegermann, Jürgen Kahrs, Stepan Kasal, John Malmberg, Chet Ramey,
Pat Rankin, Andrew Schorr, Corinna Vinschen, and Eli Zaretskii (in alphabetical order)
make up the current gawk “crack portability team.” Without their hard work and help,
gawk would not be nearly the robust, portable program it is today. It has been and continues
to be a pleasure working with this team of fine people.
Notable code and documentation contributions were made by a number of people. See
Section A.9 [Major Contributors to gawk], page 488, for the full list.
Thanks to Michael Brennan for the Forewords.
Thanks to Patrice Dumas for the new makeinfo program. Thanks to Karl Berry for his
past work on Texinfo, and to Gavin Smith, who continues to work to improve the Texinfo
markup language.

Preface 13

Robert P.J. Day, Michael Brennan, and Brian Kernighan kindly acted as reviewers forthe 2015 edition of this book. Their feedback helped improve the final work.
I would also like to thank Brian Kernighan for his invaluable assistance during the testingand debugging of gawk, and for his ongoing help and advice in clarifying numerous points
about the language. We could not have done nearly as good a job on either gawk or its
documentation without his help.
Brian is in a class by himself as a programmer and technical author. I have to thank him(yet again) for his ongoing friendship and for being a role model to me for over 30 years!
Having him as a reviewer is an exciting privilege. It has also been extremely humbling. . .
I must thank my wonderful wife, Miriam, for her patience through the many versions of
this project, for her proofreading, and for sharing me with the computer. I would like tothank my parents for their love, and for the grace with which they raised and educated me.
Finally, I also must acknowledge my gratitude to G-d, for the many opportunities He has
sent my way, as well as for the gifts He has given me with which to take advantage of thoseopportunities.

Arnold Robbins
Nof Ayalon
IsraelMarch, 2020

Part I:
The awk Language

Chapter 1: Getting Started with awk

17

1 Getting Started with awk
The basic function of awk is to search files for lines (or other units of text) that containcertain patterns. When a line matches one of the patterns, awk performs specified actions
on that line. awk continues to process input lines in this way until it reaches the end of the
input files.
Programs in awk are different from programs in most other languages, because awk
programs are data driven (i.e., you describe the data you want to work with and then what
to do when you find it). Most other languages are procedural; you have to describe, in great
detail, every step the program should take. When working with procedural languages, it isusually much harder to clearly describe the data your program will process. For this reason,
awk programs are often refreshingly easy to read and write.
When you run awk, you specify an awk program that tells awk what to do. The program
consists of a series of rules (it may also contain function definitions, an advanced featurethat we will ignore for now; see Section 9.2 [User-Defined Functions], page 221). Each rule
specifies one pattern to search for and one action to perform upon finding the pattern.
Syntactically, a rule consists of a pattern followed by an action. The action is enclosed
in braces to separate it from the pattern. Newlines usually separate rules. Therefore, an
awk program looks like this:pattern { action }pattern { action }...

1.1 How to Run awk Programs
There are several ways to run an awk program. If the program is short, it is easiest to
include it in the command that runs awk, like this:awk 'program' input-file1 input-file2 ...
When the program is long, it is usually more convenient to put it in a file and run itwith a command like this:awk -f program-file input-file1 input-file2 ...
This section discusses both mechanisms, along with several variations of each.

1.1.1 One-Shot Throwaway awk ProgramsOnce you are familiar with awk, you will often type in simple programs the moment you want
to use them. Then you can write the program as the first argument of the awk command,
like this:awk 'program' input-file1 input-file2 ...
where program consists of a series of patterns and actions, as described earlier.
This command format instructs the shell, or command interpreter, to start awk and use
the program to process records in the input file(s). There are single quotes around program
so the shell won’t interpret any awk characters as special shell characters. The quotes also
cause the shell to treat all of program as a single argument for awk, and allow program tobe more than one line long.

18 GAWK: Effective AWK Programming

This format is also useful for running short or medium-sized awk programs from shell
scripts, because it avoids the need for a separate file for the awk program. A self-contained
shell script is more reliable because there are no other files to misplace.
Later in this chapter, in Section 1.3 [Some Simple Examples], page 24, we’ll see examplesof several short, self-contained programs.

1.1.2 Running awk Without Input Files
You can also run awk without any input files. If you type the following command line:
awk 'program'awk applies the program to the standard input, which usually means whatever you typeon the keyboard. This continues until you indicate end-of-file by typing Ctrl-d. (Onnon-POSIX operating systems, the end-of-file character may be different.)
As an example, the following program prints a friendly piece of advice (from Douglas
Adams’s The Hitchhiker’s Guide to the Galaxy), to keep you from worrying about thecomplexities of computer programming:$ awk 'BEGIN { print "Don\47t Panic!" }'a Don't Panic!
awk executes statements associated with BEGIN before reading any input. If there areno other statements in your program, as is the case here, awk just stops, instead of trying
to read input it doesn’t know how to process. The ‘\47’ is a magic way (explained later)of getting a single quote into the program, without having to engage in ugly shell quoting
tricks.
NOTE: If you use Bash as your shell, you should execute the command ‘set +H’before running this program interactively, to disable the C shell-style command
history, which treats ‘!’ as a special character. We recommend putting thiscommand into your personal startup file.
This next simple awk program emulates the cat utility; it copies whatever you type on
the keyboard to its standard output (why this works is explained shortly):$ awk '{ print }'Now is the time for all good men
a Now is the time for all good men
to come to the aid of their country.
a to come to the aid of their country.
Four score and seven years ago, ...
a Four score and seven years ago, ...
What, me worry?
a What, me worry?
Ctrl-d

1.1.3 Running Long Programs
Sometimes awk programs are very long. In these cases, it is more convenient to put the
program into a separate file. In order to tell awk to use that file for its program, you type:awk -f source-file input-file1 input-file2 ...

Chapter 1: Getting Started with awk

19

The -f instructs the awk utility to get the awk program from the file source-file (seeSection 2.2 [Command-Line Options], page 33). Any file name can be used for source-file.
For example, you could put the program:
BEGIN { print "Don't Panic!" }into the file advice. Then this command:awk -f advice
does the same thing as this one:awk 'BEGIN { print "Don\47t Panic!" }'This was explained earlier (see Section 1.1.2 [Running awk Without Input Files], page 18).
Note that you don’t usually need single quotes around the file name that you specify with-f, because most file names don’t contain any of the shell’s special characters. Notice that inadvice, the awk program did not have single quotes around it. The quotes are only needed
for programs that are provided on the awk command line. (Also, placing the program in afile allows us to use a literal single quote in the program text, instead of the magic ‘\47’.)
If you want to clearly identify an awk program file as such, you can add the extension.awk to the file name. This doesn’t affect the execution of the awk program but it does
make “housekeeping” easier.

1.1.4 Executable awk ProgramsOnce you have learned awk, you may want to write self-contained awk scripts, using the ‘#!’script mechanism. You can do this on many systems.1 For example, you could update thefile advice to look like this:#! /bin/awk -f
BEGIN { print "Don't Panic!" }After making this file executable (with the chmod utility), simply type ‘advice’ at the shelland the system arranges to run awk as if you had typed ‘awk -f advice’:$ chmod +x advice$ ./advice
a Don't Panic!
Self-contained awk scripts are useful when you want to write a program that users can
invoke without their having to know that the program is written in awk.

1

The ‘#!’ mechanism works on GNU/Linux systems, BSD-based systems, and commercial Unix systems.

20 GAWK: Effective AWK Programming

Understanding ‘#!’awk is an interpreted language. This means that the awk utility reads your program andthen processes your data according to the instructions in your program. (This is different
from a compiled language such as C, where your program is first compiled into machinecode that is executed directly by your system’s processor.) The awk utility is thus termed
an interpreter. Many modern languages are interpreted.
The line beginning with ‘#!’ lists the full file name of an interpreter to run and asingle optional initial command-line argument to pass to that interpreter. The operating
system then runs the interpreter with the given argument and the full argument list of theexecuted program. The first argument in the list is the full file name of the awk program.
The rest of the argument list contains either options to awk, or data files, or both. (Notethat on many systems awk is found in /usr/bin instead of in /bin.)
Some systems limit the length of the interpreter name to 32 characters. Often, thiscan be dealt with by using a symbolic link.
You should not put more than one argument on the ‘#!’ line after the path to awk. It
does not work. The operating system treats the rest of the line as a single argument andpasses it to awk. Doing this leads to confusing behavior—most likely a usage diagnostic
of some sort from awk.
Finally, the value of ARGV[0] (see Section 7.5 [Predefined Variables], page 163) varies
depending upon your operating system. Some systems put ‘awk’ there, some put the fullpathname of awk (such as /bin/awk), and some put the name of your script (‘advice’).
Don’t rely on the value of ARGV[0] to provide your script name.

1.1.5 Comments in awk Programs
A comment is some text that is included in a program for the sake of human readers; it
is not really an executable part of the program. Comments can explain what the program
does and how it works. Nearly all programming languages have provisions for comments,
as programs are typically hard to understand without them.
In the awk language, a comment starts with the number sign character (‘#’) and continues
to the end of the line. The ‘#’ does not have to be the first character on the line. The awk
language ignores the rest of a line following a number sign. For example, we could have put
the following into advice:# This program prints a nice, friendly message. It helps# keep novice users from being afraid of the computer.
BEGIN{ print "Don't Panic!" }You can put comment lines into keyboard-composed throwaway awk programs, but thisusually isn’t very useful; the purpose of a comment is to help you or another person understand the program when reading it at a later time.
CAUTION: As mentioned in Section 1.1.1 [One-Shot Throwaway awk Programs], page 17, you can enclose short to medium-sized programs in single
quotes, in order to keep your shell scripts self-contained. When doing so, don’t
put an apostrophe (i.e., a single quote) into a comment (or anywhere else inyour program). The shell interprets the quote as the closing quote for the en-Chapter 1: Getting Started with awk

21tire program. As a result, usually the shell prints a message about mismatched
quotes, and if awk actually runs, it will probably print strange messages about
syntax errors. For example, look at the following:$ awk 'BEGIN { print "hello" } # let's be cute'>The shell sees that the first two quotes match, and that a new quoted object
begins at the end of the command line. It therefore prompts with the secondary
prompt, waiting for more input. With Unix awk, closing the quoted string
produces this result:$ awk '{ print "hello" } # let's be cute'> 'error awk: can't open file besource line number 1
error
Putting a backslash before the single quote in ‘let's’ wouldn’t help, because
backslashes are not special inside single quotes. The next subsection describes
the shell’s quoting rules.

1.1.6 Shell Quoting Issues
For short to medium-length awk programs, it is most convenient to enter the program on
the awk command line. This is best done by enclosing the entire program in single quotes.
This is true whether you are entering the program interactively at the shell prompt, or
writing it as part of a larger shell script:
awk 'program text' input-file1 input-file2 ...
Once you are working with the shell, it is helpful to have a basic knowledge of shellquoting rules. The following rules apply only to POSIX-compliant, Bourne-style shells(such as Bash, the GNU Bourne-Again Shell). If you use the C shell, you’re on your own.
Before diving into the rules, we introduce a concept that appears throughout this book,which is that of the null, or empty, string.
The null string is character data that has no value. In other words, it is empty. It
is written in awk programs like this: "". In the shell, it can be written using single or
double quotes: "" or ''. Although the null string has no characters in it, it does exist. For
example, consider this command:$ echo ""Here, the echo utility receives a single argument, even though that argument has no characters in it. In the rest of this book, we use the terms null string and empty string interchangeably. Now, on to the quoting rules:• Quoted items can be concatenated with nonquoted items as well as with other quoted
items. The shell turns everything into one argument for the command.
• Preceding any single character with a backslash (‘\’) quotes that character. The shell
removes the backslash and passes the quoted character on to the command.
• Single quotes protect everything between the opening and closing quotes. The shell
does no interpretation of the quoted text, passing it on verbatim to the command. It is
impossible to embed a single quote inside single-quoted text. Refer back to Section 1.1.5[Comments in awk Programs], page 20, for an example of what happens if you try.

22 GAWK: Effective AWK Programming• Double quotes protect most things between the opening and closing quotes. The shell
does at least variable and command substitution on the quoted text. Different shells
may do additional kinds of processing on double-quoted text.
Because certain characters within double-quoted text are processed by the shell, theymust be escaped within the text. Of note are the characters ‘$’, ‘`’, ‘\’, and ‘"’, allof which must be preceded by a backslash within double-quoted text if they are to bepassed on literally to the program. (The leading backslash is stripped first.) Thus, theexample seen previously in Section 1.1.2 [Running awk Without Input Files], page 18:
awk 'BEGIN { print "Don\47t Panic!" }'could instead be written this way:
$ awk "BEGIN { print \"Don't Panic!\" }"a Don't Panic!
Note that the single quote is not special within double quotes.
• Null strings are removed when they occur as part of a non-null command-line argument,
while explicit null objects are kept. For example, to specify that the field separator FS
should be set to the null string, use:
awk -F "" 'program' files # correct
Don’t use this:awk -F"" 'program' files # wrong!
In the second case, awk attempts to use the text of the program as the value of FS, andthe first file name as the text of the program! This results in syntax errors at best, andconfusing behavior at worst.
Mixing single and double quotes is difficult. You have to resort to shell quoting tricks,
like this:$ awk 'BEGIN { print "Here is a single quote <'"'"'>" }'a Here is a single quote <'>This program consists of three concatenated quoted strings. The first and the third are
single-quoted, and the second is double-quoted.
This can be “simplified” to:$ awk 'BEGIN { print "Here is a single quote <'\''>" }'a Here is a single quote <'>Judge for yourself which of these two is the more readable.
Another option is to use double quotes, escaping the embedded, awk-level double quotes:$ awk "BEGIN { print \"Here is a single quote <'>\" }"a Here is a single quote <'>This option is also painful, because double quotes, backslashes, and dollar signs are very
common in more advanced awk programs.
A third option is to use the octal escape sequence equivalents (see Section 3.2 [EscapeSequences], page 52) for the single- and double-quote characters, like so:$ awk 'BEGIN { print "Here is a single quote <\47>" }'a Here is a single quote <'>$ awk 'BEGIN { print "Here is a double quote <\42>" }'a Here is a double quote <">Chapter 1: Getting Started with awk

23This works nicely, but you should comment clearly what the escape sequences mean.
A fourth option is to use command-line variable assignment, like this:$ awk -v sq="'" 'BEGIN { print "Here is a single quote <" sq ">" }'a Here is a single quote <'>(Here, the two string constants and the value of sq are concatenated into a single stringthat is printed by print.)
If you really need both single and double quotes in your awk program, it is probably best
to move it into a separate file, where the shell won’t be part of the picture and you can say
what you mean.

1.1.6.1 Quoting in MS-Windows Batch Files
Although this book generally only worries about POSIX systems and the POSIX shell, thefollowing issue arises often enough for many users that it is worth addressing.
The “shells” on Microsoft Windows systems use the double-quote character for quoting, and make it difficult or impossible to include an escaped double-quote character in
a command-line script. The following example, courtesy of Jeroen Brink, shows how toescape the double quotes from this one liner script that prints all lines in a file surroundedby double quotes:{ print "\"" $0 "\"" }In an MS-Windows command-line the one-liner script above may be passed as follows:
gawk "{ print \"\042\" $0 \"\042\" }" fileIn this example the ‘\042’ is the octal code for a double-quote; gawk converts it into areal double-quote for output by the print statement.
In MS-Windows escaping double-quotes is a little tricky because you use backslashes toescape double-quotes, but backslashes themselves are not escaped in the usual way; indeed
they are either duplicated or not, depending upon whether there is a subsequent doublequote. The MS-Windows rule for double-quoting a string is the following:
1. For each double quote in the original string, let N be the number of backslash(es)before it, N might be zero. Replace these N backslash(es) by 2 × N + 1 backslash(es)2. Let N be the number of backslash(es) tailing the original string, N might be zero.
Replace these N backslash(es) by 2 × N backslash(es)3. Surround the resulting string by double-quotes.
So to double-quote the one-liner script ‘{ print "\"" $0 "\"" }’ from the previous example you would do it this way:
gawk "{ print \"\\\"\" $0 \"\\\"\" }" file
However, the use of ‘\042’ instead of ‘\\\"’ is also possible and easier to read, because
backslashes that are not followed by a double-quote don’t need duplication.

1.2 Data files for the Examples
Many of the examples in this book take their input from two sample data files. The first,
mail-list, represents a list of peoples’ names together with their email addresses andinformation about those people. The second data file, called inventory-shipped, contains
information about monthly shipments. In both files, each line is considered to be one record.

24 GAWK: Effective AWK Programming

In mail-list, each record contains the name of a person, his/her phone number, his/heremail address, and a code for his/her relationship with the author of the list. The columns
are aligned using spaces. An ‘A’ in the last column means that the person is an acquaintance.
An ‘F’ in the last column means that the person is a friend. An ‘R’ means that the person
is a relative:
AmeliaAnthony
Becky
Bill
Broderick
CamillaFabiusJulie
MartinSamuelJean-Paul

555-5553
555-3412
555-7685555-1675555-0542
555-2912
555-1234555-6699
555-6480
555-3430555-2127

amelia.zodiacusque@gmail.com
F
anthony.asserturo@hotmail.com
A
becky.algebrarum@gmail.com
Abill.drowning@hotmail.com
A
broderick.aliquotiens@yahoo.com R
camilla.infusarum@skynet.beRfabius.undevicesimus@ucb.edu
Fjulie.perscrutabor@skeeve.com
Fmartin.codicibus@hotmail.com
Asamuel.lanceolis@shu.edu
A
jeanpaul.campanorum@nyu.edu
R

The data file inventory-shipped represents information about shipments during theyear. Each record contains the month, the number of green crates shipped, the number of
red boxes shipped, the number of orange bags shipped, and the number of blue packages
shipped, respectively. There are 16 entries, covering the 12 months of last year and the first
four months of the current year. An empty line separates the data for the two years:
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec

13
15
15
31
16
31
24
15
13
29
20
17

25
32
24
52
34
42
34
34
55
54
87
35

15 11524 22634 22863 42029 20875 49267 43647 31637 27768 52582 57761 401

Jan
Feb
Mar
Apr

21
26
24
21

36
58
75
70

64 62080 65270 49574 514The sample files are included in the gawk distribution, in the directory awklib/eg/data.

1.3 Some Simple Examples
The following command runs a simple awk program that searches the input file mail-list
for the character string ‘li’ (a grouping of characters is usually called a string; the term
string is based on similar usage in English, such as “a string of pearls” or “a string of cars
in a train”):

Chapter 1: Getting Started with awk

25

awk '/li/ { print $0 }' mail-list
When lines containing ‘li’ are found, they are printed because ‘print $0’ means print thecurrent line. (Just ‘print’ by itself means the same thing, so we could have written thatinstead.)You will notice that slashes (‘/’) surround the string ‘li’ in the awk program. The slashes
indicate that ‘li’ is the pattern to search for. This type of pattern is called a regularexpression, which is covered in more detail later (see Chapter 3 [Regular Expressions],
page 51). The pattern is allowed to match parts of words. There are single quotes aroundthe awk program so that the shell won’t interpret any of it as special shell characters.
Here is what this program prints:$ awk '/li/ { print $0 }' mail-list555-5553
amelia.zodiacusque@gmail.com
F
a Amelia
555-0542
broderick.aliquotiens@yahoo.com R
a Broderick
555-6699
julie.perscrutabor@skeeve.com
F
a Julie
Samuel
555-3430
samuel.lanceolis@shu.edu
Aa
In an awk rule, either the pattern or the action can be omitted, but not both. If thepattern is omitted, then the action is performed for every input line. If the action is omitted,
the default action is to print all lines that match the pattern.
Thus, we could leave out the action (the print statement and the braces) in the previous
example and the result would be the same: awk prints all lines matching the pattern ‘li’.
By comparison, omitting the print statement but retaining the braces makes an empty
action that does nothing (i.e., no lines are printed).
Many practical awk programs are just a line or two long. Following is a collection of
useful, short programs to get you started. Some of these programs contain constructs thathaven’t been covered yet. (The description of the program will give you a good idea of what
is going on, but you’ll need to read the rest of the book to become an awk expert!) Mostof the examples use a data file named data. This is just a placeholder; if you use these
programs yourself, substitute your own file names for data.
Some of the following examples use the output of ‘ls -l’ as input. ls is a systemcommand that gives you a listing of the files in a directory. With the -l option, this listing
includes each file’s size and the date the file was last modified. Its output looks like this:-rw-r--r--rw-r--r--rw-r--r--rw-r--r--rw-r--r--rw-r--r--rw-r--r--rw-r--r--1 arnold
1 arnold
1 arnold
1 arnold
1 arnold
1 arnold
1 arnold
1 arnold

user
user
user
user
user
user
user
user

1933 Nov 7 13:05 Makefile10809 Nov 7 13:03 awk.h
983 Apr 13 12:14 awk.tab.h31869 Jun 15 12:20 awkgram.y22414 Nov 7 13:03 awk1.c
37455 Nov 7 13:03 awk2.c
27511 Dec 9 13:07 awk3.c
7989 Nov 7 13:03 awk4.c

The first field contains read-write permissions, the second field contains the number of links
to the file, and the third field identifies the file’s owner. The fourth field identifies the file’s
group. The fifth field contains the file’s size in bytes. The sixth, seventh, and eighth fields
contain the month, day, and time, respectively, that the file was last modified. Finally, theninth field contains the file name.

26 GAWK: Effective AWK Programming

For future reference, note that there is often more than one way to do things in awk. At
some point, you may want to look back at these examples and see if you can come up withdifferent ways to do the same things shown here:• Print every line that is longer than 80 characters:
awk 'length($0) > 80' data
The sole rule has a relational expression as its pattern and has no action—so it uses
the default action, printing the record.
• Print the length of the longest input line:
awk '{ if (length($0) > max) max = length($0) }END { print max }' data
The code associated with END executes after all input has been read; it’s the other sideof the coin to BEGIN.
• Print the length of the longest line in data:
expand data | awk '{ if (x < length($0)) x = length($0) }END { print "maximum line length is " x }'This example differs slightly from the previous one: the input is processed by the
expand utility to change TABs into spaces, so the widths compared are actually theright-margin columns, as opposed to the number of input characters on each line.
• Print every line that has at least one field:
awk 'NF > 0' data
This is an easy way to delete blank lines from a file (or rather, to create a new filesimilar to the old file but from which the blank lines have been removed).
• Print seven random numbers from 0 to 100, inclusive:
awk 'BEGIN { for (i = 1; i <= 7; i++)print int(101 * rand()) }'• Print the total number of bytes used by files:
ls -l files | awk '{ x += $5 }END { print "total bytes: " x }'• Print the total number of kilobytes used by files:
ls -l files | awk '{ x += $5 }END { print "total K-bytes:", x / 1024 }'• Print a sorted list of the login names of all users:
awk -F: '{ print $1 }' /etc/passwd | sort
• Count the lines in a file:
awk 'END { print NR }' data
• Print the even-numbered lines in the data file:
awk 'NR % 2 == 0' data
If you used the expression ‘NR % 2 == 1’ instead, the program would print the oddnumbered lines.

Chapter 1: Getting Started with awk

27

1.4 An Example with Two Rules
The awk utility reads the input files one line at a time. For each line, awk tries the patternsof each rule. If several patterns match, then several actions execute in the order in which
they appear in the awk program. If no patterns match, then no actions run.
After processing all the rules that match the line (and perhaps there are none), awk
reads the next line. (However, see Section 7.4.8 [The next Statement], page 160, and also
see Section 7.4.9 [The nextfile Statement], page 161.) This continues until the program
reaches the end of the file. For example, the following awk program contains two rules:
/12/
/21/

{ print $0 }{ print $0 }The first rule has the string ‘12’ as the pattern and ‘print $0’ as the action. The second
rule has the string ‘21’ as the pattern and also has ‘print $0’ as the action. Each rule’s
action is enclosed in its own pair of braces.
This program prints every line that contains the string ‘12’ or the string ‘21’. If a line
contains both strings, it is printed twice, once by each rule.
This is what happens if we run this program on our two sample data files, mail-list
and inventory-shipped:$ awk '/12/ { print $0 }>/21/ { print $0 }' mail-list inventory-shipped555-3412
anthony.asserturo@hotmail.com
a Anthony555-2912
camilla.infusarum@skynet.bea Camilla
555-1234
fabius.undevicesimus@ucb.edu
a Fabius
555-2127
jeanpaul.campanorum@nyu.edu
a Jean-Paul
555-2127
jeanpaul.campanorum@nyu.edu
a Jean-Paul
Jan
21
36
64
620
a
a Apr 21 70 74 514

A
R
F
R
R

Note how the line beginning with ‘Jean-Paul’ in mail-list was printed twice, once foreach rule.

1.5 A More Complex Example
Now that we’ve mastered some simple tasks, let’s look at what typical awk programs do.
This example shows how awk can be used to summarize, select, and rearrange the output of
another utility. It uses features that haven’t been covered yet, so don’t worry if you don’t
understand all the details:ls -l | awk '$6 == "Nov" { sum += $5 }END { print sum }'This command prints the total number of bytes in all the files in the current directorythat were last modified in November (of any year).
As a reminder, the output of ‘ls -l’ gives you a listing of the files in a directory, including
each file’s size and the date the file was last modified. The first field contains read-write
permissions, the second field contains the number of links to the file, and the third field
identifies the file’s owner. The fourth field identifies the file’s group. The fifth field contains
the file’s size in bytes. The sixth, seventh, and eighth fields contain the month, day, and28 GAWK: Effective AWK Programming

time, respectively, that the file was last modified. Finally, the ninth field contains the filename.
The ‘$6 == "Nov"’ in our awk program is an expression that tests whether the sixth fieldof the output from ‘ls -l’ matches the string ‘Nov’. Each time a line has the string ‘Nov’for its sixth field, awk performs the action ‘sum += $5’. This adds the fifth field (the file’ssize) to the variable sum. As a result, when awk has finished reading all the input lines, sumis the total of the sizes of the files whose lines matched the pattern. (This works because
awk variables are automatically initialized to zero.)After the last line of output from ls has been processed, the END rule executes and prints
the value of sum. In this example, the value of sum is 80600.
These more advanced awk techniques are covered in later sections (see Section 7.3 [Actions], page 153). Before you can move on to more advanced awk programming, you have toknow how awk interprets your input and displays your output. By manipulating fields andusing print statements, you can produce some very useful and impressive-looking reports.

1.6 awk Statements Versus Lines
Most often, each line in an awk program is a separate statement or separate rule, like this:awk '/12/ { print $0 }/21/ { print $0 }' mail-list inventory-shipped
However, gawk ignores newlines after any of the following symbols and keywords:
,
{
?
:
||
&&
do
else
A newline at any other point is considered the end of the statement.2
If you would like to split a single statement into two lines at a point where a newline
would terminate it, you can continue it by ending the first line with a backslash character(‘\’). The backslash must be the final character on the line in order to be recognized asa continuation character. A backslash followed by a newline is allowed anywhere in thestatement, even in the middle of a string or regular expression. For example:
awk '/This regular expression is too long, so continue it\on the next line/ { print $1 }'We have generally not used backslash continuation in our sample programs. gawk places
no limit on the length of a line, so backslash continuation is never strictly necessary; it justmakes programs more readable. For this same reason, as well as for clarity, we have kept
most statements short in the programs presented throughout the book.
Backslash continuation is most useful when your awk program is in a separate source fileinstead of entered from the command line. You should also note that many awk implementations are more particular about where you may use backslash continuation. For example,
they may not allow you to split a string constant using backslash continuation. Thus, formaximum portability of your awk programs, it is best not to split your lines in the middle
of a regular expression or a string.
CAUTION: Backslash continuation does not work as described with the C shell.
It works for awk programs in files and for one-shot programs, provided you are2

The ‘?’ and ‘:’ referred to here is the three-operand conditional expression described in Section 6.3.4[Conditional Expressions], page 141. Splitting lines after ‘?’ and ‘:’ is a minor gawk extension; if --posixis specified (see Section 2.2 [Command-Line Options], page 33), then this extension is disabled.

Chapter 1: Getting Started with awk

29using a POSIX-compliant shell, such as the Unix Bourne shell or Bash. But
the C shell behaves differently! There you must use two backslashes in a row,followed by a newline. Note also that when using the C shell, every newline inyour awk program must be escaped with a backslash. To illustrate:% awk 'BEGIN { \?
print \\?
"hello, world" \? }'a hello, world
Here, the ‘%’ and ‘?’ are the C shell’s primary and secondary prompts, analogous
to the standard shell’s ‘$’ and ‘>’.
Compare the previous example to how it is done with a POSIX-compliant shell:$ awk 'BEGIN {>print \>
"hello, world"> }'a hello, world
awk is a line-oriented language. Each rule’s action has to begin on the same line as
the pattern. To have the pattern and action on separate lines, you must use backslash
continuation; there is no other option.
Another thing to keep in mind is that backslash continuation and comments do not mix.
As soon as awk sees the ‘#’ that starts a comment, it ignores everything on the rest of theline. For example:$ gawk 'BEGIN { print "dont panic" # a friendly \>BEGIN rule
> }'BEGIN rule
error gawk: cmd. line:2:^ syntax error
error gawk: cmd. line:2:
In this case, it looks like the backslash would continue the comment onto the next line.
However, the backslash-newline combination is never even noticed because it is “hidden”
inside the comment. Thus, the BEGIN is noted as a syntax error.
Backslash continuation comes into play in an additional, unexpected situation. Consider:gawk -F'\
a' '...'
This command line assigns a value to FS. But what value? There are several possibilities,
and in fact different versions of awk do different things. gawk treats this as if it were written:
BEGIN { FS = "\a"}...
In short, the backslash and newline are removed, assigning "a" to FS. This same treatment
applies to variable assignments made with the -v option (see Section 2.2 [Command-Line
Options], page 33) and to regular command-line variable assignments (see Section 6.1.3.2[Assigning Variables on the Command Line], page 125).

30 GAWK: Effective AWK Programming

If you’re interested, see https://lists.gnu.org/archive/html/bug-gawk/2022-10/
msg00025.html for a source code patch that allows lines to be continued when inside parentheses. This patch was not added to gawk since it would quietly decrease the portability of
awk programs.
When awk statements within one rule are short, you might want to put more than one ofthem on a line. This is accomplished by separating the statements with a semicolon (‘;’).
This also applies to the rules themselves. Thus, the program shown at the start of thissection could also be written this way:
/12/ { print $0 } ; /21/ { print $0 }NOTE: The requirement that states that rules on the same line must be separated with a semicolon was not in the original awk language; it was added forconsistency with the treatment of statements within an action.

1.7 Other Features of awk
The awk language provides a number of predefined, or built-in, variables that your programs
can use to get information from awk. There are other variables your program can set aswell to control how awk processes your data.
In addition, awk provides a number of built-in functions for doing common computational and string-related operations. gawk provides built-in functions for working withtimestamps, performing bit manipulation, for runtime string translation (internationalization), determining the type of a variable, and array sorting.
As we develop our presentation of the awk language, we will introduce most of thevariables and many of the functions. They are described systematically in Section 7.5[Predefined Variables], page 163, and in Section 9.1 [Built-in Functions], page 193.

1.8 When to Use awk
Now that you’ve seen some of what awk can do, you might wonder how awk could be
useful for you. By using utility programs, advanced patterns, field separators, arithmetic
statements, and other selection criteria, you can produce much more complex output. The
awk language is very useful for producing reports from large amounts of raw data, such assummarizing information from the output of other utility programs like ls. (See Section 1.5[A More Complex Example], page 27.)
Programs written with awk are usually much smaller than they would be in other languages. This makes awk programs easy to compose and use. Often, awk programs canbe quickly composed at your keyboard, used once, and thrown away. Because awk programs are interpreted, you can avoid the (usually lengthy) compilation part of the typicaledit-compile-test-debug cycle of software development.
Complex programs have been written in awk, including a complete retargetable assembler for eight-bit microprocessors (see [Glossary], page 529, for more information), and a
microcode assembler for a special-purpose Prolog computer. The original awk’s capabilities
were strained by tasks of such complexity, but modern versions are more capable.
If you find yourself writing awk scripts of more than, say, a few hundred lines, you might
consider using a different programming language. The shell is good at string and patternChapter 1: Getting Started with awk

31matching; in addition, it allows powerful use of the system utilities. Python offers a nicebalance between high-level ease of programming and access to system facilities.3

1.9 Summary• Programs in awk consist of pattern–action pairs.
• An action without a pattern always runs. The default action for a pattern without oneis ‘{ print $0 }’.
• Use either ‘awk 'program' files’ or ‘awk -f program-file files’ to run awk.
• You may use the special ‘#!’ header line to create awk programs that are directly
executable.
• Comments in awk programs start with ‘#’ and continue to the end of the same line.
• Be aware of quoting issues when writing awk programs as part of a larger shell script(or MS-Windows batch file).
• You may use backslash continuation to continue a source line. Lines are automaticallycontinued after a comma, open brace, question mark, colon, ‘||’, ‘&&’, do, and else.

3

Other popular scripting languages include Ruby and Perl.

Chapter 2: Running awk and gawk 33

2 Running awk and gawk
This chapter covers how to run awk, both POSIX-standard and gawk-specific command-line
options, and what awk and gawk do with nonoption arguments. It then proceeds to cover
how gawk searches for source files, reading standard input along with other files, gawk’s environment variables, gawk’s exit status, using include files, and obsolete and undocumented
options and/or features.
Many of the options and features described here are discussed in more detail later in thebook; feel free to skip over things in this chapter that don’t interest you right now.

2.1 Invoking awk
There are two ways to run awk—with an explicit program or with one or more program
files. Here are templates for both of them; items enclosed in [. . . ] in these templates are
optional:
awk [options] -f progfile [--] file . . .
awk [options] [--] 'program' file . . .
In addition to traditional one-letter POSIX-style options, gawk also supports GNU longoptions.
It is possible to invoke awk with an empty program:awk '' datafile1 datafile2
Doing so makes little sense, though; awk exits silently when given an empty program. If--lint has been specified on the command line, gawk issues a warning that the program isempty.

2.2 Command-Line Options
Options begin with a dash and consist of a single character. GNU-style long options consistof two dashes and a keyword. The keyword can be abbreviated, as long as the abbreviation
allows the option to be uniquely identified. If the option takes an argument, either the
keyword is immediately followed by an equals sign (‘=’) and the argument’s value, or the
keyword and the argument’s value are separated by whitespace (spaces or TABs). If aparticular option with a value is given more than once, it is (usually) the last value thatcounts.
Each long option for gawk has a corresponding POSIX-style short option. The longand short options are interchangeable in all contexts. The following list describes options
mandated by the POSIX standard:-F fs--field-separator fs
Set the FS variable to fs (see Section 4.5 [Specifying How Fields Are Separated],
page 73).
-f source-file--file source-file
Read the awk program source from source-file instead of in the first nonoption
argument. This option may be given multiple times; the awk program consistsof the concatenation of the contents of each specified source-file.

34 GAWK: Effective AWK Programming

Files named with -f are treated as if they had ‘@namespace "awk"’ at their
beginning. See Section 15.4 [Changing The Namespace], page 384, for more
information on this advanced feature.
-v var=val--assign var=val
Set the variable var to the value val before execution of the program begins.
Such variable values are available inside the BEGIN rule (see Section 2.3 [Other
Command-Line Arguments], page 40).
The -v option can only set one variable, but it can be used more than once,
setting another variable each time, like this: ‘awk -v foo=1 -v bar=2 ...’.
CAUTION: Using -v to set the values of the built-in variables maylead to surprising results. awk will reset the values of those variables
as it needs to, possibly ignoring any initial value you may havegiven.
-W gawk-opt
Provide an implementation-specific option. This is the POSIX convention forproviding implementation-specific options. These options also have corresponding GNU-style long options. Note that the long options may be abbreviated, aslong as the abbreviations remain unique. The full list of gawk-specific options
is provided next.
--

Signal the end of the command-line options. The following arguments are nottreated as options even if they begin with ‘-’. This interpretation of -- follows
the POSIX argument parsing conventions.
This is useful if you have file names that start with ‘-’, or in shell scripts, if
you have file names that will be specified by the user that could start with ‘-’.
It is also useful for passing options on to the awk program; see Section 10.4
[Processing Command-Line Options], page 259.
The following list describes gawk-specific options:-b--characters-as-bytes
Cause gawk to treat all input data as single-byte characters. In addition, alloutput written with print or printf is treated as single-byte characters.
Normally, gawk follows the POSIX standard and attempts to process its input
data according to the current locale (see Section 6.6 [Where You Are Makes a
Difference], page 145). This can often involve converting multibyte characters
into wide characters (internally), and can lead to problems or confusion if the
input data does not contain valid multibyte characters. This option is an easy
way to tell gawk, “Hands off my data!”-c--traditional
Specify compatibility mode, in which the GNU extensions to the awk language
are disabled, so that gawk behaves just like BWK awk. See Section A.5 [Extensions in gawk Not in POSIX awk], page 475, which summarizes the extensions.
Also see Section C.1 [Downward Compatibility and Debugging], page 515.

Chapter 2: Running awk and gawk 35-C--copyrightPrint the short version of the General Public License and then exit.
-d[file]--dump-variables[=file]
Print a sorted list of global variables, their types, and final values to file. If
no file is provided, print this list to a file named awkvars.out in the current
directory. No space is allowed between the -d and file, if file is supplied.
Having a list of all global variables is a good way to look for typographicalerrors in your programs. You would also use this option if you have a large
program with a lot of functions, and you want to be sure that your functions
don’t inadvertently use global variables that you meant to be local. (This is aparticularly easy mistake to make with simple variable names like i, j, etc.)-D[file]--debug[=file]
Enable debugging of awk programs (see Section 14.1 [Introduction to the gawk
Debugger], page 367). By default, the debugger reads commands interactively
from the keyboard (standard input). The optional file argument allows you to
specify a file with a list of commands for the debugger to execute noninteractively. No space is allowed between the -D and file, if file is supplied.
-e program-text--source program-text
Provide program source code in the program-text. This option allows you to mix
source code in files with source code that you enter on the command line. This isparticularly useful when you have library functions that you want to use fromyour command-line programs (see Section 2.5.1 [The AWKPATH Environment
Variable], page 42).
Note that gawk treats each string as if it ended with a newline character (evenif it doesn’t). This makes building the total program easier.
CAUTION: Prior to version 5.0, there was no requirement that eachprogram-text be a full syntactic unit. I.e., the following worked:$ gawk -e 'BEGIN { a = 5 ;' -e 'print a }'a 5However, this is no longer true. If you have any scripts that rely
upon this feature, you should revise them.
This is because each program-text is treated as if it had‘@namespace "awk"’ at its beginning. See Section 15.4 [Changing
The Namespace], page 384, for more information.
-E file--exec file
Similar to -f, read awk program text from file. There are two differences from-f:
• This option terminates option processing; anything else on the commandline is passed on directly to the awk program.

36 GAWK: Effective AWK Programming• Command-line variable assignments of the form ‘var=value’ are disallowed.
This option is particularly necessary for World Wide Web CGI applications
that pass arguments through the URL; using this option prevents a malicious(or other) user from passing in options, assignments, or awk source code (via-e) to the CGI application.1 This option should be used with ‘#!’ scripts (seeSection 1.1.4 [Executable awk Programs], page 19), like so:#! /usr/local/bin/gawk -E
awk program here ...
-g--gen-pot
Analyze the source program and generate a GNU gettext portable object
template file on standard output for all string constants that have been marked
for translation. See Chapter 13 [Internationalization with gawk], page 357, for
information about this option.
-h--helpPrint a “usage” message summarizing the short- and long-style options thatgawk accepts and then exit.

-i source-file--include source-file
Read an awk source library from source-file. This option is completely equivalent
to using the @include directive inside your program. It is very similar to the-f option, but there are two important differences. First, when -i is used, theprogram source is not loaded if it has been previously loaded, whereas with-f, gawk always loads the file. Second, because this option is intended to be
used with code libraries, gawk does not recognize such files as constituting main
program input. Thus, after processing an -i argument, gawk still expects tofind the main source code via the -f option or on the command line.
Files named with -i are treated as if they had ‘@namespace "awk"’ at their
beginning. See Section 15.4 [Changing The Namespace], page 384, for more
information.
-I--trace

-k
--csv

1

Print the internal byte code names as they are executed when running the
program. The trace is printed to standard error. Each “op code” is precededby a + sign in the output.
Enable special processing for files with comma separated values (CSV). See
Section 4.5.4 [Working With Comma Separated Value Files], page 76. This
option cannot be used with --posix. Attempting to do causes a fatal error.

For more detail, please see Section 4.4 of RFC 3875. Also see the explanatory note sent to the gawk bug
mailing list.

Chapter 2: Running awk and gawk 37-l ext--load ext
Load a dynamic extension named ext. Extensions are stored as system sharedlibraries. This option searches for the library using the AWKLIBPATH environment
variable. The correct library suffix for your platform will be supplied by default,
so it need not be specified in the extension name. The extension initialization
routine should be named dl_load(). An alternative is to use the @load keyword
inside the program to load a shared library. This advanced feature is describedin detail in Chapter 17 [Writing Extensions for gawk], page 405.
-L[value]--lint[=value]
Warn about constructs that are dubious or nonportable to other awk implementations. No space is allowed between the -L and value, if value is supplied.
Some warnings are issued when gawk first reads your program. Others are issued at runtime, as your program executes. The optional argument may be oneof the following:
fatal

Cause lint warnings become fatal errors. This may be drastic, but
its use will certainly encourage the development of cleaner awk programs.

invalid

Only issue warnings about things that are actually invalid are issued. (This is not fully implemented yet.)no-extDisable warnings about gawk extensions.

Some warnings are only printed once, even if the dubious constructs they warn
about occur multiple times in your awk program. Thus, when eliminating problems pointed out by --lint, you should take care to search for all occurrencesof each inappropriate construct. As awk programs are usually short, doing so
is not burdensome.
-M--bignumSelect arbitrary-precision arithmetic on numbers. This option has no effect if
gawk is not compiled to use the GNU MPFR and MP libraries (see Chapter 16[Arithmetic and Arbitrary-Precision Arithmetic with gawk], page 389).
As of version 5.2, the arbitrary precision arithmetic features in gawk are “onparole.” The primary maintainer is no longer willing to support this feature,
but another member of the development team has stepped up to take it over.
As long as this situation remains stable, MPFR will be supported. If it changes,
the MPFR support will be removed from gawk.

-n--non-decimal-data
Enable automatic interpretation of octal and hexadecimal values in input data
(see Section 12.1 [Allowing Nondecimal Input Data], page 335).
CAUTION: This option can severely break old programs. Use withcare. Also note that this option may disappear in a future versionof gawk.

38 GAWK: Effective AWK Programming-N--use-lc-numeric
Force the use of the locale’s decimal point character when parsing numeric input
data (see Section 6.6 [Where You Are Makes a Difference], page 145).
-o[file]--pretty-print[=file]
Enable pretty-printing of awk programs. Implies --no-optimize. By default,
the output program is created in a file named awkprof.out (see Section 12.6
[Profiling Your awk Programs], page 348). The optional file argument allows
you to specify a different file name for the output. No space is allowed between
the -o and file, if file is supplied.
NOTE: In the past, this option would also execute your program.
This is no longer the case.
-O--optimize
Enable gawk’s default optimizations on the internal representation of the program. At the moment, this includes just simple constant folding.
Optimization is enabled by default. This option remains primarily for backwards compatibility. However, it may be used to cancel the effect of an earlier-s option (see later in this list).
-p[file]--profile[=file]
Enable profiling of awk programs (see Section 12.6 [Profiling Your awk Programs], page 348). Implies --no-optimize. By default, profiles are created ina file named awkprof.out. The optional file argument allows you to specify adifferent file name for the profile file. No space is allowed between the -p andfile, if file is supplied.
The profile contains execution counts for each statement in the program in theleft margin, and function call counts for each function.
-P--posix

Operate in strict POSIX mode. This disables all gawk extensions (just like--traditional) and disables all extensions not allowed by POSIX. See
Section A.7 [Common Extensions Summary], page 486, for a summary of
the extensions in gawk that are disabled by this option. Also, the following
additional restrictions apply:• Newlines are not allowed after ‘?’ or ‘:’ (see Section 6.3.4 [Conditional
Expressions], page 141).
• Specifying ‘-Ft’ on the command line does not set the value of FS to bea single TAB character (see Section 4.5 [Specifying How Fields Are Separated], page 73).
• The locale’s decimal point character is used for parsing input data (seeSection 6.6 [Where You Are Makes a Difference], page 145).
If you supply both --traditional and --posix on the command line, --posix
takes precedence. gawk issues a warning if both options are supplied.

Chapter 2: Running awk and gawk 39-r--re-interval
Allow interval expressions (see Section 3.3 [Regular Expression Operators],
page 54) in regexps. This is now gawk’s default behavior. Nevertheless, thisoption remains for backward compatibility.
-s--no-optimize
Disable gawk’s default optimizations on the internal representation of the program.
-S--sandbox
Disable the system() function, input redirections with getline, output redirections with print and printf, and dynamic extensions. Also, disallow adding
file names to ARGV that were not there when gawk started running. This is particularly useful when you want to run awk scripts from questionable sources andneed to make sure the scripts can’t access your system (other than the specified
input data files).
-t--lint-old
Warn about constructs that are not available in the original version of awk from
Version 7 Unix (see Section A.1 [Major Changes Between V7 and SVR3.1],
page 473).
-V--version
Print version information for this particular copy of gawk. This allows you todetermine if your copy of gawk is up to date with respect to whatever the Free
Software Foundation is currently distributing. It is also useful for bug reports(see Section B.4 [Reporting Problems and Bugs], page 505).
--

Mark the end of all options. Any command-line arguments following -- areplaced in ARGV, even if they start with a minus sign.

In compatibility mode, as long as program text has been supplied, any other options are
flagged as invalid with a warning message but are otherwise ignored.
In compatibility mode, as a special case, if the value of fs supplied to the -F option is‘t’, then FS is set to the TAB character ("\t"). This is true only for --traditional andnot for --posix (see Section 4.5 [Specifying How Fields Are Separated], page 73).
The -f option may be used more than once on the command line. If it is, awk reads
its program source from all of the named files, as if they had been concatenated together
into one big file. This is useful for creating libraries of awk functions. These functions canbe written once and then retrieved from a standard place, instead of having to be includedin each individual program. The -i option is similar in this regard. (As mentioned in
Section 9.2.1 [Function Definition Syntax], page 221, function names must be unique.)With standard awk, library functions can still be used, even if the program is entered at
the keyboard, by specifying ‘-f /dev/tty’. After typing your program, type Ctrl-d (theend-of-file character) to terminate it. (You may also use ‘-f -’ to read program source from

40 GAWK: Effective AWK Programming

the standard input, but then you will not be able to also use the standard input as a sourceof data.)
Because it is clumsy using the standard awk mechanisms to mix source file and commandline awk programs, gawk provides the -e option. This does not require you to preempt thestandard input for your source code, and it allows you to easily mix command-line andlibrary source code (see Section 2.5.1 [The AWKPATH Environment Variable], page 42). As
with -f, the -e and -i options may also be used multiple times on the command line.
If no -f option (or -e option for gawk) is specified, then awk uses the first nonoptioncommand-line argument as the text of the program source code. Arguments on the command line that follow the program text are entered into the ARGV array; awk does notcontinue to parse the command line looking for options.
If the environment variable POSIXLY_CORRECT exists, then gawk behaves in strict POSIX
mode, exactly as if you had supplied --posix. Many GNU programs look for this environment variable to suppress extensions that conflict with POSIX, but gawk behaves differently:
it suppresses all extensions, even those that do not conflict with POSIX, and behaves instrict POSIX mode. If --lint is supplied on the command line and gawk turns on POSIX
mode because of POSIXLY_CORRECT, then it issues a warning message indicating that POSIX
mode is in effect. You would typically set this variable in your shell’s startup file. For a
Bourne-compatible shell (such as Bash), you would add these lines to the .profile file inyour home directory:
POSIXLY_CORRECT=true
export POSIXLY_CORRECT
For a C shell-compatible shell,2 you would add this line to the .login file in your home
directory:
setenv POSIXLY_CORRECT true
Having POSIXLY_CORRECT set is not recommended for daily use, but it is good for testing
the portability of your programs to other environments.

2.3 Other Command-Line Arguments
Any additional arguments on the command line are normally treated as input files to beprocessed in the order specified. However, an argument that has the form var=value, assigns the value value to the variable var—it does not specify a file at all. (See Section 6.1.3.2[Assigning Variables on the Command Line], page 125.) In the following example, ‘count=1’
is a variable assignment, not a file name:awk -f program.awk file1 count=1 file2
As a side point, should you really need to have awk process a file named count=1 (or any
file whose name looks like a variable assignment), precede the file name with ‘./’, like so:
awk -f program.awk file1 ./count=1 file2
All the command-line arguments are made available to your awk program in the ARGV
array (see Section 7.5 [Predefined Variables], page 163). Command-line options and theprogram text (if present) are omitted from ARGV. All other arguments, including variableassignments, are included. As each element of ARGV is processed, gawk sets ARGIND to the2

Not recommended.

Chapter 2: Running awk and gawk 41

index in ARGV of the current element. (gawk makes the full command line, including program
text and options, available in PROCINFO["argv"]; see Section 7.5.2 [Built-in Variables That
Convey Information], page 165.)
Changing ARGC and ARGV in your awk program lets you control how awk processes the
input files; this is described in more detail in Section 7.5.3 [Using ARGC and ARGV], page 173.
The distinction between file name arguments and variable-assignment arguments is madewhen awk is about to open the next input file. At that point in execution, it checks the filename to see whether it is really a variable assignment; if so, awk sets the variable insteadof reading a file.
Therefore, the variables actually receive the given values after all previously specified
files have been read. In particular, the values of variables assigned in this fashion are
not available inside a BEGIN rule (see Section 7.1.4 [The BEGIN and END Special Patterns],
page 150), because such rules are run before awk begins scanning the argument list.
The variable values given on the command line are processed for escape sequences (seeSection 3.2 [Escape Sequences], page 52).
In some very early implementations of awk, when a variable assignment occurred before
any file names, the assignment would happen before the BEGIN rule was executed. awk’s
behavior was thus inconsistent; some command-line assignments were available inside the
BEGIN rule, while others were not. Unfortunately, some applications came to depend uponthis “feature.” When awk was changed to be more consistent, the -v option was added toaccommodate applications that depended upon the old behavior.
The variable assignment feature is most useful for assigning to variables such as RS, OFS,
and ORS, which control input and output formats, before scanning the data files. It is also
useful for controlling state if multiple passes are needed over a data file. For example:
awk 'pass == 1 { pass 1 stuff }pass == 2 { pass 2 stuff }' pass=1 mydata pass=2 mydata
Given the variable assignment feature, the -F option for setting the value of FS is notstrictly necessary. It remains for historical compatibility.

42 GAWK: Effective AWK Programming

Quoting Shell Variables On The awk Command Line
Small awk programs are often embedded in larger shell scripts, so it’s worthwhile tounderstand some shell basics. Consider the following:f=""awk '{ print("hi") }' $f
In this case, awk reads from standard input instead of trying to open any commandline files. To the unwary, this looks like awk is hanging.
However awk doesn’t see an explicit empty string. When a variable expansion is thenull string, and it’s not quoted, the shell simply removes it from the command line. To
demonstrate:$ f=""$ awk 'BEGIN { print ARGC }' $f
a 1$ awk 'BEGIN { print ARGC }' "$f"a 2

2.4 Naming Standard Input
Often, you may wish to read standard input together with other files. For example, you
may wish to read one file, read standard input coming from a pipe, and then read anotherfile.
The way to name the standard input, with all versions of awk, is to use a single, standalone minus sign or dash, ‘-’. For example:
some_command | awk -f myprog.awk file1 - file2
Here, awk first reads file1, then it reads the output of some command, and finally it reads
file2.
You may also use "-" to name standard input when reading files with getline (seeSection 4.10.3 [Using getline from a File], page 91). And, you can even use "-" with the-f option to read program source code from standard input (see Section 2.2 [Command-Line
Options], page 33).
In addition, gawk allows you to specify the special file name /dev/stdin, both on thecommand line and with getline. Some other versions of awk also support this, but it is notstandard. (Some operating systems provide a /dev/stdin file in the filesystem; however,gawk always processes this file name itself.)

2.5 The Environment Variables gawk Uses
A number of environment variables influence how gawk behaves.

2.5.1 The AWKPATH Environment Variable
In most awk implementations, you must supply a precise pathname for each program file,unless the file is in the current directory. But with gawk, if the file name supplied to the-f or -i options does not contain a directory separator ‘/’, then gawk searches a list of
directories (called the search path) one by one, looking for a file with the specified name.

Chapter 2: Running awk and gawk 43

The search path is a string consisting of directory names separated by colons.3 gawk
gets its search path from the AWKPATH environment variable. If that variable does not exist,
or if it has an empty value, gawk uses a default path (described shortly).
The search path feature is particularly helpful for building libraries of useful awk functions. The library files can be placed in a standard directory in the default path and thenspecified on the command line with a short file name. Otherwise, you would have to typethe full file name for each file.
By using the -i or -f options, your command-line awk programs can use facilities in awk
library files (see Chapter 10 [A Library of awk Functions], page 241). Path searching is notdone if gawk is in compatibility mode. This is true for both --traditional and --posix.
See Section 2.2 [Command-Line Options], page 33.
If the source code file is not found after the initial search, the path is searched again
after adding the suffix ‘.awk’ to the file name.
gawk’s path search mechanism is similar to the shell’s. (See The Bourne-Again SHellmanual.) It treats a null entry in the path as indicating the current directory. (A null entry
is indicated by starting or ending the path with a colon or by placing two colons next toeach other [‘::’].)
NOTE: To include the current directory in the path, either place . as an entry
in the path or write a null entry in the path.
Different past versions of gawk would also look explicitly in the current directory, either before or after the path search. As of version 4.1.2, this no longer
happens; if you wish to look in the current directory, you must include . either
as a separate entry or as a null entry in the search path.
The default value for AWKPATH is ‘.:/usr/local/share/awk’.4 Since . is included at the
beginning, gawk searches first in the current directory and then in /usr/local/share/awk.
In practice, this means that you will rarely need to change the value of AWKPATH.
See Section B.2.2 [Shell Startup Files], page 496, for information on functions that help
to manipulate the AWKPATH variable.
gawk places the value of the search path that it used into ENVIRON["AWKPATH"]. This
provides access to the actual search path value from within an awk program.
Although you can change ENVIRON["AWKPATH"] within your awk program, this has noeffect on the running program’s behavior. This makes sense: the AWKPATH environment
variable is used to find the program source files. Once your program is running, all the files
have been found, and gawk no longer needs to use AWKPATH.

2.5.2 The AWKLIBPATH Environment Variable
The AWKLIBPATH environment variable is similar to the AWKPATH variable, but it is used tosearch for loadable extensions (stored as system shared libraries) specified with the -l option
rather than for source files. If the extension is not found, the path is searched again after
3
4

Semicolons on MS-Windows.
Your version of gawk may use a different directory; it will depend upon how gawk was built and installed.
The actual directory is the value of $(pkgdatadir) generated when gawk was configured. (For moredetail, see the INSTALL file in the source distribution, and see Section B.2.1 [Compiling gawk for UnixLike Systems], page 495. You probably don’t need to worry about this, though.)

44 GAWK: Effective AWK Programming

adding the appropriate shared library suffix for the platform. For example, on GNU/Linuxsystems, the suffix ‘.so’ is used. The search path specified is also used for extensions loaded
via the @load keyword (see Section 2.8 [Loading Dynamic Extensions into Your Program],
page 47).
If AWKLIBPATH does not exist in the environment, or if it has an empty value, gawk uses
a default path; this is typically ‘/usr/local/lib/gawk’, although it can vary depending
upon how gawk was built.5
See Section B.2.2 [Shell Startup Files], page 496, for information on functions that help
to manipulate the AWKLIBPATH variable.
gawk places the value of the search path that it used into ENVIRON["AWKLIBPATH"]. This
provides access to the actual search path value from within an awk program.
Although you can change ENVIRON["AWKLIBPATH"] within your awk program, this has noeffect on the running program’s behavior. This makes sense: the AWKLIBPATH environment
variable is used to find any requested extensions, and they are loaded before the program
starts to run. Once your program is running, all the extensions have been found, and gawk
no longer needs to use AWKLIBPATH.

2.5.3 Other Environment Variables
A number of other environment variables affect gawk’s behavior, but they are more specialized. Those in the following list are meant to be used by regular users:
GAWK_MSEC_SLEEPSpecifies the interval between connection retries, in milliseconds. On systems
that do not support the usleep() system call, the value is rounded up to an
integral number of seconds.
GAWK_PERSIST_FILE
Specifies the backing file to use for persistent storage of gawk’s variables andarrays. See Section 12.7 [Preserving Data Between Runs], page 352.
GAWK_READ_TIMEOUT
Specifies the time, in milliseconds, for gawk to wait for input before returningwith an error. See Section 4.11 [Reading Input with a Timeout], page 95.
GAWK_SOCK_RETRIESControls the number of times gawk attempts to retry a two-way TCP/IP(socket) connection before giving up. See Section 12.5 [Using gawk for
Network Programming], page 346. Note that when nonfatal I/O is enabled(see Section 5.11 [Enabling Nonfatal Output], page 116), gawk only tries toopen a TCP/IP socket once.
PMA_VERBOSITY
Controls the verbosity of the persistent memory allocator. See Section 12.7
[Preserving Data Between Runs], page 352.
5Your version of gawk may use a different directory; it will depend upon how gawk was built and installed.
The actual directory is the value of $(pkgextensiondir) generated when gawk was configured. (Formore detail, see the INSTALL file in the source distribution, and see Section B.2.1 [Compiling gawk forUnix-Like Systems], page 495. You probably don’t need to worry about this, though.)Chapter 2: Running awk and gawk 45

POSIXLY_CORRECT
Causes gawk to switch to POSIX-compatibility mode, disabling all traditional
and GNU extensions. See Section 2.2 [Command-Line Options], page 33.
The environment variables in the following list are meant for use by the gawk developers
for testing and tuning. They are subject to change. The variables are:AWKBUFSIZE
This variable only affects gawk on POSIX-compliant systems. With a value of‘exact’, gawk uses the size of each input file as the size of the memory buffer
to allocate for I/O. Otherwise, the value should be a number, and gawk usesthat number as the size of the buffer to allocate. (When this variable is not
set, gawk uses the smaller of the file’s size and the “default” blocksize, which isusually the filesystem’s I/O blocksize.)
AWK_HASH

If this variable exists with a value of ‘gst’, gawk switches to using the hashfunction from GNU Smalltalk for managing arrays. With a value of ‘fnv1a’,gawk uses the FNV1-A hash function. These functions may be marginally fasterthan the standard function.

AWKREADFUNC
If this variable exists, gawk switches to reading source files one line at a time,
instead of reading in blocks. This exists for debugging problems on filesystems
on non-POSIX operating systems where I/O is performed in records, not inblocks.
GAWK_MSG_SRC
If this variable exists, gawk includes the file name and line number within thegawk source code from which warning and/or fatal messages are generated. Its
purpose is to help isolate the source of a message, as there are multiple places
that produce the same warning or error message.
GAWK_LOCALE_DIR
Specifies the location of compiled message object files for gawk itself. This ispassed to the bindtextdomain() function when gawk starts up.
GAWK_NO_DFA
If this variable exists, gawk does not use the DFA regexp matcher for “does itmatch” kinds of tests. This can cause gawk to be slower. Its purpose is to help
isolate differences between the two regexp matchers that gawk uses internally.
(There aren’t supposed to be differences, but occasionally theory and practice
don’t coordinate with each other.)
GAWK_STACKSIZE
This specifies the amount by which gawk should grow its internal evaluation
stack, when needed.
INT_CHAIN_MAX
This specifies intended maximum number of items gawk will maintain on a hashchain for managing arrays indexed by integers.

46 GAWK: Effective AWK Programming

STR_CHAIN_MAX
This specifies intended maximum number of items gawk will maintain on a hashchain for managing arrays indexed by strings.
TIDYMEM

If this variable exists, gawk uses the mtrace() library calls from the GNU C
library to help track down possible memory leaks. This cannot be used together
with the persistent memory allocator.

2.6 gawk’s Exit Status
If the exit statement is used with a value (see Section 7.4.10 [The exit Statement],
page 162), then gawk exits with the numeric value given to it.
Otherwise, if there were no problems during execution, gawk exits with the value of the
C constant EXIT_SUCCESS. This is usually zero.
If an error occurs, gawk exits with the value of the C constant EXIT_FAILURE. This is
usually one.
If gawk exits because of a fatal error, the exit status is two. On non-POSIX systems,
this value may be mapped to EXIT_FAILURE.

2.7 Including Other Files into Your ProgramThis section describes a feature that is specific to gawk.
The @include keyword can be used to read external awk source files. This gives you theability to split large awk source files into smaller, more manageable pieces, and also lets you
reuse common awk code from various awk scripts. In other words, you can group together
awk functions used to carry out specific tasks into external files. These files can be usedjust like function libraries, using the @include keyword in conjunction with the AWKPATH
environment variable. Note that source files may also be included using the -i option.
Let’s see an example. We’ll start with two (trivial) awk scripts, namely test1 and test2.
Here is the test1 script:
BEGIN {
print "This is script test1."}and here is test2:@include "test1"
BEGIN {
print "This is script test2."}
Running gawk with test2 produces the following result:$ gawk -f test2
a This is script test1.
a This is script test2.
gawk runs the test2 script, which includes test1 using the @include keyword. So, toinclude external awk source files, you just use @include followed by the name of the file to
be included, enclosed in double quotes.

Chapter 2: Running awk and gawk 47

NOTE: Keep in mind that this is a language construct and the file name cannot
be a string variable, but rather just a literal string constant in double quotes.
The files to be included may be nested; e.g., given a third script, namely test3:@include "test2"
BEGIN {
print "This is script test3."}
Running gawk with the test3 script produces the following results:$ gawk -f test3
a This is script test1.
a This is script test2.
a This is script test3.
The file name can, of course, be a pathname. For example:@include "../io_funcs"
and:@include "/usr/awklib/network"
are both valid. The AWKPATH environment variable can be of great value when using@include. The same rules for the use of the AWKPATH variable in command-line file searches(see Section 2.5.1 [The AWKPATH Environment Variable], page 42) apply to @include also.
This is very helpful in constructing gawk function libraries. If you have a large scriptwith useful, general-purpose awk functions, you can break it down into library files and put
those files in a special directory. You can then include those “libraries,” either by using thefull pathnames of the files, or by setting the AWKPATH environment variable accordingly andthen using @include with just the file part of the full pathname. Of course, you can keep
library files in more than one directory; the more complex the working environment is, themore directories you may need to organize the files to be included.
Given the ability to specify multiple -f options, the @include mechanism is not strictly
necessary. However, the @include keyword can help you in constructing self-contained
gawk programs, thus reducing the need for writing complex and tedious command lines. In
particular, @include is very useful for writing CGI scripts to be run from web pages.
The @include directive and the -i/--include command line option are completely
equivalent. An included program source is not loaded if it has been previously loaded.
The rules for finding a source file described in Section 2.5.1 [The AWKPATH Environment
Variable], page 42, also apply to files loaded with @include.
Finally, files included with @include are treated as if they had ‘@namespace "awk"’ attheir beginning. See Section 15.4 [Changing The Namespace], page 384, for more information.

2.8 Loading Dynamic Extensions into Your ProgramThis section describes a feature that is specific to gawk.
The @load keyword can be used to read external awk extensions (stored as system sharedlibraries). This allows you to link in compiled code that may offer superior performance
and/or give you access to extended capabilities not supported by the awk language. The

48 GAWK: Effective AWK ProgrammingAWKLIBPATH variable is used to search for the extension. Using @load is completely equivalent to using the -l command-line option.
If the extension is not initially found in AWKLIBPATH, another search is conducted after
appending the platform’s default shared library suffix to the file name. For example, on
GNU/Linux systems, the suffix ‘.so’ is used:$ gawk '@load "ordchr"; BEGIN {print chr(65)}'
a A
This is equivalent to the following example:$ gawk -lordchr 'BEGIN {print chr(65)}'
a A
For command-line usage, the -l option is more convenient, but @load is useful for embedding
inside an awk source file that requires access to an extension.
Chapter 17 [Writing Extensions for gawk], page 405, describes how to write extensions(in C or C++) that can be loaded with either @load or the -l option. It also describes theordchr extension.

2.9 Obsolete Options and/or Features
This section describes features and/or command-line options from previous releases of gawk
that either are not available in the current version or are still supported but deprecated(meaning that they will not be in a future release).
As of gawk version 5.2. the arbitrary precision arithmetic feature is “on parole.” Thisfeature is now being supported by a volunteer in the development team and not by the
primary maintainer. If this situation changes, then the feature will be removed. For more
information see Section 16.3.1 [Arbitrary Precision Arithmetic is On Parole!], page 392.

2.10 Undocumented Options and Features
Use the Source, Luke!
—Obi-Wan
This section intentionally left blank.

2.11 Summary• gawk parses arguments on the command line, left to right, to determine if they shouldbe treated as options or as non-option arguments.
• gawk recognizes several options which control its operation, as described in Section 2.2[Command-Line Options], page 33. All options begin with ‘-’.
• Any argument that is not recognized as an option is treated as a non-option argument,even if it begins with ‘-’.
− However, when an option itself requires an argument, and the option is separated
from that argument on the command line by at least one space, the space isignored, and the argument is considered to be related to the option. Thus, in the
invocation, ‘gawk -F x’, the ‘x’ is treated as belonging to the -F option, not as aseparate non-option argument.

Chapter 2: Running awk and gawk 49• Once gawk finds a non-option argument, it stops looking for options. Therefore, allfollowing arguments are also non-option arguments, even if they resemble recognized
options.
• If no -e or -f options are present, gawk expects the program text to be in the first
non-option argument.
• All non-option arguments, except program text provided in the first non-option argument, are placed in ARGV as explained in Section 7.5.3 [Using ARGC and ARGV],
page 173, and are processed as described in Section 2.3 [Other Command-Line Arguments], page 40. Adjusting ARGC and ARGV affects how awk processes input.
• The three standard options for all versions of awk are -f, -F, and -v. gawk supplies
these and many others, as well as corresponding GNU-style long options.
• Nonoption command-line arguments are usually treated as file names, unless they havethe form ‘var=value’, in which case they are taken as variable assignments to be
performed at that point in processing the input.
• You can use a single minus sign (‘-’) to refer to standard input on the command line.
gawk also lets you use the special file name /dev/stdin.
• gawk pays attention to a number of environment variables. AWKPATH, AWKLIBPATH, and
POSIXLY_CORRECT are the most important ones.
• gawk’s exit status conveys information to the program that invoked it. Use the exit
statement from within an awk program to set the exit status.
• gawk allows you to include other awk source files into your program using the @include
statement and/or the -i and -f command-line options.
• gawk allows you to load additional functions written in C or C++ using the @load statement and/or the -l option. (This advanced feature is described later, in Chapter 17[Writing Extensions for gawk], page 405.)

Chapter 3: Regular Expressions 51

3 Regular Expressions
A regular expression, or regexp, is a way of describing a set of strings. Because regularexpressions are such a fundamental part of awk programming, their format and use deservea separate chapter.
A regular expression enclosed in slashes (‘/’) is an awk pattern that matches every input
record whose text belongs to that set. The simplest regular expression is a sequence ofletters, numbers, or both. Such a regexp matches any string that contains that sequence.
Thus, the regexp ‘foo’ matches any string containing ‘foo’. Thus, the pattern /foo/
matches any input record containing the three adjacent characters ‘foo’ anywhere in therecord. Other kinds of regexps let you specify more complicated classes of strings.
Initially, the examples in this chapter are simple. As we explain more about how regularexpressions work, we present more complicated instances.

3.1 How to Use Regular Expressions
A regular expression can be used as a pattern by enclosing it in slashes. Then the regularexpression is tested against the entire text of each record. (Normally, it only needs to match
some part of the text in order to succeed.) For example, the following prints the second
field of each record where the string ‘li’ appears anywhere in the record:$ awk '/li/ { print $2 }' mail-list
a 555-5553
a 555-0542
a 555-6699
a 555-3430
Regular expressions can also be used in matching expressions. These expressions allow
you to specify the string to match against; it need not be the entire current input record.
The two operators ‘~’ and ‘!~’ perform regular expression comparisons. Expressions using
these operators can be used as patterns, or in if, while, for, and do statements. (See
Section 7.4 [Control Statements in Actions], page 154.) For example, the following is true
if the expression exp (taken as a string) matches regexp:
exp ~ /regexp/
This example matches, or selects, all input records with the uppercase letter ‘J’ somewherein the first field:$ awk '$1 ~ /J/' inventory-shipped
a Jan 13 25 15 115
a Jun 31 42 75 492
a Jul 24 34 67 436
a Jan 21 36 64 620
So does this:awk '{ if ($1 ~ /J/) print }' inventory-shipped
This next example is true if the expression exp (taken as a character string) does notmatch regexp:
exp !~ /regexp/

52 GAWK: Effective AWK Programming

The following example matches, or selects, all input records whose first field does notcontain the uppercase letter ‘J’:
$ awk '$1 !~ /J/' inventory-shippeda Feb 15 32 24 226
a Mar 15 24 34 228
a Apr 31 52 63 420
a May 16 34 29 208...
When a regexp is enclosed in slashes, such as /foo/, we call it a regexp constant, much
like 5.27 is a numeric constant and "foo" is a string constant.

3.2 Escape Sequences
Some characters cannot be included literally in string constants ("foo") or regexp constants(/foo/). Instead, they should be represented with escape sequences, which are character
sequences beginning with a backslash (‘\’). One use of an escape sequence is to include a
double-quote character in a string constant. Because a plain double quote ends the string,
you must use ‘\"’ to represent an actual double-quote character as a part of the string. For
example:$ awk 'BEGIN { print "He said \"hi!\" to her." }'a He said "hi!" to her.
The backslash character itself is another character that cannot be included normally;
you must write ‘\\’ to put one backslash in the string or regexp. Thus, the string whose
contents are the two characters ‘"’ and ‘\’ must be written "\"\\".
Other escape sequences represent unprintable characters such as TAB or newline. Thereis nothing to stop you from entering most unprintable characters directly in a string constantor regexp constant, but they may look ugly.
The following list presents all the escape sequences used in awk and what they represent.
Unless noted otherwise, all these escape sequences apply to both string constants and regexp
constants:\\

A literal backslash, ‘\’.

\a

The “alert” character, Ctrl-g, ASCII code 7 (BEL). (This often makes some
sort of audible noise.)\b

Backspace, Ctrl-h, ASCII code 8 (BS).

\f

Formfeed, Ctrl-l, ASCII code 12 (FF).

\nNewline, Ctrl-j, ASCII code 10 (LF).

\r

Carriage return, Ctrl-m, ASCII code 13 (CR).

\t

Horizontal TAB, Ctrl-i, ASCII code 9 (HT).

\v

Vertical TAB, Ctrl-k, ASCII code 11 (VT).

\nnn

The octal value nnn, where nnn stands for 1 to 3 digits between ‘0’ and ‘7’. For
example, the code for the ASCII ESC (escape) character is ‘\033’.

Chapter 3: Regular Expressions 53\xhh...

The hexadecimal value hh, where hh stands for a sequence of hexadecimal digits(‘0’–‘9’, and either ‘A’–‘F’ or ‘a’–‘f’). A maximum of two digits are allowed
after the ‘\x’. Any further hexadecimal digits are treated as simple letters or
numbers. (c.e.) (The ‘\x’ escape sequence is not allowed in POSIX awk.)
CAUTION: In ISO C, the escape sequence continues until the first
nonhexadecimal digit is seen. For many years, gawk would continue incorporating hexadecimal digits into the value until a nonhexadecimal digit or the end of the string was encountered. However, using more than two hexadecimal digits produced undefined
results. As of version 4.2, only two digits are processed.

\uhh...

The hexadecimal value hh, where hh stands for a sequence of one or morehexadecimal digits (‘0’–‘9’, and either ‘A’–‘F’ or ‘a’–‘f’). A maximum of eight
digits are allowed after the ‘\u’. Any further hexadecimal digits are treated as
simple letters or numbers. (c.e.) (The ‘\u’ escape sequence is not allowed in
POSIX awk.)
This escape sequence is intended for designating a character in the current locale’s character set.1 gawk first converts the given digits into an integer and thentranslates the given “wide character” value into the current locale’s multibyte
encoding. If the wide character value does not represent a valid character, or if
the character is valid but cannot be encoded into the current locale’s multibyte
encoding, the value becomes "?". gawk issues a warning message when thishappens.

\/

A literal slash (should be used for regexp constants only). This sequence isused when you want to write a regexp constant that contains a slash (suchas /.*:\/home\/[[:alnum:]]+:.*/; the ‘[[:alnum:]]’ notation is discussed
in Section 3.4 [Using Bracket Expressions], page 58). Because the regexp is
delimited by slashes, you need to escape any slash that is part of the pattern,in order to tell awk to keep processing the rest of the regexp.

\"

A literal double quote (should be used for string constants only). This sequenceis used when you want to write a string constant that contains a double quote(such as "He said \"hi!\" to her."). Because the string is delimited by double quotes, you need to escape any quote that is part of the string, in order totell awk to keep processing the rest of the string.

In gawk, a number of additional two-character sequences that begin with a backslash havespecial meaning in regexps. See Section 3.7 [gawk-Specific Regexp Operators], page 61.
In a regexp, a backslash before any character that is not in the previous list and not listed
in Section 3.7 [gawk-Specific Regexp Operators], page 61, means that the next charactershould be taken literally, even if it would normally be a regexp operator. For example,/a\+b/ matches the three characters ‘a+b’.
For complete portability, do not use a backslash before any character not shown in theprevious list or that is not an operator.
1

Typically, this is a Unicode-based locale, but it doesn’t have to be.

54 GAWK: Effective AWK ProgrammingBackslash Before Regular Characters
If you place a backslash in a string constant before something that is not one of thecharacters previously listed, POSIX awk purposely leaves what happens as undefined.
There are two choices:
Strip the backslash out
This is what BWK awk and gawk both do. For example, "a\qc" is the sameas "aqc". (Because this is such an easy bug both to introduce and to miss,gawk warns you about it.) Consider ‘FS = "[ \t]+\|[ \t]+"’ to use verticalbars surrounded by whitespace as the field separator. There should be twobackslashes in the string: ‘FS = "[ \t]+\\|[ \t]+"’.)
Leave the backslash alone
Some other awk implementations do this. In such implementations, typing"a\qc" is the same as typing "a\\qc".
To summarize:• The escape sequences in the preceding list are always processed first, for both string
constants and regexp constants. This happens very early, as soon as awk reads yourprogram.
• gawk processes both regexp constants and dynamic regexps (see Section 3.6 [Using
Dynamic Regexps], page 60), for the special operators listed in Section 3.7 [gawkSpecific Regexp Operators], page 61.
• A backslash before any other character means to treat that character literally.
Escape Sequences for Metacharacters
Suppose you use an octal or hexadecimal escape to represent a regexp metacharacter.
(See Section 3.3 [Regular Expression Operators], page 54.) Does awk treat the characteras a literal character or as a regexp operator?
Historically, such characters were taken literally.
However, the POSIX standard
indicates that they should be treated as real metacharacters, which is what gawk does. In
compatibility mode (see Section 2.2 [Command-Line Options], page 33), gawk treats thecharacters represented by octal and hexadecimal escape sequences literally when used inregexp constants. Thus, /a\52b/ is equivalent to /a\*b/.

3.3 Regular Expression Operators
You can combine regular expressions with special characters, called regular expression operators or metacharacters, to increase the power and versatility of regular expressions.

3.3.1 Regexp Operators in awk
The escape sequences described earlier in Section 3.2 [Escape Sequences], page 52, arevalid inside a regexp. They are introduced by a ‘\’ and are recognized and converted into
corresponding real characters as the very first step in processing regexps.

Chapter 3: Regular Expressions 55

Here is a list of metacharacters. All characters that are not escape sequences and thatare not listed here stand for themselves:\

This suppresses the special meaning of a character when matching. For example, ‘\$’ matches the character ‘$’.

^This matches the beginning of a string. ‘^@chapter’ matches ‘@chapter’ atthe beginning of a string, for example, and can be used to identify chapter
beginnings in Texinfo source files. The ‘^’ is known as an anchor, because it
anchors the pattern to match only at the beginning of the string.
It is important to realize that ‘^’ does not match the beginning of a line (thepoint right after a ‘\n’ newline character) embedded in a string. The condition
is not true in the following example:
if ("line1\nLINE 2" ~ /^L/) ...
This is similar to ‘^’, but it matches only at the end of a string. For example,‘p$’ matches a record that ends with a ‘p’. The ‘$’ is an anchor and does
not match the end of a line (the point right before a ‘\n’ newline character)embedded in a string. The condition in the following example is not true:$

if ("line1\nLINE 2" ~ /1$/) ...
. (period) This matches any single character, including the newline character. For example, ‘.P’ matches any single character followed by a ‘P’ in a string. Using
concatenation, we can make a regular expression such as ‘U.A’, which matches
any three-character sequence that begins with ‘U’ and ends with ‘A’.
In strict POSIX mode (see Section 2.2 [Command-Line Options], page 33), ‘.’does not match the nul character, which is a character with all bits equal tozero. Otherwise, nul is just another character. Other versions of awk may not
be able to match the nul character.
[. . . ]This is called a bracket expression.2 It matches any one of the characters thatare enclosed in the square brackets. For example, ‘[MVX]’ matches any oneof the characters ‘M’, ‘V’, or ‘X’ in a string. A full discussion of what can beinside the square brackets of a bracket expression is given in Section 3.4 [Using
Bracket Expressions], page 58.

[^. . . ]This is a complemented bracket expression. The first character after the ‘[’must be a ‘^’. It matches any characters except those in the square brackets.
For example, ‘[^awk]’ matches any character that is not an ‘a’, ‘w’, or ‘k’.

|

This is the alternation operator and it is used to specify alternatives. The ‘|’
has the lowest precedence of all the regular expression operators. For example,‘^P|[aeiouy]’ matches any string that matches either ‘^P’ or ‘[aeiouy]’. Thismeans it matches any string that starts with ‘P’ or contains (anywhere withinit) a lowercase English vowel.
The alternation applies to the largest possible regexps on either side.
2

In other literature, you may see a bracket expression referred to as either a character set, a characterclass, or a character list.

56 GAWK: Effective AWK Programming(. . . )Parentheses are used for grouping in regular expressions, as in arithmetic. They
can be used to concatenate regular expressions containing the alternation operator, ‘|’. For example, ‘@(samp|code)\{[^}]+\}’ matches both ‘@code{foo}’and ‘@samp{bar}’. (These are Texinfo formatting control sequences. The ‘+’ isexplained further on in this list.)
The left or opening parenthesis is always a metacharacter; to match one literally,
precede it with a backslash. However, the right or closing parenthesis is only
special when paired with a left parenthesis; an unpaired right parenthesis is(silently) treated as a regular character.

*

This symbol means that the preceding regular expression should be repeatedas many times as necessary to find a match. For example, ‘ph*’ applies the ‘*’
symbol to the preceding ‘h’ and looks for matches of one ‘p’ followed by any
number of ‘h’s. This also matches just ‘p’ if no ‘h’s are present.
There are two subtle points to understand about how ‘*’ works. First, the ‘*’applies only to the single preceding regular expression component (e.g., in ‘ph*’,it applies just to the ‘h’). To cause ‘*’ to apply to a larger subexpression, use
parentheses: ‘(ph)*’ matches ‘ph’, ‘phph’, ‘phphph’, and so on.
Second, ‘*’ finds as many repetitions as possible. If the text to be matched is‘phhhhhhhhhhhhhhooey’, ‘ph*’ matches all of the ‘h’s.

+

This symbol is similar to ‘*’, except that the preceding expression must be
matched at least once. This means that ‘wh+y’ would match ‘why’ and ‘whhy’,
but not ‘wy’, whereas ‘wh*y’ would match all three.

?

This symbol is similar to ‘*’, except that the preceding expression can bematched either once or not at all. For example, ‘fe?d’ matches ‘fed’ and‘fd’, but nothing else.

{n}
{n,}
{n,m}

One or two numbers inside braces denote an interval expression. If there is one
number in the braces, the preceding regexp is repeated n times. If there are
two numbers separated by a comma, the preceding regexp is repeated n to mtimes. If there is one number followed by a comma, then the preceding regexp
is repeated at least n times:wh{3}yMatches ‘whhhy’, but not ‘why’ or ‘whhhhy’.

wh{3,5}yMatches ‘whhhy’, ‘whhhhy’, or ‘whhhhhy’ only.

wh{2,}yMatches ‘whhy’, ‘whhhy’, and so on.

In regular expressions, the ‘*’, ‘+’, and ‘?’ operators, as well as the braces ‘{’ and ‘}’,have the highest precedence, followed by concatenation, and finally by ‘|’. As in arithmetic,
parentheses can change how operators are grouped.
According to the POSIX specification, when ‘*’, ‘+’, ‘?’, or ‘{’ are not preceded by acharacter, the behavior is “undefined.” In practice, for gawk, the ‘*’, ‘+’, ‘?’ and ‘{’ operators
stand for themselves when there is nothing in the regexp that precedes them. For example,/+/ matches a literal plus sign. However, many other versions of awk treat such a usage asa syntax error.

Chapter 3: Regular Expressions 57

What About The Empty Regexp?
We describe here an advanced regexp usage. Feel free to skip it upon first reading.
You can supply an empty regexp constant (‘//’) in all places where a regexp is expected.
Is this useful? What does it match?
It is useful. It matches the (invisible) empty string at the start and end of a stringof characters, as well as the empty string between characters. This is best illustrated
with the gsub() function, which makes global substitutions in a string (see Section 9.1.4[String-Manipulation Functions], page 196). Normal usage of gsub() is like so:$ awk '> BEGIN {>x = "ABC_CBA">
gsub(/B/, "bb", x)>print x> }'a AbbC_CbbA
We can use gsub() to see where the empty strings are that match the empty regexp:$ awk '> BEGIN {>x = "ABC">gsub(//, "x", x)>print x> }'a xAxBxCx

3.3.2 Some Notes On Interval Expressions
Interval expressions were not traditionally available in awk. They were added as part of the
POSIX standard to make awk and egrep consistent with each other.
Initially, because old programs may use ‘{’ and ‘}’ in regexp constants, gawk did notmatch interval expressions in regexps.
However, beginning with version 4.0, gawk does match interval expressions by default.
This is because compatibility with POSIX has become more important to most gawk users
than compatibility with old programs.
For programs that use ‘{’ and ‘}’ in regexp constants, it is good practice to always escapethem with a backslash. Then the regexp constants are valid and work the way you wantthem to, using any version of awk.3
When ‘{’ and ‘}’ appear in regexp constants in a way that cannot be interpreted as aninterval expression (such as /q{a}/), then they stand for themselves.
As mentioned, interval expressions were not traditionally available in awk. In March of
2019, BWK awk (finally) acquired them. Starting with version 5.2, gawk’s --traditional
option no longer disables interval expressions in regular expressions.
3

Use two backslashes if you’re using a string constant with a regexp operator or function.

58 GAWK: Effective AWK Programming

POSIX says that interval expressions containing repetition counts greater than 255 produce unspecified results.
In the manual for GNU grep, Paul Eggert notes the following:
Interval expressions may be implemented internally via repetition.
For example, ‘^(a|bc){2,4}$’ might be implemented as‘^(a|bc)(a|bc)((a|bc)(a|bc)?)?$’. A large repetition count may exhaust
memory or greatly slow matching. Even small counts can cause problems if
cascaded; for example, ‘grep -E ".*{10,}{10,}{10,}{10,}{10,}"’ is likely
to overflow a stack. Fortunately, regular expressions like these are typicallyartificial, and cascaded repetitions do not conform to POSIX so cannot be
used in portable programs anyway.
This same caveat applies to gawk.

3.4 Using Bracket Expressions
As mentioned earlier, a bracket expression matches any character among those listed between the opening and closing square brackets.
Within a bracket expression, a range expression consists of two characters separated by ahyphen. It matches any single character that sorts between the two characters, based upon
the system’s native character set. For example, ‘[0-9]’ is equivalent to ‘[0123456789]’.
(See Section A.8 [Regexp Ranges and Locales: A Long Sad Story], page 486, for an explanation of how the POSIX standard and gawk have changed over time. This is mainly ofhistorical interest.)
With the increasing popularity of the Unicode character standard, there is an additional
wrinkle to consider. Octal and hexadecimal escape sequences inside bracket expressions aretaken to represent only single-byte characters (characters whose values fit within the range
0–256). To match a range of characters where the endpoints of the range are larger than256, enter the multibyte encodings of the characters directly.
To include one of the characters ‘\’, ‘]’, ‘-’, or ‘^’ in a bracket expression, put a ‘\’ infront of it. For example:[d\]]matches either ‘d’ or ‘]’. Additionally, if you place ‘]’ right after the opening ‘[’, the closing
bracket is treated as one of the characters to be matched.
The treatment of ‘\’ in bracket expressions is compatible with other awk implementations
and is also mandated by POSIX. The regular expressions in awk are a superset of the POSIX
specification for Extended Regular Expressions (EREs). POSIX EREs are based on theregular expressions accepted by the traditional egrep utility.
Character classes are a feature introduced in the POSIX standard. A character class isa special notation for describing lists of characters that have a specific attribute, but theactual characters can vary from country to country and/or from character set to characterset. For example, the notion of what is an alphabetic character differs between the UnitedStates and France.
A character class is only valid in a regexp inside the brackets of a bracket expression.
Character classes consist of ‘[:’, a keyword denoting the class, and ‘:]’. Table 3.1 lists thecharacter classes defined by the POSIX standard.

Chapter 3: Regular Expressions 59

Class[:alnum:][:alpha:][:blank:][:cntrl:][:digit:][:graph:][:lower:][:print:][:punct:][:space:][:upper:][:xdigit:]

MeaningAlphanumeric characters
Alphabetic characters
Space and TAB characters
Control characters
Numeric characters
Characters that are both printable and visible (a space is printable but notvisible, whereas an ‘a’ is both)
Lowercase alphabetic characters
Printable characters (characters that are not control characters)
Punctuation characters (characters that are not letters, digits, control characters, or space characters)
Space characters (these are: space, TAB, newline, carriage return, formfeed
and vertical tab)Uppercase alphabetic characters
Characters that are hexadecimal digits

Table 3.1: POSIX character classes
For example, before the POSIX standard, you had to write /[A-Za-z0-9]/ to matchalphanumeric characters. If your character set had other alphabetic characters in it, thiswould not match them. With the POSIX character classes, you can write /[[:alnum:]]/to match the alphabetic and numeric characters in your character set.
Some utilities that match regular expressions provide a nonstandard ‘[:ascii:]’ character class; awk does not. However, you can simulate such a construct using ‘[\x00-\x7F]’.
This matches all values numerically between zero and 127, which is the defined range of the
ASCII character set. Use a complemented character list (‘[^\x00-\x7F]’) to match any
single-byte characters that are not in the ASCII range.
NOTE: Some older versions of Unix awk treat [:blank:] like [:space:], incorrectly matching more characters than they should. Caveat Emptor.
Two additional special sequences can appear in bracket expressions. These apply to
non-ASCII character sets, which can have single symbols (called collating elements) thatare represented with more than one character. They can also have several characters thatare equivalent for collating, or sorting, purposes. (For example, in French, a plain “e” anda grave-accented “è” are equivalent.) These sequences are:Collating symbols
Multicharacter collating elements enclosed between ‘[.’ and ‘.]’. For example,
if ‘ch’ is a collating element, then ‘[[.ch.]]’ is a regexp that matches thiscollating element, whereas ‘[ch]’ is a regexp that matches either ‘c’ or ‘h’.
Equivalence classes
Locale-specific names for a list of characters that are equal. The name is enclosed between ‘[=’ and ‘=]’. For example, the name ‘e’ might be used torepresent all of “e,” “^e,” “è,” and “é.” In this case, ‘[[=e=]]’ is a regexp thatmatches any of ‘e’, ‘^e’, ‘é’, or ‘è’.
These features are very valuable in non-English-speaking locales.

60 GAWK: Effective AWK Programming

CAUTION: The library functions that gawk uses for regular expression matching currently recognize only POSIX character classes; they do not recognize
collating symbols or equivalence classes.
Inside a bracket expression, an opening bracket (‘[’) that does not start a characterclass, collating element or equivalence class is taken literally. This is also true of ‘.’ and ‘*’.

3.5 How Much Text Matches?
Consider the following:echo aaaabcd | awk '{ sub(/a+/, "<A>"); print }'This example uses the sub() function to make a change to the input record. (sub()replaces the first instance of any text matched by the first argument with the string provided
as the second argument; see Section 9.1.4 [String-Manipulation Functions], page 196.) Here,the regexp /a+/ indicates “one or more ‘a’ characters,” and the replacement text is ‘<A>’.
The input contains four ‘a’ characters. awk (and POSIX) regular expressions always
match the leftmost, longest sequence of input characters that can match. Thus, all four ‘a’characters are replaced with ‘<A>’ in this example:$ echo aaaabcd | awk '{ sub(/a+/, "<A>"); print }'a <A>bcd
For simple match/no-match tests, this is not so important. But when doing text matching and substitutions with the match(), sub(), gsub(), and gensub() functions, it is very
important. Understanding this principle is also important for regexp-based record and field
splitting (see Section 4.1 [How Input Is Split into Records], page 65, and also see Section 4.5[Specifying How Fields Are Separated], page 73).

3.6 Using Dynamic Regexps
The righthand side of a ‘~’ or ‘!~’ operator need not be a regexp constant (i.e., a stringof characters between slashes). It may be any expression. The expression is evaluated andconverted to a string if necessary; the contents of the string are then used as the regexp. A
regexp computed in this way is called a dynamic regexp or a computed regexp:
BEGIN { digits_regexp = "[[:digit:]]+" }$0 ~ digits_regexp{ print }This sets digits_regexp to a regexp that describes one or more digits, and tests whether
the input record matches this regexp.
NOTE: When using the ‘~’ and ‘!~’ operators, be aware that there is a difference
between a regexp constant enclosed in slashes and a string constant enclosed indouble quotes. If you are going to use a string constant, you have to understandthat the string is, in essence, scanned twice: the first time when awk reads yourprogram, and the second time when it goes to match the string on the lefthandside of the operator with the pattern on the right. This is true of any stringvalued expression (such as digits_regexp, shown in the previous example),not just string constants.
What difference does it make if the string is scanned twice? The answer has to dowith escape sequences, and particularly with backslashes. To get a backslash into a regularexpression inside a string, you have to type two backslashes.

Chapter 3: Regular Expressions 61

For example, /\*/ is a regexp constant for a literal ‘*’. Only one backslash is needed.
To do the same thing with a string, you have to type "\\*". The first backslash escapes
the second one so that the string actually contains the two characters ‘\’ and ‘*’.
Given that you can use both regexp and string constants to describe regular expressions,which should you use? The answer is “regexp constants,” for several reasons:• String constants are more complicated to write and more difficult to read. Using regexp
constants makes your programs less error-prone. Not understanding the difference
between the two kinds of constants is a common source of errors.
• It is more efficient to use regexp constants. awk can note that you have supplied aregexp and store it internally in a form that makes pattern matching more efficient.
When using a string constant, awk must first convert the string into this internal form
and then perform the pattern matching.
• Using regexp constants is better form; it shows clearly that you intend a regexp match.
Using \n in Bracket Expressions of Dynamic Regexps
Some older versions of awk do not allow the newline character to be used inside abracket expression for a dynamic regexp:$ awk '$0 ~ "[ \t\n]"'error awk: newline in character class [error ]...
source line number 1
error
context iserror$0 ~ "[ >>> \t\n]" <<<error
But a newline in a regexp constant works with no problem:$ awk '$0 ~ /[ \t\n]/'here is a sample line
a here is a sample line
Ctrl-dgawk does not have this problem, and it isn’t likely to occur often in practice, but it’sworth noting for future reference.

3.7 gawk-Specific Regexp Operators
GNU software that deals with regular expressions provides a number of additional regexp
operators. These operators are described in this section and are specific to gawk; they are
not available in other awk implementations. Most of the additional operators deal withword matching. For our purposes, a word is a sequence of one or more letters, digits, orunderscores (‘_’):\s

Matches any space character as defined by the current locale. Think of it asshorthand for ‘[[:space:]]’.

\S

Matches any character that is not a space, as defined by the current locale.
Think of it as shorthand for ‘[^[:space:]]’.

62 GAWK: Effective AWK Programming\w

Matches any word-constituent character—that is, it matches any letter, digit,
or underscore. Think of it as shorthand for ‘[[:alnum:]_]’.

\W

Matches any character that is not word-constituent. Think of it as shorthandfor ‘[^[:alnum:]_]’.

\<

Matches the empty string at the beginning of a word. For example, /\<away/
matches ‘away’ but not ‘stowaway’.

\>

Matches the empty string at the end of a word. For example, /stow\>/ matches‘stow’ but not ‘stowaway’.

\yMatches the empty string at either the beginning or the end of a word (i.e., theword boundary). For example, ‘\yballs?\y’ matches either ‘ball’ or ‘balls’,
as a separate word.

\B

Matches the empty string that occurs between two word-constituent characters.
For example, /\Brat\B/ matches ‘crate’, but it does not match ‘dirty rat’.
‘\B’ is essentially the opposite of ‘\y’. Another way to think of this is that ‘\B’matches the empty string provided it’s not at the edge of a word.

There are two other operators that work on buffers. In Emacs, a buffer is, naturally, an
Emacs buffer. Other GNU programs, including gawk, consider the entire string to matchas the buffer. The operators are:\`Matches the empty string at the beginning of a buffer (string)\'

Matches the empty string at the end of a buffer (string)

Because ‘^’ and ‘$’ always work in terms of the beginning and end of strings, these
operators don’t add any new capabilities for awk. They are provided for compatibility withother GNU software.
In other GNU software, the word-boundary operator is ‘\b’. However, that conflicts
with the awk language’s definition of ‘\b’ as backspace, so gawk uses a different letter. An
alternative method would have been to require two backslashes in the GNU operators, but
this was deemed too confusing. The current method of using ‘\y’ for the GNU ‘\b’ appears
to be the lesser of two evils.
The various command-line options (see Section 2.2 [Command-Line Options], page 33)
control how gawk interprets characters in regexps:
No options
In the default case, gawk provides all the facilities of POSIX regexps and thepreviously described GNU regexp operators.
--posix

Match only POSIX regexps; the GNU operators are not special (e.g., ‘\w’matches a literal ‘w’). Interval expressions are allowed.

--traditional
Match traditional Unix awk regexps. The GNU operators are not special. Because BWK awk supports them, the POSIX character classes (‘[[:alnum:]]’,etc.) are available. So too, interval expressions are allowed. Characters described by octal and hexadecimal escape sequences are treated literally, even if
they represent regexp metacharacters.

Chapter 3: Regular Expressions 63--re-intervalThis option remains for backwards compatibility but no longer has any realeffect.

3.8 Case Sensitivity in Matching
Case is normally significant in regular expressions, both when matching ordinary characters(i.e., not metacharacters) and inside bracket expressions. Thus, a ‘w’ in a regular expression
matches only a lowercase ‘w’ and not an uppercase ‘W’.
The simplest way to do a case-independent match is to use a bracket expression—for
example, ‘[Ww]’. However, this can be cumbersome if you need to use it often, and it canmake the regular expressions harder to read. There are two alternatives that you might
prefer.
One way to perform a case-insensitive match at a particular point in the program is
to convert the data to a single case, using the tolower() or toupper() built-in string
functions (which we haven’t discussed yet; see Section 9.1.4 [String-Manipulation Functions],
page 196). For example:
tolower($1) ~ /foo/{ ... }

converts the first field to lowercase before matching against it. This works in any POSIXcompliant awk.
Another method, specific to gawk, is to set the variable IGNORECASE to a nonzero value(see Section 7.5 [Predefined Variables], page 163). When IGNORECASE is not zero, all regexp
and string operations ignore case.
Changing the value of IGNORECASE dynamically controls the case sensitivity of the program as it runs. Case is significant by default because IGNORECASE (like most variables) is
initialized to zero:x = "aB"
if (x ~ /ab/) ...

# this test will fail

IGNORECASE = 1
if (x ~ /ab/) ...

# now it will succeed

In general, you cannot use IGNORECASE to make certain rules case insensitive and otherrules case sensitive, as there is no straightforward way to set IGNORECASE just for the patternof a particular rule.4 To do this, use either bracket expressions or tolower(). However,
one thing you can do with IGNORECASE only is dynamically turn case sensitivity on or off
for all the rules at once.
IGNORECASE can be set on the command line or in a BEGIN rule (see Section 2.3 [Other
Command-Line Arguments], page 40; also see Section 7.1.4.1 [Startup and Cleanup Actions],
page 150). Setting IGNORECASE from the command line is a way to make a program caseinsensitive without having to edit it.
4

Experienced C and C++ programmers will note that it is possible, using something like ‘IGNORECASE =1 && /foObAr/ { ... }’ and ‘IGNORECASE = 0 || /foobar/ { ... }’. However, this is somewhat obscure
and we don’t recommend it.

64 GAWK: Effective AWK Programming

In multibyte locales, the equivalences between upper- and lowercase characters are tested
based on the wide-character values of the locale’s character set. Prior to version 5.0, singlebyte characters were tested based on the ISO-8859-1 (ISO Latin-1) character set. However,as of version 5.0, single-byte characters are also tested based on the values of the locale’scharacter set.5
The value of IGNORECASE has no effect if gawk is in compatibility mode (see Section 2.2[Command-Line Options], page 33). Case is always significant in compatibility mode.

3.9 Summary• Regular expressions describe sets of strings to be matched. In awk, regular expression
constants are written enclosed between slashes: /. . . /.
• Regexp constants may be used standalone in patterns and in conditional expressions,or as part of matching expressions using the ‘~’ and ‘!~’ operators.
• Escape sequences let you represent nonprintable characters and also let you represent
regexp metacharacters as literal characters to be matched.
• Regexp operators provide grouping, alternation, and repetition.
• Bracket expressions give you a shorthand for specifying sets of characters that canmatch at a particular point in a regexp. Within bracket expressions, POSIX characterclasses let you specify certain groups of characters in a locale-independent fashion.
• Regular expressions match the leftmost longest text in the string being matched. Thismatters for cases where you need to know the extent of the match, such as for text
substitution and when the record separator is a regexp.
• Matching expressions may use dynamic regexps (i.e., string values treated as regularexpressions).
• gawk’s IGNORECASE variable lets you control the case sensitivity of regexp matching. In
other awk versions, use tolower() or toupper().

5

If you don’t understand this, don’t worry about it; it just means that gawk does the right thing.

Chapter 4: Reading Input Files

65

4 Reading Input Files
In the typical awk program, awk reads all input either from the standard input (by default,
this is the keyboard, but often it is a pipe from another command) or from files whose names
you specify on the awk command line. If you specify input files, awk reads them in order,
processing all the data from one before going on to the next. The name of the current input
file can be found in the predefined variable FILENAME (see Section 7.5 [Predefined Variables],page 163).
The input is read in units called records, and is processed by the rules of your program
one record at a time. By default, each record is one line. Each record is automatically split
into chunks called fields. This makes it more convenient for programs to work on the partsof a record.
On rare occasions, you may need to use the getline command. The getline commandis valuable both because it can do explicit input from any number of files, and because
the files used with it do not have to be named on the awk command line (see Section 4.10
[Explicit Input with getline], page 89).

4.1 How Input Is Split into Records
awk divides the input for your program into records and fields. It keeps track of the numberof records that have been read so far from the current input file. This value is stored ina predefined variable called FNR, which is reset to zero every time a new file is started.
Another predefined variable, NR, records the total number of input records read so far fromall data files. It starts at zero, but is never automatically reset to zero.
Normally, records are separated by newline characters. You can control how records are
separated by assigning values to the built-in variable RS. If RS is any single character, thatcharacter separates records. Otherwise (in gawk), RS is treated as a regular expression. Thismechanism is explained in greater detail shortly.
NOTE: When gawk is invoked with the --csv option, nothing in this section applies. See Section 4.5.4 [Working With Comma Separated Value Files], page 76,
for the details.

4.1.1 Record Splitting with Standard awk
Records are separated by a character called the record separator. By default, the record
separator is the newline character. This is why records are, by default, single lines. To use
a different character for the record separator, simply assign that character to the predefined
variable RS.
Like any other variable, the value of RS can be changed in the awk program with theassignment operator, ‘=’ (see Section 6.2.3 [Assignment Expressions], page 130). The newrecord-separator character should be enclosed in quotation marks, which indicate a string
constant. Often, the right time to do this is at the beginning of execution, before any
input is processed, so that the very first record is read with the proper separator. To dothis, use the special BEGIN pattern (see Section 7.1.4 [The BEGIN and END Special Patterns],
page 150). For example:
awk 'BEGIN { RS = "u" }{ print $0 }' mail-list

66 GAWK: Effective AWK Programmingchanges the value of RS to ‘u’, before reading any input. The new value is a string whose
first character is the letter “u”; as a result, records are separated by the letter “u”. Thenthe input file is read, and the second rule in the awk program (the action with no pattern)
prints each record. Because each print statement adds a newline at the end of its output,
this awk program copies the input with each ‘u’ changed to a newline. Here are the resultsof running the program on mail-list:$ awk 'BEGIN { RS = "u" }>{ print $0 }' mail-list555-5553
amelia.zodiac
a Amelia
a sqF
a e@gmail.com
555-3412
anthony.assert
a AnthonyA
a ro@hotmail.com
555-7685
becky.algebrara BeckyA
a m@gmail.com
555-1675
bill.drowning@hotmail.com
a Bill
555-0542
broderick.aliq
a Brodericka otiens@yahoo.com R
555-2912
camilla.inf
a Camillaa sarRa m@skynet.bea Fabi555-1234
fabi
a s
a s.
a ndevicesim
a s@a cb.edFaa J
555-6699ja lielie.perscraFa tabor@skeeve.com
555-6480
martin.codiciba MartinA
a s@hotmail.com
a Sam
555-3430sam
a el
a el.lanceolis@sh
a .ed
A
a
a Jean-Pa
555-2127
jeanpaa la l.campanor
a m@ny
a .ed
R
a
a

A

Chapter 4: Reading Input Files

67

Note that the entry for the name ‘Bill’ is not split. In the original data file (see Section 1.2[Data files for the Examples], page 23), the line looks like this:Bill

555-1675

bill.drowning@hotmail.com

A

It contains no ‘u’, so there is no reason to split the record, unlike the others, which eachhave one or more occurrences of the ‘u’. In fact, this record is treated as part of the previous
record; the newline separating them in the output is the original newline in the data file,
not the one added by awk when it printed the record!
Another way to change the record separator is on the command line, using the variableassignment feature (see Section 2.3 [Other Command-Line Arguments], page 40):
awk '{ print $0 }' RS="u" mail-list
This sets RS to ‘u’ before processing mail-list.
Using an alphabetic character such as ‘u’ for the record separator is highly likely to
produce strange results. Using an unusual character such as ‘/’ is more likely to produce
correct behavior in the majority of cases, but there are no guarantees. The moral is: KnowYour Data.
gawk allows RS to be a full regular expression (discussed shortly; see Section 4.1.2 [Record
Splitting with gawk], page 67). Even so, using a regular expression metacharacter, such as‘.’ as the single character in the value of RS has no special effect: it is treated literally. Thisis required for backwards compatibility with both Unix awk and with POSIX.
Reaching the end of an input file terminates the current input record, even if the last
character in the file is not the character in RS.
The empty string "" (a string without any characters) has a special meaning as the valueof RS. It means that records are separated by one or more blank lines and nothing else. See
Section 4.9 [Multiple-Line Records], page 86, for more details.
If you change the value of RS in the middle of an awk run, the new value is used todelimit subsequent records, but the record currently being processed, as well as records
already processed, are not affected.
After the end of the record has been determined, gawk sets the variable RT to the text
in the input that matched RS.

4.1.2 Record Splitting with gawk
When using gawk, the value of RS is not limited to a one-character string. If it contains morethan one character, it is treated as a regular expression (see Chapter 3 [Regular Expressions],
page 51). (c.e.) In general, each record ends at the next string that matches the regularexpression; the next record starts at the end of the matching string. This general rule isactually at work in the usual case, where RS contains just a newline: a record ends at the
beginning of the next matching string (the next newline in the input), and the following
record starts just after the end of this string (at the first character of the following line).
The newline, because it matches RS, is not part of either record.
When RS is a single character, RT contains the same single character. However, when RS isa regular expression, RT contains the actual input text that matched the regular expression.
If the input file ends without any text matching RS, gawk sets RT to the null string.

68 GAWK: Effective AWK Programming

The following example illustrates both of these features. It sets RS equal to a regularexpression that matches either a newline or a series of one or more uppercase letters with
optional leading and/or trailing whitespace:$ echo record 1 AAAA record 2 BBBB record 3 |> gawk 'BEGIN { RS = "\n|( *[[:upper:]]+ *)" }>{ print "Record =", $0,"and RT = [" RT "]" }'a Record = record 1 and RT = [ AAAA ]a Record = record 2 and RT = [ BBBB ]a Record = record 3 and RT = [a ]The square brackets delineate the contents of RT, letting you see the leading and trailingwhitespace. The final value of RT is a newline. See Section 11.3.8 [A Simple Stream Editor],
page 321, for a more useful example of RS as a regexp and RT.

If you set RS to a regular expression that allows optional trailing text, such as ‘RS ="abc(XYZ)?"’, it is possible, due to implementation constraints, that gawk may match theleading part of the regular expression, but not the trailing part, particularly if the input
text that could match the trailing part is fairly long. gawk attempts to avoid this problem,
but currently, there’s no guarantee that this will never happen.
Caveats When Using Regular Expressions for RS
Remember that in awk, the ‘^’ and ‘$’ anchor metacharacters match the beginning andend of a string, and not the beginning and end of a line. As a result, something like ‘RS= "^[[:upper:]]"’ can only match at the beginning of a file. This is because gawk views
the input file as one long string that happens to contain newline characters. It is thus best
to avoid anchor metacharacters in the value of RS.
Record splitting with regular expressions works differently than regexp matching withthe sub(), gsub(), and gensub() (see Section 9.1.4 [String-Manipulation Functions],
page 196). Those functions allow a regexp to match the empty string; record splitting
does not. Thus, for example ‘RS = "()"’ does not split records between characters.

The use of RS as a regular expression and the RT variable are gawk extensions; they are
not available in compatibility mode (see Section 2.2 [Command-Line Options], page 33). In
compatibility mode, only the first character of the value of RS determines the end of therecord.

mawk has allowed RS to be a regexp for decades. As of October, 2019, BWK awk also
supports it. Neither version supplies RT, however.

Chapter 4: Reading Input Files69

RS = "\0" Is Not Portable
There are times when you might want to treat an entire data file as a single record.
The only way to make this happen is to give RS a value that you know doesn’t occur inthe input file. This is hard to do in a general way, such that a program always works forarbitrary input files.
You might think that for text files, the nul character, which consists of a characterwith all bits equal to zero, is a good value to use for RS in this case:
BEGIN { RS = "\0" } # whole file becomes one record?
gawk in fact accepts this, and uses the nul character for the record separator. This
works for certain special files, such as /proc/environ on GNU/Linux systems, where thenul character is in fact the record separator. However, this usage is not portable to mostother awk implementations.
Almost all other awk implementations1 store strings internally as C-style strings. C
strings use the nul character as the string terminator. In effect, this means that ‘RS ="\0"’ is the same as ‘RS = ""’.
It happens that recent versions of mawk can use the nul character as a record separator.
However, this is a special case: mawk does not allow embedded nul characters in strings.
(This may change in a future version of mawk.)
See Section 10.2.8 [Reading a Whole File at Once], page 251, for an interesting way
to read whole files. If you are using gawk, see Section 17.7.10 [Reading an Entire File],
page 466, for another option.

4.2 Examining Fields
When awk reads an input record, the record is automatically parsed or separated by the awk
utility into chunks called fields. By default, fields are separated by whitespace, like words ina line. Whitespace in awk means any string of one or more spaces, TABs, or newlines; othercharacters that are considered whitespace by other languages (such as formfeed, verticaltab, etc.) are not considered whitespace by awk.
The purpose of fields is to make it more convenient for you to refer to these pieces of therecord. You don’t have to use them—you can operate on the whole record if you want—but
fields are what make simple awk programs so powerful.
You use a dollar sign (‘$’) to refer to a field in an awk program, followed by the numberof the field you want. Thus, $1 refers to the first field, $2 to the second, and so on. (Unlikein the Unix shells, the field numbers are not limited to single digits. $127 is the 127th fieldin the record.) For example, suppose the following is a line of input:
This seems like a pretty nice example.
Here the first field, or $1, is ‘This’, the second field, or $2, is ‘seems’, and so on. Note thatthe last field, $7, is ‘example.’. Because there is no space between the ‘e’ and the ‘.’, theperiod is considered part of the seventh field.
1

At least that we know about.

70 GAWK: Effective AWK Programming

NF is a predefined variable whose value is the number of fields in the current record. awk
automatically updates the value of NF each time it reads a record. No matter how many
fields there are, the last field in a record can be represented by $NF. So, $NF is the same as$7, which is ‘example.’. If you try to reference a field beyond the last one (such as $8 when
the record has only seven fields), you get the empty string. If used in a numeric operation,
you get zero.2
The use of $0, which looks like a reference to the “zeroth” field, is a special case: it
represents the whole input record. Use it when you are not interested in specific fields.
Here are some more examples:$ awk '$1 ~ /li/ { print $0 }' mail-list555-5553
amelia.zodiacusque@gmail.com
a Amelia
555-6699
julie.perscrutabor@skeeve.com
a Julie

F
F

This example prints each record in the file mail-list whose first field contains the string‘li’.
By contrast, the following example looks for ‘li’ in the entire record and prints the firstand last fields for each matching input record:$ awk '/li/ { print $1, $NF }' mail-lista Amelia F
a Broderick R
a Julie F
a Samuel A

4.3 Nonconstant Field Numbers
A field number need not be a constant. Any expression in the awk language can be used
after a ‘$’ to refer to a field. The value of the expression specifies the field number. If thevalue is a string, rather than a number, it is converted to a number. Consider this example:
awk '{ print $NR }'Recall that NR is the number of records read so far: one in the first record, two in the second,and so on. So this example prints the first field of the first record, the second field of thesecond record, and so on. For the twentieth record, field number 20 is printed; most likely,
the record has fewer than 20 fields, so this prints a blank line. Here is another example ofusing expressions as field numbers:
awk '{ print $(2*2) }' mail-list
awk evaluates the expression ‘(2*2)’ and uses its value as the number of the field to print.
The ‘*’ represents multiplication, so the expression ‘2*2’ evaluates to four. The parentheses
are used so that the multiplication is done before the ‘$’ operation; they are necessary
whenever there is a binary operator3 in the field-number expression. This example, then,prints the type of relationship (the fourth field) for every line of the file mail-list. (Allof the awk operators are listed, in order of decreasing precedence, in Section 6.5 [Operator
Precedence (How Operators Nest)], page 144.)2
3

In either case, with the --lint option, gawk warns that you are referencing an uninitialized field.
A binary operator, such as ‘*’ for multiplication, is one that takes two operands. The distinction isrequired because awk also has unary (one-operand) and ternary (three-operand) operators.

Chapter 4: Reading Input Files71

If the field number you compute is zero, you get the entire record. Thus, ‘$(2-2)’ has thesame value as $0. Negative field numbers are not allowed; trying to reference one usuallyterminates the program. (The POSIX standard does not define what happens when you
reference a negative field number. gawk notices this and terminates your program. Otherawk implementations may behave differently.)
As mentioned in Section 4.2 [Examining Fields], page 69, awk stores the current record’s
number of fields in the built-in variable NF (also see Section 7.5 [Predefined Variables],page 163). Thus, the expression $NF is not a special feature—it is the direct consequenceof evaluating NF and using its value as a field number.

4.4 Changing the Contents of a Field
The contents of a field, as seen by awk, can be changed within an awk program; this changes
what awk perceives as the current input record. (The actual input is untouched; awk nevermodifies the input file.) Consider the following example and its output:$ awk '{ nboxes = $3 ; $3 = $3 - 10>
print nboxes, $3 }' inventory-shippeda 25 15
a 32 22a 24 14...
The program first saves the original value of field three in the variable nboxes. The ‘-’sign represents subtraction, so this program reassigns field three, $3, as the original valueof field three minus ten: ‘$3 - 10’. (See Section 6.2.1 [Arithmetic Operators], page 128.)
Then it prints the original and new values for field three. (Someone in the warehouse madea consistent mistake while inventorying the red boxes.)
For this to work, the text in $3 must make sense as a number; the string of charactersmust be converted to a number for the computer to do arithmetic on it. The number
resulting from the subtraction is converted back to a string of characters that then becomes
field three. See Section 6.1.4 [Conversion of Strings and Numbers], page 126.
When the value of a field is changed (as perceived by awk), the text of the input record
is recalculated to contain the new field where the old one was. In other words, $0 changes
to reflect the altered field. Thus, this program prints a copy of the input file, with 10
subtracted from the second field of each line:$ awk '{ $2 = $2 - 10; print $0 }' inventory-shipped
a Jan 3 25 15 115
a Feb 5 32 24 226
a Mar 5 24 34 228...
It is also possible to assign contents to fields that are out of range. For example:$ awk '{ $6 = ($5 + $4 + $3 + $2)>
print $6 }' inventory-shipped
a 168
a 297
a 301

72 GAWK: Effective AWK Programming...
We’ve just created $6, whose value is the sum of fields $2, $3, $4, and $5. The ‘+’ sign
represents addition. For the file inventory-shipped, $6 represents the total number ofparcels shipped for a particular month.
Creating a new field changes awk’s internal copy of the current input record, which isthe value of $0. Thus, if you do ‘print $0’ after adding a field, the record printed includes
the new field, with the appropriate number of field separators between it and the previously
existing fields.
This recomputation affects and is affected by NF (the number of fields; see Section 4.2[Examining Fields], page 69). For example, the value of NF is set to the number of thehighest field you create. The exact format of $0 is also affected by a feature that has
not been discussed yet: the output field separator, OFS, used to separate the fields (seeSection 5.3 [Output Separators], page 101).
Note, however, that merely referencing an out-of-range field does not change the valueof either $0 or NF. Referencing an out-of-range field only produces an empty string. For
example:if ($(NF+1) != "")print "can't happen"
else
print "everything is normal"
should print ‘everything is normal’, because NF+1 is certain to be out of range. (See
Section 7.4.1 [The if-else Statement], page 154, for more information about awk’s if-else
statements. See Section 6.3.2 [Variable Typing and Comparison Expressions], page 135, formore information about the ‘!=’ operator.)
It is important to note that making an assignment to an existing field changes the valueof $0 but does not change the value of NF, even when you assign the empty string to a field.
For example:$ echo a b c d | awk '{ OFS = ":"; $2 = "">print $0; print NF }'a a::c:da 4
The field is still there; it just has an empty value, delimited by the two colons between ‘a’and ‘c’. This example shows what happens if you create a new field:$ echo a b c d | awk '{ OFS = ":"; $2 = ""; $6 = "new">print $0; print NF }'a a::c:d::newa 6
The intervening field, $5, is created with an empty value (indicated by the second pair ofadjacent colons), and NF is updated with the value six.
Decrementing NF throws away the values of the fields after the new value of NF andrecomputes $0. Here is an example:$ echo a b c d e f | awk '{ print "NF =", NF;>
NF = 3; print $0 }'a NF = 6

Chapter 4: Reading Input Files73

a a b cCAUTION: Some versions of awk don’t rebuild $0 when NF is decremented.
Until August, 2018, this included BWK awk; fortunately his version now handles
this correctly.
Finally, there are times when it is convenient to force awk to rebuild the entire record,using the current values of the fields and OFS. To do this, use the seemingly innocuous
assignment:$1 = $1
print $0

# force record to be reconstituted# or whatever else with $0

This forces awk to rebuild the record. It does help to add a comment, as we’ve shown here.
There is a flip side to the relationship between $0 and the fields. Any assignment to $0
causes the record to be reparsed into fields using the current value of FS. This also applies
to any built-in function that updates $0, such as sub() and gsub() (see Section 9.1.4[String-Manipulation Functions], page 196).


Understanding $0
It is important to remember that $0 is the full record, exactly as it was read fromthe input. This includes any leading or trailing whitespace, and the exact whitespace (orother characters) that separates the fields.
It is a common error to try to change the field separators in a record simply by setting
FS and OFS, and then expecting a plain ‘print’ or ‘print $0’ to print the modified record.
But this does not work, because nothing was done to change the record itself. Instead,
you must force the record to be rebuilt, typically with a statement such as ‘$1 = $1’, as
described earlier.

4.5 Specifying How Fields Are Separated
The field separator, which is either a single character or a regular expression, controls theway awk splits an input record into fields. awk scans the input record for character sequencesthat match the separator; the fields themselves are the text between the matches.
In the examples that follow, we use the bullet symbol (•) to represent spaces in the
output. If the field separator is ‘oo’, then the following line:
moo goo gai pan
is split into three fields: ‘m’, ‘•g’, and ‘•gai•pan’. Note the leading spaces in the values ofthe second and third fields.
The field separator is represented by the predefined variable FS. Shell programmers takenote: awk does not use the name IFS that is used by the POSIX-compliant shells (such as
the Unix Bourne shell, sh, or Bash).
The value of FS can be changed in the awk program with the assignment operator, ‘=’(see Section 6.2.3 [Assignment Expressions], page 130). Often, the right time to do this isat the beginning of execution before any input has been processed, so that the very firstrecord is read with the proper separator. To do this, use the special BEGIN pattern (see

74 GAWK: Effective AWK Programming

Section 7.1.4 [The BEGIN and END Special Patterns], page 150). For example, here we set
the value of FS to the string ":":
awk 'BEGIN { FS = ":" } ; { print $2 }'Given the input line:
John Q. Smith: 29 Oak St.: Walamazoo: MI 42139
this awk program extracts and prints the string ‘•29•Oak•St.’.
Sometimes the input data contains separator characters that don’t separate fields theway you thought they would. For instance, the person’s name in the example we just used
might have a title or suffix attached, such as:
John Q. Smith: LXIX: 29 Oak St.: Walamazoo: MI 42139
The same program would extract ‘•LXIX’ instead of ‘•29•Oak•St.’. If you were expecting
the program to print the address, you would be surprised. The moral is to choose your datalayout and separator characters carefully to prevent such problems. (If the data is not in aform that is easy to process, perhaps you can massage it first with a separate awk program.)

4.5.1 Whitespace Normally Separates Fields
Fields are normally separated by whitespace sequences (spaces, TABs, and newlines), notby single spaces. Two spaces in a row do not delimit an empty field. The default value ofthe field separator FS is a string containing a single space, " ". If awk interpreted this value
in the usual way, each space character would separate fields, so two spaces in a row would
make an empty field between them. The reason this does not happen is that a single spaceas the value of FS is a special case—it is taken to specify the default manner of delimiting
fields.
If FS is any other single character, such as ",", then each occurrence of that character
separates two fields. Two consecutive occurrences delimit an empty field. If the characteroccurs at the beginning or the end of the line, that too delimits an empty field. The spacecharacter is the only single character that does not follow these rules.

4.5.2 Using Regular Expressions to Separate Fields
The previous subsection discussed the use of single characters or simple strings as the valueof FS. More generally, the value of FS may be a string containing any regular expression. In
this case, each match in the record for the regular expression separates fields. For example,
the assignment:
FS = ", \t"makes every area of an input line that consists of a comma followed by a space and a TAB
into a field separator.
For a less trivial example of a regular expression, try using single spaces to separate
fields the way single commas are used. FS can be set to "[ ]" (left bracket, space, rightbracket). This regular expression matches a single space and nothing else (see Chapter 3[Regular Expressions], page 51).
There is an important difference between the two cases of ‘FS = " "’ (a single space) and‘FS = "[ \t\n]+"’ (a regular expression matching one or more spaces, TABs, or newlines).
For both values of FS, fields are separated by runs (multiple adjacent occurrences) of spaces,
TABs, and/or newlines. However, when the value of FS is " ", awk first strips leading andChapter 4: Reading Input Files

75

trailing whitespace from the record and then decides where the fields are. For example, thefollowing pipeline prints ‘b’:
$ echo ' a b c d ' | awk '{ print $2 }'a bHowever, this pipeline prints ‘a’ (note the extra spaces around each letter):$ echo ' a b c d ' | awk 'BEGIN { FS = "[ \t\n]+" }>{ print $2 }'a aIn this case, the first field is null, or empty.
The stripping of leading and trailing whitespace also comes into play whenever $0 isrecomputed. For instance, study this pipeline:$ echo 'a b c d' | awk '{ print; $2 = $2; print }'a b c daa a b c d
The first print statement prints the record as it was read, with leading whitespace intact.
The assignment to $2 rebuilds $0 by concatenating $1 through $NF together, separated by
the value of OFS (which is a space by default). Because the leading whitespace was ignored
when finding $1, it is not part of the new $0. Finally, the last print statement prints the
new $0.
There is an additional subtlety to be aware of when using regular expressions for field
splitting. It is not well specified in the POSIX standard, or anywhere else, what ‘^’ means
when splitting fields. Does the ‘^’ match only at the beginning of the entire record? Or
is each field separator a new string? It turns out that different awk versions answer thisquestion differently, and you should not rely on any specific behavior in your programs.
As a point of information, BWK awk allows ‘^’ to match only at the beginning of therecord. gawk also works this way. For example:$ echo 'xxAA xxBxx C' |> gawk -F '(^x+)|( +)' '{ for (i = 1; i <= NF; i++)>printf "-->%s<--\n", $i }'a --><-a -->AA<-a -->xxBxx<-a -->C<-Finally, field splitting with regular expressions works differently than regexp matchingwith the sub(), gsub(), and gensub() (see Section 9.1.4 [String-Manipulation Functions],
page 196). Those functions allow a regexp to match the empty string; field splitting doesnot. Thus, for example ‘FS = "()"’ does not split fields between characters.

4.5.3 Making Each Character a Separate Field
There are times when you may want to examine each character of a record separately. Thiscan be done in gawk by simply assigning the null string ("") to FS. (c.e.) In this case, eachindividual character in the record becomes a separate field. For example:$ echo a b | gawk 'BEGIN { FS = "" }>{

76 GAWK: Effective AWK Programming>>>a Field 1 is aa Field 2 isa Field 3 is bfor (i = 1; i <= NF; i = i + 1)
print "Field", i, "is", $i}'

Traditionally, the behavior of FS equal to "" was not defined. In this case, most versionsof Unix awk simply treat the entire record as only having one field. In compatibility mode(see Section 2.2 [Command-Line Options], page 33), if FS is the null string, then gawk also
behaves this way.

4.5.4 Working With Comma Separated Value Files
Many commonly-used tools use a comma to separate fields, instead of whitespace. Thisis particularly true of popular spreadsheet programs. There is no universally accepted
standard for the format of these files, although RFC 4180 lists the common practices.
For decades, anyone wishing to work with CSV files and awk had to “roll their own”
solution. (For an example, see Section 4.7 [Defining Fields by Content], page 83). In 2023,
Brian Kernighan decided to add CSV support to his version of awk. In order to keep up,
gawk too provides the same support as his version. To use CSV data, invoke gawk witheither of the -k or --csv options.
Fields in CSV files are separated by commas. In order to allow a comma to appear
inside a field (i.e., as data), the field may be quoted by beginning and ending it with double
quotes. In order to allow a double quote inside a field, the field must be quoted, and two
double quotes represent an actual double quote. The double quote that starts a quoted fieldmust be the first character after the comma. Table 4.1 shows some examples.
Input
abc def"quoted data""quoted, data""She said ""Stop!""."

Field Contents
abc def
quoted data
quoted, data
She said "Stop!".

Table 4.1: Examples of CSV data
Additionally, and here’s where it gets messy, newlines are also allowed inside doublequoted fields! In order to deal with such things, when processing CSV files, gawk scans the
input data looking for newlines that are not enclosed in double quotes. Thus, use of the--csv option totally overrides normal record processing with RS (see Section 4.1 [How Input
Is Split into Records], page 65), as well as field splitting with any of FS, FIELDWIDTHS, or
FPAT.

Chapter 4: Reading Input Files

77



Carriage-Return–Line-Feed Line Endings In CSV Files\r\n is the invention of the devil.
—Brian Kernighan
Many CSV files are imported from systems where the line terminator for text files
is a carriage-return–line-feed pair (CR-LF, ‘\r’ followed by ‘\n’). For ease of use, whenprocessing CSV files, gawk converts CR-LF pairs into a single newline. That is, the ‘\r’
is removed.
This occurs only when a CR is paired with an LF; a standalone CR is left alone.
This behavior is consistent with Windows systems which automatically convert CR-LF infiles into a plain LF in memory, and also with the commonly available unix2dos utility
program.
The behavior of the split() function (not formally discussed yet, see Section 9.1.4[String-Manipulation Functions], page 196) differs slightly when processing CSV files. Whencalled with two arguments (‘split(string, array)’), split() does CSV-based splitting.
Otherwise, it behaves normally.
If --csv has been used, PROCINFO["CSV"] will exist. Otherwise, it will not.
Section 7.5.2 [Built-in Variables That Convey Information], page 165.

See

Finally, if --csv has been used, assigning a value to any of FS, FIELDWIDTHS, FPAT, or
RS generates a warning message.
To be clear, gawk takes RFC 4180 as its specification for CSV input data. There are nomechanisms for accepting nonstandard CSV data, such as files that use a semicolon insteadof a comma as the separator.

4.5.5 Setting FS from the Command LineFS can be set on the command line. Use the -F option to do so. For example:
awk -F, 'program' input-files
sets FS to the ‘,’ character. Notice that the option uses an uppercase ‘F’ instead of alowercase ‘f’. The latter option (-f) specifies a file containing an awk program.
The value used for the argument to -F is processed in exactly the same way as assignments to the predefined variable FS. Any special characters in the field separator must be
escaped appropriately. For example, to use a ‘\’ as the field separator on the command
line, you would have to type:# same as FS = "\\"
awk -F\\\\ '...' files ...
Because ‘\’ is used for quoting in the shell, awk sees ‘-F\\’. Then awk processes the ‘\\’ forescape characters (see Section 3.2 [Escape Sequences], page 52), finally yielding a single ‘\’
to use for the field separator.
As a special case, in compatibility mode (see Section 2.2 [Command-Line Options],
page 33), if the argument to -F is ‘t’, then FS is set to the TAB character. If you type‘-F\t’ at the shell, without any quotes, the ‘\’ gets deleted, so awk figures that you reallywant your fields to be separated with TABs and not ‘t’s. Use ‘-v FS="t"’ or ‘-F"[t]"’78 GAWK: Effective AWK Programmingon the command line if you really do want to separate your fields with ‘t’s. Use ‘-F '\t'’when not in compatibility mode to specify that TABs separate fields.
As an example, let’s use an awk program file called edu.awk that contains the pattern/edu/ and the action ‘print $1’:
/edu/

{ print $1 }Let’s also set FS to be the ‘-’ character and run the program on the file mail-list.
The following command prints a list of the names of the people that work at or attend auniversity, and the first three digits of their phone numbers:$ awk -F- -f edu.awk mail-list
555
a Fabius
555
a Samuela JeanNote the third line of output. The third line in the original file looked like this:Jean-Paul

555-2127

jeanpaul.campanorum@nyu.edu

RThe ‘-’ as part of the person’s name was used as the field separator, instead of the ‘-’in the phone number that was originally intended. This demonstrates why you have to becareful in choosing your field and record separators.
Perhaps the most common use of a single character as the field separator occurs whenprocessing the Unix system password file. On many Unix systems, each user has a separate
entry in the system password file, with one line per user. The information in these lines
is separated by colons. The first field is the user’s login name and the second is the user’s
encrypted or shadow password. (A shadow password is indicated by the presence of a single‘x’ in the second field.) A password file entry might look like this:arnold:x:2076:10:Arnold Robbins:/home/arnold:/bin/bash
The following program searches the system password file and prints the entries for users
whose full name is not indicated:
awk -F: '$5 == ""' /etc/passwd

4.5.6 Making the Full Line Be a Single Field
Occasionally, it’s useful to treat the whole input line as a single field. This can be done
easily and portably simply by setting FS to "\n" (a newline):4
awk -F'\n' 'program' files ...
When you do this, $1 is the same as $0.

4

Thanks to Andrew Schorr for this tip.

Chapter 4: Reading Input Files79



Changing FS Does Not Affect the Fields
According to the POSIX standard, awk is supposed to behave as if each record is split
into fields at the time it is read. In particular, this means that if you change the value ofFS after a record is read, the values of the fields (i.e., how they were split) should reflect
the old value of FS, not the new one.
However, many older implementations of awk do not work this way. Instead, they defer
splitting the fields until a field is actually referenced. The fields are split using the current
value of FS! This behavior can be difficult to diagnose. The following example illustrates
the difference between the two methods:
sed 1q /etc/passwd | awk '{ FS = ":" ; print $1 }'which usually prints:root
on an incorrect implementation of awk, while gawk prints the full first line of the file,something like:root:x:0:0:Root:/:(The sed5 command prints just the first line of /etc/passwd.)

4.5.7 Field-Splitting Summary
It is important to remember that when you assign a string constant as the value of FS,
it undergoes normal awk string processing. For example, with Unix awk and gawk, theassignment ‘FS = "\.."’ assigns the character string ".." to FS (the backslash is stripped).
This creates a regexp meaning “fields are separated by occurrences of any two characters.”
If instead you want fields to be separated by a literal period followed by any single character,use ‘FS = "\\.."’.
The following list summarizes how fields are split, based on the value of FS (‘==’ means“is equal to”):
gawk was invoked with --csv
Field splitting follows the rules given in Section 4.5.4 [Working With Comma
Separated Value Files], page 76. The value of FS is ignored.
FS == " "Fields are separated by runs of whitespace. Leading and trailing whitespace
are ignored. This is the default.

FS == any other single character
Fields are separated by each occurrence of the character. Multiple successive
occurrences delimit empty fields, as do leading and trailing occurrences. The
character can even be a regexp metacharacter; it does not need to be escaped.
FS == regexp
Fields are separated by occurrences of characters that match regexp. Leading
and trailing matches of regexp delimit empty fields.
5

The sed utility is a “stream editor.” Its behavior is also defined by the POSIX standard.

80 GAWK: Effective AWK Programming

FS == ""

Each individual character in the record becomes a separate field. (This is acommon extension; it is not specified by the POSIX standard.)FS and IGNORECASEThe IGNORECASE variable (see Section 7.5.1 [Built-in Variables That Control awk],
page 163) affects field splitting only when the value of FS is a regexp. It has no effect when FS is a single character, even if that character is a letter. Thus, in the following
code:
FS = "c"IGNORECASE = 1$0 = "aCa"
print $1
The output is ‘aCa’. If you really want to split fields on an alphabetic character while
ignoring case, use a regexp that will do it for you (e.g., ‘FS = "[c]"’). In this case,
IGNORECASE will take effect.

4.6 Reading Fixed-Width Data
This section discusses an advanced feature of gawk. If you are a novice awk user, you mightwant to skip it on the first reading.
gawk provides a facility for dealing with fixed-width fields with no distinctive field separator. We discuss this feature in the following subsections.

4.6.1 Processing Fixed-Width Data
An example of fixed-width data would be the input for old Fortran programs where numbers
are run together, or the output of programs that did not anticipate the use of their output
as input for other programs.
An example of the latter is a table where all the columns are lined up by the use ofa variable number of spaces and empty fields are just spaces. Clearly, awk’s normal field
splitting based on FS does not work well in this case. Although a portable awk program
can use a series of substr() calls on $0 (see Section 9.1.4 [String-Manipulation Functions],
page 196), this is awkward and inefficient for a large number of fields.
The splitting of an input record into fixed-width fields is specified by assigning a string
containing space-separated numbers to the built-in variable FIELDWIDTHS. Each number
specifies the width of the field, including columns between fields. If you want to ignore thecolumns between fields, you can specify the width as a separate field that is subsequently
ignored. It is a fatal error to supply a field width that has a negative value.
The following data is the output of the Unix w utility. It is useful to illustrate the use of
FIELDWIDTHS:

Chapter 4: Reading Input Files

10:06pm up 21 days, 14:04, 23 users
User
ttylogin idle
JCPU
hzuo
ttyV0
8:58pm
9
hzang
ttyV3
6:37pm
50
eklye
ttyV5
9:53pm7
dportein ttyV6
8:17pm 1:47
gierd
ttyD3
10:00pm
1
dave
ttyD4
9:47pm
4
brent
ttyp0
26Jun91 4:46 26:46
dave
ttyq4
26Jun9115days
46

81

PCPU what
5 vi p24.tex-csh
1 em thes.tex
-csh
elm
4 w
4:41 bash
46 wnewmail

The following program takes this input, converts the idle time to number of seconds,
and prints out the first two fields and the calculated idle time:
BEGIN { FIELDWIDTHS = "9 6 10 6 7 7 35" }NR > 2 {idle = $4
sub(/^ +/, "", idle)# strip leading spaces
if (idle == "")idle = 0
if (idle ~ /:/) {# hh:mmsplit(idle, t, ":")
idle = t[1] * 60 + t[2]}if (idle ~ /days/)idle *= 24 * 60 * 60
print $1, $2, idle}
NOTE: The preceding program uses a number of awk features that haven’t been
introduced yet.
Running the program on the data produces the following results:
hzuo
hzang
eklye
dportein
gierd
dave
brent
dave

ttyV0
ttyV3
ttyV5
ttyV6
ttyD3
ttyD4
ttyp0
ttyq4

0
50
0
107
1
0
286
1296000

Another (possibly more practical) example of fixed-width input data is the input froma deck of balloting cards. In some parts of the United States, voters mark their choices bypunching holes in computer cards. These cards are then processed to count the votes forany particular candidate or on any particular issue. Because a voter may choose not to voteon some issue, any column on the card may be empty. An awk program for processing suchdata could use the FIELDWIDTHS feature to simplify reading the data. (Of course, getting
gawk to run on a system with card readers is another story!)

82 GAWK: Effective AWK Programming

4.6.2 Skipping Intervening Fields
Starting in version 4.2, each field width may optionally be preceded by a colon-separated
value specifying the number of characters to skip before the field starts. Thus, the preceding
program could be rewritten to specify FIELDWIDTHS like so:
BEGIN { FIELDWIDTHS = "8 1:5 4:7 6 1:6 1:6 2:33" }This strips away some of the white space separating the fields. With such a change, theprogram produces the following results:hzang
ttyV3 50
eklye
ttyV5 0dportein ttyV6 107
gierd
ttyD3 1
dave
ttyD4 0brent
ttyp0 286
dave
ttyq4 1296000

4.6.3 Capturing Optional Trailing Data
There are times when fixed-width data may be followed by additional data that has no fixed
length. Such data may or may not be present, but if it is, it should be possible to get at itfrom an awk program.
Starting with version 4.2, in order to provide a way to say “anything else in the record
after the defined fields,” gawk allows you to add a final ‘*’ character to the value of
FIELDWIDTHS. There can only be one such character, and it must be the final non-whitespacecharacter in FIELDWIDTHS. For example:$ cat fw.awk
Show the programa BEGIN { FIELDWIDTHS = "2 2 *" }a { print NF, $1, $2, $3 }$ cat fw.in
Show sample input
a 1234abcdefghi$ gawk -f fw.awk fw.in
Run the programa 3 12 34 abcdefghi

4.6.4 Field Values With Fixed-Width Data
So far, so good. But what happens if there isn’t as much data as there should be based on
the contents of FIELDWIDTHS? Or, what happens if there is more data than expected?
For many years, what happens in these cases was not well defined. Starting with version
4.2, the rules are as follows:
Enough data for some fields
For example, if FIELDWIDTHS is set to "2 3 4" and the input record is ‘aabbb’.
In this case, NF is set to two.
Not enough data for a field
For example, if FIELDWIDTHS is set to "2 3 4" and the input record is ‘aab’.
In this case, NF is set to two and $2 has the value "b". The idea is that eventhough there aren’t as many characters as were expected, there are some, so
the data should be made available to the program.

Chapter 4: Reading Input Files83

Too much data
For example, if FIELDWIDTHS is set to "2 3 4" and the input record is‘aabbbccccddd’. In this case, NF is set to three and the extra characters(‘ddd’) are ignored. If you want gawk to capture the extra characters, supply a
final ‘*’ in the value of FIELDWIDTHS.
Too much data, but with ‘*’ supplied
For example, if FIELDWIDTHS is set to "2 3 4 *" and the input record is‘aabbbccccddd’. In this case, NF is set to four, and $4 has the value "ddd".

4.7 Defining Fields by Content
NOTE: This whole section needs rewriting now that gawk has built-in CSV
parsing. Sigh.
This section discusses an advanced feature of gawk. If you are a novice awk user, you
might want to skip it on the first reading.
Normally, when using FS, gawk defines the fields as the parts of the record that occur inbetween each field separator. In other words, FS defines what a field is not, instead of whata field is. However, there are times when you really want to define the fields by what they
are, and not by what they are not.
The most notorious such case is comma-separated values (CSV) data. Many spreadsheet
programs, for example, can export their data into text files, where each record is terminatedwith a newline, and fields are separated by commas. If commas only separated the data,
there wouldn’t be an issue. The problem comes when one of the fields contains an embeddedcomma. In such cases, most programs embed the field in double quotes.6 So, we might havedata like this:Robbins,Arnold,"1234 A Pretty Street, NE",MyTown,MyState,12345-6789,USA
The FPAT variable offers a solution for cases like this. The value of FPAT should be astring that provides a regular expression. This regular expression describes the contents ofeach field.
In the case of CSV data as presented here, each field is either “anything that is not
a comma,” or “a double quote, anything that is not a double quote, and a closing double
quote.” (There are more complicated definitions of CSV data, treated shortly.) If written asa regular expression constant (see Chapter 3 [Regular Expressions], page 51), we would have/([^,]+)|("[^"]+")/. Writing this as a string requires us to escape the double quotes,
leading to:
FPAT = "([^,]+)|(\"[^\"]+\")"
Putting this to use, here is a simple program to parse the data:
BEGIN {FPAT = "([^,]+)|(\"[^\"]+\")"}
6

The CSV format lacked a formal standard definition for many years. RFC 4180 standardizes the mostcommon practices.

84 GAWK: Effective AWK Programming{
print "NF = ", NF
for (i = 1; i <= NF; i++) {printf("$%d = <%s>\n", i, $i)}}
When run, we get the following:$ gawk -f simple-csv.awk addresses.csv
NF = 7$1 = <Robbins>$2 = <Arnold>$3 = <"1234 A Pretty Street, NE">
$4 = <MyTown>$5 = <MyState>$6 = <12345-6789>$7 = <USA>
Note the embedded comma in the value of $3.
A straightforward improvement when processing CSV data of this sort would be toremove the quotes when they occur, with something like this:if (substr($i, 1, 1) == "\"") {len = length($i)$i = substr($i, 2, len - 2)
# Get text within the two quotes}
NOTE: Some programs export CSV data that contains embedded newlines
between the double quotes. gawk provides no way to deal with this. Even
though a formal specification for CSV data exists, there isn’t much more to
be done; the FPAT mechanism provides an elegant solution for the majority ofcases, and the gawk developers are satisfied with that.
As written, the regexp used for FPAT requires that each field contain at least one character. A straightforward modification (changing the first ‘+’ to ‘*’) allows fields to be empty:
FPAT = "([^,]*)|(\"[^\"]+\")"
As with FS, the IGNORECASE variable (see Section 7.5.1 [Built-in Variables That Controlawk], page 163) affects field splitting with FPAT.
Assigning a value to FPAT overrides field splitting with FS and with FIELDWIDTHS.
Finally, the patsplit() function makes the same functionality available for splitting
regular strings (see Section 9.1.4 [String-Manipulation Functions], page 196).
NOTE: Given that gawk now has built-in CSV parsing (see Section 4.5.4 [Working With Comma Separated Value Files], page 76), the examples presented here
are obsolete. Nonetheless, it remains useful as an example of what FPAT-based
field parsing can do.

4.7.1 More on CSV Files
Manuel Collado notes that in addition to commas, a CSV field can also contains quotes,that have to be escaped by doubling them. The previously described regexps fail to accept

Chapter 4: Reading Input Files

85

quoted fields with both commas and quotes inside. He suggests that the simplest FPAT
expression that recognizes this kind of fields is /([^,]*)|("([^"]|"")+")/. He provides
the following input data to test these variants:
p,"q,r",sp,"q""r",sp,"q,""r",sp,"",sp,,sAnd here is his test program:
BEGIN {
fp[0] = "([^,]+)|(\"[^\"]+\")"
fp[1] = "([^,]*)|(\"[^\"]+\")"
fp[2] = "([^,]*)|(\"([^\"]|\"\")+\")"FPAT = fp[fpat+0]}{
print "<" $0 ">"printf("NF = %s ", NF)
for (i = 1; i <= NF; i++) {printf("<%s>", $i)}
print ""}
When run on the third variant, it produces:$ gawk -v fpat=2 -f test-csv.awk sample.csv
a <p,"q,r",s>a NF = 3 <p><"q,r">
a <p,"q""r",s>a NF = 3 <p><"q""r">
a <p,"q,""r",s>a NF = 3 <p><"q,""r">
a <p,"",s>a NF = 3 <p><"">
a <p,,s>a NF = 3 <p><>

In general, using FPAT to do your own CSV parsing is like having a bed with a blanket
that’s not quite big enough. There’s always a corner that isn’t covered. We recommend,
instead, that you use Manuel Collado’s CSVMODE library for gawk.

4.7.2 FS Versus FPAT: A Subtle Difference
As we discussed earlier, FS describes the data between fields (“what fields are not”) andFPAT describes the fields themselves (“what fields are”). This leads to a subtle difference in
how fields are found when using regexps as the value for FS or FPAT.

86 GAWK: Effective AWK Programming

In order to distinguish one field from another, there must be a non-empty separator
between each field. This makes intuitive sense—otherwise one could not distinguish fields
from separators.
Thus, regular expression matching as done when splitting fields with FS is not allowed
to match the null string; it must always match at least one character, in order to be able
to proceed through the entire record.
On the other hand, regular expression matching with FPAT can match the null string,and the non-matching intervening characters function as the separators.
This same difference is reflected in how matching is done with the split() andpatsplit() functions (see Section 9.1.4 [String-Manipulation Functions], page 196).

4.8 Checking How gawk Is Splitting Records
As we’ve seen, gawk provides three independent methods to split input records into fields.
The mechanism used is based on which of the three variables—FS, FIELDWIDTHS, or FPAT—
was last assigned to. In addition, an API input parser may choose to override the record
parsing mechanism; please refer to Section 17.4.6.4 [Customized Input Parsers], page 420,
for further information about this feature.
To restore normal field splitting after using FIELDWIDTHS and/or FPAT, simply assign avalue to FS. You can use ‘FS = FS’ to do this, without having to know the current value ofFS.
In order to tell which kind of field splitting is in effect, use PROCINFO["FS"] (seeSection 7.5.2 [Built-in Variables That Convey Information], page 165). The value is "FS"if regular field splitting is being used, "FIELDWIDTHS" if fixed-width field splitting is being
used, or "FPAT" if content-based field splitting is being used:
if ("CSV" in PROCINFO)
CSV-based field splitting ...
else if (PROCINFO["FS"] == "FS")regular field splitting ...
else if (PROCINFO["FS"] == "FIELDWIDTHS")
fixed-width field splitting ...
else if (PROCINFO["FS"] == "FPAT")
content-based field splitting ...
else
API input parser field splitting ... (advanced feature)
This information is useful when writing a function that needs to temporarily change
FS, FIELDWIDTHS, or FPAT, read some records, and then restore the original settings (seeSection 10.5 [Reading the User Database], page 266, for an example of such a function).

4.9 Multiple-Line Records
In some databases, a single line cannot conveniently hold all the information in one entry.
In such cases, you can use multiline records. The first step in doing this is to choose yourdata format.
One technique is to use an unusual character or string to separate records. For example,
you could use the formfeed character (written ‘\f’ in awk, as in C) to separate them, making

Chapter 4: Reading Input Files

87each record a page of the file. To do this, just set the variable RS to "\f" (a string containing
the formfeed character). Any other character could equally well be used, as long as it won’t
be part of the data in a record.
Another technique is to have blank lines separate records. By a special dispensation, an
empty string as the value of RS indicates that records are separated by one or more blanklines. When RS is set to the empty string, each record always ends at the first blank line
encountered. The next record doesn’t start until the first nonblank line that follows. No
matter how many blank lines appear in a row, they all act as one record separator. (Blanklines must be completely empty; lines that contain only whitespace do not count.)
You can achieve the same effect as ‘RS = ""’ by assigning the string "\n\n+" to RS. This
regexp matches the newline at the end of the record and one or more blank lines after therecord. In addition, a regular expression always matches the longest possible sequence when
there is a choice (see Section 3.5 [How Much Text Matches?], page 60). So, the next record
doesn’t start until the first nonblank line that follows—no matter how many blank lines
appear in a row, they are considered one record separator.
However, there is an important difference between ‘RS = ""’ and ‘RS = "\n\n+"’. In thefirst case, leading newlines in the input data file are ignored, and if a file ends without extra
blank lines after the last record, the final newline is removed from the record. In the second
case, this special processing is not done.
Now that the input is separated into records, the second step is to separate the fields
in the records. One way to do this is to divide each of the lines into fields in the normalmanner. This happens by default as the result of a special feature. When RS is set to theempty string and FS is set to a single character, the newline character always acts as a field
separator. This is in addition to whatever field separations result from FS.
NOTE: When FS is the null string ("") or a regexp, this special feature of RS
does not apply. It does apply to the default field separator of a single space:‘FS = " "’.
Note that language in the POSIX specification implies that this special feature
should apply when FS is a regexp. However, Unix awk has never behaved thatway, nor has gawk. This is essentially a bug in POSIX.
The original motivation for this special exception was probably to provide useful behavior
in the default case (i.e., FS is equal to " "). This feature can be a problem if you really
don’t want the newline character to separate fields, because there is no way to prevent
it. However, you can work around this by using the split() function to break up therecord manually (see Section 9.1.4 [String-Manipulation Functions], page 196). If you havea single-character field separator, you can work around the special feature in a different way,by making FS into a regexp for that single character. For example, if the field separator isa percent character, instead of ‘FS = "%"’, use ‘FS = "[%]"’.
Another way to separate fields is to put each field on a separate line: to do this, justset the variable FS to the string "\n". (This single-character separator matches a single
newline.) A practical example of a data file organized this way might be a mailing list,
where blank lines separate the entries. Consider a mailing list in a file named addresses,
which looks like this:Jane Doe
123 Main Street

88 GAWK: Effective AWK Programming

Anywhere, SE 12345-6789
John Smith
456 Tree-lined Avenue
Smallville, MW 98765-4321...
A simple program to process this file is as follows:# addrs.awk --- simple mailing list program# Records are separated by blank lines.
# Each line is one field.
BEGIN { RS = "" ; FS = "\n" }{print "Name is:", $1
print "Address is:", $2
print "City and State are:", $3
print ""}
Running the program produces the following output:$ awk -f addrs.awk addressesa Name is: Jane Doea Address is: 123 Main Street
a City and State are: Anywhere, SE 12345-6789
aa Name is: John Smitha Address is: 456 Tree-lined Avenue
a City and State are: Smallville, MW 98765-4321
a...
See Section 11.3.4 [Printing Mailing Labels], page 313, for a more realistic program
dealing with address lists. The following list summarizes how records are split, based on
the value of RS:
RS == "\n"Records are separated by the newline character (‘\n’). In effect, every line in
the data file is a separate record, including blank lines. This is the default.
RS == any single character
Records are separated by each occurrence of the character. Multiple successive
occurrences delimit empty records.
RS == ""

Records are separated by runs of blank lines. When FS is a single character, then
the newline character always serves as a field separator, in addition to whatevervalue FS may have. Leading and trailing newlines in a file are ignored.

Chapter 4: Reading Input Files89

RS == regexp
Records are separated by occurrences of characters that match regexp. Leading
and trailing matches of regexp delimit empty records. (This is a gawk extension;
it is not specified by the POSIX standard.)
If not in compatibility mode (see Section 2.2 [Command-Line Options], page 33), gawk
sets RT to the input text that matched the value specified by RS. But if the input file ended
without any text that matches RS, then gawk sets RT to the null string.

4.10 Explicit Input with getline
So far we have been getting our input data from awk’s main input stream—either the
standard input (usually your keyboard, sometimes the output from another program) or
the files specified on the command line. The awk language has a special built-in command
called getline that can be used to read input under your explicit control.
The getline command is used in several different ways and should not be used by
beginners. The examples that follow the explanation of the getline command include
material that has not been covered yet. Therefore, come back and study the getline
command after you have reviewed the rest of Parts I and II and have a good knowledge of
how awk works.
The getline command returns 1 if it finds a record and 0 if it encounters the end of the
file. If there is some error in getting a record, such as a file that cannot be opened, thengetline returns −1. In this case, gawk sets the variable ERRNO to a string describing theerror that occurred.
If ERRNO indicates that the I/O operation may be retried, and PROCINFO["input","RETRY"] is set, then getline returns −2 instead of −1, and further calls to getline may
be attempted. See Section 4.12 [Retrying Reads After Certain Input Errors], page 97, forfurther information about this feature.
In the following examples, command stands for a string value that represents a shellcommand.
NOTE: When --sandbox is specified (see Section 2.2 [Command-Line Options],
page 33), reading lines from files, pipes, and coprocesses is disabled.

4.10.1 Using getline with No Arguments
The getline command can be used without arguments to read input from the current
input file. All it does in this case is read the next input record and split it up into fields.
This is useful if you’ve finished processing the current record, but want to do some special
processing on the next record right now. For example:# Remove text between /* and */, inclusive{
while ((start = index($0, "/*")) != 0) {out = substr($0, 1, start - 1) # leading part of the string
rest = substr($0, start + 2)
# ... */ ...
while ((end = index(rest, "*/")) == 0) { # is */ in trailing part?
# get more text
if (getline <= 0) {

90 GAWK: Effective AWK Programming

print("unexpected EOF or error:", ERRNO) > "/dev/stderr"exit}# build up the line using string concatenation
rest = rest $0}
rest = substr(rest, end + 2) # remove comment# build up the output line using string concatenation$0 = out rest}
print $0}
This awk program deletes C-style comments (‘/* ... */’) from the input. It uses anumber of features we haven’t covered yet, including string concatenation (see Section 6.2.2[String Concatenation], page 129) and the index() and substr() built-in functions (seeSection 9.1.4 [String-Manipulation Functions], page 196). By replacing the ‘print $0’ withother statements, you could perform more complicated processing on the decommented
input, such as searching for matches of a regular expression.
Here is some sample input:
mon/*comment*/key
rab/*commen
t*/bithorse /*comment*/more text
part 1 /*comment*/part 2 /*comment*/part 3
no comment
When run, the output is:$ awk -f strip_comments.awk example_text
a monkeya rabbita horse more text
a part 1 part 2 part 3
a no comment
This form of the getline command sets NF, NR, FNR, RT, and the value of $0.
NOTE: The new value of $0 is used to test the patterns of any subsequent
rules. The original value of $0 that triggered the rule that executed getline
is lost. By contrast, the next statement reads a new record but immediately
begins processing it normally, starting with the first rule in the program. See
Section 7.4.8 [The next Statement], page 160.

4.10.2 Using getline into a Variable
You can use ‘getline var’ to read the next record from awk’s input into the variable var.
No other processing is done. For example, suppose the next line is a comment or a special
string, and you want to read it without triggering any rules. This form of getline allows you
to read that line and store it in a variable so that the main read-a-line-and-check-each-ruleloop of awk never sees it. The following example swaps every two lines of input:

Chapter 4: Reading Input Files91{
if ((getline tmp) > 0) {print tmp
print $0
} elseprint $0}
It takes the following list:
wan
tew
free
phore
and produces these results:
tew
wan
phore
free
The getline command used in this way sets only the variables NR, FNR, and RT (and,of course, var). The record is not split into fields, so the values of the fields (including $0)
and the value of NF do not change.

4.10.3 Using getline from a File
Use ‘getline < file’ to read the next record from file. Here, file is a string-valued expression that specifies the file name. ‘< file’ is called a redirection because it directs input
to come from a different place. For example, the following program reads its input record
from the file secondary.input when it encounters a first field with a value equal to 10 in
the current input file:{if ($1 == 10) {getline < "secondary.input"
print} elseprint}
Because the main input stream is not used, the values of NR and FNR are not changed.
However, the record it reads is split into fields in the normal manner, so the values of $0
and the other fields are changed, resulting in a new value of NF. RT is also set.
According to POSIX, ‘getline < expression’ is ambiguous if expression contains unparenthesized operators other than ‘$’; for example, ‘getline < dir "/" file’ is ambiguous
because the concatenation operator (not discussed yet; see Section 6.2.2 [String Concatenation], page 129) is not parenthesized. You should write it as ‘getline < (dir "/" file)’ if
you want your program to be portable to all awk implementations.

92 GAWK: Effective AWK Programming

4.10.4 Using getline into a Variable from a File
Use ‘getline var < file’ to read input from the file file, and put it in the variable var. As
earlier, file is a string-valued expression that specifies the file from which to read.
In this version of getline, none of the predefined variables are changed and the record is
not split into fields. The only variable changed is var.7 For example, the following program
copies all the input files to the output, except for records that say ‘@include filename’.
Such a record is replaced by the contents of the file filename:{if (NF == 2 && $1 == "@include") {while ((getline line < $2) > 0)
print line
close($2)} elseprint}
Note here how the name of the extra input file is not built into the program; it is taken
directly from the data, specifically from the second field on the @include line.
The close() function is called to ensure that if two identical @include lines appear
in the input, the entire specified file is included twice. See Section 5.9 [Closing Input and
Output Redirections], page 113.
One deficiency of this program is that it does not process nested @include statements(i.e., @include statements in included files) the way a true macro preprocessor would. See
Section 11.3.9 [An Easy Way to Use Library Functions], page 322, for a program that does
handle nested @include statements.

4.10.5 Using getline from a Pipe
Omniscience has much to recommend it. Failing that, attention to details would
be useful.
—Brian Kernighan
The output of a command can also be piped into getline, using ‘command | getline’.
In this case, the string command is run as a shell command and its output is piped into awk
to be used as input. This form of getline reads one record at a time from the pipe. For
example, the following program copies its input to its output, except for lines that begin
with ‘@execute’, which are replaced by the output produced by running the rest of the line
as a shell command:{
if ($1 == "@execute") {tmp = substr($0, 10)# Remove "@execute"
while ((tmp | getline) > 0)
print
close(tmp)} elseprint}
7This is not quite true. RT could be changed if RS is a regular expression.

Chapter 4: Reading Input Files

93

The close() function is called to ensure that if two identical ‘@execute’ lines appear in
the input, the command is run for each one. Given the input:
foo
bar
baz
@execute who
bletchthe program might produce:
foo
bar
baz
arnoldmiriambill
bletch

ttyv0
ttyp0
ttyp1

Jul 13 14:22
Jul 13 14:23
Jul 13 14:23(murphy:0)(murphy:0)

Notice that this program ran the command who and printed the result. (If you try thisprogram yourself, you will of course get different results, depending upon who is logged inon your system.)This variation of getline splits the record into fields, sets the value of NF, and recomputes the value of $0. The values of NR and FNR are not changed. RT is set.
According to POSIX, ‘expression | getline’ is ambiguous if expression contains unparenthesized operators other than ‘$’—for example, ‘"echo " "date" | getline’ is ambiguous because the concatenation operator is not parenthesized. You should write it as‘("echo " "date") | getline’ if you want your program to be portable to all awk implementations.
NOTE: Unfortunately, gawk has not been consistent in its treatment of a construct like ‘"echo " "date" | getline’. Most versions, including the current
version, treat it as ‘("echo " "date") | getline’. (This is also how BWK
awk behaves.) Some versions instead treat it as ‘"echo " ("date" | getline)’.
(This is how mawk behaves.) In short, always use explicit parentheses, and then
you won’t have to worry.

4.10.6 Using getline into a Variable from a Pipe
When you use ‘command | getline var’, the output of command is sent through a pipe togetline and into the variable var. For example, the following program reads the current
date and time into the variable current_time, using the date utility, and then prints it:
BEGIN {
"date" | getline current_time
close("date")
print "Report printed on " current_time}
In this version of getline, none of the predefined variables are changed and the record
is not split into fields. However, RT is set.

94 GAWK: Effective AWK Programming

4.10.7 Using getline from a Coprocess
Reading input into getline from a pipe is a one-way operation. The command that isstarted with ‘command | getline’ only sends data to your awk program.
On occasion, you might want to send data to another program for processing and thenread the results back. gawk allows you to start a coprocess, with which two-way communications are possible. This is done with the ‘|&’ operator. Typically, you write data to the
coprocess first and then read the results back, as shown in the following:
print "some query" |& "db_server""db_server" |& getline
which sends a query to db_server and then reads the results.
The values of NR and FNR are not changed, because the main input stream is not used.
However, the record is split into fields in the normal manner, thus changing the values of$0, of the other fields, and of NF and RT.
Coprocesses are an advanced feature. They are discussed here only because this is thesection on getline. See Section 12.4 [Two-Way Communications with Another Process],
page 343, where coprocesses are discussed in more detail.

4.10.8 Using getline into a Variable from a Coprocess
When you use ‘command |& getline var’, the output from the coprocess command is sent
through a two-way pipe to getline and into the variable var.
In this version of getline, none of the predefined variables are changed and the record
is not split into fields. The only variable changed is var. However, RT is set.

4.10.9 Points to Remember About getline
Here are some miscellaneous points about getline that you should bear in mind:• When getline changes the value of $0 and NF, awk does not automatically jump to thestart of the program and start testing the new record against every pattern. However,the new record is tested against any subsequent rules.
• Some very old awk implementations limit the number of pipelines that an awk program
may have open to just one. In gawk, there is no such limit. You can open as many
pipelines (and coprocesses) as the underlying operating system permits.
• An interesting side effect occurs if you use getline without a redirection inside a BEGIN
rule. Because an unredirected getline reads from the command-line data files, thefirst getline command causes awk to set the value of FILENAME. Normally, FILENAME
does not have a value inside BEGIN rules, because you have not yet started to process
the command-line data files.
(See Section 7.1.4 [The BEGIN and END Special Patterns], page 150; also see Section 7.5.2 [Built-in Variables That Convey Information],
page 165.)• Using FILENAME with getline (‘getline < FILENAME’) is likely to be a source of confusion. awk opens a separate input stream from the current input file. However, by
not using a variable, $0 and NF are still updated. If you’re doing this, it’s probably by
accident, and you should reconsider what it is you’re trying to accomplish.
• Section 4.10.10 [Summary of getline Variants], page 95, presents a table summarizing
the getline variants and which variables they can affect. It is worth noting that thoseChapter 4: Reading Input Files

95

variants that do not use redirection can cause FILENAME to be updated if they cause
awk to start reading a new input file.
• If the variable being assigned is an expression with side effects, different versions of
awk behave differently upon encountering end-of-file. Some versions don’t evaluate theexpression; many versions (including gawk) do. Here is an example, courtesy of Duncan
Moore:BEGIN {
system("echo 1 > f")while ((getline a[++c] < "f") > 0) { }print c}Here, the side effect is the ‘++c’. Is c incremented if end-of-file is encountered before
the element in a is assigned?
gawk treats getline like a function call, and evaluates the expression ‘a[++c]’ before
attempting to read from f. However, some versions of awk only evaluate the expression
once they know that there is a string value to be assigned.

4.10.10 Summary of getline Variants
Table 4.2 summarizes the eight variants of getline, listing which predefined variables are
set by each one, and whether the variant is standard or a gawk extension. Note: for eachvariant, gawk sets the RT predefined variable.
Variant
getlinegetline var
getline < filegetline var < filecommand | getline
command | getline var
command |& getline
command |& getline var

Effect
Sets $0, NF, FNR, NR, and RT
Sets var, FNR, NR, and RT
Sets $0, NF, and RT
Sets var and RT
Sets $0, NF, and RT
Sets var and RT
Sets $0, NF, and RT
Sets var and RT

awk / gawk
awk
awk
awk
awk
awk
awk
gawk
gawk

Table 4.2: getline variants and what they set

4.11 Reading Input with a Timeout
This section describes a feature that is specific to gawk.
You may specify a timeout in milliseconds for reading input from the keyboard, a pipe,or two-way communication, including TCP/IP sockets. This can be done on a per-input,
per-command, or per-connection basis, by setting a special element in the PROCINFO array(see Section 7.5.2 [Built-in Variables That Convey Information], page 165):
PROCINFO["input_name", "READ_TIMEOUT"] = timeout in milliseconds
When set, this causes gawk to time out and return failure if no data is available to read
within the specified timeout period. For example, a TCP client can decide to give up onreceiving any response from the server after a certain amount of time:96 GAWK: Effective AWK Programming

Service = "/inet/tcp/0/localhost/daytime"
PROCINFO[Service, "READ_TIMEOUT"] = 100
if ((Service |& getline) > 0)
print $0
else if (ERRNO != "")print ERRNO
Here is how to read interactively from the user8 without waiting for more than fiveseconds:
PROCINFO["/dev/stdin", "READ_TIMEOUT"] = 5000
while ((getline < "/dev/stdin") > 0)
print $0
gawk terminates the read operation if input does not arrive after waiting for the timeout
period, returns failure, and sets ERRNO to an appropriate string value. A negative or zero
value for the timeout is the same as specifying no timeout at all.
A timeout can also be set for reading from the keyboard in the implicit loop that reads
input records and matches them against patterns, like so:$ gawk 'BEGIN { PROCINFO["-", "READ_TIMEOUT"] = 5000 }> { print "You entered: " $0 }'gawk
a You entered: gawk
In this case, failure to respond within five seconds results in the following error message:error

gawk: cmd. line:2: (FILENAME=- FNR=1) fatal: error reading input file `-': Conne

The timeout can be set or changed at any time, and will take effect on the next attempt
to read from the input device. In the following example, we start with a timeout value of
one second, and progressively reduce it by one-tenth of a second until we wait indefinitely
for the input to arrive:
PROCINFO[Service, "READ_TIMEOUT"] = 1000
while ((Service |& getline) > 0) {print $0
PROCINFO[Service, "READ_TIMEOUT"] -= 100}
NOTE: You should not assume that the read operation will block exactly after
the tenth record has been printed. It is possible that gawk will read and buffer
more than one record’s worth of data the first time. Because of this, changing
the value of timeout like in the preceding example is not very useful.
If the PROCINFO element is not present and the GAWK_READ_TIMEOUT environment variable exists, gawk uses its value to initialize the timeout value. The exclusive use of theenvironment variable to specify timeout has the disadvantage of not being able to control
it on a per-command or per-connection basis.
gawk considers a timeout event to be an error even though the attempt to read from theunderlying device may succeed in a later attempt. This is a limitation, and it also meansthat you cannot use this to multiplex input from two or more sources. See Section 4.12
8This assumes that standard input is the keyboard.

Chapter 4: Reading Input Files

97[Retrying Reads After Certain Input Errors], page 97, for a way to enable later I/O attempts
to succeed.
Assigning a timeout value prevents read operations from being blocked indefinitely. But
bear in mind that there are other ways gawk can stall waiting for an input device to beready. A network client can sometimes take a long time to establish a connection before it
can start reading any data, or the attempt to open a FIFO special file for reading can beblocked indefinitely until some other process opens it for writing.

4.12 Retrying Reads After Certain Input Errors
This section describes a feature that is specific to gawk.
When gawk encounters an error while reading input, by default getline returns −1, andsubsequent attempts to read from that file result in an end-of-file indication. However, you
may optionally instruct gawk to allow I/O to be retried when certain errors are encounteredby setting a special element in the PROCINFO array (see Section 7.5.2 [Built-in Variables
That Convey Information], page 165):
PROCINFO["input_name", "RETRY"] = 1
When this element exists, gawk checks the value of the system (C language) errno
variable when an I/O error occurs. If errno indicates a subsequent I/O attempt may
succeed, getline instead returns −2 and further calls to getline may succeed. Thisapplies to the errno values EAGAIN, EWOULDBLOCK, EINTR, or ETIMEDOUT.
This feature is useful in conjunction with PROCINFO["input_name", "READ_TIMEOUT"]
or situations where a file descriptor has been configured to behave in a non-blocking fashion.

4.13 Directories on the Command Line
According to the POSIX standard, files named on the awk command line must be text files;
it is a fatal error if they are not. Most versions of awk treat a directory on the commandline as a fatal error.
By default, gawk produces a warning for a directory on the command line, but otherwise
ignores it. This makes it easier to use shell wildcards with your awk program:$ gawk -f whizprog.awk *Directories could kill this program
If either of the --posix or --traditional options is given, then gawk reverts to treatinga directory on the command line as a fatal error.
See Section 17.7.6 [Reading Directories], page 463, for a way to treat directories as usable
data from an awk program.

4.14 Summary• Input is split into records based on the value of RS. The possibilities are as follows:
Value of RS
Records are split on . . .
awk / gawk
Any single character
That character
awk
The empty string Runs of two or more awk("")
newlines
A regexp
Text that matches the gawk
regexp98 GAWK: Effective AWK Programming• FNR indicates how many records have been read from the current input file; NR indicates
how many records have been read in total.
• gawk sets RT to the text matched by RS.
• After splitting the input into records, awk further splits the records into individual
fields, named $1, $2, and so on. $0 is the whole record, and NF indicates how many
fields there are. The default way to split fields is between whitespace characters.
• Fields may be referenced using a variable, as in $NF. Fields may also be assigned values,
which causes the value of $0 to be recomputed when it is later referenced. Assigning toa field with a number greater than NF creates the field and rebuilds the record, using
OFS to separate the fields. Incrementing NF does the same thing. Decrementing NF
throws away fields and rebuilds the record.
• Field splitting is more complicated than record splitting:Field separator value
Fields are split . . .
awk / gawk
FS == " "On runs of whitespaceawk
FS == any single characterOn that characterawk
FS == regexp
On text matching the regexp
awk
FS == ""Such that each individual char- gawk
acter is a separate field
FIELDWIDTHS == list of columns Based on character position
gawk
FPAT == regexp
On the text surrounding text gawk
matching the regexp• Using ‘FS = "\n"’ causes the entire record to be a single field (assuming that newlines
separate records).
• FS may be set from the command line using the -F option. This can also be done usingcommand-line variable assignment.
• Use PROCINFO["FS"] to see how fields are being split.
• Use getline in its various forms to read additional records from the default input
stream, from a file, or from a pipe or coprocess.
• Use PROCINFO[file, "READ_TIMEOUT"] to cause reads to time out for file.
• Directories on the command line are fatal for standard awk; gawk ignores them if not
in POSIX mode.

4.15 Exercises
1. Using the FIELDWIDTHS variable (see Section 4.6 [Reading Fixed-Width Data], page 80),
write a program to read election data, where each record represents one voter’s votes.
Come up with a way to define which columns are associated with each ballot item, andprint the total votes, including abstentions, for each item.

Chapter 5: Printing Output 99

5 Printing Output
One of the most common programming actions is to print, or output, some or all of the
input. Use the print statement for simple output, and the printf statement for fancier
formatting. The print statement is not limited when computing which values to print.
However, with two exceptions, you cannot specify how to print them—how many columns,
whether to use exponential notation or not, and so on. (For the exceptions, see Section 5.3[Output Separators], page 101, and Section 5.4 [Controlling Numeric Output with print],
page 102.) For printing with specifications, you need the printf statement (see Section 5.5[Using printf Statements for Fancier Printing], page 102).
Besides basic and formatted printing, this chapter also covers I/O redirections to files
and pipes, introduces the special file names that gawk processes internally, and discusses
the close() built-in function.

5.1 The print Statement
Use the print statement to produce output with simple, standardized formatting. You
specify only the strings or numbers to print, in a list separated by commas. They are
output, separated by single spaces, followed by a newline. The statement looks like this:print item1, item2, ...
The entire list of items may be optionally enclosed in parentheses. The parentheses arenecessary if any of the item expressions uses the ‘>’ relational operator; otherwise it could
be confused with an output redirection (see Section 5.6 [Redirecting Output of print andprintf], page 108).
The items to print can be constant strings or numbers, fields of the current record (suchas $1), variables, or any awk expression. Numeric values are converted to strings and thenprinted.
The simple statement ‘print’ with no items is equivalent to ‘print $0’: it prints theentire current record. To print a blank line, use ‘print ""’. To print a fixed piece of text,
use a string constant, such as "Don't Panic", as one item. If you forget to use the doublequote characters, your text is taken as an awk expression, and you will probably get anerror. Keep in mind that a space is printed between any two items.
Note that the print statement is a statement and not an expression—you can’t use it
in the pattern part of a pattern–action statement, for example.

5.2 print Statement Examples
Each print statement makes at least one line of output. However, it isn’t limited to only
one line. If an item value is a string containing a newline, the newline is output along withthe rest of the string. A single print statement can make any number of lines this way.
The following is an example of printing a string that contains embedded newlines:$ awk 'BEGIN { print "line one\nline two\nline three" }'a line one
a line two
a line three

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The next example, which is run on the inventory-shipped file, prints the first two fields
of each input record, with a space between them:$ awk '{ print $1, $2 }' inventory-shipped
a Jan 13a Feb 15a Mar 15...
A common mistake in using the print statement is to omit the comma between two
items. This often has the effect of making the items run together in the output, withno space. The reason for this is that juxtaposing two string expressions in awk means toconcatenate them. Here is the same program, without the comma:$ awk '{ print $1 $2 }' inventory-shipped
a Jan13a Feb15a Mar15...
To someone unfamiliar with the inventory-shipped file, neither example’s output
makes much sense. A heading line at the beginning would make it clearer. Let’s add
some headings to our table of months ($1) and green crates shipped ($2). We do this usinga BEGIN rule (see Section 7.1.4 [The BEGIN and END Special Patterns], page 150) so that theheadings are only printed once:
awk 'BEGIN {{print "Month Crates"
print "----- ------" }print $1, $2 }' inventory-shipped

When run, the program prints the following:Month Crates----- -----Jan 13
Feb 15
Mar 15...
The only problem, however, is that the headings and the table data don’t line up! We canfix this by printing some spaces between the two fields:
awk 'BEGIN { print "Month Crates"
print "----- ------" }{ print $1, "", $2 }' inventory-shipped
Lining up columns this way can get pretty complicated when there are many columns to
fix. Counting spaces for two or three columns is simple, but any more than this can take up
a lot of time. This is why the printf statement was created (see Section 5.5 [Using printf
Statements for Fancier Printing], page 102); one of its specialties is lining up columns ofdata.
NOTE: You can continue either a print or printf statement simply by putting
a newline after any comma (see Section 1.6 [awk Statements Versus Lines],
page 28).

Chapter 5: Printing Output 101

5.3 Output Separators
As mentioned previously, a print statement contains a list of items separated by commas.
In the output, the items are normally separated by single spaces. However, this doesn’t
need to be the case; a single space is simply the default. Any string of characters may be
used as the output field separator by setting the predefined variable OFS. The initial valueof this variable is the string " " (i.e., a single space).
The output from an entire print statement is called an output record. Each printstatement outputs one output record, and then outputs a string called the output record
separator (or ORS). The initial value of ORS is the string "\n" (i.e., a newline character).
Thus, each print statement normally makes a separate line.
In order to change how output fields and records are separated, assign new values to thevariables OFS and ORS. The usual place to do this is in the BEGIN rule (see Section 7.1.4[The BEGIN and END Special Patterns], page 150), so that it happens before any input isprocessed. It can also be done with assignments on the command line, before the namesof the input files, or using the -v command-line option (see Section 2.2 [Command-Line
Options], page 33). The following example prints the first and second fields of each input
record, separated by a semicolon, with a blank line added after each newline:$ awk 'BEGIN { OFS = ";"; ORS = "\n\n" }>{ print $1, $2 }' mail-lista Amelia;555-5553
aa Anthony;555-3412
aa Becky;555-7685
aa Bill;555-1675
aa Broderick;555-0542
aa Camilla;555-2912
aa Fabius;555-1234
aa Julie;555-6699
aa Martin;555-6480
aa Samuel;555-3430
aa Jean-Paul;555-2127
a
If the value of ORS does not contain a newline, the program’s output runs together on asingle line.

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5.4 Controlling Numeric Output with print
When printing numeric values with the print statement, awk internally converts each number to a string of characters and prints that string. awk uses the sprintf() function to
do this conversion (see Section 9.1.4 [String-Manipulation Functions], page 196). For now,
it suffices to say that the sprintf() function accepts a format specification that tells it
how to format numbers (or strings), and that there are a number of different ways in which
numbers can be formatted. The different format specifications are discussed more fully in
Section 5.5.2 [Format-Control Letters], page 103.
The predefined variable OFMT contains the format specification that print uses withsprintf() when it wants to convert a number to a string for printing. The default valueof OFMT is "%.6g". The way print prints numbers can be changed by supplying a different
format specification for the value of OFMT, as shown in the following example:$ awk 'BEGIN {>OFMT = "%.0f" # print numbers as integers (rounds)>
print 17.23, 17.54 }'a 17 18
More detail on how awk converts numeric values into strings is provided in Section 6.1.4.1[How awk Converts Between Strings and Numbers], page 126. In particular, for print, awk
uses the value of OFMT instead of that of CONVFMT, but otherwise behaves exactly the sameas described in that section.
According to the POSIX standard, awk’s behavior is undefined if OFMT contains anything
but a floating-point conversion specification.

5.5 Using printf Statements for Fancier Printing
For more precise control over the output format than what is provided by print, use printf.
With printf you can specify the width to use for each item, as well as various formatting
choices for numbers (such as what output base to use, whether to print an exponent, whether
to print a sign, and how many digits to print after the decimal point).

5.5.1 Introduction to the printf Statement
A simple printf statement looks like this:printf format, item1, item2, ...
As for print, the entire list of arguments may optionally be enclosed in parentheses. Heretoo, the parentheses are necessary if any of the item expressions uses the ‘>’ relational operator; otherwise, it can be confused with an output redirection (see Section 5.6 [Redirecting
Output of print and printf], page 108).
The difference between printf and print is the format argument. This is an expression
whose value is taken as a string; it specifies how to output each of the other arguments. It
is called the format string.
The format string is very similar to that in the ISO C library function printf(). Mostof format is text to output verbatim. Scattered among this text are format specifiers—one
per item. Each format specifier says to output the next item in the argument list at thatplace in the format.

Chapter 5: Printing Output 103

The printf statement does not automatically append a newline to its output. It outputs
only what the format string specifies. So if a newline is needed, you must include one in
the format string. The output separator variables OFS and ORS have no effect on printf
statements. For example:$ awk 'BEGIN {>ORS = "\nOUCH!\n"; OFS = "+">msg = "Don\47t Panic!">
printf "%s\n", msg> }'a Don't Panic!
Here, neither the ‘+’ nor the ‘OUCH!’ appears in the output message.

5.5.2 Format-Control Letters
A format specifier starts with the character ‘%’ and ends with a format-control letter—it
tells the printf statement how to output one item. The format-control letter specifies whatkind of value to print. The rest of the format specifier is made up of optional modifiers that
control how to print the value, such as the field width. Here is a list of the format-control
letters:%a, %AA floating point number of the form [-]0xh.hhhhp+-dd (C99 hexadecimal floating point format). For %A, uppercase letters are used instead of lowercase ones.
NOTE: The current POSIX standard requires support for %a and%A in awk. As far as we know, besides gawk, the only other versionof awk that actually implements it is BWK awk. It’s use is thus
highly nonportable!
Furthermore, these formats are not available on any system where
the underlying C library printf() function does not support them.
As of this writing, among current systems, only OpenVMS is known
to not support them.

%cPrint a number as a character; thus, ‘printf "%c", 65’ outputs the letter ‘A’.
The output for a string value is the first character of the string.
NOTE: The POSIX standard says the first character of a string
is printed. In locales with multibyte characters, gawk attempts toconvert the leading bytes of the string into a valid wide characterand then to print the multibyte encoding of that character. Similarly, when printing a numeric value, gawk allows the value to bewithin the numeric range of values that can be held in a wide character. If the conversion to multibyte encoding fails, gawk uses thelow eight bits of the value as the character to print.
Other awk versions generally restrict themselves to printing the first
byte of a string or to numeric values within the range of a single
byte (0–255).

%d, %iPrint a decimal integer. The two control letters are equivalent. (The ‘%i’
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Print a number in scientific (exponential) notation. For example:
printf "%4.3e\n", 1950
prints ‘1.950e+03’, with a total of four significant figures, three of which follow
the decimal point. (The ‘4.3’ represents two modifiers, discussed in the next
subsection.) ‘%E’ uses ‘E’ instead of ‘e’ in the output.

%fPrint a number in floating-point notation. For example:
printf "%4.3f", 1950
prints ‘1950.000’, with a minimum of four significant figures, three of which
follow the decimal point. (The ‘4.3’ represents two modifiers, discussed in thenext subsection.)
On systems supporting IEEE 754 floating-point format, values representing
negative infinity are formatted as ‘-inf’ or ‘-infinity’, and positive infinity
as ‘inf’ or ‘infinity’. The special “not a number” value formats as ‘-nan’ or‘nan’ (see Section 16.4.1.4 [Floating Point Values They Didn’t Talk About In
School], page 395).

%F

Like ‘%f’, but the infinity and “not a number” values are spelled using uppercase
letters.
The ‘%F’ format is a POSIX extension to ISO C; not all systems support it. On
those that don’t, gawk uses ‘%f’ instead.

%g, %G

Print a number in either scientific notation or in floating-point notation, whichever uses fewer characters; if the result is printed in scientific notation, ‘%G’ uses
‘E’ instead of ‘e’.

%oPrint an unsigned octal integer (see Section 6.1.1.2 [Octal and Hexadecimal
Numbers], page 120).

%sPrint a string.

%uPrint an unsigned decimal integer. (This format is of marginal use, because
all numbers in awk are floating point; it is provided primarily for compatibility
with C.)%x, %X

Print an unsigned hexadecimal integer; ‘%X’ uses the letters ‘A’ through ‘F’instead of ‘a’ through ‘f’ (see Section 6.1.1.2 [Octal and Hexadecimal Numbers],
page 120).

%%

Print a single ‘%’. This does not consume an argument and it ignores any
modifiers.
NOTE: When using the integer format-control letters for values that are outside
the range of the widest C integer type, gawk switches to the ‘%g’ format specifier.
If --lint is provided on the command line (see Section 2.2 [Command-Line
Options], page 33), gawk warns about this. Other versions of awk may print
invalid values or do something else entirely.
NOTE: The IEEE 754 standard for floating-point arithmetic allows for special
values that represent “infinity” (positive and negative) and values that are “nota number” (NaN).

Chapter 5: Printing Output 105

Input and output of these values occurs as text strings. This is somewhat
problematic for the awk language, which predates the IEEE standard. Further details are provided in Section 16.7 [Standards Versus Existing Practice],
page 401; please see there.

5.5.3 Modifiers for printf Formats
A format specification can also include modifiers that can control how much of the item’svalue is printed, as well as how much space it gets. The modifiers come between the ‘%’and the format-control letter. We use the bullet symbol “•” in the following examples torepresent spaces in the output. Here are the possible modifiers, in the order in which they
may appear:N$

An integer constant followed by a ‘$’ is a positional specifier. Normally, format
specifications are applied to arguments in the order given in the format string.
With a positional specifier, the format specification is applied to a specificargument, instead of what would be the next argument in the list. Positional
specifiers begin counting with one. Thus:
printf "%s %s\n", "don't", "panic"
printf "%2$s %1$s\n", "panic", "don't"
prints the famous friendly message twice.
At first glance, this feature doesn’t seem to be of much use. It is in facta gawk extension, intended for use in translating messages at runtime. See
Section 13.4.2 [Rearranging printf Arguments], page 362, which describes how
and why to use positional specifiers. For now, we ignore them.

- (Minus) The minus sign, used before the width modifier (see later on in this list), says
to left-justify the argument within its specified width. Normally, the argument
is printed right-justified in the specified width. Thus:
printf "%-4s", "foo"prints ‘foo•’.
space

For numeric conversions, prefix positive values with a space and negative values
with a minus sign.

+

The plus sign, used before the width modifier (see later on in this list), says
to always supply a sign for numeric conversions, even if the data to format is
positive. The ‘+’ overrides the space modifier.

#

Use an “alternative form” for certain control letters. For ‘%o’, supply a leading
zero. For ‘%x’ and ‘%X’, supply a leading ‘0x’ or ‘0X’ for a nonzero result. For‘%e’, ‘%E’, ‘%f’, and ‘%F’, the result always contains a decimal point. For ‘%g’
and ‘%G’, trailing zeros are not removed from the result.

0

A leading ‘0’ (zero) acts as a flag indicating that output should be padded withzeros instead of spaces. This applies only to the numeric output formats. This
flag only has an effect when the field width is wider than the value to print.

'

A single quote or apostrophe character is a POSIX extension to ISO C. It
indicates that the integer part of a floating-point value, or the entire part of an

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integer decimal value, should have a thousands-separator character in it. This
only works in locales that support such characters. For example:$ cat thousands.awk
Show source programa BEGIN { printf "%'d\n", 1234567 }$ LC_ALL=C gawk -f thousands.awk
Results in "C" locale
a 1234567$ LC_ALL=en_US.UTF-8 gawk -f thousands.awk
Results in US English UTF locale
a 1,234,567
For more information about locales and internationalization issues, see
Section 6.6 [Where You Are Makes a Difference], page 145.
NOTE: The ‘'’ flag is a nice feature, but its use complicates things:
it becomes difficult to use it in command-line programs. For information on appropriate quoting tricks, see Section 1.1.6 [Shell
Quoting Issues], page 21.
widthThis is a number specifying the desired minimum width of a field. Inserting
any number between the ‘%’ sign and the format-control character forces thefield to expand to this width. The default way to do this is to pad with spaceson the left. For example:
printf "%4s", "foo"prints ‘•foo’.
The value of width is a minimum width, not a maximum. If the item value
requires more than width characters, it can be as wide as necessary. Thus, thefollowing:
printf "%4s", "foobar"
prints ‘foobar’.
Preceding the width with a minus sign causes the output to be padded withspaces on the right, instead of on the left.

.prec

A period followed by an integer constant specifies the precision to use whenprinting. The meaning of the precision varies by control letter:%d, %i, %o, %u, %x, %X
Minimum number of digits to print.
%e, %E, %f, %F
Number of digits to the right of the decimal point.
%g, %G

Maximum number of significant digits.

%s

Maximum number of characters from the string that should print.

Thus, the following:
printf "%.4s", "foobar"
prints ‘foob’.
The C library printf’s dynamic width and prec capability (e.g., "%*.*s") is supported.
Instead of supplying explicit width and/or prec values in the format string, they are passedin the argument list. For example:
w = 5

Chapter 5: Printing Output 107

p = 3
s = "abcdefg"
printf "%*.*s\n", w, p, s
is exactly equivalent to:s = "abcdefg"
printf "%5.3s\n", sBoth programs output ‘••abc’. Earlier versions of awk did not support this capability. If
you must use such a version, you may simulate this feature by using concatenation to build
up the format string, like so:
w = 5
p = 3
s = "abcdefg"
printf "%" w "." p "s\n", sThis is not particularly easy to read, but it does work.
C programmers may be used to supplying additional modifiers (‘h’, ‘j’, ‘l’, ‘L’, ‘t’, and‘z’) in printf format strings. These are not valid in awk. Most awk implementations silently
ignore them. If --lint is provided on the command line (see Section 2.2 [Command-Line
Options], page 33), gawk warns about their use. If --posix is supplied, their use is a fatal
error.

5.5.4 Examples Using printf
The following simple example shows how to use printf to make an aligned table:
awk '{ printf "%-10s %s\n", $1, $2 }' mail-list
This command prints the names of the people ($1) in the file mail-list as a string of 10
characters that are left-justified. It also prints the phone numbers ($2) next on the line.
This produces an aligned two-column table of names and phone numbers, as shown here:$ awk '{ printf "%-10s %s\n", $1, $2 }' mail-list555-5553
a Amelia
Anthony555-3412a
555-7685
a Becky555-1675
a Billa Broderick 555-0542
555-2912
a Camilla
555-1234
a Fabius
555-6699
a Julie555-6480
a Martin555-3430
a Samuela Jean-Paul 555-2127
In this case, the phone numbers had to be printed as strings because the numbers are
separated by dashes. Printing the phone numbers as numbers would have produced justthe first three digits: ‘555’. This would have been pretty confusing.
It wasn’t necessary to specify a width for the phone numbers because they are last on
their lines. They don’t need to have spaces after them.

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The table could be made to look even nicer by adding headings to the tops of the columns.
This is done using a BEGIN rule (see Section 7.1.4 [The BEGIN and END Special Patterns],
page 150) so that the headers are only printed once, at the beginning of the awk program:awk 'BEGIN { print "Name
Number"print "---------" }{ printf "%-10s %s\n", $1, $2 }' mail-list
The preceding example mixes print and printf statements in the same program. Using
just printf statements can produce the same results:
awk 'BEGIN { printf "%-10s %s\n", "Name", "Number"printf "%-10s %s\n", "----", "------" }{ printf "%-10s %s\n", $1, $2 }' mail-list
Printing each column heading with the same format specification used for the column elements ensures that the headings are aligned just like the columns.
The fact that the same format specification is used three times can be emphasized bystoring it in a variable, like this:awk 'BEGIN { format = "%-10s %s\n"printf format, "Name", "Number"printf format, "----", "------" }{ printf format, $1, $2 }' mail-list

5.6 Redirecting Output of print and printf
So far, the output from print and printf has gone to the standard output, usually thescreen. Both print and printf can also send their output to other places. This is called
redirection.
NOTE: When --sandbox is specified (see Section 2.2 [Command-Line Options],
page 33), redirecting output to files, pipes, and coprocesses is disabled.
A redirection appears after the print or printf statement. Redirections in awk are
written just like redirections in shell commands, except that they are written inside the awk
program.
There are four forms of output redirection: output to a file, output appended to a file,
output through a pipe to another command, and output to a coprocess. We show them all
for the print statement, but they work identically for printf:
print items > output-file
This redirection prints the items into the output file named output-file. The
file name output-file can be any expression. Its value is changed to a string andthen used as a file name (see Chapter 6 [Expressions], page 119).
When this type of redirection is used, the output-file is erased before the first
output is written to it. Subsequent writes to the same output-file do not erase
output-file, but append to it. (This is different from how you use redirectionsin shell scripts.) If output-file does not exist, it is created. For example, here
is how an awk program can write a list of peoples’ names to one file named
name-list, and a list of phone numbers to another file named phone-list:$ awk '{ print $2 > "phone-list"

Chapter 5: Printing Output 109>
print $1 > "name-list" }' mail-list$ cat phone-list
a 555-5553
a 555-3412...
$ cat name-lista Amelia
a Anthony...
Each output file contains one name or number per line.
print items >> output-file
This redirection prints the items into the preexisting output file named outputfile. The difference between this and the single-‘>’ redirection is that the oldcontents (if any) of output-file are not erased. Instead, the awk output is appended to the file. If output-file does not exist, then it is created.
print items | command
It is possible to send output to another program through a pipe instead of into
a file. This redirection opens a pipe to command, and writes the values of items
through this pipe to another process created to execute command.
The redirection argument command is actually an awk expression. Its value isconverted to a string whose contents give the shell command to be run. For
example, the following produces two files, one unsorted list of peoples’ names,
and one list sorted in reverse alphabetical order:
awk '{ print $1 > "names.unsorted"command = "sort -r > names.sorted"
print $1 | command }' mail-list
The unsorted list is written with an ordinary redirection, while the sorted list
is written by piping through the sort utility.
The next example uses redirection to mail a message to the mailing list bugsystem. This might be useful when trouble is encountered in an awk script run
periodically for system maintenance:
report = "mail bug-system"
print("Awk script failed:", $0) | reportprint("at record number", FNR, "of", FILENAME) | reportclose(report)
The close() function is called here because it’s a good idea to close the pipe
as soon as all the intended output has been sent to it. See Section 5.9 [Closing
Input and Output Redirections], page 113, for more information.
This example also illustrates the use of a variable to represent a file or command—it is not necessary to always use a string constant. Using a variable
is generally a good idea, because (if you mean to refer to that same file orcommand) awk requires that the string value be written identically every time.

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print items |& command
This redirection prints the items to the input of command. The difference
between this and the single-‘|’ redirection is that the output from command
can be read with getline. Thus, command is a coprocess, which works together
with but is subsidiary to the awk program.
This feature is a gawk extension, and is not available in POSIX awk. See
Section 4.10.7 [Using getline from a Coprocess], page 94, for a brief discussion.
See Section 12.4 [Two-Way Communications with Another Process], page 343,
for a more complete discussion.

Redirecting output using ‘>’, ‘>>’, ‘|’, or ‘|&’ asks the system to open a file, pipe, or
coprocess only if the particular file or command you specify has not already been written
to by your program or if it has been closed since it was last written to. In other words, files,
pipes, and coprocesses remain open until explicitly closed. All further print and printf
statements continue to write to the same open file, pipe, or coprocess.

In the shell, when you are building up a file a line at a time, you first use ‘>’ to create
the file, and then you use ‘>>’ for subsequent additions to it, like so:

echo Name: Arnold Robbins > data
echo Street Address: 1234 A Pretty Street, NE >> data
echo City and State: MyTown, MyState 12345-6789 >> data

In awk, the ‘>’ and ‘>>’ operators are subtly different. The operator you use the firsttime you write to a file determines how awk will open (or create) the file. If you use ‘>’, thefile is truncated, and then all subsequent output appends data to the file, even if additional
print or printf statements continue to use ‘>’. If you use ‘>>’ the first time, then existingdata is not truncated, and all subsequent print or printf statements append data to thefile.

You should be consistent and always use the same operator for all output to the same
file. (You can mix ‘>’ and ‘>>’, and nothing bad will happen, but mixing the operators is
considered to be bad style in awk. If invoked with the --lint option, gawk issues a warningwhen it encounters both operators being used for the same open file.)

As mentioned earlier (see Section 4.10.9 [Points to Remember About getline], page 94),
many older awk implementations limit the number of pipelines that an awk program may
have open to just one! In gawk, there is no such limit. gawk allows a program to open as
many pipelines as the underlying operating system permits.

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

Piping into sh
A particularly powerful way to use redirection is to build command lines and pipe theminto the shell, sh. For example, suppose you have a list of files brought over from a system
where all the file names are stored in uppercase, and you wish to rename them to havenames in all lowercase. The following program is both simple and efficient:{ printf("mv %s %s\n", $0, tolower($0)) | "sh" }END { close("sh") }The tolower() function returns its argument string with all uppercase charactersconverted to lowercase (see Section 9.1.4 [String-Manipulation Functions], page 196). The
program builds up a list of command lines, using the mv utility to rename the files. It thensends the list to the shell for execution.
See Section 10.2.9 [Quoting Strings to Pass to the Shell], page 252, for a function that
can help in generating command lines to be fed to the shell.

5.7 Special Files for Standard Preopened Data Streams
Running programs conventionally have three input and output streams already available tothem for reading and writing. These are known as the standard input, standard output,
and standard error output. These open streams (and any other open files or pipes) areoften referred to by the technical term file descriptors.
These streams are, by default, connected to your keyboard and screen, but they areoften redirected with the shell, via the ‘<’, ‘<<’, ‘>’, ‘>>’, ‘>&’, and ‘|’ operators. Standard
error is typically used for writing error messages; the reason there are two separate streams,
standard output and standard error, is so that they can be redirected separately.
In traditional implementations of awk, the only way to write an error message to standard
error in an awk program is as follows:
print "Serious error detected!" | "cat 1>&2"This works by opening a pipeline to a shell command that can access the standard error
stream that it inherits from the awk process. This is far from elegant, and it also requires
a separate process. So people writing awk programs often don’t do this. Instead, they send
the error messages to the screen, like this:print "Serious error detected!" > "/dev/tty"(/dev/tty is a special file supplied by the operating system that is connected to yourkeyboard and screen. It represents the “terminal,”1 which on modern systems is a keyboard
and screen, not a serial console.) This generally has the same effect, but not always:
although the standard error stream is usually the screen, it can be redirected; when that
happens, writing to the screen is not correct. In fact, if awk is run from a background job,
it may not have a terminal at all. Then opening /dev/tty fails.
gawk, BWK awk, and mawk provide special file names for accessing the three standard
streams. If the file name matches one of these special names when gawk (or one of the1

The “tty” in /dev/tty stands for “Teletype,” a serial terminal.

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ported to, not just those that are POSIX-compliant:/dev/stdin
The standard input (file descriptor 0).
/dev/stdout
The standard output (file descriptor 1).
/dev/stderr
The standard error output (file descriptor 2).
With these facilities, the proper way to write an error message then becomes:
print "Serious error detected!" > "/dev/stderr"
Note the use of quotes around the file name. Like with any other redirection, the value
must be a string. It is a common error to omit the quotes, which leads to confusing results.
gawk does not treat these file names as special when in POSIX-compatibility mode.
However, because BWK awk supports them, gawk does support them even when invokedwith the --traditional option (see Section 2.2 [Command-Line Options], page 33).

5.8 Special File names in gawk
Besides access to standard input, standard output, and standard error, gawk provides accessto any open file descriptor. Additionally, there are special file names reserved for TCP/IP
networking.

5.8.1 Accessing Other Open Files with gawk
Besides the /dev/stdin, /dev/stdout, and /dev/stderr special file names mentioned
earlier, gawk provides syntax for accessing any other inherited open file:/dev/fd/N
The file associated with file descriptor N. Such a file must be opened by the
program initiating the awk execution (typically the shell). Unless special pains
are taken in the shell from which gawk is invoked, only descriptors 0, 1, and 2
are available.
The file names /dev/stdin, /dev/stdout, and /dev/stderr are essentially aliases for/dev/fd/0, /dev/fd/1, and /dev/fd/2, respectively. However, those names are more selfexplanatory.
Note that using close() on a file name of the form "/dev/fd/N", for file descriptor
numbers above two, does actually close the given file descriptor.

5.8.2 Special Files for Network Communications
gawk programs can open a two-way TCP/IP connection, acting as either a client or a server.
This is done using a special file name of the form:/net-type/protocol/local-port/remote-host/remote-port
The net-type is one of ‘inet’, ‘inet4’, or ‘inet6’. The protocol is one of ‘tcp’ or ‘udp’,
and the other fields represent the other essential pieces of information for making a networking connection. These file names are used with the ‘|&’ operator for communicating with

Chapter 5: Printing Output 113

a coprocess (see Section 12.4 [Two-Way Communications with Another Process], page 343).
This is an advanced feature, mentioned here only for completeness. Full discussion is delayed
until Section 12.5 [Using gawk for Network Programming], page 346.

5.8.3 Special File name Caveats
Here are some things to bear in mind when using the special file names that gawk provides:• Recognition of the file names for the three standard preopened files is disabled only in
POSIX mode.
• Recognition of the other special file names is disabled if gawk is in compatibility mode(either --traditional or --posix; see Section 2.2 [Command-Line Options], page 33).
• gawk always interprets these special file names. For example, using ‘/dev/fd/4’ for
output actually writes on file descriptor 4, and not on a new file descriptor that is
dup()ed from file descriptor 4. Most of the time this does not matter; however, it isimportant to not close any of the files related to file descriptors 0, 1, and 2. Doing so
results in unpredictable behavior.

5.9 Closing Input and Output Redirections
If the same file name or the same shell command is used with getline more than once
during the execution of an awk program (see Section 4.10 [Explicit Input with getline],
page 89), the file is opened (or the command is executed) the first time only. At that time,
the first record of input is read from that file or command. The next time the same file orcommand is used with getline, another record is read from it, and so on.
Similarly, when a file or pipe is opened for output, awk remembers the file name or command associated with it, and subsequent writes to the same file or command are appended
to the previous writes. The file or pipe stays open until awk exits.
This implies that special steps are necessary in order to read the same file again from the
beginning, or to rerun a shell command (rather than reading more output from the samecommand). The close() function makes these things possible:
close(filename)or:close(command)
The argument filename or command can be any expression. Its value must exactly matchthe string that was used to open the file or start the command (spaces and other “irrelevant”
characters included). For example, if you open a pipe with this:"sort -r names" | getline foothen you must close it with this:close("sort -r names")Once this function call is executed, the next getline from that file or command, or the
next print or printf to that file or command, reopens the file or reruns the command.
Because the expression that you use to close a file or pipeline must exactly match theexpression used to open the file or run the command, it is good practice to use a variable
to store the file name or command. The previous example becomes the following:sortcom = "sort -r names"
sortcom | getline foo

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close(sortcom)
This helps avoid hard-to-find typographical errors in your awk programs. Here are some of
the reasons for closing an output file:• To write a file and read it back later on in the same awk program. Close the file after
writing it, then begin reading it with getline.
• To write numerous files, successively, in the same awk program. If the files aren’t closed,
eventually awk may exceed a system limit on the number of open files in one process.
It is best to close each one when the program has finished writing it.
• To make a command finish. When output is redirected through a pipe, the command
reading the pipe normally continues to try to read input as long as the pipe is open.
Often this means the command cannot really do its work until the pipe is closed. For
example, if output is redirected to the mail program, the message is not actually sent
until the pipe is closed.
• To run the same program a second time, with the same arguments. This is not thesame thing as giving more input to the first run!
For example, suppose a program pipes output to the mail program. If it outputs
several lines redirected to this pipe without closing it, they make a single message ofseveral lines. By contrast, if the program closes the pipe after each line of output, theneach line makes a separate message.
If you use more files than the system allows you to have open, gawk attempts to multiplex
the available open files among your data files. gawk’s ability to do this depends upon thefacilities of your operating system, so it may not always work. It is therefore both goodpractice and good portability advice to always use close() on your files when you are donewith them. In fact, if you are using a lot of pipes, it is essential that you close commands
when done. For example, consider something like this:{...
command = ("grep " $1 " /some/file | my_prog -q " $3)
while ((command | getline) > 0) {process output of command}# need close(command) here}
This example creates a new pipeline based on data in each record. Without the call toclose() indicated in the comment, awk creates child processes to run the commands, until
it eventually runs out of file descriptors for more pipelines.
Even though each command has finished (as indicated by the end-of-file return statusfrom getline), the child process is not terminated;2 more importantly, the file descriptor
for the pipe is not closed and released until close() is called or awk exits.
2

The technical terminology is rather morbid. The finished child is called a “zombie,” and cleaning up
after it is referred to as “reaping.”

Chapter 5: Printing Output 115

close() silently does nothing if given an argument that does not represent a file, pipe,or coprocess that was opened with a redirection. In such a case, it returns a negative value,indicating an error. In addition, gawk sets ERRNO to a string indicating the error.
Note also that ‘close(FILENAME)’ has no “magic” effects on the implicit loop that reads
through the files named on the command line. It is, more likely, a close of a file that wasnever opened with a redirection, so awk silently does nothing, except return a negativevalue.
When using the ‘|&’ operator to communicate with a coprocess, it is occasionally useful
to be able to close one end of the two-way pipe without closing the other. This is doneby supplying a second argument to close(). As in any other call to close(), the firstargument is the name of the command or special file used to start the coprocess. The
second argument should be a string, with either of the values "to" or "from". Case does
not matter. As this is an advanced feature, discussion is delayed until Section 12.4 [TwoWay Communications with Another Process], page 343, which describes it in more detailand gives an example.

5.9.1 Using close()’s Return Value
In many older versions of Unix awk, the close() function is actually a statement. It is a
syntax error to try and use the return value from close():command = "..."
command | getline info
retval = close(command)# syntax error in many Unix awks

gawk treats close() as a function. The return value is −1 if the argument names
something that was never opened with a redirection, or if there is a system problem closing
the file or process. In these cases, gawk sets the predefined variable ERRNO to a string
describing the problem.
In gawk, starting with version 4.2, when closing a pipe or coprocess (input or output),
the return value is the exit status of the command, as described in Table 5.1.3 Otherwise, itis the return value from the system’s close() or fclose() C functions when closing input
or output files, respectively. This value is zero if the close succeeds, or −1 if it fails. Recent
versions of BWK awk also return the same values from close().
Situation
Normal exit of command
Death by signal of command
Death by signal of command with core dump
Some kind of error

Return value from close()
Command’s exit status
256 + number of murderous signal
512 + number of murderous signal−1

Table 5.1: Return values from close() of a pipe
The POSIX standard is very vague; it says that close() returns zero on success anda nonzero value otherwise. In general, different implementations vary in what they reportwhen closing pipes; thus, the return value cannot be used portably. In POSIX mode (seeSection 2.2 [Command-Line Options], page 33), gawk just returns zero when closing a pipe.
3

Prior to version 4.2, the return value from closing a pipe or co-process was the full 16-bit exit value as
defined by the wait() system call.

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5.10 Speeding Up Pipe Output
This section describes a gawk-specific feature.
Normally, when you send data down a pipeline to a command with print or printf,
gawk flushes the output down the pipe. That is, output is not buffered, but written directly.
This assures, that pipeline output intermixed with gawk’s output comes out in the expectedorder:
print "something"# goes to standard output
print "something else" | "some-command" # also to standard output
print "more stuff"# and this too
There can be a price to pay for this; flushing data down the pipeline uses more CPU
time, and in certain environments this can become expensive.
You can tell gawk not to flush buffered data in one of two ways:• Set PROCINFO["BUFFERPIPE"] to any value. When this is done, gawk will buffer data
for all pipelines.
• Set PROCINFO["command", "BUFFERPIPE"] to any value. In this case, only command’sdata will be fully buffered.
You must create one or the other of these elements in PROCINFO before the first print
or printf to the pipeline. Doing so after output has already been sent is too late.
Be aware that using this feature may change the output behavior of your programs, so
exercise caution.

5.11 Enabling Nonfatal Output
This section describes a gawk-specific feature.
In standard awk, output with print or printf to a nonexistent file, or some other I/Oerror (such as filling up the disk) is a fatal error.
$ gawk 'BEGIN { print "hi" > "/no/such/file" }'error gawk: cmd. line:1: fatal: can't redirect to `/no/such/file' (Noerror such file or directory)
gawk makes it possible to detect that an error has occurred, allowing you to possibly
recover from the error, or at least print an error message of your choosing before exiting.
You can do this in one of two ways:• For all output files, by assigning any value to PROCINFO["NONFATAL"].
• On a per-file basis, by assigning any value to PROCINFO[filename, "NONFATAL"]. Here,filename is the name of the file to which you wish output to be nonfatal.
Once you have enabled nonfatal output, you must check ERRNO after every relevant print
or printf statement to see if something went wrong. It is also a good idea to initialize
ERRNO to zero before attempting the output. For example:$ gawk '> BEGIN {>PROCINFO["NONFATAL"] = 1>
ERRNO = 0>
print "hi" > "/no/such/file"Chapter 5: Printing Output 117

>
>
>
>
> }'

if (ERRNO) {print("Output failed:", ERRNO) > "/dev/stderr"
exit 1}error

Output failed: No such file or directory

Here, gawk did not produce a fatal error; instead it let the awk program code detect the
problem and handle it.
This mechanism works also for standard output and standard error. For standard output,
you may use PROCINFO["-", "NONFATAL"] or PROCINFO["/dev/stdout", "NONFATAL"].
For standard error, use PROCINFO["/dev/stderr", "NONFATAL"].
When attempting to open a TCP/IP socket (see Section 12.5 [Using gawk for Network
Programming], page 346), gawk tries multiple times. The GAWK_SOCK_RETRIES environment
variable (see Section 2.5.3 [Other Environment Variables], page 44) allows you to override
gawk’s builtin default number of attempts. However, once nonfatal I/O is enabled for agiven socket, gawk only retries once, relying on awk-level code to notice that there was a
problem.

5.12 Summary• The print statement prints comma-separated expressions. Each expression is separated by the value of OFS and terminated by the value of ORS. OFMT provides theconversion format for numeric values for the print statement.
• The printf statement provides finer-grained control over output, with format-control
letters for different data types and various flags that modify the behavior of the formatcontrol letters.
• Output from both print and printf may be redirected to files, pipes, and coprocesses.
• gawk provides special file names for access to standard input, output, and error, andfor network communications.
• Use close() to close open file, pipe, and coprocess redirections. For coprocesses, it ispossible to close only one direction of the communications.
• Normally errors with print or printf are fatal. gawk lets you make output errors be
nonfatal either for all files or on a per-file basis. You must then check for errors after
every relevant output statement.

5.13 Exercises
1. Rewrite the program:awk 'BEGIN { print "Month Crates"
print "----- ------" }{ print $1, "", $2 }' inventory-shippedfrom Section 5.3 [Output Separators], page 101, by using a new value of OFS.
2. Use the printf statement to line up the headings and table data for the inventoryshipped example that was covered in Section 5.1 [The print Statement], page 99.

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3. What happens if you forget the double quotes when redirecting output, as follows:
BEGIN { print "Serious error detected!" > /dev/stderr }Chapter 6: Expressions 119

6 Expressions
Expressions are the basic building blocks of awk patterns and actions. An expression evaluates to a value that you can print, test, or pass to a function. Additionally, an expression
can assign a new value to a variable or a field by using an assignment operator.
An expression can serve as a pattern or action statement on its own. Most other kindsof statements contain one or more expressions that specify the data on which to operate.
As in other languages, expressions in awk can include variables, array references, constants,
and function calls, as well as combinations of these with various operators.

6.1 Constants, Variables, and Conversions
Expressions are built up from values and the operations performed upon them. This section
describes the elementary objects that provide the values used in expressions.

6.1.1 Constant Expressions
The simplest type of expression is the constant, which always has the same value. There
are three types of constants: numeric, string, and regular expression.
Each is used in the appropriate context when you need a data value that isn’t going tochange. Numeric constants can have different forms, but are internally stored in an identicalmanner.

6.1.1.1 Numeric and String Constants
A numeric constant stands for a number. This number can be an integer, a decimal fraction,
or a number in scientific (exponential) notation.1 Here are some examples of numeric
constants that all have the same value:105
1.05e+2
1050e-1
A string constant consists of a sequence of characters enclosed in double quotation marks.
For example:"parrot"
represents the string whose contents are ‘parrot’. Strings in gawk can be of any length,
and they can contain any of the possible eight-bit ASCII characters, including ASCII nul(character code zero). Other awk implementations may have difficulty with some charactercodes.
Some languages allow you to continue long strings across multiple lines by ending theline with a backslash. For example in C:#include <stdio.h>
int main(){
1

The internal representation of all numbers, including integers, uses double-precision floating-point numbers. On most modern systems, these are in IEEE 754 standard format. See Chapter 16 [Arithmetic
and Arbitrary-Precision Arithmetic with gawk], page 389, for much more information.

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printf("hello, \world\n");return 0;}
In such a case, the C compiler removes both the backslash and the newline, producing astring as if it had been typed ‘"hello, world\n"’. This is useful when a single string needs
to contain a large amount of text.
The POSIX standard says explicitly that newlines are not allowed inside string constants.
And indeed, all awk implementations report an error if you try to do so. For example:$ gawk 'BEGIN { print "hello,> world" }'a gawk: cmd. line:1: BEGIN { print "hello,^ unterminated stringa gawk: cmd. line:1:
a gawk: cmd. line:1: BEGIN { print "hello,^ syntax error
a gawk: cmd. line:1:
Although POSIX doesn’t define what happens if you use an escaped newline, as in theprevious C example, all known versions of awk allow you to do so. Unfortunately, what
each one does with such a string varies. gawk, mawk, and the OpenSolaris POSIX awk (seeSection B.5 [Other Freely Available awk Implementations], page 510) elide the backslashand newline, as in C:$ gawk 'BEGIN { print "hello, \> world" }'a hello, world
In POSIX mode (see Section 2.2 [Command-Line Options], page 33), gawk does not allow
escaped newlines. Otherwise, it behaves as just described.
BWK awk2 and BusyBox awk remove the backslash but leave the newline intact, as partof the string:$ nawk 'BEGIN { print "hello, \> world" }'a hello,a world

6.1.1.2 Octal and Hexadecimal Numbers
In awk, all numbers are in decimal (i.e., base 10). Many other programming languages
allow you to specify numbers in other bases, often octal (base 8) and hexadecimal (base16). In octal, the numbers go 0, 1, 2, 3, 4, 5, 6, 7, 10, 11, 12, and so on. Just as ‘11’ indecimal is 1 times 10 plus 1, so ‘11’ in octal is 1 times 8 plus 1. This equals 9 in decimal.
In hexadecimal, there are 16 digits. Because the everyday decimal number system only
has ten digits (‘0’–‘9’), the letters ‘a’ through ‘f’ represent the rest. (Case in the letters is
usually irrelevant; hexadecimal ‘a’ and ‘A’ have the same value.) Thus, ‘11’ in hexadecimalis 1 times 16 plus 1, which equals 17 in decimal.
Just by looking at plain ‘11’, you can’t tell what base it’s in. So, in C, C++, and otherlanguages derived from C, there is a special notation to signify the base. Octal numbers
start with a leading ‘0’, and hexadecimal numbers start with a leading ‘0x’ or ‘0X’:
2

In all examples throughout this book, nawk is BWK awk.

Chapter 6: Expressions 121

11

Decimal value 11011

Octal 11, decimal value 9

0x11

Hexadecimal 11, decimal value 17

This example shows the difference:$ gawk 'BEGIN { printf "%d, %d, %d\n", 011, 11, 0x11 }'a 9, 11, 17
Being able to use octal and hexadecimal constants in your programs is most usefulwhen working with data that cannot be represented conveniently as characters or as regularnumbers, such as binary data of various sorts.
gawk allows the use of octal and hexadecimal constants in your program text. However,such numbers in the input data are not treated differently; doing so by default would breakold programs. (If you really need to do this, use the --non-decimal-data commandline option; see Section 12.1 [Allowing Nondecimal Input Data], page 335.) If you haveoctal or hexadecimal data, you can use the strtonum() function (see Section 9.1.4 [StringManipulation Functions], page 196) to convert the data into a number. Most of the time,
you will want to use octal or hexadecimal constants when working with the built-in bitmanipulation functions; see Section 9.1.7 [Bit-Manipulation Functions], page 216, for more
information.
Unlike in some early C implementations, ‘8’ and ‘9’ are not valid in octal constants. For
example, gawk treats ‘018’ as decimal 18:$ gawk 'BEGIN { print "021 is", 021 ; print 018 }'a 021 is 17
a 18
Octal and hexadecimal source code constants are a gawk extension. If gawk is in compatibility mode (see Section 2.2 [Command-Line Options], page 33), they are not available.


A Constant’s Base Does Not Affect Its Value
Once a numeric constant has been converted internally into a number, gawk no longer
remembers what the original form of the constant was; the internal value is always used.
This has particular consequences for conversion of numbers to strings:$ gawk 'BEGIN { printf "0x11 is <%s>\n", 0x11 }'a 0x11 is <17>

6.1.1.3 Regular Expression Constants
A regexp constant is a regular expression description enclosed in slashes, such as/^beginning and end$/. Most regexps used in awk programs are constant, but the ‘~’ and‘!~’ matching operators can also match computed or dynamic regexps (which are typicallyjust ordinary strings or variables that contain a regexp, but could be more complex
expressions).

6.1.2 Using Regular Expression Constants
Regular expression constants consist of text describing a regular expression enclosed inslashes (such as /the +answer/). This section describes how such constants work in POSIX122

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awk and gawk, and then goes on to describe strongly typed regexp constants, which are agawk extension.

6.1.2.1 Standard Regular Expression Constants
When used on the righthand side of the ‘~’ or ‘!~’ operators, a regexp constant merely
stands for the regexp that is to be matched. However, regexp constants (such as /foo/)
may be used like simple expressions. When a regexp constant appears by itself, it has
the same meaning as if it appeared in a pattern (i.e., ‘($0 ~ /foo/)’). See Section 7.1.2[Expressions as Patterns], page 147. This means that the following two code segments:
if ($0 ~ /barfly/ || $0 ~ /camelot/)
print "found"and:if (/barfly/ || /camelot/)
print "found"are exactly equivalent. One rather bizarre consequence of this rule is that the following
Boolean expression is valid, but does not do what its author probably intended:# Note that /foo/ is on the left of the ~if (/foo/ ~ $1) print "found foo"This code is “obviously” testing $1 for a match against the regexp /foo/. But in fact, theexpression ‘/foo/ ~ $1’ really means ‘($0 ~ /foo/) ~ $1’. In other words, first match the
input record against the regexp /foo/. The result is either zero or one, depending upon
the success or failure of the match. That result is then matched against the first field in
the record. Because it is unlikely that you would ever really want to make this kind of test,
gawk issues a warning when it sees this construct in a program. Another consequence of
this rule is that the assignment statement:
matches = /foo/assigns either zero or one to the variable matches, depending upon the contents of thecurrent input record.
Constant regular expressions are also used as the first argument for the gensub(), sub(),and gsub() functions, as the second argument of the match() function, and as the thirdargument of the split() and patsplit() functions (see Section 9.1.4 [String-Manipulation
Functions], page 196). Modern implementations of awk, including gawk, allow the thirdargument of split() to be a regexp constant, but some older implementations do not.
Because some built-in functions accept regexp constants as arguments, confusion canarise when attempting to use regexp constants as arguments to user-defined functions (seeSection 9.2 [User-Defined Functions], page 221). For example:
function mysub(pat, repl, str, global){
if (global)
gsub(pat, repl, str)else
sub(pat, repl, str)
return str}

Chapter 6: Expressions 123{...
text = "hi! hi yourself!"
mysub(/hi/, "howdy", text, 1)...
}
In this example, the programmer wants to pass a regexp constant to the user-defined
function mysub(), which in turn passes it on to either sub() or gsub(). However, whatreally happens is that the pat parameter is assigned a value of either one or zero, depending
upon whether or not $0 matches /hi/. gawk issues a warning when it sees a regexp constantused as a parameter to a user-defined function, because passing a truth value in this way isprobably not what was intended.

6.1.2.2 Strongly Typed Regexp Constants
This section describes a gawk-specific feature.
As we saw in the previous section, regexp constants (/.../) hold a strange position in
the awk language. In most contexts, they act like an expression: ‘$0 ~ /.../’. In othercontexts, they denote only a regexp to be matched. In no case are they really a “first classcitizen” of the language. That is, you cannot define a scalar variable whose type is “regexp”
in the same sense that you can define a variable to be a number or a string:num = 42
Numeric variable
str = "hi"String variable
re = /foo/Wrong! re is the result of $0 ~ /foo/
For a number of more advanced use cases, it would be nice to have regexp constants thatare strongly typed; in other words, that denote a regexp useful for matching, and not an
expression.
gawk provides this feature. A strongly typed regexp constant looks almost like a regularregexp constant, except that it is preceded by an ‘@’ sign:
re = @/foo/Regexp variable
Strongly typed regexp constants cannot be used everywhere that a regular regexp constant can, because this would make the language even more confusing. Instead, you may
use them only in certain contexts:• On the righthand side of the ‘~’ and ‘!~’ operators: ‘some_var ~ @/foo/’ (seeSection 3.1 [How to Use Regular Expressions], page 51).
• In the case part of a switch statement (see Section 7.4.5 [The switch Statement],
page 157).
• As an argument to one of the built-in functions that accept regexp constants:
gensub(), gsub(), match(), patsplit(), split(), and sub() (see Section 9.1.4[String-Manipulation Functions], page 196).
• As a parameter in a call to a user-defined function (see Section 9.2 [User-Defined
Functions], page 221).
• As the return value of a user-defined function.
• On the righthand side of an assignment to a variable: ‘some_var = @/foo/’. In thiscase, the type of some_var is regexp. Additionally, some_var can be used with ‘~’ and124

GAWK: Effective AWK Programming‘!~’, passed to one of the built-in functions listed above, or passed as a parameter to auser-defined function.
You may use the -v option (see Section 2.2 [Command-Line Options], page 33) to assign
a strongly-typed regexp constant to a variable on the command line, like so:gawk -v pattern='@/something(interesting)+/' ...
You may also make such assignments as regular command-line arguments (see Section 2.3[Other Command-Line Arguments], page 40).
You may use the typeof() built-in function (see Section 9.1.8 [Getting Type Information], page 219) to determine if a variable or function parameter is a regexp variable.
The true power of this feature comes from the ability to create variables that have regexp
type. Such variables can be passed on to user-defined functions, without the confusing
aspects of computed regular expressions created from strings or string constants. They
may also be passed through indirect function calls (see Section 9.3 [Indirect Function Calls],
page 231) and on to the built-in functions that accept regexp constants.
When used in numeric conversions, strongly typed regexp variables convert to zero.
When used in string conversions, they convert to the string value of the original regexp
text.
There is an additional, interesting corner case. When used as the third argument tosub() or gsub(), they retain their type. Thus, if you have something like this:re = @/don't panic/sub(/don't/, "do", re)print typeof(re), rethen re retains its type, but now attempts to match the string ‘do panic’. This provides a(very indirect) way to create regexp-typed variables at runtime.

6.1.3 Variables
Variables are ways of storing values at one point in your program for use later in anotherpart of your program. They can be manipulated entirely within the program text, and they
can also be assigned values on the awk command line.

6.1.3.1 Using Variables in a Program
Variables let you give names to values and refer to them later. Variables have already beenused in many of the examples. The name of a variable must be a sequence of letters, digits,
or underscores, and it may not begin with a digit. Here, a letter is any one of the 52
upper- and lowercase English letters. Other characters that may be defined as letters in
non-English locales are not valid in variable names. Case is significant in variable names; aand A are distinct variables.
A variable name is a valid expression by itself; it represents the variable’s current value.
Variables are given new values with assignment operators, increment operators, and decrement operators (see Section 6.2.3 [Assignment Expressions], page 130). In addition, thesub() and gsub() functions can change a variable’s value, and the match(), split(), andpatsplit() functions can change the contents of their array parameters (see Section 9.1.4[String-Manipulation Functions], page 196).

Chapter 6: Expressions 125

A few variables have special built-in meanings, such as FS (the field separator) and NF(the number of fields in the current input record). See Section 7.5 [Predefined Variables],page 163, for a list of the predefined variables. These predefined variables can be used andassigned just like all other variables, but their values are also used or changed automaticallyby awk. All predefined variables’ names are entirely uppercase.
Variables in awk can be assigned either numeric or string values. The kind of value a
variable holds can change over the life of a program. By default, variables are initialized
to the empty string, which is zero if converted to a number. There is no need to explicitly
initialize a variable in awk, which is what you would do in C and in most other traditional
languages.

6.1.3.2 Assigning Variables on the Command LineAny awk variable can be set by including a variable assignment among the arguments on
the command line when awk is invoked (see Section 2.3 [Other Command-Line Arguments],
page 40). Such an assignment has the following form:
variable=text
With it, a variable is set either at the beginning of the awk run or in between input files.
When the assignment is preceded with the -v option, as in the following:-v variable=text
the variable is set at the very beginning, even before the BEGIN rules execute. The -v
option and its assignment must precede all the file name arguments, as well as the program
text. (See Section 2.2 [Command-Line Options], page 33, for more information about the-v option.) Otherwise, the variable assignment is performed at a time determined by its
position among the input file arguments—after the processing of the preceding input fileargument. For example:
awk '{ print $n }' n=4 inventory-shipped n=2 mail-list
prints the value of field number n for all input records. Before the first file is read, thecommand line sets the variable n equal to four. This causes the fourth field to be printed inlines from inventory-shipped. After the first file has finished, but before the second fileis started, n is set to two, so that the second field is printed in lines from mail-list:$ awk '{ print $n }' n=4 inventory-shipped n=2 mail-list
a 15
a 24
...
a 555-5553
a 555-3412...
Command-line arguments are made available for explicit examination by the awk program in the ARGV array (see Section 7.5.3 [Using ARGC and ARGV], page 173). awk processes
the values of command-line assignments for escape sequences (see Section 3.2 [Escape Sequences], page 52).
Normally, variables assigned on the command line (with or without the -v option) aretreated as strings. When such variables are used as numbers, awk’s normal automaticconversion of strings to numbers takes place, and everything “just works.”

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However, gawk supports variables whose types are “regexp”. You can assign variables ofthis type using the following syntax:
gawk -v 're1=@/foo|bar/' '...' /path/to/file1 're2=@/baz|quux/' /path/to/file2
Strongly typed regexps are an advanced feature (see Section 6.1.2.2 [Strongly Typed Regexp
Constants], page 123). We mention them here only for completeness.

6.1.4 Conversion of Strings and Numbers
Number-to-string and string-to-number conversion are generally straightforward. There canbe subtleties to be aware of; this section discusses this important facet of awk.

6.1.4.1 How awk Converts Between Strings and Numbers
Strings are converted to numbers and numbers are converted to strings, if the context of the
awk program demands it. For example, if the value of either foo or bar in the expression‘foo + bar’ happens to be a string, it is converted to a number before the addition is
performed. If numeric values appear in string concatenation, they are converted to strings.
Consider the following:
two = 2; three = 3
print (two three) + 4
This prints the (numeric) value 27. The numeric values of the variables two and three areconverted to strings and concatenated together. The resulting string is converted back to
the number 23, to which 4 is then added.
If, for some reason, you need to force a number to be converted to a string, concatenatethat number with the empty string, "". To force a string to be converted to a number, add
zero to that string. A string is converted to a number by interpreting any numeric prefixof the string as numerals: "2.5" converts to 2.5, "1e3" converts to 1,000, and "25fix" hasa numeric value of 25. Strings that can’t be interpreted as valid numbers convert to zero.
The exact manner in which numbers are converted into strings is controlled by theawk predefined variable CONVFMT (see Section 7.5 [Predefined Variables], page 163). Numbers are converted using the sprintf() function with CONVFMT as the format specifier (seeSection 9.1.4 [String-Manipulation Functions], page 196).
CONVFMT’s default value is "%.6g", which creates a value with at most six significant
digits. For some applications, you might want to change it to specify more precision. On
most modern machines, 17 digits is usually enough to capture a floating-point number’svalue exactly.3
Strange results can occur if you set CONVFMT to a string that doesn’t tell sprintf() how
to format floating-point numbers in a useful way. For example, if you forget the ‘%’ in theformat, awk converts all numbers to the same constant string.
As a special case, if a number is an integer, then the result of converting it to a string isalways an integer, no matter what the value of CONVFMT may be. Given the following codefragment:
CONVFMT = "%2.2f"
a = 12
3

Pathological cases can require up to 752 digits (!), but we doubt that you need to worry about this.

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127

b = a ""b has the value "12", not "12.00".
Pre-POSIX awk Used OFMT for String Conversion
Prior to the POSIX standard, awk used the value of OFMT for converting numbers
to strings. OFMT specifies the output format to use when printing numbers with print.
CONVFMT was introduced in order to separate the semantics of conversion from the semantics of printing. Both CONVFMT and OFMT have the same default value: "%.6g". In the vast
majority of cases, old awk programs do not change their behavior. See Section 5.1 [Theprint Statement], page 99, for more information on the print statement.

6.1.4.2 Locales Can Influence Conversion
Where you are can matter when it comes to converting between numbers and strings. The
local character set and language—the locale—can affect numeric formats. In particular, forawk programs, it affects the decimal point character and the thousands-separator character.
The "C" locale, and most English-language locales, use the period character (‘.’) as thedecimal point and don’t have a thousands separator. However, many (if not most) European
and non-English locales use the comma (‘,’) as the decimal point character. European
locales often use either a space or a period as the thousands separator, if they have one.
The POSIX standard says that awk always uses the period as the decimal point whenreading the awk program source code, and for command-line variable assignments (seeSection 2.3 [Other Command-Line Arguments], page 40). However, when interpreting input
data, for print and printf output, and for number-to-string conversion, the local decimal
point character is used. In all cases, numbers in source code and in input data cannot
have a thousands separator. Here are some examples indicating the difference in behavior,
on a GNU/Linux system:$ export POSIXLY_CORRECT=1
Force POSIX behavior$ gawk 'BEGIN { printf "%g\n", 3.1415927 }'a 3.14159$ LC_ALL=en_DK.utf-8 gawk 'BEGIN { printf "%g\n", 3.1415927 }'a 3,14159$ echo 4,321 | gawk '{ print $1 + 1 }'a 5$ echo 4,321 | LC_ALL=en_DK.utf-8 gawk '{ print $1 + 1 }'a 5,321
The en_DK.utf-8 locale is for English in Denmark, where the comma acts as the decimal
point separator. In the normal "C" locale, gawk treats ‘4,321’ as 4, while in the Danish
locale, it’s treated as the full number including the fractional part, 4.321.
Some earlier versions of gawk fully complied with this aspect of the standard. However, many users in non-English locales complained about this behavior, because their data
used a period as the decimal point, so the default behavior was restored to use a period as
the decimal point character. You can use the --use-lc-numeric option (see Section 2.2[Command-Line Options], page 33) to force gawk to use the locale’s decimal point charac-

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ter. (gawk also uses the locale’s decimal point character when in POSIX mode, either via--posix or the POSIXLY_CORRECT environment variable, as shown previously.)
Table 6.1 describes the cases in which the locale’s decimal point character is used andwhen a period is used. Some of these features have not been described yet.
Feature
%'g
%g
Input
strtonum()

Default
Use locale
Use period
Use period
Use period--posix or --use-lc-numeric
Use locale
Use locale
Use locale
Use locale

Table 6.1: Locale decimal point versus a period
Finally, modern-day formal standards and the IEEE standard floating-point representation can have an unusual but important effect on the way gawk converts some special stringvalues to numbers. The details are presented in Section 16.7 [Standards Versus Existing
Practice], page 401.

6.2 Operators: Doing Something with Values
This section introduces the operators that make use of the values provided by constantsand variables.

6.2.1 Arithmetic Operators
The awk language uses the common arithmetic operators when evaluating expressions. Allof these arithmetic operators follow normal precedence rules and work as you would expect
them to.
The following example uses a file named grades, which contains a list of student names
as well as three test scores per student (it’s a small class):
Pat
100 97 58
Sandy 84 72 93
Chris 72 92 89
This program takes the file grades and prints the average of the scores:$ awk '{ sum = $2 + $3 + $4 ; avg = sum / 3>
print $1, avg }' grades
a Pat 85
a Sandy 83
a Chris 84.3333
The following list provides the arithmetic operators in awk, in order from the highest
precedence to the lowest:x^yx ** yExponentiation; x raised to the y power. ‘2 ^ 3’ has the value eight; the character sequence ‘**’ is equivalent to ‘^’. (c.e.)-x

Negation.

+x

Unary plus; the expression is converted to a number.

Chapter 6: Expressions 129

x*yMultiplication.

x/yDivision; because all numbers in awk are floating-point numbers, the result is
not rounded to an integer—‘3 / 4’ has the value 0.75. (It is a common mistake,especially for C programmers, to forget that all numbers in awk are floating
point, and that division of integer-looking constants produces a real number,not an integer.)x%yRemainder; further discussion is provided in the text, just after this list.

x+y

Addition.

x-y

Subtraction.

Unary plus and minus have the same precedence, the multiplication operators all havethe same precedence, and addition and subtraction have the same precedence.
When computing the remainder of ‘x % y’, the quotient is rounded toward zero to an
integer and multiplied by y. This result is subtracted from x; this operation is sometimes
known as “trunc-mod.” The following relation always holds:b * int(a / b) + (a % b) == a
One possibly undesirable effect of this definition of remainder is that ‘x % y’ is negative
if x is negative. Thus:-17 % 8 = -1
This definition is compliant with the POSIX standard, which says that the % operator
produces results equivalent to using the standard C fmod() function, and that function inturn works as just described.
In other awk implementations, the signedness of the remainder may be machinedependent.
NOTE: The POSIX standard only specifies the use of ‘^’ for exponentiation.
For maximum portability, do not use the ‘**’ operator.

6.2.2 String Concatenation
It seemed like a good idea at the time.
—Brian Kernighan
There is only one string operation: concatenation. It does not have a specific operator
to represent it. Instead, concatenation is performed by writing expressions next to one
another, with no operator. For example:$ awk '{ print "Field number one: " $1 }' mail-lista Field number one: Ameliaa Field number one: Anthony...
Without the space in the string constant after the ‘:’, the line runs together. For
example:$ awk '{ print "Field number one:" $1 }' mail-lista Field number one:Ameliaa Field number one:Anthony
...

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Because string concatenation does not have an explicit operator, it is often necessary
to ensure that it happens at the right time by using parentheses to enclose the items toconcatenate. For example, you might expect that the following code fragment concatenatesfile and name:file = "file"name = "name"print "something meaningful" > file name
This produces a syntax error with some versions of Unix awk.4 It is necessary to use thefollowing:
print "something meaningful" > (file name)
Parentheses should be used around concatenation in all but the most common contexts,
such as on the righthand side of ‘=’. Be careful about the kinds of expressions used in string
concatenation. In particular, the order of evaluation of expressions used for concatenationis undefined in the awk language. Consider this example:
BEGIN {
a = "don't"
print (a " " (a = "panic"))}
It is not defined whether the second assignment to a happens before or after the value of ais retrieved for producing the concatenated value. The result could be either ‘don't panic’,or ‘panic panic’.
The precedence of concatenation, when mixed with other operators, is often counterintuitive. Consider this example:$ awk 'BEGIN { print -12 " " -24 }'a -12-24
This “obviously” is concatenating −12, a space, and −24. But where did the space disappear to? The answer lies in the combination of operator precedences and awk’s automaticconversion rules. To get the desired result, write the program this way:
$ awk 'BEGIN { print -12 " " (-24) }'a -12 -24
This forces awk to treat the ‘-’ on the ‘-24’ as unary. Otherwise, it’s parsed as follows:−12 (" " − 24)⇒ −12 (0 − 24)⇒ −12 (−24)⇒ −12−24
As mentioned earlier, when mixing concatenation with other operators, parenthesize.
Otherwise, you’re never quite sure what you’ll get.

6.2.3 Assignment Expressions
An assignment is an expression that stores a (usually different) value into a variable. For
example, let’s assign the value one to the variable z:z = 1
4It happens that BWK awk, gawk, and mawk all “get it right,” but you should not rely on this.

Chapter 6: Expressions 131

After this expression is executed, the variable z has the value one. Whatever old value
z had before the assignment is forgotten.
Assignments can also store string values. For example, the following stores the value"this food is good" in the variable message:
thing = "food"predicate = "good"message = "this " thing " is " predicate
This also illustrates string concatenation. The ‘=’ sign is called an assignment operator. It
is the simplest assignment operator because the value of the righthand operand is stored
unchanged. Most operators (addition, concatenation, and so on) have no effect except
to compute a value. If the value isn’t used, there’s no reason to use the operator. An
assignment operator is different; it does produce a value, but even if you ignore it, theassignment still makes itself felt through the alteration of the variable. We call this a side
effect.
The lefthand operand of an assignment need not be a variable (see Section 6.1.3 [Variables], page 124); it can also be a field (see Section 4.4 [Changing the Contents of a Field],page 71) or an array element (see Chapter 8 [Arrays in awk], page 177). These are all calledlvalues, which means they can appear on the lefthand side of an assignment operator. The
righthand operand may be any expression; it produces the new value that the assignment
stores in the specified variable, field, or array element. (Such values are called rvalues.)
It is important to note that variables do not have permanent types. A variable’s typeis simply the type of whatever value was last assigned to it. In the following programfragment, the variable foo has a numeric value at first, and a string value later on:
foo = 1
print foo
foo = "bar"print foo
When the second assignment gives foo a string value, the fact that it previously had anumeric value is forgotten.
String values that do not begin with a digit have a numeric value of zero. After executing
the following code, the value of foo is five:foo = "a string"foo = foo + 5
NOTE: Using a variable as a number and then later as a string can be confusing
and is poor programming style. The previous two examples illustrate how awk
works, not how you should write your programs!
An assignment is an expression, so it has a value—the same value that is assigned. Thus,‘z = 1’ is an expression with the value one. One consequence of this is that you can write
multiple assignments together, such as:
x = y = z = 5This example stores the value five in all three variables (x, y, and z). It does so because
the value of ‘z = 5’, which is five, is stored into y and then the value of ‘y = z = 5’, which isfive, is stored into x.

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Assignments may be used anywhere an expression is called for. For example, it is valid
to write ‘x != (y = 1)’ to set y to one, and then test whether x equals one. But this styletends to make programs hard to read; such nesting of assignments should be avoided, except
perhaps in a one-shot program.
Aside from ‘=’, there are several other assignment operators that do arithmetic with theold value of the variable. For example, the operator ‘+=’ computes a new value by adding
the righthand value to the old value of the variable. Thus, the following assignment adds
five to the value of foo:
foo += 5This is equivalent to the following:
foo = foo + 5
Use whichever makes the meaning of your program clearer.
There are situations where using ‘+=’ (or any assignment operator) is not the same as
simply repeating the lefthand operand in the righthand expression. For example:# Thanks to Pat Rankin for this example
BEGIN {
foo[rand()] += 5
for (x in foo)
print x, foo[x]bar[rand()] = bar[rand()] + 5
for (x in bar)
print x, bar[x]}The indices of bar are practically guaranteed to be different, because rand() returns different values each time it is called. (Arrays and the rand() function haven’t been covered
yet. See Chapter 8 [Arrays in awk], page 177, and see Section 9.1.3 [Numeric Functions],
page 194, for more information.) This example illustrates an important fact about assignment operators: the lefthand expression is only evaluated once.
It is up to the implementation as to which expression is evaluated first, the lefthand or
the righthand. Consider this example:i = 1
a[i += 2] = i + 1
The value of a[3] could be either two or four.
Table 6.2 lists the arithmetic assignment operators. In each case, the righthand operandis an expression whose value is converted to a number.

Chapter 6: Expressions 133

Operatorlvalue += increment
lvalue -= decrement
lvalue *= coefficientlvalue /= divisor
lvalue %= modulus
lvalue ^= power
lvalue **= power

Effect
Add increment to the value of lvalue.
Subtract decrement from the value of lvalue.
Multiply the value of lvalue by coefficient.
Divide the value of lvalue by divisor.
Set lvalue to its remainder by modulus.
Raise lvalue to the power power.
Raise lvalue to the power power. (c.e.)

Table 6.2: Arithmetic assignment operators
NOTE: Only the ‘^=’ operator is specified by POSIX. For maximum portability,
do not use the ‘**=’ operator.


Syntactic Ambiguities Between ‘/=’ and Regular Expressions
There is a syntactic ambiguity between the /= assignment operator and regexp constants whose first character is an ‘=’.
This is most notable in some commercial awk
versions. For example:$ awk /==/ /dev/null
error awk: syntax error at source line 1
context iserror>>> /= <<<error
error awk: bailing out at source line 1
A workaround is:
awk '/[=]=/' /dev/nullgawk does not have this problem; BWK awk and mawk also do not.

6.2.4 Increment and Decrement Operators
Increment and decrement operators increase or decrease the value of a variable by one. An
assignment operator can do the same thing, so the increment operators add no power tothe awk language; however, they are convenient abbreviations for very common operations.
The operator used for adding one is written ‘++’. It can be used to increment a variable
either before or after taking its value. To pre-increment a variable v, write ‘++v’. This adds
one to the value of v—that new value is also the value of the expression. (The assignment
expression ‘v += 1’ is completely equivalent.) Writing the ‘++’ after the variable specifies
post-increment. This increments the variable value just the same; the difference is that thevalue of the increment expression itself is the variable’s old value. Thus, if foo has the valuefour, then the expression ‘foo++’ has the value four, but it changes the value of foo to five.
In other words, the operator returns the old value of the variable, but with the side effectof incrementing it.
The post-increment ‘foo++’ is nearly the same as writing ‘(foo += 1) - 1’. It is not
perfectly equivalent because all numbers in awk are floating point—in floating point, ‘foo
+ 1 - 1’ does not necessarily equal foo. But the difference is minute as long as you stick tonumbers that are fairly small (less than 1012 ).

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Fields and array elements are incremented just like variables. (Use ‘$(i++)’ when you
want to do a field reference and a variable increment at the same time. The parentheses
are necessary because of the precedence of the field reference operator ‘$’.)The decrement operator ‘--’ works just like ‘++’, except that it subtracts one insteadof adding it. As with ‘++’, it can be used before the lvalue to pre-decrement or after it topost-decrement. Following is a summary of increment and decrement expressions:++lvalue

Increment lvalue, returning the new value as the value of the expression.

lvalue++Increment lvalue, returning the old value of lvalue as the value of the expression.

--lvalue

Decrement lvalue, returning the new value as the value of the expression. (Thisexpression is like ‘++lvalue’, but instead of adding, it subtracts.)

lvalue--Decrement lvalue, returning the old value of lvalue as the value of the expression. (This expression is like ‘lvalue++’, but instead of adding, it subtracts.)
Operator Evaluation Order
Doctor, it hurts when I do this!
Then don’t do that!
—Groucho Marx
What happens for something like the following?
b = 6
print b += b++
Or something even stranger?
b = 6b += ++b + b++
print bIn other words, when do the various side effects prescribed by the postfix operators(‘b++’) take effect? When side effects happen is implementation-defined. In other words,
it is up to the particular version of awk. The result for the first example may be 12 or 13,
and for the second, it may be 22 or 23.
In short, doing things like this is not recommended and definitely not anything thatyou can rely upon for portability. You should avoid such things in your own programs.

6.3 Truth Values and Conditions
In certain contexts, expression values also serve as “truth values”; i.e., they determine whatshould happen next as the program runs. This section describes how awk defines “true”
and “false” and how values are compared.

6.3.1 True and False in awk
Many programming languages have a special representation for the concepts of “true” and“false.” Such languages usually use the special constants true and false, or perhaps their
uppercase equivalents. However, awk is different. It borrows a very simple concept of trueand false from C. In awk, any nonzero numeric value or any nonempty string value is true.

Chapter 6: Expressions 135

Any other value (zero or the null string, "") is false. The following program prints ‘A
strange truth value’ three times:
BEGIN {
if (3.1415927)
print "A strange truth value"if ("Four Score And Seven Years Ago")
print "A strange truth value"
if (j = 57)
print "A strange truth value"}
There is a surprising consequence of the “nonzero or non-null” rule: the string constant"0" is actually true, because it is non-null.

6.3.2 Variable Typing and Comparison Expressions
The Guide is definitive. Reality is frequently inaccurate.
—Douglas Adams, The Hitchhiker’s Guide to the Galaxy
Unlike in other programming languages, in awk variables do not have a fixed type.
Instead, they can be either a number or a string, depending upon the value that is assigned
to them. We look now at how variables are typed, and how awk compares variables.

6.3.2.1 String Type versus Numeric Type
Scalar objects in awk (variables, array elements, and fields) are dynamically typed. Thismeans their type can change as the program runs, from untyped before any use,5 to stringor number, and then from string to number or number to string, as the program progresses.
(gawk also provides regexp-typed scalars, but let’s ignore that for now; see Section 6.1.2.2[Strongly Typed Regexp Constants], page 123.)
You can’t do much with untyped variables, other than tell that they are untyped. The
following program tests a against "" and 0; the test succeeds when a has never been assigned
a value. It also uses the built-in typeof() function (not presented yet; see Section 9.1.8[Getting Type Information], page 219) to show a’s type:$ gawk 'BEGIN { print (a == "" && a == 0 ?
> "a is untyped" : "a has a type!") ; print typeof(a) }'a a is untyped
a unassigned
A scalar has numeric type when assigned a numeric value, such as from a numeric
constant, or from another scalar with numeric type:$ gawk 'BEGIN { a = 42 ; print typeof(a)> b = a ; print typeof(b) }'numbernumberSimilarly, a scalar has string type when assigned a string value, such as from a string
constant, or from another scalar with string type:$ gawk 'BEGIN { a = "forty two" ; print typeof(a)5gawk calls this unassigned, as the following example shows.

136

GAWK: Effective AWK Programming> b = a ; print typeof(b) }'stringstring
So far, this is all simple and straightforward. What happens, though, when awk has toprocess data from a user? Let’s start with field data. What should the following commandproduce as output?
echo hello | awk '{ printf("%s %s < 42\n", $1,($1 < 42 ? "is" : "is not")) }'Since ‘hello’ is alphabetic data, awk can only do a string comparison. Internally, it converts
42 into "42" and compares the two string values "hello" and "42". Here’s the result:$ echo hello | awk '{ printf("%s %s < 42\n", $1,>($1 < 42 ? "is" : "is not")) }'
hello
is
not
<
42
a
However, what happens when data from a user looks like a number? On the one hand,in reality, the input data consists of characters, not binary numeric values. But, on theother hand, the data looks numeric, and awk really ought to treat it as such. And indeed,
it does:$ echo 37 | awk '{ printf("%s %s < 42\n", $1,>($1 < 42 ? "is" : "is not")) }'a 37 is < 42
Here are the rules for when awk treats data as a number, and for when it treats data asa string.
The POSIX standard uses the term numeric string for input data that looks numeric.
The ‘37’ in the previous example is a numeric string. So what is the type of a numericstring? Answer: numeric.
The type of a variable is important because the types of two variables determine how
they are compared. Variable typing follows these definitions and rules:• A numeric constant or the result of a numeric operation has the numeric attribute.
• A string constant or the result of a string operation has the string attribute.
• Fields, getline input, FILENAME, ARGV elements, ENVIRON elements, and the elementsof an array created by match(), split(), and patsplit() that are numeric strings
have the strnum attribute.6 Otherwise, they have the string attribute. Uninitialized
variables also have the strnum attribute.
• Attributes propagate across assignments but are not changed by any use.
The last rule is particularly important. In the following program, a has numeric type,even though it is later used in a string operation:
BEGIN {
a = 12.345
b = a " is a cute number"print b}6

Thus, a POSIX numeric string and gawk’s strnum are the same thing.

Chapter 6: Expressions 137

When two operands are compared, either string comparison or numeric comparison may
be used. This depends upon the attributes of the operands, according to the following
symmetric matrix:

STRINGNUMERICSTRNUM

STRINGNUMERICSTRNUM

stringstringstringstringnumeric
numeric

stringnumeric
numeric

The basic idea is that user input that looks numeric—and only user input—should betreated as numeric, even though it is actually made of characters and is therefore also astring. Thus, for example, the string constant " +3.14", when it appears in program source
code, is a string—even though it looks numeric—and is never treated as a number forcomparison purposes.
In short, when one operand is a “pure” string, such as a string constant, then a string
comparison is performed. Otherwise, a numeric comparison is performed. (The primarydifference between a number and a strnum is that for strnums gawk preserves the original
string value that the scalar had when it came in.)This point bears additional emphasis: Input that looks numeric is numeric. All other
input is treated as strings.
Thus, the six-character input string ‘ +3.14’ receives the strnum attribute. In contrast,
the eight characters " +3.14" appearing in program text comprise a string constant. The
following examples print ‘1’ when the comparison between the two different constants istrue, and ‘0’ otherwise:$ echo ' +3.14' | awk '{ print($0 == " +3.14") }'a 1$ echo ' +3.14' | awk '{ print($0 == "+3.14") }'a 0$ echo ' +3.14' | awk '{ print($0 == "3.14") }'a 0$ echo ' +3.14' | awk '{ print($0 == 3.14) }'a 1$ echo ' +3.14' | awk '{ print($1 == " +3.14") }'a 0$ echo ' +3.14' | awk '{ print($1 == "+3.14") }'a 1$ echo ' +3.14' | awk '{ print($1 == "3.14") }'a 0$ echo ' +3.14' | awk '{ print($1 == 3.14) }'
a 1

True
False
False
True
False
True
False
True

You can see the type of an input field (or other user input) using typeof():$ echo hello 37 | gawk '{ print typeof($1), typeof($2) }'a string strnum

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6.3.2.2 Comparison Operators
Comparison expressions compare strings or numbers for relationships such as equality. They
are written using relational operators, which are a superset of those in C. Table 6.3 describes
them.
Expressionx <yx <= yx >yx >= yx == yx != yx ~yx !~ ysubscript in array

ResultTrue if x is less than yTrue if x is less than or equal to yTrue if x is greater than yTrue if x is greater than or equal to yTrue if x is equal to yTrue if x is not equal to yTrue if the string x matches the regexp denoted by yTrue if the string x does not match the regexp denoted by yTrue if the array array has an element with the subscript subscript

Table 6.3: Relational operators
Comparison expressions have the value one if true and zero if false. When comparing
operands of mixed types, numeric operands are converted to strings using the value of
CONVFMT (see Section 6.1.4 [Conversion of Strings and Numbers], page 126).
Strings are compared by comparing the first character of each, then the second characterof each, and so on. Thus, "10" is less than "9". If there are two strings where one is aprefix of the other, the shorter string is less than the longer one. Thus, "abc" is less than"abcd".
It is very easy to accidentally mistype the ‘==’ operator and leave off one of the ‘=’characters. The result is still valid awk code, but the program does not do what is intended:
if (a = b)
...
else
...

# oops! should be a == b

Unless b happens to be zero or the null string, the if part of the test always succeeds.
Because the operators are so similar, this kind of error is very difficult to spot when scanning
the source code.
The following list of expressions illustrates the kinds of comparisons awk performs, aswell as what the result of each comparison is:
1.5 <= 2.0
Numeric comparison (true)"abc" >= "xyz"String comparison (false)
1.5 != " +2"
String comparison (true)"1e2" < "3"
String comparison (true)Chapter 6: Expressions 139

a = 2; b = "2"a == bString comparison (true)a = 2; b = " +2"a == bString comparison (false)
In this example:$ echo 1e2 3 | awk '{ print ($1 < $2) ? "true" : "false" }'a falsethe result is ‘false’ because both $1 and $2 are user input. They are numeric strings—
therefore both have the strnum attribute, dictating a numeric comparison. The purpose of
the comparison rules and the use of numeric strings is to attempt to produce the behavior
that is “least surprising,” while still “doing the right thing.”
String comparisons and regular expression comparisons are very different. For example:
x == "foo"has the value one, or is true if the variable x is precisely ‘foo’. By contrast:x ~ /foo/
has the value one if x contains ‘foo’, such as "Oh, what a fool am I!".
The righthand operand of the ‘~’ and ‘!~’ operators may be either a regexp constant(/. . . /) or an ordinary expression. In the latter case, the value of the expression as a stringis used as a dynamic regexp (see Section 3.1 [How to Use Regular Expressions], page 51;
also see Section 3.6 [Using Dynamic Regexps], page 60).
A constant regular expression in slashes by itself is also an expression. /regexp/ is anabbreviation for the following comparison expression:$0 ~ /regexp/
One special place where /foo/ is not an abbreviation for ‘$0 ~ /foo/’ is when it is therighthand operand of ‘~’ or ‘!~’. See Section 6.1.2 [Using Regular Expression Constants],
page 121, where this is discussed in more detail.

6.3.2.3 String Comparison Based on Locale Collating Order
The POSIX standard used to say that all string comparisons are performed based on thelocale’s collating order. This is the order in which characters sort, as defined by the locale(for more discussion, see Section 6.6 [Where You Are Makes a Difference], page 145). Thisorder is usually very different from the results obtained when doing straight byte-by-bytecomparison.7
Because this behavior differs considerably from existing practice, gawk only implemented
it when in POSIX mode (see Section 2.2 [Command-Line Options], page 33). Here is an
example to illustrate the difference, in an en_US.UTF-8 locale:$ gawk 'BEGIN { printf("ABC < abc = %s\n",>("ABC" < "abc" ? "TRUE" : "FALSE")) }'a ABC < abc = TRUE
$ gawk --posix 'BEGIN { printf("ABC < abc = %s\n",7

Technically, string comparison is supposed to behave the same way as if the strings were compared with
the C strcoll() function.

140

GAWK: Effective AWK Programming>a ABC < abc = FALSE("ABC" < "abc" ? "TRUE" : "FALSE")) }'Fortunately, as of August 2016, comparison based on locale collating order is no longer
required for the == and != operators.8 However, comparison based on locales is still requiredfor <, <=, >, and >=. POSIX thus recommends as follows:
Since the == operator checks whether strings are identical, not whether theycollate equally, applications needing to check whether strings collate equally
can use:a <= b && a >= b
As of version 4.2, gawk continues to use locale collating order for <, <=, >, and >= only
in POSIX mode.

6.3.3 Boolean Expressions
A Boolean expression is a combination of comparison expressions or matching expressions,using the Boolean operators “or” (‘||’), “and” (‘&&’), and “not” (‘!’), along with parentheses
to control nesting. The truth value of the Boolean expression is computed by combining
the truth values of the component expressions. Boolean expressions are also referred to as
logical expressions. The terms are equivalent.
Boolean expressions can be used wherever comparison and matching expressions can beused. They can be used in if, while, do, and for statements (see Section 7.4 [Control
Statements in Actions], page 154). They have numeric values (one if true, zero if false)that come into play if the result of the Boolean expression is stored in a variable or used inarithmetic.
In addition, every Boolean expression is also a valid pattern, so you can use one as apattern to control the execution of rules. The Boolean operators are:boolean1 && boolean2
True if both boolean1 and boolean2 are true. For example, the following statement prints the current input record if it contains both ‘edu’ and ‘li’:
if ($0 ~ /edu/ && $0 ~ /li/) print
The subexpression boolean2 is evaluated only if boolean1 is true. This canmake a difference when boolean2 contains expressions that have side effects. In
the case of ‘$0 ~ /foo/ && ($2 == bar++)’, the variable bar is not incremented
if there is no substring ‘foo’ in the record.
boolean1 || boolean2
True if at least one of boolean1 or boolean2 is true. For example, the following
statement prints all records in the input that contain either ‘edu’ or ‘li’:
if ($0 ~ /edu/ || $0 ~ /li/) print
The subexpression boolean2 is evaluated only if boolean1 is false. This canmake a difference when boolean2 contains expressions that have side effects.
(Thus, this test never really distinguishes records that contain both ‘edu’ and‘li’—as soon as ‘edu’ is matched, the full test succeeds.)
8

See the Austin Group website.

Chapter 6: Expressions 141! boolean True if boolean is false. For example, the following program prints ‘no home!’in the unusual event that the HOME environment variable is not defined:
BEGIN { if (! ("HOME" in ENVIRON))
print "no home!" }(The in operator is described in Section 8.1.2 [Referring to an Array Element],
page 179.)
The ‘&&’ and ‘||’ operators are called short-circuit operators because of the way theywork. Evaluation of the full expression is “short-circuited” if the result can be determined
partway through its evaluation.
Statements that end with ‘&&’ or ‘||’ can be continued simply by putting a newline after
them. But you cannot put a newline in front of either of these operators without usingbackslash continuation (see Section 1.6 [awk Statements Versus Lines], page 28).
The actual value of an expression using the ‘!’ operator is either one or zero, depending
upon the truth value of the expression it is applied to. The ‘!’ operator is often useful forchanging the sense of a flag variable from false to true and back again. For example, thefollowing program is one way to print lines in between special bracketing lines:$1 == "START"interested$1 == "END"{ interested = ! interested; next }{ print }{ interested = ! interested; next }The variable interested, as with all awk variables, starts out initialized to zero, which isalso false. When a line is seen whose first field is ‘START’, the value of interested is toggled
to true, using ‘!’. The next rule prints lines as long as interested is true. When a line isseen whose first field is ‘END’, interested is toggled back to false.9
Most commonly, the ‘!’ operator is used in the conditions of if and while statements,
where it often makes more sense to phrase the logic in the negative:if (! some condition || some other condition) {... do whatever processing ...
}NOTE: The next statement is discussed in Section 7.4.8 [The next Statement],
page 160. next tells awk to skip the rest of the rules, get the next record, andstart processing the rules over again at the top. The reason it’s there is to avoid
printing the bracketing ‘START’ and ‘END’ lines.

6.3.4 Conditional Expressions
A conditional expression is a special kind of expression that has three operands. It allows
you to use one expression’s value to select one of two other expressions. The conditional
expression in awk is the same as in the C language, as shown here:
selector ? if-true-exp : if-false-exp
There are three subexpressions. The first, selector, is always computed first. If it is “true”(not zero or not null), then if-true-exp is computed next, and its value becomes the value ofthe whole expression. Otherwise, if-false-exp is computed next, and its value becomes the9This program has a bug; it prints lines starting with ‘END’. How would you fix it?

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value of the whole expression. For example, the following expression produces the absolutevalue of x:x >= 0 ? x : -x
Each time the conditional expression is computed, only one of if-true-exp and if-false-exp
is used; the other is ignored. This is important when the expressions have side effects. For
example, this conditional expression examines element i of either array a or array b, andincrements i:x == y ? a[i++] : b[i++]This is guaranteed to increment i exactly once, because each time only one of the two
increment expressions is executed and the other is not. See Chapter 8 [Arrays in awk],
page 177, for more information about arrays.
As a minor gawk extension, a statement that uses ‘?:’ can be continued simply by putting
a newline after either character. However, putting a newline in front of either character
does not work without using backslash continuation (see Section 1.6 [awk Statements Versus
Lines], page 28). If --posix is specified (see Section 2.2 [Command-Line Options], page 33),
this extension is disabled.

6.4 Function Calls
A function is a name for a particular calculation. This enables you to ask for it by name atany point in the program. For example, the function sqrt() computes the square root of
a number.
A fixed set of functions are built in, which means they are available in every awk program.
The sqrt() function is one of these. See Section 9.1 [Built-in Functions], page 193, for alist of built-in functions and their descriptions. In addition, you can define functions foruse in your program. See Section 9.2 [User-Defined Functions], page 221, for instructions
on how to do this. Finally, gawk lets you write functions in C or C++ that may be calledfrom your program (see Chapter 17 [Writing Extensions for gawk], page 405).
The way to use a function is with a function call expression, which consists of thefunction name followed immediately by a list of arguments in parentheses. The arguments
are expressions that provide the raw materials for the function’s calculations. When there
is more than one argument, they are separated by commas. If there are no arguments, justwrite ‘()’ after the function name. The following examples show function calls with andwithout arguments:
sqrt(x^2 + y^2)
atan2(y, x)rand()

one argument
two arguments
no arguments

CAUTION: Do not put any space between the function name and the opening parenthesis! A user-defined function name looks just like the name of avariable—a space would make the expression look like concatenation of a variable with an expression inside parentheses. With built-in functions, space before
the parenthesis is harmless, but it is best not to get into the habit of using space
to avoid mistakes with user-defined functions.

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Each function expects a particular number of arguments. For example, the sqrt()
function must be called with a single argument, the number of which to take the squareroot:
sqrt(argument)
Some of the built-in functions have one or more optional arguments. If those arguments
are not supplied, the functions use a reasonable default value. See Section 9.1 [Built-in
Functions], page 193, for full details. If arguments are omitted in calls to user-defined
functions, then those arguments are treated as local variables. Such local variables act likethe empty string if referenced where a string value is required, and like zero if referenced
where a numeric value is required (see Section 9.2 [User-Defined Functions], page 221).
As an advanced feature, gawk provides indirect function calls, which is a way to choose
the function to call at runtime, instead of when you write the source code to your program.
We defer discussion of this feature until later; see Section 9.3 [Indirect Function Calls],
page 231.
Like every other expression, the function call has a value, often called the return value,
which is computed by the function based on the arguments you give it. In this example, thereturn value of ‘sqrt(argument)’ is the square root of argument. The following program
reads numbers, one number per line, and prints the square root of each one:
$ awk '{ print "The square root of", $1, "is", sqrt($1) }'1
a The square root of 1 is 1
3
a The square root of 3 is 1.73205
5
a The square root of 5 is 2.23607
Ctrl-dA function can also have side effects, such as assigning values to certain variables or doing
I/O. This program shows how the match() function (see Section 9.1.4 [String-Manipulation
Functions], page 196) changes the variables RSTART and RLENGTH:{
if (match($1, $2))
print RSTART, RLENGTH
else
print "no match"}
Here is a sample run:$ awk -f matchit.awk
aaccdd c+
a 3 2
foo
bar
a no matchabcdefg e
a 5 1

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6.5 Operator Precedence (How Operators Nest)
Operator precedence determines how operators are grouped when different operators appear
close by in one expression. For example, ‘*’ has higher precedence than ‘+’; thus, ‘a + b *c’ means to multiply b and c, and then add a to the product (i.e., ‘a + (b * c)’).
The normal precedence of the operators can be overruled by using parentheses. Think of
the precedence rules as saying where the parentheses are assumed to be. In fact, it is wise
to always use parentheses whenever there is an unusual combination of operators, because
other people who read the program may not remember what the precedence is in this case.
Even experienced programmers occasionally forget the exact rules, which leads to mistakes.
Explicit parentheses help prevent any such mistakes.
When operators of equal precedence are used together, the leftmost operator groups
first, except for the assignment, conditional, and exponentiation operators, which group in
the opposite order. Thus, ‘a - b + c’ groups as ‘(a - b) + c’ and ‘a = b = c’ groups as ‘a =(b = c)’.
Normally the precedence of prefix unary operators does not matter, because there is only
one way to interpret them: innermost first. Thus, ‘$++i’ means ‘$(++i)’ and ‘++$x’ means‘++($x)’. However, when another operator follows the operand, then the precedence of theunary operators can matter. ‘$x^2’ means ‘($x)^2’, but ‘-x^2’ means ‘-(x^2)’, because ‘-’
has lower precedence than ‘^’, whereas ‘$’ has higher precedence. Also, operators cannot be
combined in a way that violates the precedence rules; for example, ‘$$0++--’ is not a valid
expression because the first ‘$’ has higher precedence than the ‘++’; to avoid the problem
the expression can be rewritten as ‘$($0++)--’.
This list presents awk’s operators, in order of highest to lowest precedence:(. . . )Grouping.

$

Field reference.

++ --Increment, decrement.

^ **Exponentiation. These operators group right to left.

+-!

Unary plus, minus, logical “not.”*/%Multiplication, division, remainder.

+-

Addition, subtraction.

String concatenation
There is no special symbol for concatenation. The operands are simply writtenside by side (see Section 6.2.2 [String Concatenation], page 129).
< <= == != > >= >> | |&Relational and redirection. The relational operators and the redirections havethe same precedence level. Characters such as ‘>’ serve both as relationals andas redirections; the context distinguishes between the two meanings.
Note that the I/O redirection operators in print and printf statements belong
to the statement level, not to expressions. The redirection does not produce anexpression that could be the operand of another operator. As a result, it does
not make sense to use a redirection operator near another operator of lower

Chapter 6: Expressions 145

precedence without parentheses. Such combinations (e.g., ‘print foo > a ? b: c’) result in syntax errors. The correct way to write this statement is ‘print
foo > (a ? b : c)’.
~ !~Matching, nonmatching.

in

Array membership.

&&

Logical “and.”||

Logical “or.”?:Conditional. This operator groups right to left.

= += -= *= /= %= ^= **=Assignment. These operators group right to left.
NOTE: The ‘|&’, ‘**’, and ‘**=’ operators are not specified by POSIX. For
maximum portability, do not use them.

6.6 Where You Are Makes a Difference
Modern systems support the notion of locales: a way to tell the system about the local
character set and language. The ISO C standard defines a default "C" locale, which is anenvironment that is typical of what many C programmers are used to.
Once upon a time, the locale setting used to affect regexp matching, but this is no longer
true (see Section A.8 [Regexp Ranges and Locales: A Long Sad Story], page 486).
Locales can affect record splitting. For the normal case of ‘RS = "\n"’, the locale islargely irrelevant. For other single-character record separators, setting ‘LC_ALL=C’ in theenvironment will give you much better performance when reading records. Otherwise, gawk
has to make several function calls, per input character, to find the record terminator.
Locales can affect how dates and times are formatted (see Section 9.1.6 [Time Functions],
page 211). For example, a common way to abbreviate the date September 4, 2015, in the
United States is “9/4/15.” In many countries in Europe, however, it is abbreviated “4.9.15.”
Thus, the ‘%x’ specification in a "US" locale might produce ‘9/4/15’, while in a "EUROPE"
locale, it might produce ‘4.9.15’.
According to POSIX, string comparison is also affected by locales (similar to regularexpressions). The details are presented in Section 6.3.2.3 [String Comparison Based on
Locale Collating Order], page 139.
Finally, the locale affects the value of the decimal point character used when gawk parses
input data. This is discussed in detail in Section 6.1.4 [Conversion of Strings and Numbers],
page 126.

6.7 Summary• Expressions are the basic elements of computation in programs. They are built fromconstants, variables, function calls, and combinations of the various kinds of values
with operators.
• awk supplies three kinds of constants: numeric, string, and regexp. gawk lets you
specify numeric constants in octal and hexadecimal (bases 8 and 16) as well as decimal

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GAWK: Effective AWK Programming(base 10). In certain contexts, a standalone regexp constant such as /foo/ has thesame meaning as ‘$0 ~ /foo/’.
• Variables hold values between uses in computations. A number of built-in variables
provide information to your awk program, and a number of others let you control how
awk behaves.
• Numbers are automatically converted to strings, and strings to numbers, as needed byawk. Numeric values are converted as if they were formatted with sprintf() using theformat in CONVFMT. Locales can influence the conversions.
• awk provides the usual arithmetic operators (addition, subtraction, multiplication, division, modulus), and unary plus and minus. It also provides comparison operators,
Boolean operators, an array membership testing operator, and regexp matching operators. String concatenation is accomplished by placing two expressions next to eachother; there is no explicit operator. The three-operand ‘?:’ operator provides an “ifelse” test within expressions.
• Assignment operators provide convenient shorthands for common arithmetic operations.
• In awk, a value is considered to be true if it is nonzero or non-null. Otherwise, thevalue is false.
• A variable’s type is set upon each assignment and may change over its lifetime. The
type determines how it behaves in comparisons (string or numeric).
• Function calls return a value that may be used as part of a larger expression. Expressions used to pass parameter values are fully evaluated before the function is called. awk
provides built-in and user-defined functions; this is described in Chapter 9 [Functions],
page 193.
• Operator precedence specifies the order in which operations are performed, unless explicitly overridden by parentheses. awk’s operator precedence is compatible with thatof C.
• Locales can affect the format of data as output by an awk program, and occasionallythe format for data read as input.

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147

7 Patterns, Actions, and Variables
As you have already seen, each awk statement consists of a pattern with an associatedaction. This chapter describes how you build patterns and actions, what kinds of things
you can do within actions, and awk’s predefined variables.
The pattern–action rules and the statements available for use within actions form thecore of awk programming. In a sense, everything covered up to here has been the foundationthat programs are built on top of. Now it’s time to start building something useful.

7.1 Pattern Elements
Patterns in awk control the execution of rules—a rule is executed when its pattern matches
the current input record. The following is a summary of the types of awk patterns:/regular expression/
A regular expression. It matches when the text of the input record fits theregular expression. (See Chapter 3 [Regular Expressions], page 51.)
expression
A single expression. It matches when its value is nonzero (if a number) or
non-null (if a string). (See Section 7.1.2 [Expressions as Patterns], page 147.)
begpat, endpat
A pair of patterns separated by a comma, specifying a range of records. The
range includes both the initial record that matches begpat and the final recordthat matches endpat. (See Section 7.1.3 [Specifying Record Ranges with Patterns], page 149.)
BEGIN
END

Special patterns for you to supply startup or cleanup actions for your awk
program. (See Section 7.1.4 [The BEGIN and END Special Patterns], page 150.)

BEGINFILEENDFILESpecial patterns for you to supply startup or cleanup actions to be done on a
per-file basis. (See Section 7.1.5 [The BEGINFILE and ENDFILE Special Patterns],
page 151.)
empty

The empty pattern matches every input record. (See Section 7.1.6 [The Empty
Pattern], page 152.)

7.1.1 Regular Expressions as Patterns
Regular expressions are one of the first kinds of patterns presented in this book. This kindof pattern is simply a regexp constant in the pattern part of a rule. Its meaning is ‘$0 ~/pattern/’. The pattern matches when the input record matches the regexp. For example:/foo|bar|baz/ { buzzwords++ }END{ print buzzwords, "buzzwords seen" }7.1.2 Expressions as Patterns
Any awk expression is valid as an awk pattern. The pattern matches if the expression’svalue is nonzero (if a number) or non-null (if a string). The expression is reevaluated each

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GAWK: Effective AWK Programmingtime the rule is tested against a new input record. If the expression uses fields such as $1,
the value depends directly on the new input record’s text; otherwise, it depends on only
what has happened so far in the execution of the awk program.
Comparison expressions, using the comparison operators described in Section 6.3.2 [Variable Typing and Comparison Expressions], page 135, are a very common kind of pattern.
Regexp matching and nonmatching are also very common expressions. The left operandof the ‘~’ and ‘!~’ operators is a string. The right operand is either a constant regularexpression enclosed in slashes (/regexp/), or any expression whose string value is used asa dynamic regular expression (see Section 3.6 [Using Dynamic Regexps], page 60). The
following example prints the second field of each input record whose first field is precisely‘li’:$ awk '$1 == "li" { print $2 }' mail-list(There is no output, because there is no person with the exact name ‘li’.) Contrast thiswith the following regular expression match, which accepts any record with a first field that
contains ‘li’:$ awk '$1 ~ /li/ { print $2 }' mail-list
a 555-5553
a 555-6699
A regexp constant as a pattern is also a special case of an expression pattern. The
expression /li/ has the value one if ‘li’ appears in the current input record. Thus, as apattern, /li/ matches any record containing ‘li’.
Boolean expressions are also commonly used as patterns. Whether the pattern matches
an input record depends on whether its subexpressions match. For example, the following
command prints all the records in mail-list that contain both ‘edu’ and ‘li’:$ awk '/edu/ && /li/' mail-list555-3430
samuel.lanceolis@shu.edu
Aa Samuel
The following command prints all records in mail-list that contain either ‘edu’ or ‘li’
(or both, of course):$ awk '/edu/ || /li/' mail-list555-5553
amelia.zodiacusque@gmail.com
F
a Amelia
555-0542
broderick.aliquotiens@yahoo.com R
a Broderick
555-1234
fabius.undevicesimus@ucb.edu
Fa Fabius
555-6699
julie.perscrutabor@skeeve.com
F
a Julie555-3430
samuel.lanceolis@shu.edu
Aa Samuel555-2127
jeanpaul.campanorum@nyu.edu
Ra Jean-PaulThe following command prints all records in mail-list that do not contain the string‘li’:$ awk '! /li/' mail-list555-3412
anthony.asserturo@hotmail.com
Aa Anthony555-7685
becky.algebrarum@gmail.com
Aa Becky555-1675
bill.drowning@hotmail.com
Aa Bill555-2912
camilla.infusarum@skynet.beRa Camilla
555-1234
fabius.undevicesimus@ucb.edu
Fa Fabius
555-6480
martin.codicibus@hotmail.com
A
a Martin555-2127
jeanpaul.campanorum@nyu.edu
Ra Jean-PaulChapter 7: Patterns, Actions, and Variables

149

The subexpressions of a Boolean operator in a pattern can be constant regular expressions, comparisons, or any other awk expressions. Range patterns are not expressions, sothey cannot appear inside Boolean patterns. Likewise, the special patterns BEGIN, END,
BEGINFILE, and ENDFILE, which never match any input record, are not expressions andcannot appear inside Boolean patterns.
The precedence of the different operators that can appear in patterns is described in
Section 6.5 [Operator Precedence (How Operators Nest)], page 144.

7.1.3 Specifying Record Ranges with Patterns
A range pattern is made of two patterns separated by a comma, in the form ‘begpat,
endpat’. It is used to match ranges of consecutive input records. The first pattern, begpat,
controls where the range begins, while endpat controls where the pattern ends. For example,
the following:
awk '$1 == "on", $1 == "off"' myfile
prints every record in myfile between ‘on’/‘off’ pairs, inclusive.
A range pattern starts out by matching begpat against every input record. When arecord matches begpat, the range pattern is turned on, and the range pattern matches thisrecord as well. As long as the range pattern stays turned on, it automatically matches every
input record read. The range pattern also matches endpat against every input record; when
this succeeds, the range pattern is turned off again for the following record. Then the range
pattern goes back to checking begpat against each record.
The record that turns on the range pattern and the one that turns it off both match therange pattern. If you don’t want to operate on these records, you can write if statementsin the rule’s action to distinguish them from the records you are interested in.
It is possible for a pattern to be turned on and off by the same record. If the record
satisfies both conditions, then the action is executed for just that record. For example,
suppose there is text between two identical markers (e.g., the ‘%’ symbol), each on its ownline, that should be ignored. A first attempt would be to combine a range pattern that
describes the delimited text with the next statement (not discussed yet, see Section 7.4.8[The next Statement], page 160). This causes awk to skip any further processing of thecurrent record and start over again with the next input record. Such a program looks likethis:/^%$/,/^%$/{ next }{ print }This program fails because the range pattern is both turned on and turned off by the firstline, which just has a ‘%’ on it. To accomplish this task, write the program in the following
manner, using a flag:/^%$/{ skip = ! skip; next }skip == 1 { next } # skip lines with `skip' set
In a range pattern, the comma (‘,’) has the lowest precedence of all the operators (i.e.,
it is evaluated last). Thus, the following program attempts to combine a range pattern withanother, simpler test:
echo Yes | awk '/1/,/2/ || /Yes/'

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The intent of this program is ‘(/1/,/2/) || /Yes/’. However, awk interprets this as‘/1/, (/2/ || /Yes/)’. This cannot be changed or worked around; range patterns do not
combine with other patterns:$ echo Yes | gawk '(/1/,/2/) || /Yes/'error gawk: cmd. line:1: (/1/,/2/) || /Yes/^ syntax error
error gawk: cmd. line:1:
As a minor point of interest, although it is poor style, POSIX allows you to put a newline
after the comma in a range pattern.

7.1.4 The BEGIN and END Special Patterns
All the patterns described so far are for matching input records. The BEGIN and END special
patterns are different. They supply startup and cleanup actions for awk programs. BEGIN
and END rules must have actions; there is no default action for these rules because there isno current record when they run. BEGIN and END rules are often referred to as “BEGIN and
END blocks” by longtime awk programmers.

7.1.4.1 Startup and Cleanup Actions
A BEGIN rule is executed once only, before the first input record is read. Likewise, an END
rule is executed once only, after all the input is read. For example:$ awk '> BEGIN { print "Analysis of \"li\"" }> /li/ { ++n }> END{ print "\"li\" appears in", n, "records." }' mail-list
a Analysis of "li"a "li" appears in 4 records.
This program finds the number of records in the input file mail-list that contain thestring ‘li’. The BEGIN rule prints a title for the report. There is no need to use the BEGIN
rule to initialize the counter n to zero, as awk does this automatically (see Section 6.1.3[Variables], page 124). The second rule increments the variable n every time a record
containing the pattern ‘li’ is read. The END rule prints the value of n at the end of the run.
The special patterns BEGIN and END cannot be used in ranges or with Boolean operators(indeed, they cannot be used with any operators). An awk program may have multiple
BEGIN and/or END rules. They are executed in the order in which they appear: all the
BEGIN rules at startup and all the END rules at termination.
BEGIN and END rules may be intermixed with other rules. This feature was added in the1987 version of awk and is included in the POSIX standard. The original (1978) version of
awk required the BEGIN rule to be placed at the beginning of the program, the END rule tobe placed at the end, and only allowed one of each. This is no longer required, but it is agood idea to follow this template in terms of program organization and readability.
Multiple BEGIN and END rules are useful for writing library functions, because each library
file can have its own BEGIN and/or END rule to do its own initialization and/or cleanup. The
order in which library functions are named on the command line controls the order in which
their BEGIN and END rules are executed. Therefore, you have to be careful when writing
such rules in library files so that the order in which they are executed doesn’t matter.
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151

functions. See Chapter 10 [A Library of awk Functions], page 241, for a number of usefullibrary functions.
If an awk program has only BEGIN rules and no other rules, then the program exits after
the BEGIN rules are run.1 However, if an END rule exists, then the input is read, even if
there are no other rules in the program. This is necessary in case the END rule checks the
FNR and NR variables, or the fields.

7.1.4.2 Input/Output from BEGIN and END Rules
There are several (sometimes subtle) points to be aware of when doing I/O from a BEGIN
or END rule. The first has to do with the value of $0 in a BEGIN rule. Because BEGIN rules
are executed before any input is read, there simply is no input record, and therefore no
fields, when executing BEGIN rules. References to $0 and the fields yield a null string orzero, depending upon the context. One way to give $0 a real value is to execute a getline
command without a variable (see Section 4.10 [Explicit Input with getline], page 89).
Another way is simply to assign a value to $0.
The second point is similar to the first, but from the other direction. Traditionally, duelargely to implementation issues, $0 and NF were undefined inside an END rule. The POSIX
standard specifies that NF is available in an END rule. It contains the number of fields fromthe last input record. Most probably due to an oversight, the standard does not say that $0
is also preserved, although logically one would think that it should be. In fact, all of BWK
awk, mawk, and gawk preserve the value of $0 for use in END rules. Be aware, however, thatsome other implementations and many older versions of Unix awk do not.
The third point follows from the first two. The meaning of ‘print’ inside a BEGIN or END
rule is the same as always: ‘print $0’. If $0 is the null string, then this prints an empty
record. Many longtime awk programmers use an unadorned ‘print’ in BEGIN and END rules
to mean ‘print ""’, relying on $0 being null. Although one might generally get away withthis in BEGIN rules, it is a very bad idea in END rules, at least in gawk. It is also poor style,because if an empty line is needed in the output, the program should print one explicitly.
Finally, the next and nextfile statements are not allowed in a BEGIN rule, because
the implicit read-a-record-and-match-against-the-rules loop has not started yet. Similarly,
those statements are not valid in an END rule, because all the input has been read. (See
Section 7.4.8 [The next Statement], page 160, and see Section 7.4.9 [The nextfile Statement], page 161.)

7.1.5 The BEGINFILE and ENDFILE Special Patterns
This section describes a gawk-specific feature.
Two special kinds of rule, BEGINFILE and ENDFILE, give you “hooks” into gawk’s
command-line file processing loop. As with the BEGIN and END rules (see the previous
section), BEGINFILE rules in a program execute in the order they are read by gawk.
Similarly, all ENDFILE rules also execute in the order they are read.
The bodies of the BEGINFILE rules execute just before gawk reads the first record froma file. FILENAME is set to the name of the current file, and FNR is set to zero.
Prior to version 5.1.1 of gawk, as an accident of the implementation, $0 and the fields
retained any previous values they had in BEGINFILE rules. Starting with version 5.1.1, $0
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The BEGINFILE rule provides you the opportunity to accomplish two tasks that would
otherwise be difficult or impossible to perform:• You can test if the file is readable. Normally, it is a fatal error if a file named on thecommand line cannot be opened for reading. However, you can bypass the fatal error
and move on to the next file on the command line.
You do this by checking if the ERRNO variable is not the empty string; if so, then gawk
was not able to open the file. In this case, your program can execute the nextfile
statement (see Section 7.4.9 [The nextfile Statement], page 161). This causes gawk
to skip the file entirely. Otherwise, gawk exits with the usual fatal error.
• If you have written extensions that modify the record handling (by inserting an “input parser”; see Section 17.4.6.4 [Customized Input Parsers], page 420), you can invoke
them at this point, before gawk has started processing the file. (This is a very advanced
feature, currently used only by the gawkextlib project.)
The ENDFILE rule is called when gawk has finished processing the last record in an input
file. For the last input file, it will be called before any END rules. The ENDFILE rule is
executed even for empty input files.
Normally, when an error occurs when reading input in the normal input-processing loop,
the error is fatal. However, if a BEGINFILE rule is present, the error becomes non-fatal, andinstead ERRNO is set. This makes it possible to catch and process I/O errors at the level ofthe awk program.
The next statement (see Section 7.4.8 [The next Statement], page 160) is not allowed
inside either a BEGINFILE or an ENDFILE rule. The nextfile statement is allowed only
inside a BEGINFILE rule, not inside an ENDFILE rule.
The getline statement (see Section 4.10 [Explicit Input with getline], page 89) is restricted inside both BEGINFILE and ENDFILE: only redirected forms of getline are allowed.
BEGINFILE and ENDFILE are gawk extensions. In most other awk implementations, or if
gawk is in compatibility mode (see Section 2.2 [Command-Line Options], page 33), they are
not special.

7.1.6 The Empty Pattern
An empty (i.e., nonexistent) pattern is considered to match every input record. For example,
the program:awk '{ print $1 }' mail-list
prints the first field of every record.

7.2 Using Shell Variables in Programs
awk programs are often used as components in larger programs written in shell. For example,
it is very common to use a shell variable to hold a pattern that the awk program searchesfor. There are two ways to get the value of the shell variable into the body of the awk
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A common method is to use shell quoting to substitute the variable’s value into theprogram inside the script. For example, consider the following program:printf "Enter search pattern: "read pattern
awk "/$pattern/ "'{ nmatches++ }END { print nmatches, "found" }' /path/to/data
The awk program consists of two pieces of quoted text that are concatenated together toform the program. The first part is double-quoted, which allows substitution of the pattern
shell variable inside the quotes. The second part is single-quoted.
Variable substitution via quoting works, but can potentially be messy. It requires a goodunderstanding of the shell’s quoting rules (see Section 1.1.6 [Shell Quoting Issues], page 21),and it’s often difficult to correctly match up the quotes when reading the program.
A better method is to use awk’s variable assignment feature (see Section 6.1.3.2 [Assigning
Variables on the Command Line], page 125) to assign the shell variable’s value to an awk
variable. Then use dynamic regexps to match the pattern (see Section 3.6 [Using Dynamic
Regexps], page 60). The following shows how to redo the previous example using thistechnique:printf "Enter search pattern: "read pattern
awk -v pat="$pattern" '$0 ~ pat { nmatches++ }END { print nmatches, "found" }' /path/to/data
Now, the awk program is just one single-quoted string.
The assignment ‘-v
pat="$pattern"’ still requires double quotes, in case there is whitespace in the valueof $pattern. The awk variable pat could be named pattern too, but that would be
more confusing. Using a variable also provides more flexibility, as the variable can beused anywhere inside the program—for printing, as an array subscript, or for any otheruse—without requiring the quoting tricks at every point in the program.

7.3 Actions
An awk program or script consists of a series of rules and function definitions interspersed.
(Functions are described later. See Section 9.2 [User-Defined Functions], page 221.) A rule
contains a pattern and an action, either of which (but not both) may be omitted. The
purpose of the action is to tell awk what to do once a match for the pattern is found. Thus,in outline, an awk program generally looks like this:[pattern] { action }pattern [{ action }]...
function name(args) { ... }...
An action consists of one or more awk statements, enclosed in braces (‘{. . . }’). Eachstatement specifies one thing to do. The statements are separated by newlines or semicolons.
The braces around an action must be used even if the action contains only one statement,
or if it contains no statements at all. However, if you omit the action entirely, omit thebraces as well. An omitted action is equivalent to ‘{ print $0 }’:/foo/ { }match foo, do nothing — empty action

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match foo, print the record — omitted action

The following types of statements are supported in awk:
Expressions
Call functions or assign values to variables (see Chapter 6 [Expressions],
page 119). Executing this kind of statement simply computes the valueof the expression. This is useful when the expression has side effects (seeSection 6.2.3 [Assignment Expressions], page 130).
Control statements
Specify the control flow of awk programs. The awk language gives you C-likeconstructs (if, for, while, and do) as well as a few special ones (see Section 7.4[Control Statements in Actions], page 154).
Compound statements
Enclose one or more statements in braces. A compound statement is used inorder to put several statements together in the body of an if, while, do, or
for statement.
Input statements
Use the getline command (see Section 4.10 [Explicit Input with getline],
page 89). Also supplied in awk are the next statement (see Section 7.4.8 [Thenext Statement], page 160) and the nextfile statement (see Section 7.4.9 [Thenextfile Statement], page 161).
Output statements
Such as print and printf. See Chapter 5 [Printing Output], page 99.
Deletion statements
For deleting array elements. See Section 8.4 [The delete Statement], page 186.

7.4 Control Statements in Actions
Control statements, such as if, while, and so on, control the flow of execution in awk
programs. Most of awk’s control statements are patterned after similar statements in C.
All the control statements start with special keywords, such as if and while, to distinguish them from simple expressions. Many control statements contain other statements.
For example, the if statement contains another statement that may or may not be executed. The contained statement is called the body. To include more than one statement in
the body, group them into a single compound statement with braces, separating them with
newlines or semicolons.

7.4.1 The if-else Statement
The if-else statement is awk’s decision-making statement. It looks like this:if (condition) then-body [else else-body]
The condition is an expression that controls what the rest of the statement does. If thecondition is true, then-body is executed; otherwise, else-body is executed. The else partof the statement is optional. The condition is considered false if its value is zero or the null
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if (x % 2 == 0)
print "x is even"
else
print "x is odd"
In this example, if the expression ‘x % 2 == 0’ is true (i.e., if the value of x is evenly
divisible by two), then the first print statement is executed; otherwise, the second printstatement is executed. If the else keyword appears on the same line as then-body andthen-body is not a compound statement (i.e., not surrounded by braces), then a semicolonmust separate then-body from the else. To illustrate this, the previous example can be
rewritten as:
if (x % 2 == 0) print "x is even"; else
print "x is odd"
If the ‘;’ is left out, awk can’t interpret the statement and it produces a syntax error. Don’t
actually write programs this way, because a human reader might fail to see the else if it is
not the first thing on its line.

7.4.2 The while Statement
In programming, a loop is a part of a program that can be executed two or more times insuccession. The while statement is the simplest looping statement in awk. It repeatedly
executes a statement as long as a condition is true. For example:
while (condition)
body
body is a statement called the body of the loop, and condition is an expression that controls
how long the loop keeps running. The first thing the while statement does is test thecondition. If the condition is true, it executes the statement body. After body has been
executed, condition is tested again, and if it is still true, body executes again. This processrepeats until the condition is no longer true. If the condition is initially false, the bodyof the loop never executes and awk continues with the statement following the loop. This
example prints the first three fields of each record, one per line:
awk '
{
i = 1
while (i <= 3) {print $ii++}}' inventory-shipped
The body of this loop is a compound statement enclosed in braces, containing two statements. The loop works in the following manner: first, the value of i is set to one. Then,the while statement tests whether i is less than or equal to three. This is true when iequals one, so the ith field is printed. Then the ‘i++’ increments the value of i and theloop repeats. The loop terminates when i reaches four.
A newline is not required between the condition and the body; however, using one makes
the program clearer unless the body is a compound statement or else is very simple. The

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newline after the open brace that begins the compound statement is not required either,
but the program is harder to read without it.

7.4.3 The do-while Statement
The do loop is a variation of the while looping statement. The do loop executes the bodyonce and then repeats the body as long as the condition is true. It looks like this:dobodywhile (condition)
Even if the condition is false at the start, the body executes at least once (and only
once, unless executing body makes condition true). Contrast this with the corresponding
while statement:
while (condition)
body
This statement does not execute the body even once if the condition is false to begin with.
The following is an example of a do statement:
{
i = 1
do {
print $0i++
} while (i <= 10)}
This program prints each input record 10 times. However, it isn’t a very realistic example,
because in this case an ordinary while would do just as well. This situation reflects actual
experience; only occasionally is there a real use for a do statement.

7.4.4 The for Statement
The for statement makes it more convenient to count iterations of a loop. The generalform of the for statement looks like this:for (initialization; condition; increment)
body
The initialization, condition, and increment parts are arbitrary awk expressions, and bodystands for any awk statement.
The for statement starts by executing initialization. Then, as long as the conditionis true, it repeatedly executes body and then increment. Typically, initialization sets avariable to either zero or one, increment adds one to it, and condition compares it against
the desired number of iterations. For example:
awk '{for (i = 1; i <= 3; i++)print $i}' inventory-shipped
This prints the first three fields of each input record, with one input field per output line.

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C and C++ programmers might expect to be able to use the comma operator to set
more than one variable in the initialization part of the for loop, or to increment multiple
variables in the increment part of the loop, like so:for (i = 0, j = length(a); i < j; i++, j--) ...

C/C++, not awk!

You cannot do this; the comma operator is not supported in awk. There are workarounds,
but they are nonobvious and can lead to code that is difficult to read and understand. It is
best, therefore, to simply write additional initializations as separate statements preceding
the for loop and to place additional increment statements at the end of the loop’s body.
Most often, increment is an increment expression, as in the earlier example. But this is
not required; it can be any expression whatsoever. For example, the following statement
prints all the powers of two between 1 and 100:for (i = 1; i <= 100; i *= 2)
print iIf there is nothing to be done, any of the three expressions in the parentheses following
the for keyword may be omitted. Thus, ‘for (; x > 0;)’ is equivalent to ‘while (x > 0)’.
If the condition is omitted, it is treated as true, effectively yielding an infinite loop (i.e., aloop that never terminates).
In most cases, a for loop is an abbreviation for a while loop, as shown here:
initialization
while (condition) {body
increment}
The only exception is when the continue statement (see Section 7.4.7 [The continue
Statement], page 160) is used inside the loop. Changing a for statement to a whilestatement in this way can change the effect of the continue statement inside the loop.
The awk language has a for statement in addition to a while statement because a forloop is often both less work to type and more natural to think of. Counting the numberof iterations is very common in loops. It can be easier to think of this counting as part oflooping rather than as something to do inside the loop.
There is an alternative version of the for loop, for iterating over all the indices of anarray:for (i in array)do something with array[i]See Section 8.1.5 [Scanning All Elements of an Array], page 181, for more information on
this version of the for loop.

7.4.5 The switch Statement
This section describes a gawk-specific feature. If gawk is in compatibility mode (seeSection 2.2 [Command-Line Options], page 33), it is not available.
The switch statement allows the evaluation of an expression and the execution of statements based on a case match. Case statements are checked for a match in the order they
are defined. If no suitable case is found, the default section is executed, if supplied.

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Each case contains a single constant, be it numeric, string, or regexp. The switch
expression is evaluated, and then each case’s constant is compared against the result inturn. The type of constant determines the comparison: numeric or string do the usualcomparisons. A regexp constant (either regular, /foo/, or strongly typed, @/foo/) does aregular expression match against the string value of the original expression. The generalform of the switch statement looks like this:switch (expression) {case value or regular expression:
case-bodydefault:
default-body}Control flow in the switch statement works as it does in C. Once a match to a given
case is made, the case statement bodies execute until a break, continue, next, nextfile,or exit is encountered, or the end of the switch statement itself. For example:
while ((c = getopt(ARGC, ARGV, "aksx")) != -1) {switch (c) {case "a":# report size of all files
all_files = TRUE;
breakcase "k":BLOCK_SIZE = 1024
# 1K block size
breakcase "s":# do sums only
sum_only = TRUE
breakcase "x":# don't cross filesystems
fts_flags = or(fts_flags, FTS_XDEV)
breakcase "?":
default:
usage()
break}}
Note that if none of the statements specified here halt execution of a matched casestatement, execution falls through to the next case until execution halts. In this example,
the case for "?" falls through to the default case, which is to call a function named
usage(). (The getopt() function being called here is described in Section 10.4 [Processing
Command-Line Options], page 259.)

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7.4.6 The break Statement
The break statement jumps out of the innermost for, while, or do loop that encloses it.
The following example finds the smallest divisor of any integer, and also identifies prime
numbers:# find smallest divisor of num{
num = $1
for (divisor = 2; divisor * divisor <= num; divisor++) {if (num % divisor == 0)
break}
if (num % divisor == 0)
printf "Smallest divisor of %d is %d\n", num, divisor
else
printf "%d is prime\n", num}
When the remainder is zero in the first if statement, awk immediately breaks out of thecontaining for loop. This means that awk proceeds immediately to the statement following
the loop and continues processing. (This is very different from the exit statement, which
stops the entire awk program. See Section 7.4.10 [The exit Statement], page 162.)
The following program illustrates how the condition of a for or while statement could
be replaced with a break inside an if:# find smallest divisor of num{
num = $1
for (divisor = 2; ; divisor++) {if (num % divisor == 0) {printf "Smallest divisor of %d is %d\n", num, divisorbreak}
if (divisor * divisor > num) {printf "%d is prime\n", num
break
}
}
}
The break statement is also used to break out of the switch statement. This is discussed
in Section 7.4.5 [The switch Statement], page 157.
The break statement has no meaning when used outside the body of a loop or switch.
However, although it was never documented, historical implementations of awk treated thebreak statement outside of a loop as if it were a next statement (see Section 7.4.8 [Thenext Statement], page 160). Recent versions of BWK awk no longer allow this usage, nordoes gawk.

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7.4.7 The continue Statement
Similar to break, the continue statement is used only inside for, while, and do loops.
It skips over the rest of the loop body, causing the next cycle around the loop to begin
immediately. Contrast this with break, which jumps out of the loop altogether.
The continue statement in a for loop directs awk to skip the rest of the body of the loop
and resume execution with the increment-expression of the for statement. The following
program illustrates this fact:
BEGIN {for (x = 0; x <= 20; x++) {if (x == 5)
continue
printf "%d ", x}print ""}
This program prints all the numbers from 0 to 20—except for 5, for which the printf isskipped. Because the increment ‘x++’ is not skipped, x does not remain stuck at 5. Contrast
the for loop from the previous example with the following while loop:
BEGIN {
x = 0
while (x <= 20) {if (x == 5)
continue
printf "%d ", xx++}print ""}
This program loops forever once x reaches 5, because the increment (‘x++’) is never reached.
The continue statement has no special meaning with respect to the switch statement,
nor does it have any meaning when used outside the body of a loop. Historical versionsof awk treated a continue statement outside a loop the same way they treated a breakstatement outside a loop: as if it were a next statement (see Section 7.4.8 [The next
Statement], page 160). Recent versions of BWK awk no longer work this way, nor does
gawk.

7.4.8 The next Statement
The next statement forces awk to immediately stop processing the current record and go
on to the next record. This means that no further rules are executed for the current record,
and the rest of the current rule’s action isn’t executed.
Contrast this with the effect of the getline function (see Section 4.10 [Explicit Input
with getline], page 89). That also causes awk to read the next record immediately, but it
does not alter the flow of control in any way (i.e., the rest of the current action executeswith a new input record).

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At the highest level, awk program execution is a loop that reads an input record andthen tests each rule’s pattern against it. If you think of this loop as a for statement whosebody contains the rules, then the next statement is analogous to a continue statement. It
skips to the end of the body of this implicit loop and executes the increment (which reads
another record).
For example, suppose an awk program works only on records with four fields, and it
shouldn’t fail when given bad input. To avoid complicating the rest of the program, write
a “weed out” rule near the beginning, in the following manner:
NF != 4 {printf("%s:%d: skipped: NF != 4\n", FILENAME, FNR) > "/dev/stderr"
next}
Because of the next statement, the program’s subsequent rules won’t see the bad record.
The error message is redirected to the standard error output stream, as error messages
should be. For more detail, see Section 5.8 [Special File names in gawk], page 112.
If the next statement causes the end of the input to be reached, then the code in any
END rules is executed. See Section 7.1.4 [The BEGIN and END Special Patterns], page 150.
The next statement is not allowed inside BEGINFILE and ENDFILE rules. See Section 7.1.5[The BEGINFILE and ENDFILE Special Patterns], page 151.
According to the POSIX standard, the behavior is undefined if the next statement is used
in a BEGIN or END rule. gawk treats it as a syntax error. Although POSIX does not disallow
it, most other awk implementations don’t allow the next statement inside function bodies(see Section 9.2 [User-Defined Functions], page 221). Just as with any other next statement,a next statement inside a function body reads the next record and starts processing it withthe first rule in the program.

7.4.9 The nextfile Statement
The nextfile statement is similar to the next statement. However, instead of abandoning
processing of the current record, the nextfile statement instructs awk to stop processing
the current data file.
Upon execution of the nextfile statement, FILENAME is updated to the name of thenext data file listed on the command line, FNR is reset to one, and processing starts overwith the first rule in the program. If the nextfile statement causes the end of the input
to be reached, then the code in any END rules is executed. An exception to this is whennextfile is invoked during execution of any statement in an END rule; in this case, it causes
the program to stop immediately. See Section 7.1.4 [The BEGIN and END Special Patterns],
page 150.
The nextfile statement is useful when there are many data files to process but it isn’t
necessary to process every record in every file. Without nextfile, in order to move on to
the next data file, a program would have to continue scanning the unwanted records. The
nextfile statement accomplishes this much more efficiently.
In gawk, execution of nextfile causes additional things to happen: any ENDFILE rules
are executed if gawk is not currently in an END rule, ARGIND is incremented, and any
BEGINFILE rules are executed. (ARGIND hasn’t been introduced yet. See Section 7.5 [Predefined Variables], page 163.)

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There is an additional, special, use case with gawk. nextfile is useful inside a BEGINFILE
rule to skip over a file that would otherwise cause gawk to exit with a fatal error. In thisspecial case, ENDFILE rules are not executed. See Section 7.1.5 [The BEGINFILE and ENDFILE
Special Patterns], page 151.
Although it might seem that ‘close(FILENAME)’ would accomplish the same as
nextfile, this isn’t true. close() is reserved for closing files, pipes, and coprocesses thatare opened with redirections. It is not related to the main processing that awk does withthe files listed in ARGV.
NOTE: For many years, nextfile was a common extension. In September
2012, it was accepted for inclusion into the POSIX standard. See the Austin
Group website.
The current version of BWK awk and mawk also support nextfile. However, they
don’t allow the nextfile statement inside function bodies (see Section 9.2 [User-Defined
Functions], page 221). gawk does; a nextfile inside a function body reads the first record
from the next file and starts processing it with the first rule in the program, just as anyother nextfile statement.

7.4.10 The exit Statement
The exit statement causes awk to immediately stop executing the current rule and to stop
processing input; any remaining input is ignored. The exit statement is written as follows:
exit [return code]
When an exit statement is executed from a BEGIN rule, the program stops processing
everything immediately. No input records are read. However, if an END rule is present, aspart of executing the exit statement, the END rule is executed (see Section 7.1.4 [The BEGIN
and END Special Patterns], page 150). If exit is used in the body of an END rule, it causes
the program to stop immediately.
An exit statement that is not part of a BEGIN or END rule stops the execution of any
further automatic rules for the current record, skips reading any remaining input records,
and executes the END rule if there is one. gawk also skips any ENDFILE rules; they do not
execute.
In such a case, if you don’t want the END rule to do its job, set a variable to a nonzero
value before the exit statement and check that variable in the END rule. See Section 10.2.2[Assertions], page 244, for an example that does this.
If an argument is supplied to exit, its value is used as the exit status code for the awk
process. If no argument is supplied, exit causes awk to return a “success” status. In thecase where an argument is supplied to a first exit statement, and then exit is called asecond time from an END rule with no argument, awk uses the previously supplied exit value.
See Section 2.6 [gawk’s Exit Status], page 46, for more information.
For example, suppose an error condition occurs that is difficult or impossible to handle.
Conventionally, programs report this by exiting with a nonzero status. An awk program
can do this using an exit statement with a nonzero argument, as shown in the following
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BEGIN {
if (("date" | getline date_now) <= 0) {print "Can't get system date" > "/dev/stderr"
exit 1}
print "current date is", date_now
close("date")}
NOTE: For full portability, exit values should be between zero and 126, inclusive. Negative values, and values of 127 or greater, may not produce consistent
results across different operating systems.

7.5 Predefined Variables
Most awk variables are available to use for your own purposes; they never change unless
your program assigns values to them, and they never affect anything unless your program
examines them. However, a few variables in awk have special built-in meanings. awk
examines some of these automatically, so that they enable you to tell awk how to do certain
things. Others are set automatically by awk, so that they carry information from the internal
workings of awk to your program.
This section documents all of gawk’s predefined variables, most of which are also documented in the chapters describing their areas of activity.

7.5.1 Built-in Variables That Control awk
The following is an alphabetical list of variables that you can change to control how awk
does certain things.
The variables that are specific to gawk are marked with a pound sign (‘#’). Thesevariables are gawk extensions. In other awk implementations or if gawk is in compatibility
mode (see Section 2.2 [Command-Line Options], page 33), they are not special. (Anyexceptions are noted in the description of each variable.)
BINMODE # On non-POSIX systems, this variable specifies use of binary mode for all I/O.
Numeric values of one, two, or three specify that input files, output files, or
all files, respectively, should use binary I/O. A numeric value less than zerois treated as zero, and a numeric value greater than three is treated as three.
Alternatively, string values of "r" or "w" specify that input files and output files,
respectively, should use binary I/O. A string value of "rw" or "wr" indicatesthat all files should use binary I/O. Any other string value is treated the same as"rw", but causes gawk to generate a warning message. BINMODE is described in
more detail in Section B.3.1.3 [Using gawk on PC Operating Systems], page 499.
mawk (see Section B.5 [Other Freely Available awk Implementations], page 510)
also supports this variable, but only using numeric values.
CONVFMTA string that controls the conversion of numbers to strings (see Section 6.1.4[Conversion of Strings and Numbers], page 126). It works by being passed, in
effect, as the first argument to the sprintf() function (see Section 9.1.4 [StringManipulation Functions], page 196). Its default value is "%.6g". CONVFMT was
introduced by the POSIX standard.

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FIELDWIDTHS #A space-separated list of columns that tells gawk how to split input with fixedcolumnar boundaries. Starting in version 4.2, each field width may optionallybe preceded by a colon-separated value specifying the number of characters toskip before the field starts. Assigning a value to FIELDWIDTHS overrides the useof FS and FPAT for field splitting. See Section 4.6 [Reading Fixed-Width Data],
page 80, for more information.
FPAT #A regular expression (as a string) that tells gawk to create the fields based on
text that matches the regular expression. Assigning a value to FPAT overrides
the use of FS and FIELDWIDTHS for field splitting. See Section 4.7 [Defining
Fields by Content], page 83, for more information.

FS

The input field separator (see Section 4.5 [Specifying How Fields Are Separated],
page 73). The value is a single-character string or a multicharacter regularexpression that matches the separations between fields in an input record. If
the value is the null string (""), then each character in the record becomes aseparate field. (This behavior is a gawk extension. POSIX awk does not specify
the behavior when FS is the null string. Nonetheless, some other versions ofawk also treat "" specially.)
The default value is " ", a string consisting of a single space. As a special
exception, this value means that any sequence of spaces, TABs, and/or newlines
is a single separator. It also causes spaces, TABs, and newlines at the beginningand end of a record to be ignored.
You can set the value of FS on the command line using the -F option:
awk -F, 'program' input-files
If gawk is using FIELDWIDTHS or FPAT for field splitting, assigning a value to FS
causes gawk to return to the normal, FS-based field splitting. An easy way to
do this is to simply say ‘FS = FS’, perhaps with an explanatory comment.

IGNORECASE #If IGNORECASE is nonzero or non-null, then all string comparisons and all regular expression matching are case-independent. This applies to regexp matching with ‘~’ and ‘!~’, the gensub(), gsub(), index(), match(), patsplit(),split(), and sub() functions, record termination with RS, and field splittingwith FS and FPAT. However, the value of IGNORECASE does not affect array
subscripting and it does not affect field splitting when using a single-character
field separator. See Section 3.8 [Case Sensitivity in Matching], page 63.
LINT #

When this variable is true (nonzero or non-null), gawk behaves as if the --lintcommand-line option is in effect (see Section 2.2 [Command-Line Options],
page 33). With a value of "fatal", lint warnings become fatal errors. Witha value of "invalid", only warnings about things that are actually invalid areissued. (This is not fully implemented yet.) Any other true value prints nonfatal
warnings. Assigning a false value to LINT turns off the lint warnings.
This variable is a gawk extension. It is not special in other awk implementations.
Unlike with the other special variables, changing LINT does affect the productionof lint warnings, even if gawk is in compatibility mode. Much as the --lint

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and --traditional options independently control different aspects of gawk’s
behavior, the control of lint warnings during program execution is independentof the flavor of awk being executed.
OFMT

A string that controls conversion of numbers to strings (see Section 6.1.4 [Conversion of Strings and Numbers], page 126) for printing with the print statement. It works by being passed as the first argument to the sprintf() function(see Section 9.1.4 [String-Manipulation Functions], page 196). Its default valueis "%.6g". Earlier versions of awk used OFMT to specify the format for converting
numbers to strings in general expressions; this is now done by CONVFMT.

OFS

The output field separator (see Section 5.3 [Output Separators], page 101). It
is output between the fields printed by a print statement. Its default value is" ", a string consisting of a single space.

ORS

The output record separator. It is output at the end of every print statement.
Its default value is "\n", the newline character. (See Section 5.3 [Output Separators], page 101.)

PREC #

The working precision of arbitrary-precision floating-point numbers, 53 bits bydefault (see Section 16.4.4 [Setting the Precision], page 397).

ROUNDMODE #The rounding mode to use for arbitrary-precision arithmetic on numbers, by
default "N" (roundTiesToEven in the IEEE 754 standard; see Section 16.4.5[Setting the Rounding Mode], page 398).
RS

The input record separator. Its default value is a string containing a single
newline character, which means that an input record consists of a single line of
text. It can also be the null string, in which case records are separated by runsof blank lines. If it is a regexp, records are separated by matches of the regexp
in the input text. (See Section 4.1 [How Input Is Split into Records], page 65.)
The ability for RS to be a regular expression is a gawk extension. In mostother awk implementations, or if gawk is in compatibility mode (see Section 2.2[Command-Line Options], page 33), just the first character of RS’s value is used.

SUBSEP

The subscript separator. It has the default value of "\034" and is used to separate the parts of the indices of a multidimensional array. Thus, the expression‘foo["A", "B"]’ really accesses foo["A\034B"] (see Section 8.5 [Multidimensional Arrays], page 188).

TEXTDOMAIN #Used for internationalization of programs at the awk level. It sets the default
text domain for specially marked string constants in the source text, as well
as for the dcgettext(), dcngettext(), and bindtextdomain() functions (seeChapter 13 [Internationalization with gawk], page 357). The default value of
TEXTDOMAIN is "messages".

7.5.2 Built-in Variables That Convey Information
The following is an alphabetical list of variables that awk sets automatically on certain
occasions in order to provide information to your program.

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The variables that are specific to gawk are marked with a pound sign (‘#’). Thesevariables are gawk extensions. In other awk implementations or if gawk is in compatibility
mode (see Section 2.2 [Command-Line Options], page 33), they are not special:
ARGC, ARGVThe command-line arguments available to awk programs are stored in an array called ARGV. ARGC is the number of command-line arguments present. See
Section 2.3 [Other Command-Line Arguments], page 40. Unlike most awk arrays, ARGV is indexed from 0 to ARGC − 1. In the following example:$ awk 'BEGIN {>for (i = 0; i < ARGC; i++)>print ARGV[i]>}' inventory-shipped mail-lista awk
a inventory-shipped
a mail-list
ARGV[0] contains ‘awk’, ARGV[1] contains ‘inventory-shipped’, and ARGV[2]contains ‘mail-list’. The value of ARGC is three, one more than the index of
the last element in ARGV, because the elements are numbered from zero.
The names ARGC and ARGV, as well as the convention of indexing the arrayfrom 0 to ARGC − 1, are derived from the C language’s method of accessing
command-line arguments.
The value of ARGV[0] can vary from system to system. Also, you should notethat the program text is not included in ARGV, nor are any of awk’s commandline options. See Section 7.5.3 [Using ARGC and ARGV], page 173, for information
about how awk uses these variables.
ARGIND #The index in ARGV of the current file being processed. Every time gawk opens
a new data file for processing, it sets ARGIND to the index in ARGV of the file
name. When gawk is processing the input files, ‘FILENAME == ARGV[ARGIND]’
is always true.
This variable is useful in file processing; it allows you to tell how far along you
are in the list of data files as well as to distinguish between successive instancesof the same file name on the command line.
While you can change the value of ARGIND within your awk program, gawk
automatically sets it to a new value when it opens the next file.

ENVIRONAn associative array containing the values of the environment. The array indices are the environment variable names; the elements are the values of theparticular environment variables. For example, ENVIRON["HOME"] might be/home/arnold.
For POSIX awk, changing this array does not affect the environment passed on
to any programs that awk may spawn via redirection or the system() function.
However, beginning with version 4.2, if not in POSIX compatibility mode, gawk
does update its own environment when ENVIRON is changed, thus changing
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especially careful if you modify ENVIRON["PATH"], which is the search path forfinding executable programs.
This can also affect the running gawk program, since some of the built-in functions may pay attention to certain environment variables. The most notableinstance of this is mktime() (see Section 9.1.6 [Time Functions], page 211),
which pays attention the value of the TZ environment variable on many systems.
Some operating systems may not have environment variables. On such
systems, the ENVIRON array is empty (except for ENVIRON["AWKPATH"] and
ENVIRON["AWKLIBPATH"]; see Section 2.5.1 [The AWKPATH Environment
Variable], page 42, and see Section 2.5.2 [The AWKLIBPATH Environment
Variable], page 43).
ERRNO #

If a system error occurs during a redirection for getline, during a read for
getline, or during a close() operation, then ERRNO contains a string describing the error.
In addition, gawk clears ERRNO before opening each command-line input file.
This enables checking if the file is readable inside a BEGINFILE pattern (seeSection 7.1.5 [The BEGINFILE and ENDFILE Special Patterns], page 151).
Otherwise, ERRNO works similarly to the C variable errno. Except for the casejust mentioned, gawk never clears it (sets it to zero or ""). Thus, you should
only expect its value to be meaningful when an I/O operation returns a failure
value, such as getline returning −1. You are, of course, free to clear it yourself
before doing an I/O operation.
If the value of ERRNO corresponds to a system error in the C errno variable,
then PROCINFO["errno"] will be set to the value of errno. For non-systemerrors, PROCINFO["errno"] will be zero.

FILENAME

The name of the current input file. When no data files are listed on the command line, awk reads from the standard input and FILENAME is set to "-".
FILENAME changes each time a new file is read (see Chapter 4 [Reading Input
Files], page 65). Inside a BEGIN rule, the value of FILENAME is "", because there
are no input files being processed yet.2 Note, though, that using getline (seeSection 4.10 [Explicit Input with getline], page 89) inside a BEGIN rule cangive FILENAME a value.

FNR

The current record number in the current file. awk increments FNR each time it
reads a new record (see Section 4.1 [How Input Is Split into Records], page 65).
awk resets FNR to zero each time it starts a new input file.

NF

The number of fields in the current input record. NF is set each time a newrecord is read, when a new field is created, or when $0 changes (see Section 4.2[Examining Fields], page 69).
Unlike most of the variables described in this subsection, assigning a value to NF
has the potential to affect awk’s internal workings. In particular, assignments

2

Some early implementations of Unix awk initialized FILENAME to "-", even if there were data files to beprocessed. This behavior was incorrect and should not be relied upon in your programs.

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to NF can be used to create fields in or remove fields from the current record.
See Section 4.4 [Changing the Contents of a Field], page 71.
FUNCTAB # An array whose indices and corresponding values are the names of all the builtin, user-defined, and extension functions in the program.
NOTE: Attempting to use the delete statement with the FUNCTAB
array causes a fatal error. Any attempt to assign to an element of
FUNCTAB also causes a fatal error.
NR

The number of input records awk has processed since the beginning of the
program’s execution (see Section 4.1 [How Input Is Split into Records], page 65).
awk increments NR each time it reads a new record.

PROCINFO #The elements of this array provide access to information about the running awk
program. The following elements (listed alphabetically) are guaranteed to beavailable:
PROCINFO["argv"]
The PROCINFO["argv"] array contains all of the command-line
arguments (after glob expansion and redirection processing on
platforms where that must be done manually by the program)with subscripts ranging from 0 through argc − 1. For example,
PROCINFO["argv"][0] will contain the name by which gawk was
invoked. Here is an example of how this feature may be used:
gawk 'BEGIN {for (i = 0; i < length(PROCINFO["argv"]); i++)print i, PROCINFO["argv"][i]}'Please note that this differs from the standard ARGV array which
does not include command-line arguments that have already beenprocessed by gawk (see Section 7.5.3 [Using ARGC and ARGV],
page 173).
PROCINFO["egid"]
The value of the getegid() system call.
PROCINFO["errno"]
The value of the C errno variable when ERRNO is set to the associated error message.
PROCINFO["euid"]
The value of the geteuid() system call.
PROCINFO["FS"]This is "FS" if field splitting with FS is in effect, "FIELDWIDTHS" iffield splitting with FIELDWIDTHS is in effect, "FPAT" if field matching with FPAT is in effect, or "API" if field splitting is controlled byan API input parser.

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PROCINFO["gid"]
The value of the getgid() system call.
PROCINFO["identifiers"]
A subarray, indexed by the names of all identifiers used in the textof the awk program. An identifier is simply the name of a variable(be it scalar or array), built-in function, user-defined function, or
extension function. For each identifier, the value of the element is
one of the following:"array"The identifier is an array.

"builtin"
The identifier is a built-in function.
"extension"
The identifier is an extension function loaded via @load
or -l.
"scalar"

The identifier is a scalar.

"untyped"
The identifier is untyped (could be used as a scalar oran array; gawk doesn’t know yet).
"user"The identifier is a user-defined function.

The values indicate what gawk knows about the identifiers after it
has finished parsing the program; they are not updated while theprogram runs.
PROCINFO["platform"]
This element gives a string indicating the platform for which gawk
was compiled. The value will be one of the following:"mingw"

Microsoft Windows, using MinGW.

"os390"OS/390 (also known as z/OS).

"posix"GNU/Linux, Cygwin, macOS, and legacy Unix systems.

"vms"

OpenVMS.

PROCINFO["pgrpid"]
The process group ID of the current process.
PROCINFO["pid"]
The process ID of the current process.
PROCINFO["pma"]
The version of the PMA memory allocator compiled into gawk. Thiselement will not be present if the PMA allocator is not available foruse. See Section 12.7 [Preserving Data Between Runs], page 352.
PROCINFO["ppid"]
The parent process ID of the current process.

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PROCINFO["strftime"]
The default time format string for strftime(). Assigning a newvalue to this element changes the default. See Section 9.1.6 [TimeFunctions], page 211.
PROCINFO["uid"]
The value of the getuid() system call.
PROCINFO["version"]
The version of gawk.
The following additional elements in the array are available to provide information about the MPFR and GMP libraries if your version of gawk supports arbitrary-precision arithmetic (see Chapter 16 [Arithmetic and ArbitraryPrecision Arithmetic with gawk], page 389):
PROCINFO["gmp_version"]
The version of the GNU MP library.
PROCINFO["mpfr_version"]
The version of the GNU MPFR library.
PROCINFO["prec_max"]
The maximum precision supported by MPFR.
PROCINFO["prec_min"]
The minimum precision required by MPFR.
The following additional elements in the array are available to provide information about the version of the extension API, if your version of gawk supports
dynamic loading of extension functions (see Chapter 17 [Writing Extensions forgawk], page 405):
PROCINFO["api_major"]
The major version of the extension API.
PROCINFO["api_minor"]
The minor version of the extension API.
On some systems, there may be elements in the array, "group1" through"groupN" for some N. N is the number of supplementary groups that theprocess has. Use the in operator to test for these elements (see Section 8.1.2[Referring to an Array Element], page 179).
The following elements allow you to change gawk’s behavior:
PROCINFO["BUFFERPIPE"]
If this element exists, all output to pipelines becomes buffered. See
Section 5.10 [Speeding Up Pipe Output], page 116.
PROCINFO["command", "BUFFERPIPE"]
Make output to command buffered. See Section 5.10 [Speeding Up
Pipe Output], page 116.
PROCINFO["NONFATAL"]
If this element exists, then I/O errors for all redirections become
nonfatal. See Section 5.11 [Enabling Nonfatal Output], page 116.

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PROCINFO["name", "NONFATAL"]
Make I/O errors for name be nonfatal. See Section 5.11 [EnablingNonfatal Output], page 116.
PROCINFO["command", "pty"]
For two-way communication to command, use a pseudo-tty insteadof setting up a two-way pipe. See Section 12.4 [Two-Way Communications with Another Process], page 343, for more information.
PROCINFO["input_name", "READ_TIMEOUT"]
Set a timeout for reading from input redirection input name. See
Section 4.11 [Reading Input with a Timeout], page 95, for more
information.
PROCINFO["input_name", "RETRY"]
If an I/O error that may be retried occurs when reading data from
input name, and this array entry exists, then getline returns −2
instead of following the default behavior of returning −1 and configuring input name to return no further data. An I/O error that may
be retried is one where errno has the value EAGAIN, EWOULDBLOCK,EINTR, or ETIMEDOUT. This may be useful in conjunction withPROCINFO["input_name", "READ_TIMEOUT"] or situations where afile descriptor has been configured to behave in a non-blocking fashion. See Section 4.12 [Retrying Reads After Certain Input Errors],
page 97, for more information.
PROCINFO["sorted_in"]
If this element exists in PROCINFO, its value controls the order inwhich array indices will be processed by ‘for (indx in array)’loops. This is an advanced feature, so we defer the full description until later; see Section 8.1.6 [Using Predefined Array Scanning
Orders with gawk], page 182.
RLENGTHThe length of the substring matched by the match() function (see Section 9.1.4[String-Manipulation Functions], page 196). RLENGTH is set by invoking thematch() function. Its value is the length of the matched string, or −1 if nomatch is found.

RSTART

The start index in characters of the substring that is matched by the match()function (see Section 9.1.4 [String-Manipulation Functions], page 196). RSTART
is set by invoking the match() function. Its value is the position of the string
where the matched substring starts, or zero if no match was found.

RT #The input text that matched the text denoted by RS, the record separator. It
is set every time a record is read.

SYMTAB #An array whose indices are the names of all defined global variables and arrays in
the program. SYMTAB makes gawk’s symbol table visible to the awk programmer.
It is built as gawk parses the program and is complete before the program starts
to run.
The array may be used for indirect access to read or write the value of a variable:
foo = 5

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SYMTAB["foo"] = 4
print foo
# prints 4
The isarray() function (see Section 9.1.8 [Getting Type Information],
page 219) may be used to test if an element in SYMTAB is an array. Also, you
may not use the delete statement with the SYMTAB array.
Prior to version 5.0 of gawk, you could use an index for SYMTAB that was not apredefined identifier:
SYMTAB["xxx"] = 5
print SYMTAB["xxx"]
This no longer works, instead producing a fatal error, as it led to rampant
confusion.
The SYMTAB array is more interesting than it looks. Andrew Schorr points outthat it effectively gives awk data pointers. Consider his example:# Indirect multiply of any variable by amount, return result
function multiply(variable, amount){return SYMTAB[variable] *= amount}
You would use it like this:BEGIN {
answer = 10.5
multiply("answer", 4)
print "The answer is", answer}
When run, this produces:$ gawk -f answer.awk
a The answer is 42
NOTE: In order to avoid severe time-travel paradoxes,3 neither
FUNCTAB nor SYMTAB is available as an element within the SYMTAB
array.

3

Not to mention difficult implementation issues.

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

Changing NR and FNR
awk increments NR and FNR each time it reads a record, instead of setting them to theabsolute value of the number of records read. This means that a program can change thesevariables and their new values are incremented for each record. The following example
shows this:$ echo '1> 2> 3> 4' | awk 'NR == 2 { NR = 17 }> { print NR }'
a 1
a 17
a 18
a 19
Before FNR was added to the awk language (see Section A.1 [Major Changes Between V7and SVR3.1], page 473), many awk programs used this feature to track the number ofrecords in a file by resetting NR to zero when FILENAME changed.

7.5.3 Using ARGC and ARGV
Section 7.5.2 [Built-in Variables That Convey Information], page 165, presented the following program describing the information contained in ARGC and ARGV:$ awk 'BEGIN {>for (i = 0; i < ARGC; i++)>print ARGV[i]>}' inventory-shipped mail-lista awk
a inventory-shipped
a mail-list
In this example, ARGV[0] contains ‘awk’, ARGV[1] contains ‘inventory-shipped’, and
ARGV[2] contains ‘mail-list’. Notice that the awk program is not entered in ARGV. The
other command-line options, with their arguments, are also not entered. This includes
variable assignments done with the -v option (see Section 2.2 [Command-Line Options],
page 33). Normal variable assignments on the command line are treated as arguments anddo show up in the ARGV array. Given the following program in a file named showargs.awk:
BEGIN {
printf "A=%d, B=%d\n", A, B
for (i = 0; i < ARGC; i++)printf "\tARGV[%d] = %s\n", i, ARGV[i]}END{ printf "A=%d, B=%d\n", A, B }Running it produces the following:$ awk -v A=1 -f showargs.awk B=2 /dev/null
a A=1, B=0

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ARGV[0] = awka
ARGV[1] = B=2a
ARGV[2] = /dev/nullaa A=1, B=2
A program can alter ARGC and the elements of ARGV. Each time awk reaches the end of
an input file, it uses the next element of ARGV as the name of the next input file. By storinga different string there, a program can change which files are read. Use "-" to represent
the standard input. Storing additional elements and incrementing ARGC causes additional
files to be read.
If the value of ARGC is decreased, that eliminates input files from the end of the list. By
recording the old value of ARGC elsewhere, a program can treat the eliminated arguments
as something other than file names.
To eliminate a file from the middle of the list, store the null string ("") into ARGV inplace of the file’s name. As a special feature, awk ignores file names that have been replacedwith the null string. Another option is to use the delete statement to remove elementsfrom ARGV (see Section 8.4 [The delete Statement], page 186).
All of these actions are typically done in the BEGIN rule, before actual processing of
the input begins. See Section 11.2.4 [Splitting a Large File into Pieces], page 293, and see
Section 11.2.5 [Duplicating Output into Multiple Files], page 298, for examples of each wayof removing elements from ARGV.
To actually get options into an awk program, end the awk options with -- and thensupply the awk program’s options, in the following manner:
awk -f myprog.awk -- -v -q file1 file2 ...
The following fragment processes ARGV in order to examine, and then remove, the previously mentioned command-line options:
BEGIN {for (i = 1; i < ARGC; i++) {if (ARGV[i] == "-v")verbose = 1
else if (ARGV[i] == "-q")debug = 1
else if (ARGV[i] ~ /^-./) {e = sprintf("%s: unrecognized option -- %c",
ARGV[0], substr(ARGV[i], 2, 1))
print e > "/dev/stderr"} elsebreak
delete ARGV[i]}}Ending the awk options with -- isn’t necessary in gawk. Unless --posix has been
specified, gawk silently puts any unrecognized options into ARGV for the awk program todeal with. As soon as it sees an unknown option, gawk stops looking for other options thatit might otherwise recognize. The previous command line with gawk would be:
gawk -f myprog.awk -q -v file1 file2 ...

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Because -q is not a valid gawk option, it and the following -v are passed on to the awk
program. (See Section 10.4 [Processing Command-Line Options], page 259, for an awk
library function that parses command-line options.)
When designing your program, you should choose options that don’t conflict with gawk’s,
because it will process any options that it accepts before passing the rest of the command line
on to your program. Using ‘#!’ with the -E option may help (see Section 1.1.4 [Executableawk Programs], page 19, and see Section 2.2 [Command-Line Options], page 33).

7.6 Summary• Pattern–action pairs make up the basic elements of an awk program. Patterns areeither normal expressions, range expressions, or regexp constants; one of the special
keywords BEGIN, END, BEGINFILE, or ENDFILE; or empty. The action executes if thecurrent record matches the pattern. Empty (missing) patterns match all records.
• I/O from BEGIN and END rules has certain constraints. This is also true, only moreso, for BEGINFILE and ENDFILE rules. The latter two give you “hooks” into gawk’s fileprocessing, allowing you to recover from a file that otherwise would cause a fatal error(such as a file that cannot be opened).
• Shell variables can be used in awk programs by careful use of shell quoting. It is easier
to pass a shell variable into awk by using the -v option and an awk variable.
• Actions consist of statements enclosed in curly braces. Statements are built up fromexpressions, control statements, compound statements, input and output statements,
and deletion statements.
• The control statements in awk are if-else, while, for, and do-while. gawk adds theswitch statement. There are two flavors of for statement: one for performing generallooping, and the other for iterating through an array.
• break and continue let you exit early or start the next iteration of a loop (or get outof a switch).
• next and nextfile let you read the next record and start over at the top of yourprogram or skip to the next input file and start over, respectively.
• The exit statement terminates your program. When executed from an action (orfunction body), it transfers control to the END statements. From an END statement
body, it exits immediately. You may pass an optional numeric value to be used as
awk’s exit status.
• Some predefined variables provide control over awk, mainly for I/O. Other variables
convey information from awk to your program.
• ARGC and ARGV make the command-line arguments available to your program. Manipulating them from a BEGIN rule lets you control how awk will process the provided data
files.

Chapter 8: Arrays in awk 177

8 Arrays in awk
An array is a table of values called elements. The elements of an array are distinguishedby their indices. Indices may be either numbers or strings.
This chapter describes how arrays work in awk, how to use array elements, how to scanthrough every element in an array, and how to remove array elements. It also describes
how awk simulates multidimensional arrays, as well as some of the less obvious points aboutarray usage. The chapter moves on to discuss gawk’s facility for sorting arrays, and ends
with a brief description of gawk’s ability to support true arrays of arrays.

8.1 The Basics of Arrays
This section presents the basics: working with elements in arrays one at a time, and traversing all of the elements in an array.

8.1.1 Introduction to Arrays
Doing linear scans over an associative array is like trying to club someone todeath with a loaded Uzi.
—Larry Wall
The awk language provides one-dimensional arrays for storing groups of related strings
or numbers. Every awk array must have a name. Array names have the same syntax asvariable names; any valid variable name would also be a valid array name. But one namecannot be used in both ways (as an array and as a variable) in the same awk program.
Arrays in awk superficially resemble arrays in other programming languages, but there
are fundamental differences. In awk, it isn’t necessary to specify the size of an array before
starting to use it. Additionally, any number or string, not just consecutive integers, may
be used as an array index.
In most other languages, arrays must be declared before use, including a specification of
how many elements or components they contain. In such languages, the declaration causes
a contiguous block of memory to be allocated for that many elements. Usually, an index
in the array must be a nonnegative integer. For example, the index zero specifies the firstelement in the array, which is actually stored at the beginning of the block of memory. Index
one specifies the second element, which is stored in memory right after the first element,
and so on. It is impossible to add more elements to the array, because it has room only
for as many elements as given in the declaration. (Some languages allow arbitrary startingand ending indices—e.g., ‘15 .. 27’—but the size of the array is still fixed when the arrayis declared.)
A contiguous array of four elements might look like Figure 8.1, conceptually, if theelement values are eight, "foo", "", and 30.

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8

"foo"

""

30

Value

0

1

2

3

Index

Figure 8.1: A contiguous array
Only the values are stored; the indices are implicit from the order of the values. Here, eightis the value at index zero, because eight appears in the position with zero elements before
it.
Arrays in awk are different—they are associative. This means that each array is acollection of pairs—an index and its corresponding array element value:
Index
3
1
0
2

Value
30
"foo"
8
""

The pairs are shown in jumbled order because their order is irrelevant.1
One advantage of associative arrays is that new pairs can be added at any time. For
example, suppose a tenth element is added to the array whose value is "number ten". The
result is:
Index
10
3
1
0
2

Value
"number ten"
30
"foo"
8
""

Now the array is sparse, which just means some indices are missing. It has elements 0–3
and 10, but doesn’t have elements 4, 5, 6, 7, 8, or 9.
Another consequence of associative arrays is that the indices don’t have to be nonnegative
integers. Any number, or even a string, can be an index. For example, the following is anarray that translates words from English to French:
Index
"dog"
"cat"
"one"
1

Value
"chien""chat""un""un"

Here we decided to translate the number one in both spelled-out and numeric form—thus
illustrating that a single array can have both numbers and strings as indices. (In fact, array1

The ordering will vary among awk implementations, which typically use hash tables to store arrayelements and values.

Chapter 8: Arrays in awk 179

subscripts are always strings. There are some subtleties to how numbers work when used asarray subscripts; this is discussed in more detail in Section 8.2 [Using Numbers to Subscript
Arrays], page 185.) Here, the number 1 isn’t double-quoted, because awk automaticallyconverts it to a string.
The value of IGNORECASE has no effect upon array subscripting. The identical string value
used to store an array element must be used to retrieve it. When awk creates an array (e.g.,with the split() built-in function), that array’s indices are consecutive integers startingat one. (See Section 9.1.4 [String-Manipulation Functions], page 196.)
awk’s arrays are efficient—the time to access an element is independent of the numberof elements in the array.

8.1.2 Referring to an Array Element
The principal way to use an array is to refer to one of its elements. An array reference isan expression as follows:array[index-expression]
Here, array is the name of an array. The expression index-expression is the index of thedesired element of the array.
The value of the array reference is the current value of that array element. For example,
foo[4.3] is an expression referencing the element of array foo at index ‘4.3’.
A reference to an array element that has no recorded value yields a value of "", the null
string. This includes elements that have not been assigned any value as well as elementsthat have been deleted (see Section 8.4 [The delete Statement], page 186).
NOTE: A reference to an element that does not exist automatically createsthat array element, with the null string as its value. (In some cases, this isunfortunate, because it might waste memory inside awk.)
Novice awk programmers often make the mistake of checking if an element exists
by checking if the value is empty:# Check if "foo" exists in a:
if (a["foo"] != "") ...

Incorrect!

This is incorrect for two reasons. First, it creates a["foo"] if it didn’t exist
before! Second, it is valid (if a bit unusual) to set an array element equal tothe empty string.
To determine whether an element exists in an array at a certain index, use the following
expression:indx in array
This expression tests whether the particular index indx exists, without the side effectof creating that element if it is not present. The expression has the value one (true) if
array[indx] exists and zero (false) if it does not exist. (We use indx here, because ‘index’is the name of a built-in function.) For example, this statement tests whether the arrayfrequencies contains the index ‘2’:
if (2 in frequencies)
print "Subscript 2 is present."

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Note that this is not a test of whether the array frequencies contains an element whose
value is two. There is no way to do that except to scan all the elements. Also, this does
not create frequencies[2], while the following (incorrect) alternative does:
if (frequencies[2] != "")print "Subscript 2 is present."

8.1.3 Assigning Array Elements
Array elements can be assigned values just like awk variables:
array[index-expression] = value
array is the name of an array. The expression index-expression is the index of the elementof the array that is assigned a value. The expression value is the value to assign to thatelement of the array.

8.1.4 Basic Array Example
The following program takes a list of lines, each beginning with a line number, and prints
them out in order of line number. The line numbers are not in order when they are first
read—instead, they are scrambled. This program sorts the lines by making an array using
the line numbers as subscripts. The program then prints out the lines in sorted order of
their numbers. It is a very simple program and gets confused upon encountering repeated
numbers, gaps, or lines that don’t begin with a number:{
if ($1 > max)
max = $1
arr[$1] = $0}
END {for (x = 1; x <= max; x++)print arr[x]}The first rule keeps track of the largest line number seen so far; it also stores each line
into the array arr, at an index that is the line’s number. The second rule runs after all the
input has been read, to print out all the lines. When this program is run with the following
input:
5 I am the Five man
2 Who are you? The new number two!
4 . . . And four on the floor
1 Who is number one?
3 I three you.
Its output is:
1 Who is number one?
2 Who are you? The new number two!
3 I three you.
4 . . . And four on the floor
5 I am the Five man

Chapter 8: Arrays in awk 181

If a line number is repeated, the last line with a given number overrides the others. Gapsin the line numbers can be handled with an easy improvement to the program’s END rule,
as follows:
END {for (x = 1; x <= max; x++)
if (x in arr)
print arr[x]}As mentioned, the program is simplistic. It can be easily confused; for example, byusing negative or nonalphabetic line numbers. The point here is merely to demonstratebasic array usage.

8.1.5 Scanning All Elements of an Array
In programs that use arrays, it is often necessary to use a loop that executes once for eachelement of an array. In other languages, where arrays are contiguous and indices are limited
to nonnegative integers, this is easy: all the valid indices can be found by counting fromthe lowest index up to the highest. This technique won’t do the job in awk, because any
number or string can be an array index. So awk has a special kind of for statement forscanning an array:for (var in array)bodyThis loop executes body once for each index in array that the program has previously used,with the variable var set to that index.
The following program uses this form of the for statement. The first rule scans the
input records and notes which words appear (at least once) in the input, by storing a one
into the array used with the word as the index. The second rule scans the elements ofused to find all the distinct words that appear in the input. It prints each word that ismore than 10 characters long and also prints the number of such words. See Section 9.1.4[String-Manipulation Functions], page 196, for more information on the built-in function
length().
# Record a 1 for each word that is used at least once{for (i = 1; i <= NF; i++)used[$i] = 1}# Find number of distinct words more than 10 characters longEND {for (x in used) {if (length(x) > 10) {++num_long_words
print x}}print num_long_words, "words longer than 10 characters"}

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GAWK: Effective AWK ProgrammingSee Section 11.3.5 [Generating Word-Usage Counts], page 314, for a more detailed exampleof this type.
The order in which elements of the array are accessed by this statement is determinedby the internal arrangement of the array elements within awk and in standard awk cannot
be controlled or changed. This can lead to problems if new elements are added to array bystatements in the loop body; it is not predictable whether the for loop will reach them.
Similarly, changing var inside the loop may produce strange results. It is best to avoid suchthings.
As a point of information, gawk sets up the list of elements to be iterated over before theloop starts, and does not change it. But not all awk versions do so. Consider this program,named loopcheck.awk:
BEGIN {
a["here"] = "here"
a["is"] = "is"a["a"] = "a"a["loop"] = "loop"for (i in a) {j++
a[j] = jprint i}}
Here is what happens when run with gawk (and mawk):$ gawk -f loopcheck.awk
a herea loopa a
a isContrast this to BWK awk:
$ nawk -f loopcheck.awk
a loopa here
a is
a a
a 1

8.1.6 Using Predefined Array Scanning Orders with gawk
This subsection describes a feature that is specific to gawk.
By default, when a for loop traverses an array, the order is undefined, meaning thatthe awk implementation determines the order in which the array is traversed. This orderis usually based on the internal implementation of arrays and will vary from one version of
awk to the next.
Often, though, you may wish to do something simple, such as “traverse the array bycomparing the indices in ascending order,” or “traverse the array by comparing the values
in descending order.” gawk provides two mechanisms that give you this control:Chapter 8: Arrays in awk 183• Set PROCINFO["sorted_in"] to one of a set of predefined values. We describe thisnow.
• Set PROCINFO["sorted_in"] to the name of a user-defined function to use for comparison of array elements. This advanced feature is described later in Section 12.3
[Controlling Array Traversal and Array Sorting], page 336.
The following special values for PROCINFO["sorted_in"] are available:"@unsorted"
Array elements are processed in arbitrary order, which is the default awk behavior.
"@ind_str_asc"
Order by indices in ascending order compared as strings; this is the most basic
sort. (Internally, array indices are always strings, so with ‘a[2*5] = 1’ the index
is "10" rather than numeric 10.)"@ind_num_asc"
Order by indices in ascending order but force them to be treated as numbersin the process. Any index with a non-numeric value will end up positioned as
if it were zero.
"@val_type_asc"
Order by element values in ascending order (rather than by indices). Ordering
is by the type assigned to the element (see Section 6.3.2 [Variable Typing andComparison Expressions], page 135). All numeric values come before all stringvalues, which in turn come before all subarrays. (Subarrays have not been
described yet; see Section 8.6 [Arrays of Arrays], page 189.)
If you choose to use this feature in traversing FUNCTAB (see Section 7.5.2 [Builtin Variables That Convey Information], page 165), then the order is built-infunctions first (see Section 9.1 [Built-in Functions], page 193), then user-defined
functions (see Section 9.2 [User-Defined Functions], page 221) next, and finallyfunctions loaded from an extension (see Chapter 17 [Writing Extensions forgawk], page 405).
"@val_str_asc"
Order by element values in ascending order (rather than by indices). Scalarvalues are compared as strings. If the string values are identical, the index
string values are compared instead. When comparing non-scalar values, "@val_
type_asc" sort ordering is used, so subarrays, if present, come out last.
"@val_num_asc"
Order by element values in ascending order (rather than by indices). Scalarvalues are compared as numbers. Non-scalar values are compared using "@val_
type_asc" sort ordering, so subarrays, if present, come out last. When numericvalues are equal, the string values are used to provide an ordering: this guarantees consistent results across different versions of the C qsort() function,22When two elements compare as equal, the C qsort() function does not guarantee that they will maintaintheir original relative order after sorting. Using the string value to provide a unique ordering when the
numeric values are equal ensures that gawk behaves consistently across different environments.

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which gawk uses internally to perform the sorting. If the string values are also
identical, the index string values are compared instead.
"@ind_str_desc"
Like "@ind_str_asc", but the string indices are ordered from high to low.
"@ind_num_desc"
Like "@ind_num_asc", but the numeric indices are ordered from high to low.
"@val_type_desc"
Like "@val_type_asc", but the element values, based on type, are ordered fromhigh to low. Subarrays, if present, come out first.
"@val_str_desc"
Like "@val_str_asc", but the element values, treated as strings, are orderedfrom high to low. If the string values are identical, the index string values arecompared instead. When comparing non-scalar values, "@val_type_desc" sortordering is used, so subarrays, if present, come out first.
"@val_num_desc"
Like "@val_num_asc", but the element values, treated as numbers, are orderedfrom high to low. If the numeric values are equal, the string values are compared instead. If they are also identical, the index string values are comparedinstead. Non-scalar values are compared using "@val_type_desc" sort ordering, so subarrays, if present, come out first.
The array traversal order is determined before the for loop starts to run. Changing
PROCINFO["sorted_in"] in the loop body does not affect the loop. For example:$ gawk '> BEGIN {>a[4] = 4>a[3] = 3>
for (i in a)>
print i, a[i]
> }'
a 4 4
a 3 3
$ gawk '> BEGIN {>PROCINFO["sorted_in"] = "@ind_str_asc">a[4] = 4>a[3] = 3>
for (i in a)>
print i, a[i]
> }'
a 3 3
a 4 4
When sorting an array by element values, if a value happens to be a subarray then it isconsidered to be greater than any string or numeric value, regardless of what the subarray

Chapter 8: Arrays in awk 185

itself contains, and all subarrays are treated as being equal to each other. Their orderrelative to each other is determined by their index strings.
Here are some additional things to bear in mind about sorted array traversal:• The value of PROCINFO["sorted_in"] is global. That is, it affects all array traversal
for loops. If you need to change it within your own code, you should see if it’s definedand save and restore the value:...
if ("sorted_in" in PROCINFO)
save_sorted = PROCINFO["sorted_in"]
PROCINFO["sorted_in"] = "@val_str_desc" # or whatever...
if (save_sorted)
PROCINFO["sorted_in"] = save_sorted• As already mentioned, the default array traversal order is represented by"@unsorted". You can also get the default behavior by assigning the null string
to PROCINFO["sorted_in"] or by just deleting the "sorted_in" element from the
PROCINFO array with the delete statement. (The delete statement hasn’t been
described yet; see Section 8.4 [The delete Statement], page 186.)
In addition, gawk provides built-in functions for sorting arrays; see Section 12.3.2 [Sorting
Array Values and Indices with gawk], page 341.

8.2 Using Numbers to Subscript Arrays
An important aspect to remember about arrays is that array subscripts are always strings.
When a numeric value is used as a subscript, it is converted to a string value before being
used for subscripting (see Section 6.1.4 [Conversion of Strings and Numbers], page 126).
This means that the value of the predefined variable CONVFMT can affect how your program
accesses elements of an array. For example:
xyz = 12.153
data[xyz] = 1
CONVFMT = "%2.2f"
if (xyz in data)
printf "%s is in data\n", xyz
else
printf "%s is not in data\n", xyz
This prints ‘12.15 is not in data’. The first statement gives xyz a numeric value. Assigning to data[xyz] subscripts data with the string value "12.153" (using the default
conversion value of CONVFMT, "%.6g"). Thus, the array element data["12.153"] is assigned the value one. The program then changes the value of CONVFMT. The test ‘(xyz indata)’ generates a new string value from xyz—this time "12.15"—because the value of
CONVFMT only allows two significant digits. This test fails, because "12.15" is different from
"12.153".
According to the rules for conversions (see Section 6.1.4 [Conversion of Strings andNumbers], page 126), integer values always convert to strings as integers, no matter what
the value of CONVFMT may happen to be. So the usual case of the following works:

186

GAWK: Effective AWK Programmingfor (i = 1; i <= maxsub; i++)do something with array[i]The “integer values always convert to strings as integers” rule has an additional consequence for array indexing. Octal and hexadecimal constants (see Section 6.1.1.2 [Octaland Hexadecimal Numbers], page 120) are converted internally into numbers, and their
original form is forgotten. This means, for example, that array[17], array[021], andarray[0x11] all refer to the same element!
As with many things in awk, the majority of the time things work as you would expect
them to. But it is useful to have a precise knowledge of the actual rules, as they cansometimes have a subtle effect on your programs.

8.3 Using Uninitialized Variables as Subscripts
Suppose it’s necessary to write a program to print the input data in reverse order. A
reasonable attempt to do so (with some test data) might look like this:$ echo 'line 1> line 2> line 3' | awk '{ l[lines] = $0; ++lines }> END {>for (i = lines - 1; i >= 0; i--)>print l[i]> }'a line 3a line 2
Unfortunately, the very first line of input data did not appear in the output!
Upon first glance, we would think that this program should have worked. The variablelines is uninitialized, and uninitialized variables have the numeric value zero. So, awk
should have printed the value of l[0].
The issue here is that subscripts for awk arrays are always strings. Uninitialized variables,
when used as strings, have the value "", not zero. Thus, ‘line 1’ ends up stored in l[""].
The following version of the program works correctly:{ l[lines++] = $0 }END {for (i = lines - 1; i >= 0; i--)print l[i]}Here, the ‘++’ forces lines to be numeric, thus making the “old value” numeric zero.
This is then converted to "0" as the array subscript.
Even though it is somewhat unusual, the null string ("") is a valid array subscript.
gawk warns about the use of the null string as a subscript if --lint is provided on thecommand line (see Section 2.2 [Command-Line Options], page 33).

8.4 The delete Statement
To remove an individual element of an array, use the delete statement:
delete array[index-expression]

Chapter 8: Arrays in awk 187

Once an array element has been deleted, any value the element once had is no longer
available. It is as if the element had never been referred to or been given a value. The
following is an example of deleting elements in an array:for (i in frequencies)
delete frequencies[i]This example removes all the elements from the array frequencies. Once an element isdeleted, a subsequent for statement to scan the array does not report that element andusing the in operator to check for the presence of that element returns zero (i.e., false):delete foo[4]if (4 in foo)
print "This will never be printed"
It is important to note that deleting an element is not the same as assigning it a nullvalue (the empty string, ""). For example:
foo[4] = ""if (4 in foo)
print "This is printed, even though foo[4] is empty"
It is not an error to delete an element that does not exist. However, if --lint isprovided on the command line (see Section 2.2 [Command-Line Options], page 33), gawk
issues a warning message when an element that is not in the array is deleted.
All the elements of an array may be deleted with a single statement by leaving off thesubscript in the delete statement, as follows:
delete array
Using this version of the delete statement is about three times more efficient than theequivalent loop that deletes each element one at a time.
This form of the delete statement is also supported by BWK awk and mawk, as well as
by a number of other implementations.
NOTE: For many years, using delete without a subscript was a common extension. In September 2012, it was accepted for inclusion into the POSIX standard.
See the Austin Group website.
The following statement provides a portable but nonobvious way to clear out an array:3
split("", array)
The split() function (see Section 9.1.4 [String-Manipulation Functions], page 196)
clears out the target array first. This call asks it to split apart the null string. Because
there is no data to split out, the function simply clears the array and then returns.
CAUTION: Deleting all the elements from an array does not change its type;
you cannot clear an array and then use the array’s name as a scalar (i.e., aregular variable). For example, the following does not work:a[1] = 3
delete aa = 3
3

Thanks to Michael Brennan for pointing this out.

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8.5 Multidimensional Arrays
A multidimensional array is an array in which an element is identified by a sequence ofindices instead of a single index. For example, a two-dimensional array requires two indices. The usual way (in many languages, including awk) to refer to an element of a
two-dimensional array named grid is with grid[x,y].
Multidimensional arrays are supported in awk through concatenation of indices into one
string. awk converts the indices into strings (see Section 6.1.4 [Conversion of Strings andNumbers], page 126) and concatenates them together, with a separator between them. This
creates a single string that describes the values of the separate indices. The combined stringis used as a single index into an ordinary, one-dimensional array. The separator used is thevalue of the built-in variable SUBSEP.
For example, suppose we evaluate the expression ‘foo[5,12] = "value"’ when the valueof SUBSEP is "@". The numbers 5 and 12 are converted to strings and concatenated with an‘@’ between them, yielding "5@12"; thus, the array element foo["5@12"] is set to "value".
Once the element’s value is stored, awk has no record of whether it was stored with a single index or a sequence of indices. The two expressions ‘foo[5,12]’ and ‘foo[5 SUBSEP 12]’are always equivalent.
The default value of SUBSEP is the string "\034", which contains a nonprinting character
that is unlikely to appear in an awk program or in most input data. The usefulness ofchoosing an unlikely character comes from the fact that index values that contain a string
matching SUBSEP can lead to combined strings that are ambiguous. Suppose that SUBSEP
is "@"; then ‘foo["a@b", "c"]’ and ‘foo["a", "b@c"]’ are indistinguishable because bothare actually stored as ‘foo["a@b@c"]’.
To test whether a particular index sequence exists in a multidimensional array, use thesame operator (in) that is used for single-dimensional arrays. Write the whole sequence ofindices in parentheses, separated by commas, as the left operand:if ((subscript1, subscript2, ...) in array)...
Here is an example that treats its input as a two-dimensional array of fields; it rotates
this array 90 degrees clockwise and prints the result. It assumes that all lines have the same
number of elements:{
if (max_nf < NF)
max_nf = NF
max_nr = NR
for (x = 1; x <= NF; x++)vector[x, NR] = $x}
END {for (x = 1; x <= max_nf; x++) {for (y = max_nr; y >= 1; --y)printf("%s ", vector[x, y])printf("\n")}Chapter 8: Arrays in awk 189}
When given the input:
1 2 3 4 5 6
2 3 4 5 6 1
3 4 5 6 1 2
4 5 6 1 2 3
the program produces the following output:
4 3 2 15 4 3 26 5 4 3
1 6 5 42 1 6 53 2 1 68.5.1 Scanning Multidimensional Arrays
There is no special for statement for scanning a “multidimensional” array. There cannot
be one, because, in truth, awk does not have multidimensional arrays or elements—there is
only a multidimensional way of accessing an array.
However, if your program has an array that is always accessed as multidimensional, you
can get the effect of scanning it by combining the scanning for statement (see Section 8.1.5[Scanning All Elements of an Array], page 181) with the built-in split() function (seeSection 9.1.4 [String-Manipulation Functions], page 196). It works in the following manner:for (combined in array) {split(combined, separate, SUBSEP)...
}
This sets the variable combined to each concatenated combined index in the array, andsplits it into the individual indices by breaking it apart where the value of SUBSEP appears.
The individual indices then become the elements of the array separate.
Thus, if a value is previously stored in array[1, "foo"], then an element with index"1\034foo" exists in array. (Recall that the default value of SUBSEP is the character withcode 034.) Sooner or later, the for statement finds that index and does an iteration withthe variable combined set to "1\034foo". Then the split() function is called as follows:
split("1\034foo", separate, "\034")
The result is to set separate[1] to "1" and separate[2] to "foo". Presto! The original
sequence of separate indices is recovered.

8.6 Arrays of Arrays
gawk goes beyond standard awk’s multidimensional array access and provides true arraysof arrays. Elements of a subarray are referred to by their own indices enclosed in square
brackets, just like the elements of the main array. For example, the following creates a
two-element subarray at index 1 of the main array a:a[1][1] = 1
a[1][2] = 2

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This simulates a true two-dimensional array. Each subarray element can contain anothersubarray as a value, which in turn can hold other arrays as well. In this way, you can create
arrays of three or more dimensions. The indices can be any awk expressions, including
scalars separated by commas (i.e., a regular awk simulated multidimensional subscript). So
the following is valid in gawk:a[1][3][1, "name"] = "barney"
Each subarray and the main array can be of different length. In fact, the elements of anarray or its subarray do not all have to have the same type. This means that the main array
and any of its subarrays can be nonrectangular, or jagged in structure. You can assign ascalar value to the index 4 of the main array a, even though a[1] is itself an array and nota scalar:a[4] = "An element in a jagged array"
The terms dimension, row, and column are meaningless when applied to such an array,but we will use “dimension” henceforth to imply the maximum number of indices needed
to refer to an existing element. The type of any element that has already been assigned
cannot be changed by assigning a value of a different type. You have to first delete thecurrent element, which effectively makes gawk forget about the element at that index:
delete a[4]a[4][5][6][7] = "An element in a four-dimensional array"
This removes the scalar value from index 4 and then inserts a three-level nested subarray
containing a scalar. You can also delete an entire subarray or subarray of subarrays:delete a[4][5]a[4][5] = "An element in subarray a[4]"But recall that you can not delete the main array a and then use it as a scalar.
The built-in functions that take array arguments can also be used with subarrays. For
example, the following code fragment uses length() (see Section 9.1.4 [String-Manipulation
Functions], page 196) to determine the number of elements in the main array a and its
subarrays:
print length(a), length(a[1]), length(a[1][3])This results in the following output for our main array a:2, 3, 1The ‘subscript in array’ expression (see Section 8.1.2 [Referring to an Array Element],
page 179) works similarly for both regular awk-style arrays and arrays of arrays. For example, the tests ‘1 in a’, ‘3 in a[1]’, and ‘(1, "name") in a[1][3]’ all evaluate to one (true)
for our array a.
The ‘for (item in array)’ statement (see Section 8.1.5 [Scanning All Elements of an
Array], page 181) can be nested to scan all the elements of an array of arrays if it isrectangular in structure. In order to print the contents (scalar values) of a two-dimensional
array of arrays (i.e., in which each first-level element is itself an array, not necessarily of thesame length), you could use the following code:for (i in array)for (j in array[i])print array[i][j]Chapter 8: Arrays in awk 191

The isarray() function (see Section 9.1.8 [Getting Type Information], page 219) lets
you test if an array element is itself an array:for (i in array) {if (isarray(array[i])) {for (j in array[i]) {print array[i][j]}}else
print array[i]}If the structure of a jagged array of arrays is known in advance, you can often devise
workarounds using control statements. For example, the following code prints the elementsof our main array a:for (i in a) {for (j in a[i]) {if (j == 3) {for (k in a[i][j])print a[i][j][k]} elseprint a[i][j]}}See Section 10.7 [Traversing Arrays of Arrays], page 274, for a user-defined function that“walks” an arbitrarily dimensioned array of arrays.

8.7 Summary• Standard awk provides one-dimensional associative arrays (arrays indexed by stringvalues). All arrays are associative; numeric indices are converted automatically tostrings.
• Array elements are referenced as array[indx]. Referencing an element creates it if it
did not exist previously.
• The proper way to see if an array has an element with a given index is to use the inoperator: ‘indx in array’.
• Use ‘for (indx in array) ...’ to scan through all the individual elements of an array.
In the body of the loop, indx takes on the value of each element’s index in turn.
• The order in which a ‘for (indx in array)’ loop traverses an array is undefined in
POSIX awk and varies among implementations. gawk lets you control the order byassigning special predefined values to PROCINFO["sorted_in"].
• Use ‘delete array[indx]’ to delete an individual element. To delete all of the elements
in an array, use ‘delete array’. This latter feature has been a common extension formany years and is now standard, but may not be supported by all commercial versionsof awk.

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GAWK: Effective AWK Programming• Standard awk simulates multidimensional arrays by separating subscript values withcommas. The values are concatenated into a single string, separated by the value of
SUBSEP. The fact that such a subscript was created in this way is not retained; thus,changing SUBSEP may have unexpected consequences. You can use ‘(sub1, sub2, ...)
in array’ to see if such a multidimensional subscript exists in array.
• gawk provides true arrays of arrays. You use a separate set of square brackets for eachdimension in such an array: data[row][col], for example. Array elements may thus
be either scalar values (number or string) or other arrays.
• Use the isarray() built-in function to determine if an array element is itself a subarray.

Chapter 9: Functions

193

9 Functions
This chapter describes awk’s built-in functions, which fall into three categories: numeric,string, and I/O. gawk provides additional groups of functions to work with values that represent time, do bit manipulation, sort arrays, provide type information, and internationalize
and localize programs.
Besides the built-in functions, awk has provisions for writing new functions that the restof a program can use. The second half of this chapter describes these user-defined functions.
Finally, we explore indirect function calls, a gawk-specific extension that lets you determine
at runtime what function is to be called.

9.1 Built-in Functions
Built-in functions are always available for your awk program to call. This section defines
all the built-in functions in awk; some of these are mentioned in other sections but aresummarized here for your convenience.

9.1.1 Calling Built-in Functions
To call one of awk’s built-in functions, write the name of the function followed by argumentsin parentheses. For example, ‘atan2(y + z, 1)’ is a call to the function atan2() and has
two arguments.
Whitespace is ignored between the built-in function name and the opening parenthesis,
but nonetheless it is good practice to avoid using whitespace there. User-defined functions
do not permit whitespace in this way, and it is easier to avoid mistakes by following a simple
convention that always works—no whitespace after a function name.
Each built-in function accepts a certain number of arguments. In some cases, arguments
can be omitted. The defaults for omitted arguments vary from function to function and aredescribed under the individual functions. In some awk implementations, extra arguments
given to built-in functions are ignored. However, in gawk, it is a fatal error to give extra
arguments to a built-in function.
When a function is called, expressions that create the function’s actual parameters areevaluated completely before the call is performed. For example, in the following codefragment:
i = 4
j = sqrt(i++)
the variable i is incremented to the value five before sqrt() is called with a value of four
for its actual parameter. The order of evaluation of the expressions used for the function’s
parameters is undefined. Thus, avoid writing programs that assume that parameters areevaluated from left to right or from right to left. For example:
i = 5
j = atan2(++i, i *= 2)
If the order of evaluation is left to right, then i first becomes six, and then 12, andatan2() is called with the two arguments six and 12. But if the order of evaluation is rightto left, i first becomes 10, then 11, and atan2() is called with the two arguments 11 and
10.

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9.1.2 Generating Boolean Values
This function is specific to gawk. It is not available in compatibility mode (see Section 2.2[Command-Line Options], page 33):
mkbool(expression)
Return a Boolean-typed value based on the regular Boolean value of expression.
Boolean “true” values have numeric value one. Boolean “false” values havenumeric zero. This is discussed in more detail in Section 12.2 [Boolean Typed
Values], page 336.

9.1.3 Numeric Functions
The following list describes all of the built-in functions that work with numbers. Optionalparameters are enclosed in square brackets ([ ]):atan2(y, x)
Return the arctangent of y / x in radians. You can use ‘pi = atan2(0, -1)’ to
retrieve the value of π.
cos(x)

Return the cosine of x, with x in radians.

exp(x)

Return the exponential of x (e ^ x) or report an error if x is out of range. The
range of values x can have depends on your machine’s floating-point representation.

int(x)

Return the nearest integer to x, located between x and zero and truncatedtoward zero. For example, int(3) is 3, int(3.9) is 3, int(-3.9) is −3, andint(-3) is −3 as well.

log(x)

Return the natural logarithm of x, if x is positive; otherwise, return NaN (“nota number”) on IEEE 754 systems. Additionally, gawk prints a warning message
when x is negative.

rand()

Return a random number. The values of rand() are uniformly distributed
between zero and one. The value could be zero but is never one.1
Often random integers are needed instead. Following is a user-defined functionthat can be used to obtain a random nonnegative integer less than n:function randint(n){return int(n * rand())}
The multiplication produces a random number greater than or equal to zeroand less than n. Using int(), this result is made into an integer between zeroand n − 1, inclusive.

1

The C version of rand() on many Unix systems is known to produce fairly poor sequences of random
numbers. However, nothing requires that an awk implementation use the C rand() to implement the awk
version of rand(). In fact, for many years, gawk used the BSD random() function, which is considerably
better than rand(), to produce random numbers. From version 4.1.4, courtesy of Nelson H.F. Beebe,gawk uses the Bayes-Durham shuffle buffer algorithm which considerably extends the period of therandom number generator, and eliminates short-range and long-range correlations that might exist in
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The following example uses a similar function to produce random integers between one and n. This program prints a new random number for each input
record:# Function to roll a simulated die.
function roll(n) { return 1 + int(rand() * n) }# Roll 3 six-sided dice and# print total number of points.
{
printf("%d points\n", roll(6) + roll(6) + roll(6))}
CAUTION: In most awk implementations, including gawk, rand()starts generating numbers from the same starting number, or seed,each time you run awk.2 Thus, a program generates the same results
each time you run it. The numbers are random within one awk run
but predictable from run to run. This is convenient for debugging,but if you want a program to do different things each time it isused, you must change the seed to a value that is different in eachrun. To do this, use srand().
sin(x)

Return the sine of x, with x in radians.

sqrt(x)Return the positive square root of x. gawk prints a warning message if x is
negative. Thus, sqrt(4) is 2.

srand([x])
Set the starting point, or seed, for generating random numbers to the value x.
Each seed value leads to a particular sequence of random numbers.3 Thus, if
the seed is set to the same value a second time, the same sequence of random
numbers is produced again.
CAUTION: Different awk implementations use different randomnumber generators internally. Don’t expect the same awk program
to produce the same series of random numbers when executed bydifferent versions of awk.
If the argument x is omitted, as in ‘srand()’, then the current date and timeof day are used for a seed. This is the way to get random numbers that are
truly unpredictable.
The return value of srand() is the previous seed. This makes it easy to keep
track of the seeds in case you need to consistently reproduce sequences of random numbers.
POSIX does not specify the initial seed; it differs among awk implementations.
2
3

mawk uses a different seed each time.
Computer-generated random numbers really are not truly random. They are technically known as
pseudorandom. This means that although the numbers in a sequence appear to be random, you can infact generate the same sequence of random numbers over and over again.

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9.1.4 String-Manipulation Functions
The functions in this section look at or change the text of one or more strings.
gawk understands locales (see Section 6.6 [Where You Are Makes a Difference], page 145)and does all string processing in terms of characters, not bytes. This distinction is particularly important to understand for locales where one character may be represented bymultiple bytes. Thus, for example, length() returns the number of characters in a string,and not the number of bytes used to represent those characters. Similarly, index() works
with character indices, and not byte indices.
CAUTION: A number of functions deal with indices into strings. For thesefunctions, the first character of a string is at position (index) one. This isdifferent from C and the languages descended from it, where the first characteris at position zero. You need to remember this when doing index calculations,particularly if you are used to C.
In the following list, optional parameters are enclosed in square brackets ([ ]). Several
functions perform string substitution; the full discussion is provided in the description of thesub() function, which comes toward the end, because the list is presented alphabetically.
Those functions that are specific to gawk are marked with a pound sign (‘#’). They are
not available in compatibility mode (see Section 2.2 [Command-Line Options], page 33):
asort(source [, dest [, how ] ]) #asorti(source [, dest [, how ] ]) #These two functions are similar in behavior, so they are described together.
NOTE: The following description ignores the third argument, how,as it requires understanding features that we have not discussed
yet. Thus, the discussion here is a deliberate simplification. (Wedo provide all the details later on; see Section 12.3.2 [Sorting Array
Values and Indices with gawk], page 341, for the full story.)
Both functions return the number of elements in the array source. For asort(),gawk sorts the values of source and replaces the indices of the sorted values ofsource with sequential integers starting with one. If the optional array dest is
specified, then source is duplicated into dest. dest is then sorted, leaving theindices of source unchanged.
When comparing strings, IGNORECASE affects the sorting (see Section 12.3.2[Sorting Array Values and Indices with gawk], page 341). If the source array
contains subarrays as values (see Section 8.6 [Arrays of Arrays], page 189), they
will come last, after all scalar values. Subarrays are not recursively sorted.
For example, if the contents of a are as follows:a["last"] = "de"a["first"] = "sac"a["middle"] = "cul"A call to asort():asort(a)results in the following contents of a:Chapter 9: Functions

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a[1] = "cul"a[2] = "de"a[3] = "sac"
The asorti() function works similarly to asort(); however, the indices are
sorted, instead of the values. Thus, in the previous example, starting with thesame initial set of indices and values in a, calling ‘asorti(a)’ would yield:a[1] = "first"
a[2] = "last"a[3] = "middle"
NOTE: You may not use either SYMTAB or FUNCTAB as the second
argument to these functions. Attempting to do so produces a fatal
error. You may use them as the first argument, but only if providing
a second array to use for the actual sorting.
You are allowed to use the same array for both the source and dest arguments,
but doing so only makes sense if you’re also supplying the third argument.
gensub(regexp, replacement, how [, target]) #Search the target string target for matches of the regular expression regexp.
If how is a string beginning with ‘g’ or ‘G’ (short for “global”), then replace
all matches of regexp with replacement. Otherwise, treat how as a number
indicating which match of regexp to replace. Treat numeric values less than
one as if they were one. If no target is supplied, use $0. Return the modified
string as the result of the function. The original target string is not changed.
The returned value is always a string, even if the original target was a number
or a regexp value.
gensub() is a general substitution function. Its purpose is to provide morefeatures than the standard sub() and gsub() functions.
gensub() provides an additional feature that is not available in sub() orgsub(): the ability to specify components of a regexp in the replacement text.
This is done by using parentheses in the regexp to mark the components andthen specifying ‘\N’ in the replacement text, where N is a digit from 1 to 9. For
example:$ gawk '> BEGIN {>a = "abc def">b = gensub(/(.+) (.+)/, "\\2 \\1", "g", a)>print b> }'a def abc
As with sub(), you must type two backslashes in order to get one into the string.
In the replacement text, the sequence ‘\0’ represents the entire matched text,
as does the character ‘&’.
The following example shows how you can use the third argument to control
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In this case, $0 is the default target string. gensub() returns the new string as
its result, which is passed directly to print for printing.
If the how argument is a string that does not begin with ‘g’ or ‘G’, or if it is anumber that is less than or equal to zero, only one substitution is performed.
If how is zero, gawk issues a warning message.
If regexp does not match target, gensub()’s return value is the original unchanged value of target. Note that, as mentioned above, the returned value isa string, even if target was not.
In the replacement string, a backslash before a non-digit character is simply
elided. For example, ‘\q’ becomes ‘q’ in the result. If the final character in thereplacement string is a backslash, it is left alone.
gsub(regexp, replacement [, target])Search target for all of the longest, leftmost, nonoverlapping matching substrings it can find and replace them with replacement. The ‘g’ in gsub() stands
for “global,” which means replace everywhere. For example:{ gsub(/Britain/, "United Kingdom"); print }replaces all occurrences of the string ‘Britain’ with ‘United Kingdom’ for all
input records.
The gsub() function returns the number of substitutions made. If the variable
to search and alter (target) is omitted, then the entire input record ($0) is used.
As in sub(), the characters ‘&’ and ‘\’ are special, and the third argument must
be assignable.
index(in, find)Search the string in for the first occurrence of the string find, and return theposition in characters where that occurrence begins in the string in. Consider
the following example:$ awk 'BEGIN { print index("peanut", "an") }'a 3
If find is not found, index() returns zero.
With BWK awk and gawk, it is a fatal error to use a regexp constant forfind. Other implementations allow it, simply treating the regexp constant as
an expression meaning ‘$0 ~ /regexp/’.
length([string])
Return the number of characters in string. If string is a number, the lengthof the digit string representing that number is returned. For example,
length("abcde") is five. By contrast, length(15 * 35) works out to three.
In this example, 15 · 35 = 525, and 525 is then converted to the string "525",
which has three characters.
If no argument is supplied, length() returns the length of $0.
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tice, although the 2008 POSIX standard explicitly allows it, to support historical practice. For programs to be maximally portable,
always supply the parentheses.
If length() is called with a variable that has not been used, gawk forces thevariable to be a scalar. Other implementations of awk leave the variable withouta type. Consider:$ gawk 'BEGIN { print length(x) ; x[1] = 1 }'a 0error gawk: fatal: attempt to use scalar `x' as array$ nawk 'BEGIN { print length(x) ; x[1] = 1 }'a 0
If --lint has been specified on the command line, gawk issues a warning about
this.
With gawk and several other awk implementations, when given an array argument, the length() function returns the number of elements in the array. (c.e.)This is less useful than it might seem at first, as the array is not guaranteed to beindexed from one to the number of elements in it. If --lint is provided on thecommand line (see Section 2.2 [Command-Line Options], page 33), gawk warns
that passing an array argument is not portable. If --posix is supplied, using
an array argument is a fatal error (see Chapter 8 [Arrays in awk], page 177).
match(string, regexp [, array])Search string for the longest, leftmost substring matched by the regular expression regexp and return the character position (index) at which that substring
begins (one, if it starts at the beginning of string). If no match is found, returnzero.
The regexp argument may be either a regexp constant (/. . . /) or a string
constant (". . . "). In the latter case, the string is treated as a regexp to bematched. See Section 3.6 [Using Dynamic Regexps], page 60, for a discussionof the difference between the two forms, and the implications for writing yourprogram correctly.
The order of the first two arguments is the opposite of most other string functions that work with regular expressions, such as sub() and gsub(). It might
help to remember that for match(), the order is the same as for the ‘~’ operator:‘string ~ regexp’.
The match() function sets the predefined variable RSTART to the index. It also
sets the predefined variable RLENGTH to the length in characters of the matchedsubstring. If no match is found, RSTART is set to zero, and RLENGTH to −1.
For example:{
if ($1 == "FIND")
regex = $2
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print "Match of", regex, "found at", where, "in", $0}}
This program looks for lines that match the regular expression stored in thevariable regex. This regular expression can be changed. If the first word on aline is ‘FIND’, regex is changed to be the second word on that line. Therefore,
if given:
FIND ru+n
My program runs
but not very quickly
FIND Melvin
JF+KM
This line is property of Reality Engineering Co.
Melvin was here.
awk prints:
Match of ru+n found at 12 in My program runs
Match of Melvin found at 1 in Melvin was here.
If array is present, it is cleared, and then the zeroth element of array is set tothe entire portion of string matched by regexp. If regexp contains parentheses,
the integer-indexed elements of array are set to contain the portion of string
matching the corresponding parenthesized subexpression. For example:$ echo foooobazbarrrrr |> gawk '{ match($0, /(fo+).+(bar*)/, arr)>
print arr[1], arr[2] }'a foooo barrrrr
In addition, multidimensional subscripts are available providing the start index
and length of each matched subexpression:$ echo foooobazbarrrrr |> gawk '{ match($0, /(fo+).+(bar*)/, arr)>
print arr[1], arr[2]>print arr[1, "start"], arr[1, "length"]>print arr[2, "start"], arr[2, "length"]> }'a foooo barrrrr
a 1 5
a 9 7
There may not be subscripts for the start and index for every parenthesized
subexpression, because they may not all have matched text; thus, they shouldbe tested for with the in operator (see Section 8.1.2 [Referring to an Array
Element], page 179).
The array argument to match() is a gawk extension. In compatibility mode(see Section 2.2 [Command-Line Options], page 33), using a third argument isa fatal error.

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patsplit(string, array [, fieldpat [, seps ] ]) #Divide string into pieces (or “fields”) defined by fieldpat and store the pieces
in array and the separator strings in the seps array. The first piece is stored
in array[1], the second piece in array[2], and so forth. The third argument,
fieldpat, is a regexp describing the fields in string (just as FPAT is a regexp
describing the fields in input records). It may be either a regexp constant ora string. If fieldpat is omitted, the value of FPAT is used. patsplit() returns
the number of elements created. seps[i] is the possibly null separator string
after array[i]. The possibly null leading separator will be in seps[0]. So
a non-null string with n fields will have n+1 separators. A null string has nofields or separators.
The patsplit() function splits strings into pieces in a manner similar to theway input lines are split into fields using FPAT (see Section 4.7 [Defining Fields
by Content], page 83).
Before splitting the string, patsplit() deletes any previously existing elementsin the arrays array and seps.
split(string, array [, fieldsep [, seps ] ])Divide string into pieces separated by fieldsep and store the pieces in array andthe separator strings in the seps array. The first piece is stored in array[1],the second piece in array[2], and so forth. The string value of the thirdargument, fieldsep, is a regexp describing where to split string (much as FS canbe a regexp describing where to split input records). If fieldsep is omitted, thevalue of FS is used. split() returns the number of elements created. seps isa gawk extension, with seps[i] being the separator string between array[i]and array[i+1]. If fieldsep is a single space, then any leading whitespace goes
into seps[0] and any trailing whitespace goes into seps[n], where n is thereturn value of split() (i.e., the number of elements in array).
The split() function splits strings into pieces in the same way that input lines
are split into fields. For example:
split("cul-de-sac", a, "-", seps)
splits the string "cul-de-sac" into three fields using ‘-’ as the separator. It
sets the contents of the array a as follows:a[1] = "cul"a[2] = "de"a[3] = "sac"and sets the contents of the array seps as follows:seps[1] = "-"seps[2] = "-"The value returned by this call to split() is three.
If gawk is invoked with --csv, then a two-argument call to split() splits
the string using the CSV parsing rules as described in Section 4.5.4 [Working
With Comma Separated Value Files], page 76. With three and four arguments,
split() works as just described. The four-argument call makes no sense, sinceeach element of seps would simply consist of a string containing a comma.

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As with input field-splitting, when the value of fieldsep is " ", leading andtrailing whitespace is ignored in values assigned to the elements of array but
not in seps, and the elements are separated by runs of whitespace. Also, aswith input field splitting, if fieldsep is the null string, each individual characterin the string is split into its own array element. (c.e.) Additionally, if fieldsep
is a single-character string, that string acts as the separator, even if its value isa regular expression metacharacter.
Note, however, that RS has no effect on the way split() works. Even though‘RS = ""’ causes the newline character to also be an input field separator, thisdoes not affect how split() splits strings.
Modern implementations of awk, including gawk, allow the third argument tobe a regexp constant (/. . . /) as well as a string. The POSIX standard allows
this as well. See Section 3.6 [Using Dynamic Regexps], page 60, for a discussionof the difference between using a string constant or a regexp constant, and theimplications for writing your program correctly.
Before splitting the string, split() deletes any previously existing elements inthe arrays array and seps.
If string is null, the array has no elements. (So this is a portable way to delete
an entire array with one statement. See Section 8.4 [The delete Statement],
page 186.)
If string does not match fieldsep at all (but is not null), array has one element
only. The value of that element is the original string.
In POSIX mode (see Section 2.2 [Command-Line Options], page 33), the fourthargument is not allowed.
sprintf(format, expression1, ...)
Return (without printing) the string that printf would have printed out withthe same arguments (see Section 5.5 [Using printf Statements for Fancier
Printing], page 102). For example:
pival = sprintf("pi = %.2f (approx.)", 22/7)
assigns the string ‘pi = 3.14 (approx.)’ to the variable pival.
strtonum(str) #Examine str and return its numeric value. If str begins with a leading ‘0’,
strtonum() assumes that str is an octal number. If str begins with a leading ‘0x’ or ‘0X’, strtonum() assumes that str is a hexadecimal number. For
example:$ echo 0x11 |> gawk '{ printf "%d\n", strtonum($1) }'a 17
Using the strtonum() function is not the same as adding zero to a string value;
the automatic coercion of strings to numbers works only for decimal data, notfor octal or hexadecimal.4
4Unless you use the --non-decimal-data option, which isn’t recommended. See Section 12.1 [Allowing
Nondecimal Input Data], page 335, for more information.

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Note also that strtonum() uses the current locale’s decimal point for recognizing numbers (see Section 6.6 [Where You Are Makes a Difference], page 145).
sub(regexp, replacement [, target])Search target, which is treated as a string, for the leftmost, longest substring
matched by the regular expression regexp. Modify the entire string by replacing
the matched text with replacement. The modified string becomes the new valueof target. Return the number of substitutions made (zero or one).
The regexp argument may be either a regexp constant (/. . . /) or a string
constant (". . . "). In the latter case, the string is treated as a regexp to bematched. See Section 3.6 [Using Dynamic Regexps], page 60, for a discussionof the difference between the two forms, and the implications for writing yourprogram correctly.
This function is peculiar because target is not simply used to compute a value,
and not just any expression will do—it must be a variable, field, or array element
so that sub() can store a modified value there. If this argument is omitted,
then the default is to use and alter $0.5 For example:
str = "water, water, everywhere"sub(/at/, "ith", str)
sets str to ‘wither, water, everywhere’, by replacing the leftmost longest
occurrence of ‘at’ with ‘ith’.
If the special character ‘&’ appears in replacement, it stands for the precise
substring that was matched by regexp. (If the regexp can match more than one
string, then this precise substring may vary.) For example:{ sub(/candidate/, "& and his wife"); print }changes the first occurrence of ‘candidate’ to ‘candidate and his wife’ oneach input line. Here is another example:$ awk 'BEGIN {>str = "daabaaa">
sub(/a+/, "C&C", str)>
print str
> }'a dCaaCbaaa
This shows how ‘&’ can represent a nonconstant string and also illustrates the“leftmost, longest” rule in regexp matching (see Section 3.5 [How Much Text
Matches?], page 60).
The effect of this special character (‘&’) can be turned off by putting a backslash
before it in the string. As usual, to insert one backslash in the string, you must
write two backslashes. Therefore, write ‘\\&’ in a string constant to include aliteral ‘&’ in the replacement. For example, the following shows how to replace
the first ‘|’ on each line with an ‘&’:{ sub(/\|/, "\\&"); print }5

Note that this means that the record will first be regenerated using the value of OFS if any fields havebeen changed, and that the fields will be updated after the substitution, even if the operation is a “no-op”
such as ‘sub(/^/, "")’.

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As mentioned, the third argument to sub() must be a variable, field, or arrayelement. Some versions of awk allow the third argument to be an expressionthat is not an lvalue. In such a case, sub() still searches for the pattern andreturns zero or one, but the result of the substitution (if any) is thrown away
because there is no place to put it. Such versions of awk accept expressions likethe following:
sub(/USA/, "United States", "the USA and Canada")
For historical compatibility, gawk accepts such erroneous code. However, using
any other nonchangeable object as the third parameter causes a fatal error andyour program will not run.
Finally, if the regexp is not a regexp constant, it is converted into a string, andthen the value of that string is treated as the regexp to match.
substr(string, start [, length ])Return a length-character-long substring of string, starting at character number
start. The first character of a string is character number one.6 For example,
substr("washington", 5, 3) returns "ing".
If length is not present, substr() returns the whole suffix of string that begins
at character number start. For example, substr("washington", 5) returns"ington". The whole suffix is also returned if length is greater than the numberof characters remaining in the string, counting from character start.
If start is less than one, substr() treats it as if it was one. (POSIX doesn’t
specify what to do in this case: BWK awk acts this way, and therefore gawk doestoo.) If start is greater than the number of characters in the string, substr()
returns the null string. Similarly, if length is present but less than or equal tozero, the null string is returned.
The string returned by substr() cannot be assigned. Thus, it is a mistake toattempt to change a portion of a string, as shown in the following example:
string = "abcdef"# try to get "abCDEf", won't work
substr(string, 3, 3) = "CDE"
It is also a mistake to use substr() as the third argument of sub() or gsub():gsub(/xyz/, "pdq", substr($0, 5, 20)) # WRONG(Some commercial versions of awk treat substr() as assignable, but doing so
is not portable.)
If you need to replace bits and pieces of a string, combine substr() with string
concatenation, in the following manner:
string = "abcdef"...
string = substr(string, 1, 2) "CDE" substr(string, 6)
tolower(string)
Return a copy of string, with each uppercase character in the string replacedwith its corresponding lowercase character. Nonalphabetic characters are left
6

This is different from C and C++, in which the first character is number zero.

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unchanged. For example, tolower("MiXeD cAsE 123") returns "mixed case123".
toupper(string)
Return a copy of string, with each lowercase character in the string replacedwith its corresponding uppercase character. Nonalphabetic characters are left
unchanged. For example, toupper("MiXeD cAsE 123") returns "MIXED CASE
123".
At first glance, the split() and patsplit() functions appear to be mirror images ofeach other. But there are differences:• split() treats its third argument like FS, with all the special rules involved for FS.
• Matching of null strings differs. This is discussed in Section 4.7.2 [FS Versus FPAT: A
Subtle Difference], page 85.
Matching the Null String
In awk, the ‘*’ operator can match the null string. This is particularly important forthe sub(), gsub(), and gensub() functions. For example:$ echo abc | awk '{ gsub(/m*/, "X"); print }'a XaXbXcX
Although this makes a certain amount of sense, it can be surprising.

9.1.4.1 More about ‘\’ and ‘&’ with sub(), gsub(), and gensub()I collect spores, molds, and fungus.
—Dr. Egon Spengler (“Ghostbusters,” 1984)
CAUTION: This subsubsection has been reported to cause headaches. Youmight want to skip it upon first reading.
When using sub(), gsub(), or gensub(), and trying to get literal backslashes andampersands into the replacement text, you need to remember that there are several levelsof escape processing going on.
First, there is the lexical level, which is when awk reads your program and builds aninternal copy of it to execute. Then there is the runtime level, which is when awk actuallyscans the replacement string to determine what to generate.
At both levels, awk looks for a defined set of characters that can come after a backslash.
At the lexical level, it looks for the escape sequences listed in Section 3.2 [Escape Sequences],
page 52. Thus, for every ‘\’ that awk processes at the runtime level, you must type twobackslashes at the lexical level. When a character that is not valid for an escape sequence
follows the ‘\’, BWK awk and gawk both simply remove the initial ‘\’ and put the next
character into the string. Thus, for example, "a\qb" is treated as "aqb".
At the runtime level, the various functions handle sequences of ‘\’ and ‘&’ differently. The
situation is (sadly) somewhat complex. Historically, the sub() and gsub() functions treated
the two-character sequence ‘\&’ specially; this sequence was replaced in the generated text
with a single ‘&’. Any other ‘\’ within the replacement string that did not precede an ‘&’
was passed through unchanged. This is illustrated in Table 9.1.

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You typesub() sees

\&
\\&
\\\&
\\\\&
\\\\\&\\\\\\&
\\q

&
\&
\&
\\&
\\&
\\\&
\q

sub() generates
The matched text
A literal ‘&’
A literal ‘&’
A literal ‘\&’A literal ‘\&’A literal ‘\\&’A literal ‘\q’

Table 9.1: Historical escape sequence processing for sub() and gsub()This table shows the lexical-level processing, where an odd number of backslashes becomesan even number at the runtime level, as well as the runtime processing done by sub(). (Forthe sake of simplicity, the rest of the following tables only show the case of even numbersof backslashes entered at the lexical level.)
The problem with the historical approach is that there is no way to get a literal ‘\’
followed by the matched text.
Several editions of the POSIX standard attempted to fix this problem but weren’t successful. The details are irrelevant at this point in time.
At one point, the gawk maintainer submitted proposed text for a revised standard thatreverts to rules that correspond more closely to the original existing practice. The proposed
rules have special cases that make it possible to produce a ‘\’ preceding the matched text.
This is shown in Table 9.2.
You typesub() sees\\\\\\&
\\\\&
\\&
\\q
\\\\

\\\&
\\&
\&
\q
\\

sub() generates
A literal ‘\&’A literal ‘\’, followed by the matched text
A literal ‘&’
A literal ‘\q’\\

Table 9.2: gawk rules for sub() and backslash
In a nutshell, at the runtime level, there are now three special sequences of characters(‘\\\&’, ‘\\&’, and ‘\&’) whereas historically there was only one. However, as in the historicalcase, any ‘\’ that is not part of one of these three sequences is not special and appears in
the output literally.
gawk 3.0 and 3.1 follow these rules for sub() and gsub(). The POSIX standard tookmuch longer to be revised than was expected. In addition, the gawk maintainer’s proposal
was lost during the standardization process. The final rules are somewhat simpler. The
results are similar except for one case.

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The POSIX rules state that ‘\&’ in the replacement string produces a literal ‘&’, ‘\\’produces a literal ‘\’, and ‘\’ followed by anything else is not special; the ‘\’ is placedstraight into the output. These rules are presented in Table 9.3.
You typesub() sees\\\\\\&
\\\\&
\\&
\\q
\\\\

\\\&
\\&
\&
\q
\\

sub() generates
A literal ‘\&’A literal ‘\’, followed by the matched text
A literal ‘&’
A literal ‘\q’\

Table 9.3: POSIX rules for sub() and gsub()The only case where the difference is noticeable is the last one: ‘\\\\’ is seen as ‘\\’ andproduces ‘\’ instead of ‘\\’.
Starting with version 3.1.4, gawk followed the POSIX rules when --posix was specified(see Section 2.2 [Command-Line Options], page 33). Otherwise, it continued to follow the
proposed rules, as that had been its behavior for many years.
When version 4.0.0 was released, the gawk maintainer made the POSIX rules the default,
breaking well over a decade’s worth of backward compatibility.7 Needless to say, this wasa bad idea, and as of version 4.0.1, gawk resumed its historical behavior, and only follows
the POSIX rules when --posix is given.
The rules for gensub() are considerably simpler. At the runtime level, whenever gawk
sees a ‘\’, if the following character is a digit, then the text that matched the corresponding
parenthesized subexpression is placed in the generated output. Otherwise, no matter whatcharacter follows the ‘\’, it appears in the generated text and the ‘\’ does not, as shown in
Table 9.4.
You type

gensub() sees

&
\\&
\\\\
\\\\&
\\\\\\&
\\q

&
\&
\\
\\&
\\\&
\q

gensub() generates
The matched text
A literal ‘&’
A literal ‘\’
A literal ‘\’, then the matched text
A literal ‘\&’A literal ‘q’

Table 9.4: Escape sequence processing for gensub()Because of the complexity of the lexical- and runtime-level processing and the special
cases for sub() and gsub(), we recommend the use of gawk and gensub() when you haveto do substitutions.
7This was rather naive of him, despite there being a note in this section indicating that the next major
version would move to the POSIX rules.

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9.1.5 Input/Output Functions
The following functions relate to input/output (I/O). Optional parameters are enclosed insquare brackets ([ ]):close(filename [, how])
Close the file filename for input or output. Alternatively, the argument may be
a shell command that was used for creating a coprocess, or for redirecting toor from a pipe; then the coprocess or pipe is closed. See Section 5.9 [Closing
Input and Output Redirections], page 113, for more information.
When closing a coprocess, it is occasionally useful to first close one end of thetwo-way pipe and then to close the other. This is done by providing a second
argument to close(). This second argument (how) should be one of the twostring values "to" or "from", indicating which end of the pipe to close. Case in
the string does not matter. See Section 12.4 [Two-Way Communications withAnother Process], page 343, which discusses this feature in more detail andgives an example.
Note that the second argument to close() is a gawk extension; it is not available
in compatibility mode (see Section 2.2 [Command-Line Options], page 33).
fflush([filename])
Flush any buffered output associated with filename, which is either a file opened
for writing or a shell command for redirecting output to a pipe or coprocess.
Many utility programs buffer their output (i.e., they save information to write
to a disk file or the screen in memory until there is enough for it to be worthwhile
to send the data to the output device). This is often more efficient than writing
every little bit of information as soon as it is ready. However, sometimes it isnecessary to force a program to flush its buffers (i.e., write the information toits destination, even if a buffer is not full). This is the purpose of the fflush()function—gawk also buffers its output, and the fflush() function forces gawk
to flush its buffers.
Brian Kernighan added fflush() to his awk in April 1992. For two decades, it
was a common extension. In December 2012, it was accepted for inclusion into
the POSIX standard. See the Austin Group website.
POSIX standardizes fflush() as follows: if there is no argument, or if theargument is the null string (""), then awk flushes the buffers for all open output
files and pipes.
NOTE: Prior to version 4.0.2, gawk would flush only the standard
output if there was no argument, and flush all output files and pipes
if the argument was the null string. This was changed in order tobe compatible with BWK awk, in the hope that standardizing thisfeature in POSIX would then be easier (which indeed proved to be
the case).
With gawk, you can use ‘fflush("/dev/stdout")’ if you wish toflush only the standard output.
fflush() returns zero if the buffer is successfully flushed; otherwise, it returns
a nonzero value. (gawk returns −1.) In the case where all buffers are flushed,Chapter 9: Functions

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the return value is zero only if all buffers were flushed successfully. Otherwise,
it is −1, and gawk warns about the problem filename.
gawk also issues a warning message if you attempt to flush a file or pipe thatwas opened for reading (such as with getline), or if filename is not an open
file, pipe, or coprocess. In such a case, fflush() returns −1, as well.
Interactive Versus Noninteractive Buffering
As a side point, buffering issues can be even more confusing if your program is interactive (i.e., communicating with a user sitting at a keyboard).8
Interactive programs generally line buffer their output (i.e., they write out every line).
Noninteractive programs wait until they have a full buffer, which may be many lines of
output. Here is an example of the difference:$ awk '{ print $1 + $2 }'
1 1
a 2
2 3
a 5
Ctrl-dEach line of output is printed immediately. Compare that behavior with this example:$ awk '{ print $1 + $2 }' | cat
1 1
2 3
Ctrl-da 2
a 5Here, no output is printed until after the Ctrl-d is typed, because it is all buffered andsent down the pipe to cat in one shot.
system(command)
Execute the operating system command command and then return to the awk
program. Return command’s exit status (see further on).
For example, if the following fragment of code is put in your awk program:END {system("date | mail -s 'awk run done' root")}
the system administrator is sent mail when the awk program finishes processing
input and begins its end-of-input processing.
Note that redirecting print or printf into a pipe is often enough to accomplish
your task. If you need to run many commands, it is more efficient to simply
print them down a pipeline to the shell:
while (more stuff to do)
8

A program is interactive if the standard output is connected to a terminal device. On modern systems,
this means your keyboard and screen.

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print command | "/bin/sh"
close("/bin/sh")However, if your awk program is interactive, system() is useful for running large
self-contained programs, such as a shell or an editor. Some operating systems
cannot implement the system() function. system() causes a fatal error if it is
not supported.
NOTE: When --sandbox is specified, the system() function is disabled (see Section 2.2 [Command-Line Options], page 33).
On POSIX systems, a command’s exit status is a 16-bit number. The exit
value passed to the C exit() function is held in the high-order eight bits. The
low-order bits indicate if the process was killed by a signal (bit 7) and if so, theguilty signal number (bits 0–6).
Traditionally, awk’s system() function has simply returned the exit status value
divided by 256. In the normal case this gives the exit status but in the case ofdeath-by-signal it yields a fractional floating-point value.9 POSIX states thatawk’s system() should return the full 16-bit value.
gawk steers a middle ground. The return values are summarized in Table 9.5.
Situation--traditional--posix
Normal exit of command
Death by signal of command
Death by signal of commandwith core dump
Some kind of error

Return value from system()
C system()’s value divided by 256
C system()’s value
Command’s exit status
256 + number of murderous signal
512 + number of murderous signal−1

Table 9.5: Return values from system()
As of August, 2018, BWK awk now follows gawk’s behavior for the return value ofsystem().

9

In private correspondence, Dr. Kernighan has indicated to me that the way this was done was probably
a mistake.

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Controlling Output Buffering with system()
The fflush() function provides explicit control over output buffering for individual
files and pipes. However, its use is not portable to many older awk implementations. An
alternative method to flush output buffers is to call system() with a null string as its
argument:
system("")# flush output
gawk treats this use of the system() function as a special case and is smart enough not
to run a shell (or other command interpreter) with the empty command. Therefore, with
gawk, this idiom is not only useful, it is also efficient. Although this method should workwith other awk implementations, it does not necessarily avoid starting an unnecessary
shell. (Other implementations may only flush the buffer associated with the standard
output and not necessarily all buffered output.)
If you think about what a programmer expects, it makes sense that system() should
flush any pending output. The following program:
BEGIN {
print "first print"
system("echo system echo")print "second print"}
must print:
first print
system echosecond print
and not:
system echo
first print
second print
If awk did not flush its buffers before calling system(), you would see the latter (undesirable) output.

9.1.6 Time Functions
awk programs are commonly used to process log files containing timestamp information,
indicating when a particular log record was written. Many programs log their timestamps
in the form returned by the time() system call, which is the number of seconds since aparticular epoch. On POSIX-compliant systems, it is the number of seconds since 197001-01 00:00:00 UTC, not counting leap seconds.10 All known POSIX-compliant systems
support timestamps from 0 through 231 − 1, which is sufficient to represent times through2038-01-19 03:14:07 UTC. Many systems support a wider range of timestamps, including
negative timestamps that represent times before the epoch.
In order to make it easier to process such log files and to produce useful reports, gawk
provides the following functions for working with timestamps. They are gawk extensions;
10

See [Glossary], page 529, especially the entries “Epoch” and “UTC.”

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they are not specified in the POSIX standard.11 However, recent versions of mawk (seeSection B.5 [Other Freely Available awk Implementations], page 510) also support thesefunctions. Optional parameters are enclosed in square brackets ([ ]):mktime(datespec [, utc-flag ])Turn datespec into a timestamp in the same form as is returned by systime().
It is similar to the function of the same name in ISO C. The argument, datespec,
is a string of the form "YYYY MM DD HH MM SS [DST]". The string consists of six
or seven numbers representing, respectively, the full year including century, themonth from 1 to 12, the day of the month from 1 to 31, the hour of the day from0 to 23, the minute from 0 to 59, the second from 0 to 60,12 and an optional
daylight-savings flag.
The values of these numbers need not be within the ranges specified; for example, an hour of −1 means 1 hour before midnight. The origin-zero Gregoriancalendar is assumed, with year 0 preceding year 1 and year −1 preceding year
0. If utc-flag is present and is either nonzero or non-null, the time is assumed
to be in the UTC time zone; otherwise, the time is assumed to be in the local
time zone. If the DST daylight-savings flag is positive, the time is assumed
to be daylight savings time; if zero, the time is assumed to be standard time;and if negative (the default), mktime() attempts to determine whether daylightsavings time is in effect for the specified time.
If datespec does not contain enough elements or if the resulting time is out ofrange, mktime() returns −1.
strftime([format [, timestamp [, utc-flag] ] ])Format the time specified by timestamp based on the contents of the format
string and return the result. It is similar to the function of the same namein ISO C. If utc-flag is present and is either nonzero or non-null, the value isformatted as UTC (Coordinated Universal Time, formerly GMT or Greenwich
Mean Time). Otherwise, the value is formatted for the local time zone. The
timestamp is in the same format as the value returned by the systime() function. If no timestamp argument is supplied, gawk uses the current time of day
as the timestamp. Without a format argument, strftime() uses the value of
PROCINFO["strftime"] as the format string (see Section 7.5 [Predefined Variables], page 163). The default string value is "%a %b %e %H:%M:%S %Z %Y". This
format string produces output that is equivalent to that of the date utility.
You can assign a new value to PROCINFO["strftime"] to change the default
format; see the following list for the various format directives.
systime()Return the current time as the number of seconds since the system epoch. On
POSIX systems, this is the number of seconds since 1970-01-01 00:00:00 UTC,
not counting leap seconds. It may be a different number on other systems.
The systime() function allows you to compare a timestamp from a log file with thecurrent time of day. In particular, it is easy to determine how long ago a particular record
11

The GNU date utility can also do many of the things described here. Its use may be preferable forsimple time-related operations in shell scripts.
12
Occasionally there are minutes in a year with a leap second, which is why the seconds can go up to 60.

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was logged. It also allows you to produce log records using the “seconds since the epoch”
format.
The mktime() function allows you to convert a textual representation of a date and time
into a timestamp. This makes it easy to do before/after comparisons of dates and times,
particularly when dealing with date and time data coming from an external source, such asa log file.
The strftime() function allows you to easily turn a timestamp into human-readable
information. It is similar in nature to the sprintf() function (see Section 9.1.4 [StringManipulation Functions], page 196), in that it copies nonformat specification charactersverbatim to the returned string, while substituting date and time values for format specifications in the format string.
strftime() is guaranteed by the 1999 ISO C standard13 to support the following dateformat specifications:%a

The locale’s abbreviated weekday name.

%A

The locale’s full weekday name.

%b

The locale’s abbreviated month name.

%B

The locale’s full month name.

%c

The locale’s “appropriate” date and time representation. (This is ‘%A %B %d %T%Y’ in the "C" locale.)%C

The century part of the current year. This is the year divided by 100 andtruncated to the next lower integer.

%d

The day of the month as a decimal number (01–31).

%D

Equivalent to specifying ‘%m/%d/%y’.

%e

The day of the month, padded with a space if it is only one digit.

%F

Equivalent to specifying ‘%Y-%m-%d’. This is the ISO 8601 date format.

%g

The year modulo 100 of the ISO 8601 week number, as a decimal number (00–99). For example, January 1, 2012, is in week 53 of 2011. Thus, the yearof its ISO 8601 week number is 2011, even though its year is 2012. Similarly,
December 31, 2012, is in week 1 of 2013. Thus, the year of its ISO week number
is 2013, even though its year is 2012.

%G

The full year of the ISO week number, as a decimal number.

%hEquivalent to ‘%b’.

%H

The hour (24-hour clock) as a decimal number (00–23).

%I

The hour (12-hour clock) as a decimal number (01–12).

%j

The day of the year as a decimal number (001–366).

%m

The month as a decimal number (01–12).

%M

The minute as a decimal number (00–59).

13

Unfortunately, not every system’s strftime() necessarily supports all of the conversions listed here.

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GAWK: Effective AWK Programming%n

A newline character (ASCII LF).

%p

The locale’s equivalent of the AM/PM designations associated with a 12-hourclock.

%r

The locale’s 12-hour clock time. (This is ‘%I:%M:%S %p’ in the "C" locale.)%R

Equivalent to specifying ‘%H:%M’.

%S

The second as a decimal number (00–60).

%t

A TAB character.

%T

Equivalent to specifying ‘%H:%M:%S’.

%u

The weekday as a decimal number (1–7). Monday is day one.

%U

The week number of the year (with the first Sunday as the first day of weekone) as a decimal number (00–53).

%V

The week number of the year (with the first Monday as the first day of week one)as a decimal number (01–53). The method for determining the week number is
as specified by ISO 8601. (To wit: if the week containing January 1 has fouror more days in the new year, then it is week one; otherwise it is the last week
[52 or 53] of the previous year and the next week is week one.)%w

The weekday as a decimal number (0–6). Sunday is day zero.

%W

The week number of the year (with the first Monday as the first day of weekone) as a decimal number (00–53).

%x

The locale’s “appropriate” date representation. (This is ‘%A %B %d %Y’ in the"C" locale.)%X

The locale’s “appropriate” time representation. (This is ‘%T’ in the "C" locale.)%y

The year modulo 100 as a decimal number (00–99).

%Y

The full year as a decimal number (e.g., 2015).

%z

The time zone offset in a ‘+HHMM’ format (e.g., the format necessary to produce
RFC 822/RFC 1036 date headers).

%Z

The time zone name or abbreviation; no characters if no time zone is determinable.

%Ec %EC %Ex %EX %Ey %EY %Od %Oe %OH%OI %Om %OM %OS %Ou %OU %OV %Ow %OW %Oy“Alternative representations” for the specifications that use only the second
letter (‘%c’, ‘%C’, and so on).14 (These facilitate compliance with the POSIX
date utility.)%%
14

A literal ‘%’.
If you don’t understand any of this, don’t worry about it; these facilities are meant to make it easier to“internationalize” programs. Other internationalization features are described in Chapter 13 [Internationalization with gawk], page 357.

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If a conversion specifier is not one of those just listed, the behavior is undefined.15
For systems that are not yet fully standards-compliant, gawk supplies a copyof strftime() from the GNU C Library. It supports all of the just-listed format
specifications. If that version is used to compile gawk (see Appendix B [Installing gawk],
page 491), then the following additional format specifications are available:%k

The hour (24-hour clock) as a decimal number (0–23). Single-digit numbers arepadded with a space.

%l

The hour (12-hour clock) as a decimal number (1–12). Single-digit numbers arepadded with a space.

%s

The time as a decimal timestamp in seconds since the epoch.

Additionally, the alternative representations are recognized but their normal representations are used.
The following example is an awk implementation of the POSIX date utility. Normally,
the date utility prints the current date and time of day in a well-known format. However,
if you provide an argument to it that begins with a ‘+’, date copies nonformat specifiercharacters to the standard output and interprets the current time according to the format
specifiers in the string. For example:$ date '+Today is %A, %B %d, %Y.'a Today is Monday, September 22, 2014.
Here is the gawk version of the date utility. It has a shell “wrapper” to handle the -u
option, which requires that date run as if the time zone is set to UTC:#! /bin/sh## date --- approximate the POSIX 'date' commandcase $1 in-u) TZ=UTC0
export TZ
shift ;;
esac

# use UTC

gawk 'BEGIN {format = PROCINFO["strftime"]
exitval = 0
if (ARGC > 2)
exitval = 1
else if (ARGC == 2) {format = ARGV[1]if (format ~ /^\+/)
format = substr(format, 2)
15# remove leading +This is because ISO C leaves the behavior of the C version of strftime() undefined and gawk uses thesystem’s version of strftime() if it’s there. Typically, the conversion specifier either does not appear in
the returned string or appears literally.

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print strftime(format)
exit exitval}' "$@"

9.1.7 Bit-Manipulation Functions
I can explain it for you, but I can’t understand it for you.
—Anonymous
Many languages provide the ability to perform bitwise operations on two integer numbers. In other words, the operation is performed on each successive pair of bits in theoperands. Three common operations are bitwise AND, OR, and XOR. The operations aredescribed in Table 9.6.

Operands
01

Bit operator
AND
OR
XOR
0
1
0
1
0
1
0
0
0
1
0
1
0
1
1
1
1
0

Table 9.6: Bitwise operations
As you can see, the result of an AND operation is 1 only when both bits are 1. The
result of an OR operation is 1 if either bit is 1. The result of an XOR operation is 1 ifeither bit is 1, but not both. The next operation is the complement; the complement of 1
is 0 and the complement of 0 is 1. Thus, this operation “flips” all the bits of a given value.
Finally, two other common operations are to shift the bits left or right. For example,
if you have a bit string ‘10111001’ and you shift it right by three bits, you end up with
‘00010111’.16 If you start over again with ‘10111001’ and shift it left by three bits, you end
up with ‘11001000’. The following list describes gawk’s built-in functions that implement
the bitwise operations. Optional parameters are enclosed in square brackets ([ ]):and(v1, v2 [, . . . ])Return the bitwise AND of the arguments. There must be at least two.
compl(val)
Return the bitwise complement of val.
lshift(val, count)
Return the value of val, shifted left by count bits.
or(v1, v2 [, . . . ])Return the bitwise OR of the arguments. There must be at least two.
rshift(val, count)
Return the value of val, shifted right by count bits.
xor(v1, v2 [, . . . ])Return the bitwise XOR of the arguments. There must be at least two.
16

This example shows that zeros come in on the left side. For gawk, this is always true, but in somelanguages, it’s possible to have the left side fill with ones.

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CAUTION: Beginning with gawk version 4.2, negative operands are not allowed
for any of these functions. A negative operand produces a fatal error. See thesidebar “Beware The Smoke and Mirrors!” for more information as to why.
Here is a user-defined function (see Section 9.2 [User-Defined Functions], page 221) that
illustrates the use of these functions:# bits2str --- turn an integer into readable ones and zeros
function bits2str(bits,{
if (bits == 0)
return "0"data, mask)

mask = 1
for (; bits != 0; bits = rshift(bits, 1))data = (and(bits, mask) ? "1" : "0") data
while ((length(data) % 8) != 0)
data = "0" data
return data}
BEGIN {
printf "123 = %s\n", bits2str(123)printf "0123 = %s\n", bits2str(0123)
printf "0x99 = %s\n", bits2str(0x99)
comp = compl(0x99)
printf "compl(0x99) = %#x = %s\n", comp, bits2str(comp)shift = lshift(0x99, 2)
printf "lshift(0x99, 2) = %#x = %s\n", shift, bits2str(shift)
shift = rshift(0x99, 2)
printf "rshift(0x99, 2) = %#x = %s\n", shift, bits2str(shift)}This program produces the following output when run:$ gawk -f testbits.awk
a 123 = 01111011
a 0123 = 01010011
a 0x99 = 10011001
a compl(0x99) = 0x3fffffffffff66 =a 00111111111111111111111111111111111111111111111101100110
a lshift(0x99, 2) = 0x264 = 0000001001100100
a rshift(0x99, 2) = 0x26 = 00100110
The bits2str() function turns a binary number into a string. Initializing mask to one
creates a binary value where the rightmost bit is set to one. Using this mask, the function
repeatedly checks the rightmost bit. ANDing the mask with the value indicates whether
the rightmost bit is one or not. If so, a "1" is concatenated onto the front of the string.

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Otherwise, a "0" is added. The value is then shifted right by one bit and the loop continues
until there are no more one bits.
If the initial value is zero, it returns a simple "0". Otherwise, at the end, it pads the value
with zeros to represent multiples of 8-bit quantities. This is typical in modern computers.
The main code in the BEGIN rule shows the difference between the decimal and octal values for the same numbers (see Section 6.1.1.2 [Octal and Hexadecimal Numbers], page 120),and then demonstrates the results of the compl(), lshift(), and rshift() functions.


Beware The Smoke and Mirrors!
It other languages, bitwise operations are performed on integer values, not floatingpoint values. As a general statement, such operations work best when performed on
unsigned integers.
gawk attempts to treat the arguments to the bitwise functions as unsigned integers.
For this reason, negative arguments produce a fatal error.
In normal operation, for all of these functions, first the double-precision floating-pointvalue is converted to the widest C unsigned integer type, then the bitwise operation is
performed. If the result cannot be represented exactly as a C double, leading nonzero
bits are removed one by one until it can be represented exactly. The result is then converted
back into a C double.17
However, when using arbitrary precision arithmetic with the -M option (see Chapter 16[Arithmetic and Arbitrary-Precision Arithmetic with gawk], page 389), the results may
differ. This is particularly noticeable with the compl() function:$ gawk 'BEGIN { print compl(42) }'a 9007199254740949$ gawk -M 'BEGIN { print compl(42) }'a -43
What’s going on becomes clear when printing the results in hexadecimal:$ gawk 'BEGIN { printf "%#x\n", compl(42) }'a 0x1fffffffffffd5$ gawk -M 'BEGIN { printf "%#x\n", compl(42) }'a 0xffffffffffffffd5
When using the -M option, under the hood, gawk uses GNU MP arbitrary precision
integers which have at least 64 bits of precision. When not using -M, gawk stores integral
values in regular double-precision floating point, which only maintain 53 bits of precision.
Furthermore, the GNU MP library treats (or at least seems to treat) the leading bit as asign bit; thus the result with -M in this case is a negative number.
In short, using gawk for any but the simplest kind of bitwise operations is probably a
bad idea; caveat emptor!

17

If you don’t understand this paragraph, the upshot is that gawk can only store a particular range of
integer values; numbers outside that range are reduced to fit within the range.

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9.1.8 Getting Type Information
gawk provides two functions that let you distinguish the type of a variable. This is necessary
for writing code that traverses every element of an array of arrays (see Section 8.6 [Arraysof Arrays], page 189), and in other contexts.
isarray(x)Return a true value if x is an array. Otherwise, return false.
typeof(x)Return one of the following strings, depending upon the type of x:"array"x is an array.

"regexp"

x is a strongly typed regexp (see Section 6.1.2.2 [Strongly Typed
Regexp Constants], page 123).

"number"

x is a number.

"number|bool"x is a Boolean typed value (see Section 12.2 [Boolean Typed Values], page 336).
"string"x is a string.

"strnum"x is a number that started life as user input, such as a field or the
result of calling split(). (I.e., x has the strnum attribute; see
Section 6.3.2.1 [String Type versus Numeric Type], page 135.)"unassigned"
x is a scalar variable that has not been assigned a value yet. For
example:BEGIN {# creates a[1] but it has no assigned valuea[1]print typeof(a[1]) # unassigned}"untyped"x has not yet been used yet at all; it can become a scalar or anarray. The typing could even conceivably differ from run to run of
the same program! For example:
BEGIN {
print "initially, typeof(v) = ", typeof(v)if ("FOO" in ENVIRON)
make_scalar(v)else
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function make_scalar(p,l) { l = p }function make_array(p) { p[1] = 1 }isarray() is meant for use in two circumstances. The first is when traversing a multidimensional array: you can test if an element is itself an array or not. The second is
inside the body of a user-defined function (not discussed yet; see Section 9.2 [User-Defined
Functions], page 221), to test if a parameter is an array or not.
NOTE: While you can use isarray() at the global level to test variables,
doing so makes no sense. Because you are the one writing the program, you
are supposed to know if your variables are arrays or not.
The typeof() function is general; it allows you to determine if a variable or function
parameter is a scalar (number, string, or strongly typed regexp) or an array.
Normally, passing a variable that has never been used to a built-in function causes it
to become a scalar variable (unassigned). However, isarray() and typeof() are different;
they do not change their arguments from untyped to unassigned.
This applies to both variables denoted by simple identifiers and array elements that comeinto existence simply by referencing them. Consider:$ gawk 'BEGIN { print typeof(x) }'a untyped$ gawk 'BEGIN { print typeof(x["foo"]) }'a untyped
Note that prior to version 5.2, array elements that come into existence simply by referencing them were different, they were automatically forced to be scalars:$ gawk-5.1.1 'BEGIN { print typeof(x) }'a untyped$ gawk-5.1.1 'BEGIN { print typeof(x["foo"]) }'a unassigned

9.1.9 String-Translation Functions
gawk provides facilities for internationalizing awk programs. These include the functions
described in the following list. The descriptions here are purposely brief. See Chapter 13[Internationalization with gawk], page 357, for the full story. Optional parameters are
enclosed in square brackets ([ ]):bindtextdomain(directory [, domain])
Set the directory in which gawk will look for message translation files, in casethey will not or cannot be placed in the “standard” locations (e.g., during
testing). It returns the directory in which domain is “bound.”
The default domain is the value of TEXTDOMAIN. If directory is the null string(""), then bindtextdomain() returns the current binding for the given domain.
dcgettext(string [, domain [, category] ])Return the translation of string in text domain domain for locale category
category. The default value for domain is the current value of TEXTDOMAIN.
The default value for category is "LC_MESSAGES".

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dcngettext(string1, string2, number [, domain [, category] ])Return the plural form used for number of the translation of string1 and string2
in text domain domain for locale category category. string1 is the English
singular variant of a message, and string2 is the English plural variant of thesame message. The default value for domain is the current value of TEXTDOMAIN.
The default value for category is "LC_MESSAGES".

9.2 User-Defined Functions
Complicated awk programs can often be simplified by defining your own functions. Userdefined functions can be called just like built-in ones (see Section 6.4 [Function Calls],
page 142), but it is up to you to define them (i.e., to tell awk what they should do).

9.2.1 Function Definition Syntax
It’s entirely fair to say that the awk syntax for local variable definitions is appallingly awful.
—Brian Kernighan
Definitions of functions can appear anywhere between the rules of an awk program.
Thus, the general form of an awk program is extended to include sequences of rules anduser-defined function definitions. There is no need to put the definition of a function before
all uses of the function. This is because awk reads the entire program before starting to
execute any of it.
The definition of a function named name looks like this:function name([parameter-list]){body-of-function}
Here, name is the name of the function to define. A valid function name is like a valid
variable name: a sequence of letters, digits, and underscores that doesn’t start with a digit.
Here too, only the 52 upper- and lowercase English letters may be used in a function name.
Within a single awk program, any particular name can only be used as a variable, array, orfunction.
parameter-list is an optional list of the function’s arguments and local variable names,
separated by commas. When the function is called, the argument names are used to holdthe argument values given in the call.
A function cannot have two parameters with the same name, nor may it have a parameterwith the same name as the function itself.
CAUTION: According to the POSIX standard, function parameters cannot
have the same name as one of the special predefined variables (see Section 7.5[Predefined Variables], page 163), nor may a function parameter have the same
name as another function.
Not all versions of awk enforce these restrictions. gawk always enforces the first
restriction. With --posix (see Section 2.2 [Command-Line Options], page 33),
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Local variables act like the empty string if referenced where a string value is required, andlike zero if referenced where a numeric value is required. This is the same as the behaviorof regular variables that have never been assigned a value. (There is more to understandabout local variables; see Section 9.2.5 [Functions and Their Effects on Variable Typing],
page 230.)
The body-of-function consists of awk statements. It is the most important part of thedefinition, because it says what the function should actually do. The argument names exist
to give the body a way to talk about the arguments; local variables exist to give the bodyplaces to keep temporary values.
Argument names are not distinguished syntactically from local variable names. Instead,the number of arguments supplied when the function is called determines how many argument variables there are. Thus, if three argument values are given, the first three names inparameter-list are arguments and the rest are local variables.
It follows that if the number of arguments is not the same in all calls to the function,some of the names in parameter-list may be arguments on some occasions and local variables
on others. Another way to think of this is that omitted arguments default to the null string.
Usually when you write a function, you know how many names you intend to use forarguments and how many you intend to use as local variables. It is conventional to placesome extra space between the arguments and the local variables, in order to document how
your function is supposed to be used.
During execution of the function body, the arguments and local variable values hide, or
shadow, any variables of the same names used in the rest of the program. The shadowed
variables are not accessible in the function definition, because there is no way to name themwhile their names have been taken away for the arguments and local variables. All othervariables used in the awk program can be referenced or set normally in the function’s body.
The arguments and local variables last only as long as the function body is executing.
Once the body finishes, you can once again access the variables that were shadowed while
the function was running.
The function body can contain expressions that call functions. They can even call thisfunction, either directly or by way of another function. When this happens, we say thefunction is recursive. The act of a function calling itself is called recursion.
All the built-in functions return a value to their caller. User-defined functions can do so
also, using the return statement, which is described in detail in Section 9.2.4 [The return
Statement], page 229. Many of the subsequent examples in this section use the returnstatement.
In many awk implementations, including gawk, the keyword function may be abbreviated func. (c.e.) However, POSIX only specifies the use of the keyword function.
This actually has some practical implications. If gawk is in POSIX-compatibility mode(see Section 2.2 [Command-Line Options], page 33), then the following statement does notdefine a function:
func foo() { a = sqrt($1) ; print a }Instead, it defines a rule that, for each record, concatenates the value of the variable ‘func’with the return value of the function ‘foo’. If the resulting string is non-null, the action

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is executed. This is probably not what is desired. (awk accepts this input as syntacticallyvalid, because functions may be used before they are defined in awk programs.18 )To ensure that your awk programs are portable, always use the keyword function whendefining a function.

9.2.2 Function Definition Examples
Here is an example of a user-defined function, called myprint(), that takes a number andprints it in a specific format:
function myprint(num){printf "%6.3g\n", num}
To illustrate, here is an awk rule that uses our myprint() function:$3 > 0{ myprint($3) }This program prints, in our special format, all the third fields that contain a positive number
in our input. Therefore, when given the following input:
1.2
3.4
5.6
7.8
9.10 11.12 -13.14 15.16
17.18 19.20 21.22 23.24
this program, using our function to format the results, prints:
5.6
21.2
This function deletes all the elements in an array (recall that the extra whitespacesignifies the start of the local variable list):function delarray(a,i){
for (i in a)delete a[i]}When working with arrays, it is often necessary to delete all the elements in an array andstart over with a new list of elements (see Section 8.4 [The delete Statement], page 186).
Instead of having to repeat this loop everywhere that you need to clear out an array,
your program can just call delarray(). (This guarantees portability. The use of ‘delete
array’ to delete the contents of an entire array is a relatively recent19 addition to the POSIX
standard.)
The following is an example of a recursive function. It takes a string as an input parameter and returns the string in reverse order. Recursive functions must always have atest that stops the recursion. In this case, the recursion terminates when the input stringis already empty:function rev(str){18
19This program won’t actually run, because foo() is undefined.
Late in 2012.

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if (str == "")return ""return (rev(substr(str, 2)) substr(str, 1, 1))}
If this function is in a file named rev.awk, it can be tested this way:
$ echo "Don't Panic!" |> gawk -e '{ print rev($0) }' -f rev.awk
a !cinaP t'noD
The C ctime() function takes a timestamp and returns it as a string, formatted ina well-known fashion. The following example uses the built-in strftime() function (seeSection 9.1.6 [Time Functions], page 211) to create an awk version of ctime():# ctime.awk## awk version of C ctime(3) function
function ctime(ts,format){format = "%a %b %e %H:%M:%S %Z %Y"
if (ts == 0)
ts = systime()# use current time as default
return strftime(format, ts)}
You might think that ctime() could use PROCINFO["strftime"] for its format string.
That would be a mistake, because ctime() is supposed to return the time formatted in astandard fashion, and user-level code could have changed PROCINFO["strftime"].

9.2.3 Calling User-Defined Functions
Calling a function means causing the function to run and do its job. A function call is anexpression and its value is the value returned by the function.

9.2.3.1 Writing a Function Call
A function call consists of the function name followed by the arguments in parentheses. awk
expressions are what you write in the call for the arguments. Each time the call is executed,
these expressions are evaluated, and the values become the actual arguments. For example,
here is a call to foo() with three arguments (the first being a string concatenation):foo(x y, "lose", 4 * z)
CAUTION: Whitespace characters (spaces and TABs) are not allowed between
the function name and the opening parenthesis of the argument list. If you
write whitespace by mistake, awk might think that you mean to concatenate avariable with an expression in parentheses. However, it notices that you useda function name and not a variable name, and reports an error.

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9.2.3.2 Controlling Variable Scope
Unlike in many languages, there is no way to make a variable local to a { . . . } block in
awk, but you can make a variable local to a function. It is good practice to do so whenevera variable is needed only in that function.
To make a variable local to a function, simply declare the variable as an argument after
the actual function arguments (see Section 9.2.1 [Function Definition Syntax], page 221).
Look at the following example, where variable i is a global variable used by both functions
foo() and bar():function bar(){for (i = 0; i < 3; i++)print "bar's i=" i}function foo(j){i = j + 1
print "foo's i=" ibar()print "foo's i=" i}
BEGIN {
i = 10
print "top's i=" ifoo(0)
print "top's i=" i}Running this script produces the following, because the i in functions foo() and bar()and at the top level refer to the same variable instance:
top's i=10
foo's i=1
bar's i=0
bar's i=1
bar's i=2
foo's i=3
top's i=3
If you want i to be local to both foo() and bar(), do as follows (the extra space before
i is a coding convention to indicate that i is a local variable, not an argument):
function bar(i){
for (i = 0; i < 3; i++)print "bar's i=" i}226

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function foo(j,i){
i = j + 1
print "foo's i=" ibar()print "foo's i=" i}
BEGIN {
i = 10
print "top's i=" ifoo(0)
print "top's i=" i}Running the corrected script produces the following:
top's i=10
foo's i=1
bar's i=0
bar's i=1
bar's i=2
foo's i=1
top's i=10
Besides scalar values (strings and numbers), you may also have local arrays. By using
a parameter name as an array, awk treats it as an array, and it is local to the function. In
addition, recursive calls create new arrays. Consider this example:
function some_func(p1,a){
if (p1++ > 3)
returna[p1] = p1
some_func(p1)printf("At level %d, index %d %s found in a\n",p1, (p1 - 1), (p1 - 1) in a ? "is" : "is not")printf("At level %d, index %d %s found in a\n",p1, p1, p1 in a ? "is" : "is not")print ""}BEGIN {
some_func(1)}When run, this program produces the following output:
At level 4, index 3 is not found in a

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At level 4, index 4 is found in a
At level 3, index 2 is not found in a
At level 3, index 3 is found in a
At level 2, index 1 is not found in a
At level 2, index 2 is found in a

9.2.3.3 Passing Function Arguments by Value Or by Reference
In awk, when you declare a function, there is no way to declare explicitly whether thearguments are passed by value or by reference.
Instead, the passing convention is determined at runtime when the function is called,according to the following rule: if the argument is an array variable, then it is passed byreference. Otherwise, the argument is passed by value.
Passing an argument by value means that when a function is called, it is given a copy ofthe value of this argument. The caller may use a variable as the expression for the argument,
but the called function does not know this—it only knows what value the argument had.
For example, if you write the following code:
foo = "bar"z = myfunc(foo)then you should not think of the argument to myfunc() as being “the variable foo.” Instead,think of the argument as the string value "bar". If the function myfunc() alters the valuesof its local variables, this has no effect on any other variables. Thus, if myfunc() does this:function myfunc(str){print str
str = "zzz"print str}
to change its first argument variable str, it does not change the value of foo in the caller.
The role of foo in calling myfunc() ended when its value ("bar") was computed. If str
also exists outside of myfunc(), the function body cannot alter this outer value, because it
is shadowed during the execution of myfunc() and cannot be seen or changed from there.
However, when arrays are the parameters to functions, they are not copied. Instead, thearray itself is made available for direct manipulation by the function. This is usually termedcall by reference. Changes made to an array parameter inside the body of a function are
visible outside that function.
NOTE: Changing an array parameter inside a function can be very dangerous
if you do not watch what you are doing. For example:
function changeit(array, ind, nvalue){
array[ind] = nvalue}
BEGIN {

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printf "a[1] = %s, a[2] = %s, a[3] = %s\n",a[1], a[2], a[3]}prints ‘a[1] = 1, a[2] = two, a[3] = 3’, because changeit() stores "two" inthe second element of a.

9.2.3.4 Other Points About Calling Functions
Some awk implementations allow you to call a function that has not been defined. They
only report a problem at runtime, when the program actually tries to call the function. For
example:BEGIN {
if (0)
foo()
else
bar()
}
function bar() { ... }# note that `foo' is not defined
Because the ‘if’ statement will never be true, it is not really a problem that foo() has notbeen defined. Usually, though, it is a problem if a program calls an undefined function.
If --lint is specified (see Section 2.2 [Command-Line Options], page 33), gawk reports
calls to undefined functions.
Some awk implementations generate a runtime error if you use either the next statement
or the nextfile statement (see Section 7.4.8 [The next Statement], page 160, and see
Section 7.4.9 [The nextfile Statement], page 161) inside a user-defined function. gawk
does not have this limitation.
You can call a function and pass it more parameters than it was declared with, like so:function foo(p1, p2){...
}
BEGIN {
foo(1, 2, 3, 4)}
Doing so is bad practice, however. The called function cannot do anything with theadditional values being passed to it, so awk evaluates the expressions but then just throws
them away.
More importantly, such a call is confusing for whoever will next read your program.20
Function parameters generally are input items that influence the computation performed
20

Said person might even be you, sometime in the future, at which point you will wonder, “what was I
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by the function. Calling a function with more parameters than it accepts gives the false
impression that those values are important to the function, when in fact they are not.
Because this is such a bad practice, gawk unconditionally issues a warning whenever it
executes such a function call. (If you don’t like the warning, fix your code! It’s incorrect,
after all.)

9.2.4 The return Statement
As seen in several earlier examples, the body of a user-defined function can contain a returnstatement. This statement returns control to the calling part of the awk program. It canalso be used to return a value for use in the rest of the awk program. It looks like this:return [expression]
The expression part is optional. Due most likely to an oversight, POSIX does not define
what the return value is if you omit the expression. Technically speaking, this makes thereturned value undefined, and therefore, unpredictable. In practice, though, all versions of
awk simply return the null string, which acts like zero if used in a numeric context.
A return statement without an expression is assumed at the end of every function
definition. So, if control reaches the end of the function body, then technically the function
returns an unpredictable value. In practice, it returns the empty string. awk does not warn
you if you use the return value of such a function.
Sometimes, you want to write a function for what it does, not for what it returns. Sucha function corresponds to a void function in C, C++, or Java, or to a procedure in Ada.
Thus, it may be appropriate to not return any value; simply bear in mind that you should
not be using the return value of such a function.
The following is an example of a user-defined function that returns a value for the largest
number among the elements of an array:function maxelt(vec,i, ret){for (i in vec) {if (ret == "" || vec[i] > ret)
ret = vec[i]}return ret}
You call maxelt() with one argument, which is an array name. The local variables i andret are not intended to be arguments; there is nothing to stop you from passing more than
one argument to maxelt() but the results would be strange. The extra space before i in
the function parameter list indicates that i and ret are local variables. You should follow
this convention when defining functions.
The following program uses the maxelt() function. It loads an array, calls maxelt(),
and then reports the maximum number in that array:function maxelt(vec,i, ret){for (i in vec) {if (ret == "" || vec[i] > ret)230

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ret = vec[i]}return ret}# Load all fields of each record into nums.
{
for(i = 1; i <= NF; i++)nums[NR, i] = $i}
END {print maxelt(nums)}
Given the following input:
1 5 23 8 16
44 3 5 2 8 26
256 291 1396 2962 100-6 467 998 1101
99385 11 0 225
the program reports (predictably) that 99,385 is the largest value in the array.

9.2.5 Functions and Their Effects on Variable Typing
It’s a desert topping!
It’s a floor wax!
—Saturday Night Live (back when it used to be funny)awk is a very fluid language. It is possible that awk can’t tell if an identifier representsa scalar variable or an array until runtime. Here is an annotated sample program:function foo(a){a[1] = 1# parameter is an array}
BEGIN {b = 1
foo(b)
foo(x)
x = 1# invalid: fatal type mismatch# x uninitialized, becomes an array dynamically# now not allowed, runtime error}
In this example, the first call to foo() generates a fatal error, so awk will not report thesecond error. If you comment out that call, though, then awk does report the second error.
Here is a more extreme example:
BEGIN {
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if (A == 0)
print "<" a ">"else
print a[1]}function funky(arr){if (A == 0)
arr = 1else
arr[1] = 1}
Here, the function uses its parameter differently depending upon the value of the global
variable A. If A is zero, the parameter arr is treated as a scalar. Otherwise it’s treated as
an array.
There are two ways this program might behave. awk could notice that in the main
program, a is subscripted, and so mark it as an array before the program even begins torun. BWK awk, mawk, and possibly others do this:$ nawk -v A=0 -f funky.awk
error nawk: can't assign to a; it's an array name.
source line number 11
error$ nawk -v A=1 -f funky.awk
a 1
Or awk could wait until runtime to set the type of a. In this case, since a was never used
before being passed to the function, how the function uses it forces the type to be resolved
to either scalar or array. gawk and the MKS awk do this:$ gawk -v A=0 -f funky.awk
a <>$ gawk -v A=1 -f funky.awk
a 1
POSIX does not specify the correct behavior, so be aware that different implementations
work differently.

9.3 Indirect Function Calls
This section describes an advanced, gawk-specific extension.
Often, you may wish to defer the choice of function to call until runtime. For example,
you may have different kinds of records, each of which should be processed differently.
Normally, you would have to use a series of if-else statements to decide which function
to call. By using indirect function calls, you can specify the name of the function to call asa string variable, and then call the function. Let’s look at an example.
Suppose you have a file with your test scores for the classes you are taking, and you wish
to get the sum and the average of your test scores. The first field is the class name. The

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following fields are the functions to call to process the data, up to a “marker” field ‘data:’.
Following the marker, to the end of the record, are the various numeric test scores.
Here is the initial file:
Biology_101 sum average data: 87.0 92.4 78.5 94.9
Chemistry_305 sum average data: 75.2 98.3 94.7 88.2
English_401 sum average data: 100.0 95.6 87.1 93.4
To process the data, you might write initially:{class = $1
for (i = 2; $i != "data:"; i++) {if ($i == "sum")sum()# processes the whole record
else if ($i == "average")average()...
# and so on}}
This style of programming works, but can be awkward. With indirect function calls, youtell gawk to use the value of a variable as the name of the function to call.
The syntax is similar to that of a regular function call: an identifier immediately followedby an opening parenthesis, any arguments, and then a closing parenthesis, with the additionof a leading ‘@’ character:the_function = "sum"result = @the_function()# calls the sum() function
Here is a full program that processes the previously shown data, using indirect function
calls:# indirectcall.awk --- Demonstrate indirect function calls# average --- return the average of the values in fields $first - $last
function average(first, last,sum, i){sum = 0;for (i = first; i <= last; i++)sum += $i
return sum / (last - first + 1)}
# sum --- return the sum of the values in fields $first - $last
function sum(first, last,ret, i){ret = 0;for (i = first; i <= last; i++)Chapter 9: Functions

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ret += $i
return ret}
These two functions expect to work on fields; thus, the parameters first and lastindicate where in the fields to start and end. Otherwise, they perform the expected computations and are not unusual:# For each record, print the class name and the requested statistics{
class_name = $1
gsub(/_/, " ", class_name) # Replace _ with spaces# find startfor (i = 1; i <= NF; i++) {if ($i == "data:") {start = i + 1
break
}
}
printf("%s:\n", class_name)for (i = 2; $i != "data:"; i++) {the_function = $i
printf("\t%s: <%s>\n", $i, @the_function(start, NF) "")}print ""}
This is the main processing for each record. It prints the class name (with underscores
replaced with spaces). It then finds the start of the actual data, saving it in start. The
last part of the code loops through each function name (from $2 up to the marker, ‘data:’),calling the function named by the field. The indirect function call itself occurs as a parameter in the call to printf. (The printf format string uses ‘%s’ as the format specifier sothat we can use functions that return strings, as well as numbers. Note that the result fromthe indirect call is concatenated with the empty string, in order to force it to be a stringvalue.)
Here is the result of running the program:$ gawk -f indirectcall.awk class_data1
a Biology 101:sum: <352.8>
aaverage: <88.2>
aaa Chemistry 305:
sum: <356.4>
aaverage: <89.1>
aaa English 401:

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average: <94.025>

The ability to use indirect function calls is more powerful than you may think at first.
The C and C++ languages provide “function pointers,” which are a mechanism for calling afunction chosen at runtime. One of the most well-known uses of this ability is the C qsort()
function, which sorts an array using the famous “quicksort” algorithm (see the Wikipedia
article for more information). To use this function, you supply a pointer to a comparison
function. This mechanism allows you to sort arbitrary data in an arbitrary fashion.
We can do something similar using gawk, like this:# quicksort.awk --- Quicksort algorithm, with user-supplied#comparison function# quicksort --- C.A.R. Hoare's quicksort algorithm. See Wikipedia#
or almost any algorithms or computer science text.
function quicksort(data, left, right, less_than,i, last){if (left >= right) # do nothing if array contains fewer
return# than two elements
quicksort_swap(data, left, int((left + right) / 2))
last = leftfor (i = left + 1; i <= right; i++)if (@less_than(data[i], data[left]))quicksort_swap(data, ++last, i)quicksort_swap(data, left, last)
quicksort(data, left, last - 1, less_than)quicksort(data, last + 1, right, less_than)}# quicksort_swap --- helper function for quicksort, should really be inline
function quicksort_swap(data, i, j,{
temp = data[i]data[i] = data[j]data[j] = temp
}

temp)

The quicksort() function receives the data array, the starting and ending indices tosort (left and right), and the name of a function that performs a “less than” comparison.
It then implements the quicksort algorithm.
To make use of the sorting function, we return to our previous example. The first thing
to do is write some comparison functions:Chapter 9: Functions

235# num_lt --- do a numeric less than comparison
function num_lt(left, right){
return ((left + 0) < (right + 0))}# num_ge --- do a numeric greater than or equal to comparison
function num_ge(left, right){
return ((left + 0) >= (right + 0))}
The num_ge() function is needed to perform a descending sort; when used to perform a“less than” test, it actually does the opposite (greater than or equal to), which yields data
sorted in descending order.
Next comes a sorting function. It is parameterized with the starting and ending
field numbers and the comparison function. It builds an array with the data and calls
quicksort() appropriately, and then formats the results as a single string:# do_sort --- sort the data according to `compare'#and return it as a string
function do_sort(first, last, compare,{delete datafor (i = 1; first <= last; first++) {data[i] = $firsti++}data, i, retval)

quicksort(data, 1, i-1, compare)retval = data[1]for (i = 2; i in data; i++)retval = retval " " data[i]return retval}
Finally, the two sorting functions call do_sort(), passing in the names of the two comparison functions:# sort --- sort the data in ascending order and return it as a string
function sort(first, last){
return do_sort(first, last, "num_lt")}

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GAWK: Effective AWK Programming# rsort --- sort the data in descending order and return it as a string
function rsort(first, last){
return do_sort(first, last, "num_ge")}
Here is an extended version of the data file:
Biology_101 sum average sort rsort data: 87.0 92.4 78.5 94.9
Chemistry_305 sum average sort rsort data: 75.2 98.3 94.7 88.2
English_401 sum average sort rsort data: 100.0 95.6 87.1 93.4
Finally, here are the results when the enhanced program is run:$ gawk -f quicksort.awk -f indirectcall.awk class_data2
a Biology 101:sum: <352.8>
aaverage: <88.2>
asort: <78.5 87.0 92.4 94.9>
arsort: <94.9 92.4 87.0 78.5>aaa Chemistry 305:
sum: <356.4>
aaverage: <89.1>
asort: <75.2 88.2 94.7 98.3>
arsort: <98.3 94.7 88.2 75.2>aaa English 401:
sum: <376.1>
aaverage: <94.025>
asort: <87.1 93.4 95.6 100.0>arsort: <100.0 95.6 93.4 87.1>
aAnother example where indirect functions calls are useful can be found in processing
arrays. This is described in Section 10.7 [Traversing Arrays of Arrays], page 274.
Remember that you must supply a leading ‘@’ in front of an indirect function call.
Starting with version 4.1.2 of gawk, indirect function calls may also be used with builtin functions and with extension functions (see Chapter 17 [Writing Extensions for gawk],
page 405). There are some limitations when calling built-in functions indirectly, as follows.
• You cannot pass a regular expression constant to a built-in function through an indirectfunction call. This applies to the sub(), gsub(), gensub(), match(), split() andpatsplit() functions. However, you can pass a strongly typed regexp constant (seeSection 6.1.2.2 [Strongly Typed Regexp Constants], page 123).
• If calling sub() or gsub(), you may only pass two arguments, since those functions are
unusual in that they update their third argument. This means that $0 will be updated.
• You cannot indirectly call built-in functions that can take $0 as a default parameter;
you must supply an argument instead. For example, you must pass an argument tolength() if calling it indirectly.

Chapter 9: Functions

237• Calling a built-in function indirectly with the wrong number of arguments for thatfunction causes a fatal error. For example, calling length() with two arguments.
These errors are found at runtime instead of when gawk parses your program, since
gawk doesn’t know until runtime if you have passed the correct number of arguments
or not.
gawk does its best to make indirect function calls efficient. For example, in the followingcase:for (i = 1; i <= n; i++)@the_function()gawk looks up the actual function to call only once.

9.4 Summary• awk provides built-in functions and lets you define your own functions.
• POSIX awk provides three kinds of built-in functions: numeric, string, and I/O. gawk
provides functions that sort arrays, work with values representing time, do bit manipulation, determine variable type (array versus scalar), and internationalize and localize
programs. gawk also provides several extensions to some of standard functions, typically in the form of additional arguments.
• Functions accept zero or more arguments and return a value. The expressions thatprovide the argument values are completely evaluated before the function is called.
Order of evaluation is not defined. The return value can be ignored.
• The handling of backslash in sub() and gsub() is not simple. It is more straightforward
in gawk’s gensub() function, but that function still requires care in its use.
• User-defined functions provide important capabilities but come with some syntactic
inelegancies. In a function call, there cannot be any space between the function nameand the opening left parenthesis of the argument list. Also, there is no provision for
local variables, so the convention is to add extra parameters, and to separate themvisually from the real parameters by extra whitespace.
• User-defined functions may call other user-defined (and built-in) functions and may callthemselves recursively. Function parameters “hide” any global variables of the samenames. You cannot use the name of a reserved variable (such as ARGC) as the name ofa parameter in user-defined functions.
• Scalar values are passed to user-defined functions by value. Array parameters are
passed by reference; any changes made by the function to array parameters are thus
visible after the function has returned.
• Use the return statement to return from a user-defined function. An optional expression becomes the function’s return value. Only scalar values may be returned by afunction.
• If a variable that has never been used is passed to a user-defined function, how thatfunction treats the variable can set its nature: either scalar or array.
• gawk provides indirect function calls using a special syntax. By setting a variable tothe name of a function, you can determine at runtime what function will be called atthat point in the program. This is equivalent to function pointers in C and C++.

Part II:
Problem Solving with awk

Chapter 10: A Library of awk Functions 241

10 A Library of awk Functions
Section 9.2 [User-Defined Functions], page 221, describes how to write your own awk functions. Writing functions is important, because it allows you to encapsulate algorithms andprogram tasks in a single place. It simplifies programming, making program development
more manageable and making programs more readable.
In their seminal 1976 book, Software Tools,1 Brian Kernighan and P.J. Plauger wrote:
Good Programming is not learned from generalities, but by seeing how significant programs can be made clean, easy to read, easy to maintain and modify,
human-engineered, efficient and reliable, by the application of common sense
and good programming practices. Careful study and imitation of good programs leads to better writing.
In fact, they felt this idea was so important that they placed this statement on the coverof their book. Because we believe strongly that their statement is correct, this chapter andChapter 11 [Practical awk Programs], page 279, provide a good-sized body of code for you
to read and, we hope, to learn from.
This chapter presents a library of useful awk functions. Many of the sample programs
presented later in this book use these functions. The functions are presented here in a
progression from simple to complex.
Section 11.3.7 [Extracting Programs from Texinfo Source Files], page 317, presents a
program that you can use to extract the source code for these example library functions
and programs from the Texinfo source for this book. (This has already been done as partof the gawk distribution.)
If you have written one or more useful, general-purpose awk functions and would liketo contribute them to the awk user community, see [How to Contribute], page 11, for more
information.
The programs in this chapter and in Chapter 11 [Practical awk Programs], page 279,
freely use gawk-specific features. Rewriting these programs for different implementations of
awk is pretty straightforward:• Diagnostic error messages are sent to /dev/stderr. Use ‘| "cat 1>&2"’ instead of ‘>"/dev/stderr"’ if your system does not have a /dev/stderr, or if you cannot use
gawk.
• Finally, some of the programs choose to ignore upper- and lowercase distinctions intheir input. They do so by assigning one to IGNORECASE. You can achieve almost thesame effect2 by adding the following rule to the beginning of the program:# ignore case
{ $0 = tolower($0) }Also, verify that all regexp and string constants used in comparisons use only lowercase
letters.
1

Sadly, over 35 years later, many of the lessons taught by this book have yet to be learned by a vast
number of practicing programmers.
2
The effects are not identical. Output of the transformed record will be in all lowercase, while IGNORECASE
preserves the original contents of the input record.

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10.1 Naming Library Function Global Variables
Due to the way the awk language evolved, variables are either global (usable by the entire program) or local (usable just by a specific function). There is no intermediate stateanalogous to static variables in C.
Library functions often need to have global variables that they can use to preserve state
information between calls to the function—for example, getopt()’s variable _opti (seeSection 10.4 [Processing Command-Line Options], page 259). Such variables are called
private, as the only functions that need to use them are the ones in the library.
When writing a library function, you should try to choose names for your private variables that will not conflict with any variables used by either another library function or auser’s main program. For example, a name like i or j is not a good choice, because user
programs often use variable names like these for their own purposes.
The example programs shown in this chapter all start the names of their private variables
with an underscore (‘_’). Users generally don’t use leading underscores in their variablenames, so this convention immediately decreases the chances that the variable names will
be accidentally shared with the user’s program.
In addition, several of the library functions use a prefix that helps indicate what function
or set of functions use the variables—for example, _pw_byname() in the user databaseroutines (see Section 10.5 [Reading the User Database], page 266). This convention isrecommended, as it even further decreases the chance of inadvertent conflict among variablenames. Note that this convention is used equally well for variable names and for private
function names.3
As a final note on variable naming, if a function makes global variables available for use
by a main program, it is a good convention to start those variables’ names with a capitalletter—for example, getopt()’s Opterr and Optind variables (see Section 10.4 [Processing
Command-Line Options], page 259). The leading capital letter indicates that it is global,
while the fact that the variable name is not all capital letters indicates that the variable is
not one of awk’s predefined variables, such as FS.
It is also important that all variables in library functions that do not need to save state
are, in fact, declared local.4 If this is not done, the variables could accidentally be used in
the user’s program, leading to bugs that are very difficult to track down:function lib_func(x, y,
l1, l2){...
# some_var should be local but by oversight is notuse variable some_var...
}
A different convention, common in the Tcl community, is to use a single associativearray to hold the values needed by the library function(s), or “package.” This significantly
3

Although all the library routines could have been rewritten to use this convention, this was not done,in order to show how our own awk programming style has evolved and to provide some basis for this
discussion.
4
gawk’s --dump-variables command-line option is useful for verifying this.

Chapter 10: A Library of awk Functions 243

decreases the number of actual global names in use. For example, the functions describedin Section 10.5 [Reading the User Database], page 266, might have used array elements
PW_data["inited"], PW_data["total"], PW_data["count"], and PW_data["awklib"],
instead of _pw_inited, _pw_awklib, _pw_total, and _pw_count.
The conventions presented in this section are exactly that: conventions. You are notrequired to write your programs this way—we merely recommend that you do so.
Beginning with version 5.0, gawk provides a powerful mechanism for solving the problems
described in this section: namespaces. Namespaces and their use are described in detail in
Chapter 15 [Namespaces in gawk], page 383.

10.2 General Programming
This section presents a number of functions that are of general programming use.

10.2.1 Converting Strings to Numbers
The strtonum() function (see Section 9.1.4 [String-Manipulation Functions], page 196) isa gawk extension. The following function provides an implementation for other versions ofawk:# mystrtonum --- convert string to number
function mystrtonum(str,ret, n, i, k, c){
if (str ~ /^0[0-7]*$/) {# octaln = length(str)ret = 0for (i = 1; i <= n; i++) {c = substr(str, i, 1)# index() returns 0 if c not in string,# includes c == "0"k = index("1234567", c)ret = ret * 8 + k}} else if (str ~ /^0[xX][[:xdigit:]]+$/) {# hexadecimal
str = substr(str, 3)# lop off leading 0x
n = length(str)ret = 0for (i = 1; i <= n; i++) {c = substr(str, i, 1)
c = tolower(c)# index() returns 0 if c not in string,# includes c == "0"k = index("123456789abcdef", c)244

GAWK: Effective AWK Programming

ret = ret * 16 + k}} else if (str ~ \/^[-+]?([0-9]+([.][0-9]*([Ee][0-9]+)?)?|([.][0-9]+([Ee][-+]?[0-9]+)?))$/) {# decimal number, possibly floating point
ret = str + 0} elseret = "NOT-A-NUMBER"
return ret}# BEGIN {# gawk test harness#
a[1] = "25"#a[2] = ".31"#
a[3] = "0123"#
a[4] = "0xdeadBEEF"#
a[5] = "123.45"#
a[6] = "1.e3"#
a[7] = "1.32"#
a[8] = "1.32E2"##
for (i = 1; i in a; i++)#print a[i], strtonum(a[i]), mystrtonum(a[i])# }The function first looks for C-style octal numbers (base 8). If the input string matches aregular expression describing octal numbers, then mystrtonum() loops through each character in the string. It sets k to the index in "1234567" of the current octal digit. The return
value will either be the same number as the digit, or zero if the character is not there, which
will be true for a ‘0’. This is safe, because the regexp test in the if ensures that only octal
values are converted.
Similar logic applies to the code that checks for and converts a hexadecimal value, whichstarts with ‘0x’ or ‘0X’. The use of tolower() simplifies the computation for finding thecorrect numeric value for each hexadecimal digit.
Finally, if the string matches the (rather complicated) regexp for a regular decimal integeror floating-point number, the computation ‘ret = str + 0’ lets awk convert the value to anumber.
A commented-out test program is included, so that the function can be tested with gawk
and the results compared to the built-in strtonum() function.

10.2.2 Assertions
When writing large programs, it is often useful to know that a condition or set of conditions
is true. Before proceeding with a particular computation, you make a statement about what
you believe to be the case. Such a statement is known as an assertion. The C language
provides an <assert.h> header file and corresponding assert() macro that a programmer
can use to make assertions. If an assertion fails, the assert() macro arranges to print a

Chapter 10: A Library of awk Functions 245

diagnostic message describing the condition that should have been true but was not, andthen it kills the program. In C, using assert() looks this:#include <assert.h>
int myfunc(int a, double b){
assert(a <= 5 && b >= 17.1);...
}
If the assertion fails, the program prints a message similar to this:prog.c:5: assertion failed: a <= 5 && b >= 17.1
The C language makes it possible to turn the condition into a string for use in printing
the diagnostic message. This is not possible in awk, so this assert() function also requiresa string version of the condition that is being tested. Following is the function:# assert --- assert that a condition is true. Otherwise, exit.
function assert(condition, string){if (! condition) {printf("%s:%d: assertion failed: %s\n",FILENAME, FNR, string) > "/dev/stderr"_assert_exit = 1
exit 1}}END {if (_assert_exit)
exit 1}
The assert() function tests the condition parameter. If it is false, it prints a message
to standard error, using the string parameter to describe the failed condition. It then sets
the variable _assert_exit to one and executes the exit statement. The exit statement
jumps to the END rule. If the END rule finds _assert_exit to be true, it exits immediately.
The purpose of the test in the END rule is to keep any other END rules from running.
When an assertion fails, the program should exit immediately. If no assertions fail, then_assert_exit is still false when the END rule is run normally, and the rest of the program’s
END rules execute. For all of this to work correctly, assert.awk must be the first source file
read by awk. The function can be used in a program in the following way:function myfunc(a, b){
assert(a <= 5 && b >= 17.1, "a <= 5 && b >= 17.1")
...
}

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If the assertion fails, you see a message similar to the following:
mydata:1357: assertion failed: a <= 5 && b >= 17.1
There is a small problem with this version of assert(). An END rule is automatically
added to the program calling assert(). Normally, if a program consists of just a BEGIN
rule, the input files and/or standard input are not read. However, now that the program has
an END rule, awk attempts to read the input data files or standard input (see Section 7.1.4.1[Startup and Cleanup Actions], page 150), most likely causing the program to hang as it
waits for input.
There is a simple workaround to this: make sure that such a BEGIN rule always ends
with an exit statement.

10.2.3 Rounding Numbers
The way printf and sprintf() (see Section 5.5 [Using printf Statements for Fancier
Printing], page 102) perform rounding often depends upon the system’s C sprintf() subroutine. On many machines, sprintf() rounding is unbiased, which means it doesn’t
always round a trailing .5 up, contrary to naive expectations. In unbiased rounding, .5
rounds to even, rather than always up, so 1.5 rounds to 2 but 4.5 rounds to 4. This meansthat if you are using a format that does rounding (e.g., "%.0f"), you should check what
your system does. The following function does traditional rounding; it might be useful if
your awk’s printf does unbiased rounding:# round.awk --- do normal rounding
function round(x,{ival = int(x)ival, aval, fraction)# integer part, int() truncates# see if fractional partif (ival == x)# no fraction
return ival
# ensure no decimals
if (x < 0) {aval = -x# absolute value
ival = int(aval)
fraction = aval - ival
if (fraction >= .5)
return int(x) - 1# -2.5 --> -3
else
return int(x)# -2.3 --> -2} else {fraction = x - ival
if (fraction >= .5)
return ival + 1
else
return ival}}

Chapter 10: A Library of awk Functions 247# test harness# { print $0, round($0) }10.2.4 The Cliff Random Number Generator
The Cliff random number generator is a very simple random number generator that “passes
the noise sphere test for randomness by showing no structure.” It is easily programmed, inless than 10 lines of awk code:# cliff_rand.awk --- generate Cliff random numbers
BEGIN { _cliff_seed = 0.1 }function cliff_rand(){_cliff_seed = (100 * log(_cliff_seed)) % 1
if (_cliff_seed < 0)
_cliff_seed = - _cliff_seed
return _cliff_seed}
This algorithm requires an initial “seed” of 0.1. Each new value uses the current seed
as input for the calculation. If the built-in rand() function (see Section 9.1.3 [Numeric
Functions], page 194) isn’t random enough, you might try using this function instead.

10.2.5 Translating Between Characters and Numbers
One commercial implementation of awk supplies a built-in function, ord(), which takes acharacter and returns the numeric value for that character in the machine’s character set.
If the string passed to ord() has more than one character, only the first one is used.
The inverse of this function is chr() (from the function of the same name in Pascal),which takes a number and returns the corresponding character. Both functions are writtenvery nicely in awk; there is no real reason to build them into the awk interpreter:# ord.awk --- do ord and chr# Global identifiers:#_ord_:
numerical values indexed by characters#_ord_init:
function to initialize _ord_
BEGIN{ _ord_init() }function _ord_init(
low, high, i, t){
low = sprintf("%c", 7) # BEL is ascii 7
if (low == "\a") {# regular ascii
low = 0high = 127} else if (sprintf("%c", 128 + 7) == "\a") {# ascii, mark parity
low = 128

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high = 255} else {low = 0high = 255}# ebcdic(!)

for (i = low; i <= high; i++) {t = sprintf("%c", i)_ord_[t] = i}}
Some explanation of the numbers used by _ord_init() is worthwhile. The most prominent character set in use today is ASCII.5 Although an 8-bit byte can hold 256 distinct
values (from 0 to 255), ASCII only defines characters that use the values from 0 to 127.6 In
the now distant past, at least one minicomputer manufacturer used ASCII, but with markparity, meaning that the leftmost bit in the byte is always 1. This means that on thosesystems, characters have numeric values from 128 to 255. Finally, large mainframe systems
use the EBCDIC character set, which uses all 256 values. There are other character sets in
use on some older systems, but they are not really worth worrying about:function ord(str,c){# only first character is of interestc = substr(str, 1, 1)
return _ord_[c]}function chr(c){# force c to be numeric by adding 0
return sprintf("%c", c + 0)}#### test code ##### BEGIN {#for (;;) {#printf("enter a character: ")#if (getline var <= 0)
#
break
#
printf("ord(%s) = %d\n", var, ord(var))
#
}
# }
5

This is changing; many systems use Unicode, a very large character set that includes ASCII as a subset.
On systems with full Unicode support, a character can occupy up to 32 bits, making simple tests suchas used here prohibitively expensive.
6
ASCII has been extended in many countries to use the values from 128 to 255 for country-specificcharacters. If your system uses these extensions, you can simplify _ord_init() to loop from 0 to 255.

Chapter 10: A Library of awk Functions 249

An obvious improvement to these functions is to move the code for the _ord_init function into the body of the BEGIN rule. It was written this way initially for ease of development.
There is a “test program” in a BEGIN rule, to test the function. It is commented out for
production use.

10.2.6 Merging an Array into a String
When doing string processing, it is often useful to be able to join all the strings in an array
into one long string. The following function, join(), accomplishes this task. It is used laterin several of the application programs (see Chapter 11 [Practical awk Programs], page 279).
Good function design is important; this function needs to be general, but it should also
have a reasonable default behavior. It is called with an array as well as the beginningand ending indices of the elements in the array to be merged. This assumes that the arrayindices are numeric—a reasonable assumption, as the array was likely created with split()(see Section 9.1.4 [String-Manipulation Functions], page 196):# join.awk --- join an array into a string
function join(array, start, end, sep,
result, i){if (sep == "")sep = " "else if (sep == SUBSEP) # magic value
sep = ""
result = array[start]for (i = start + 1; i <= end; i++)result = result sep array[i]return result}
An optional additional argument is the separator to use when joining the strings back
together. If the caller supplies a nonempty value, join() uses it; if it is not supplied, it hasa null value. In this case, join() uses a single space as a default separator for the strings.
If the value is equal to SUBSEP, then join() joins the strings with no separator between
them. SUBSEP serves as a “magic” value to indicate that there should be no separation
between the component strings.7

10.2.7 Managing the Time of Day
The systime() and strftime() functions described in Section 9.1.6 [Time Functions],
page 211, provide the minimum functionality necessary for dealing with the time of day
in human-readable form. Although strftime() is extensive, the control formats are notnecessarily easy to remember or intuitively obvious when reading a program.
The following function, getlocaltime(), populates a user-supplied array with preformatted time information. It returns a string with the current time formatted in the same
way as the date utility:# getlocaltime.awk --- get the time of day in a usable format
7

It would be nice if awk had an assignment operator for concatenation. The lack of an explicit operator
for concatenation makes string operations more difficult than they really need to be.

250

GAWK: Effective AWK Programming# Returns a string in the format of output of date(1)# Populates the array argument time with individual values:#time["second"]-- seconds (0 - 59)#
time["minute"]-- minutes (0 - 59)#
time["hour"]-- hours (0 - 23)#time["althour"]-- hours (0 - 12)#time["monthday"]-- day of month (1 - 31)#time["month"]-- month of year (1 - 12)#time["monthname"]-- name of the month#time["shortmonth"]-- short name of the month#time["year"]-- year modulo 100 (0 - 99)#
time["fullyear"]-- full year#
time["weekday"]-- day of week (Sunday = 0)#time["altweekday"]-- day of week (Monday = 0)#time["dayname"]-- name of weekday#
time["shortdayname"] -- short name of weekday#
time["yearday"]-- day of year (0 - 365)#
time["timezone"]-- abbreviation of timezone name#time["ampm"]-- AM or PM designation#time["weeknum"]-- week number, Sunday first day#
time["altweeknum"]-- week number, Monday first day
function getlocaltime(time,ret, now, i){# get time once, avoids unnecessary system calls
now = systime()# return date(1)-style output
ret = strftime("%a %b %e %H:%M:%S %Z %Y", now)
# clear out target array
delete time
# fill in values, force numeric values to be# numeric by adding 0
time["second"]= strftime("%S", now) + 0
time["minute"]= strftime("%M", now) + 0
time["hour"]= strftime("%H", now) + 0
time["althour"]= strftime("%I", now) + 0
time["monthday"]= strftime("%d", now) + 0
time["month"]= strftime("%m", now) + 0
time["monthname"]= strftime("%B", now)time["shortmonth"]= strftime("%b", now)time["year"]= strftime("%y", now) + 0
time["fullyear"]= strftime("%Y", now) + 0
time["weekday"]= strftime("%w", now) + 0

Chapter 10: A Library of awk Functions 251

time["altweekday"]= strftime("%u", now) + 0
time["dayname"]= strftime("%A", now)time["shortdayname"] = strftime("%a", now)time["yearday"]= strftime("%j", now) + 0
time["timezone"]= strftime("%Z", now)time["ampm"]= strftime("%p", now)time["weeknum"]= strftime("%U", now) + 0
time["altweeknum"]= strftime("%W", now) + 0
return ret}
The string indices are easier to use and read than the various formats required bystrftime(). The alarm program presented in Section 11.3.2 [An Alarm Clock Program],
page 308, uses this function. A more general design for the getlocaltime() function would
have allowed the user to supply an optional timestamp value to use instead of the current
time.

10.2.8 Reading a Whole File at Once
Often, it is convenient to have the entire contents of a file available in memory as a single
string. A straightforward but naive way to do that might be as follows:
function readfile1(file,tmp, contents){
if ((getline tmp < file) < 0)returncontents = tmp RT
while ((getline tmp < file) > 0)contents = contents tmp RT
close(file)
return contents}
This function reads from file one record at a time, building up the full contents of thefile in the local variable contents. It works, but is not necessarily efficient.
The following function, based on a suggestion by Denis Shirokov, reads the entire contents of the named file in one shot:# readfile.awk --- read an entire file at once
function readfile(file,{save_rs = RS
RS = "^$"
getline tmp < fileclose(file)
RS = save_rs

tmp, save_rs)

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return tmp}
It works by setting RS to ‘^$’, a regular expression that will never match if the file hascontents. gawk reads data from the file into tmp, attempting to match RS. The match fails
after each read, but fails quickly, such that gawk fills tmp with the entire contents of the
file. (See Section 4.1 [How Input Is Split into Records], page 65, for information on RT andRS.)
In the case that file is empty, the return value is the null string. Thus, calling code
may use something like:
contents = readfile("/some/path")
if (length(contents) == 0)# file was empty ...
This tests the result to see if it is empty or not. An equivalent test would be‘contents == ""’.
See Section 17.7.10 [Reading an Entire File], page 466, for an extension function that
also reads an entire file into memory.

10.2.9 Quoting Strings to Pass to the Shell
Michael Brennan offers the following programming pattern, which he uses frequently:#! /bin/sh
awkp='...
'
input_program | awk "$awkp" | /bin/shFor example, a program of his named flac-edit has this form:$ flac-edit -song="Whoope! That's Great" file.flac
It generates the following output, which is to be piped to the shell (/bin/sh):chmod +w file.flac
metaflac --remove-tag=TITLE file.flac
LANG=en_US.88591 metaflac --set-tag=TITLE='Whoope! That'"'"'s Great' file.flac
chmod -w file.flac
Note the need for shell quoting. The function shell_quote() does it. SINGLE is the
one-character string "'" and QSINGLE is the three-character string "\"'\"":# shell_quote --- quote an argument for passing to the shellfunction shell_quote(s,SINGLE, QSINGLE, i, X, n, ret){if (s == "")return "\"\""
SINGLE = "\x27"# single quote# parameter# locals

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QSINGLE = "\"\x27\""n = split(s, X, SINGLE)
ret = SINGLE X[1] SINGLE
for (i = 2; i <= n; i++)ret = ret QSINGLE SINGLE X[i] SINGLE
return ret}

10.2.10 Checking Whether A Value Is Numeric
A frequent programming question is how to ascertain whether a value is numeric. Thiscan be solved by using this example function isnumeric(), which employs the trick ofconverting a string value to user input by using the split() function:# isnumeric --- check whether a value is numericfunction isnumeric(x, f){switch (typeof(x)) {case "strnum":case "number":
return 1
case "string":
return (split(x, f, " ") == 1) && (typeof(f[1]) == "strnum")
default:
return 0}}
Please note that leading or trailing white space is disregarded in deciding whether avalue is numeric or not, so if it matters to you, you may want to add an additional checkfor that.
Traditionally, it has been recommended to check for numeric values using the test ‘x+0== x’. This function is superior in two ways: it will not report that unassigned variables
contain numeric values; and it recognizes string values with numeric contents where CONVFMT
does not yield the original string. On the other hand, it uses the typeof() function (seeSection 9.1.8 [Getting Type Information], page 219), which is specific to gawk.

10.2.11 Producing CSV Data
gawk’s --csv option causes gawk to process CSV data (see Section 4.5.4 [Working With
Comma Separated Value Files], page 76).
But what if you have regular data that you want to output in CSV format? This section
provides functions for doing that.
The first function, tocsv(), takes an array of data fields as input. The array should beindexed starting from one. The optional second parameter is the separator to use. If none
is supplied, the default is a comma.

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The function takes care to quote fields that contain double quotes, newlines, or the
separator character. It then builds up the final CSV record and returns it.
# tocsv.awk --- convert data to CSV format
function tocsv(fields, sep,{
if (length(fields) == 0)
return ""i, j, nfields, result)if (sep == "")sep = ","
delete nfieldsfor (i = 1; i in fields; i++) {nfields[i] = fields[i]if (nfields[i] ~ /["\n]/ || index(nfields[i], sep) != 0) {gsub(/"/, "\"\"", nfields[i])# double up quotes
nfields[i] = "\"" nfields[i] "\""# wrap in quotes}}
result = nfields[1]j = length(nfields)for (i = 2; i <= j; i++)result = result sep nfields[i]return result}
The next function, tocsv_rec() is a wrapper around tocsv(). Its intended use is forwhen you want to convert the current input record to CSV format. The function itself
simply copies the fields into an array to pass to tocsv() which does the work. It accepts anoptional separator character as its first parameter, which it simply passes on to tocsv().
function tocsv_rec(sep,i, fields){
delete fieldsfor (i = 1; i <= NF; i++)fields[i] = $i
return tocsv(fields, sep)}10.3 Data file Management
This section presents functions that are useful for managing command-line data files.

10.3.1 Noting Data file Boundaries
The BEGIN and END rules are each executed exactly once, at the beginning and end ofyour awk program, respectively (see Section 7.1.4 [The BEGIN and END Special Patterns],

Chapter 10: A Library of awk Functions 255

page 150). We (the gawk authors) once had a user who mistakenly thought that the BEGIN
rules were executed at the beginning of each data file and the END rules were executed at
the end of each data file.
When informed that this was not the case, the user requested that we add new special
patterns to gawk, named BEGIN_FILE and END_FILE, that would have the desired behavior.
He even supplied us the code to do so.
Adding these special patterns to gawk wasn’t necessary; the job can be done cleanly
in awk itself, as illustrated by the following library program. It arranges to call two usersupplied functions, beginfile() and endfile(), at the beginning and end of each data
file. Besides solving the problem in only nine(!) lines of code, it does so portably; this works
with any implementation of awk:
# transfile.awk## Give the user a hook for filename transitions## The user must supply functions beginfile() and endfile()# that each take the name of the file being started or# finished, respectively.
FILENAME != _oldfilename {if (_oldfilename != "")endfile(_oldfilename)_oldfilename = FILENAME
beginfile(FILENAME)}
END { endfile(FILENAME) }This file must be loaded before the user’s “main” program, so that the rule it supplies
is executed first.
This rule relies on awk’s FILENAME variable, which automatically changes for each newdata file. The current file name is saved in a private variable, _oldfilename. If FILENAME
does not equal _oldfilename, then a new data file is being processed and it is necessary tocall endfile() for the old file. Because endfile() should only be called if a file has beenprocessed, the program first checks to make sure that _oldfilename is not the null string.
The program then assigns the current file name to _oldfilename and calls beginfile()
for the file. Because, like all awk variables, _oldfilename is initialized to the null string,
this rule executes correctly even for the first data file.
The program also supplies an END rule to do the final processing for the last file. Because
this END rule comes before any END rules supplied in the “main” program, endfile() is calledfirst. Once again, the value of multiple BEGIN and END rules should be clear.
If the same data file occurs twice in a row on the command line, then endfile() andbeginfile() are not executed at the end of the first pass and at the beginning of the second
pass. The following version solves the problem:# ftrans.awk --- handle datafile transitions#

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FNR == 1 {if (_filename_ != "")endfile(_filename_)_filename_ = FILENAME
beginfile(FILENAME)}
END { endfile(_filename_) }Section 11.2.7 [Counting Things], page 304, shows how this library function can be usedand how it simplifies writing the main program.


So Why Does gawk Have BEGINFILE and ENDFILE?
You are probably wondering, if beginfile() and endfile() functions can do the job,
why does gawk have BEGINFILE and ENDFILE patterns?
Good question. Normally, if awk cannot open a file, this causes an immediate fatal
error. In this case, there is no way for a user-defined function to deal with the problem, as
the mechanism for calling it relies on the file being open and at the first record. Thus, themain reason for BEGINFILE is to give you a “hook” to catch files that cannot be processed.
ENDFILE exists for symmetry, and because it provides an easy way to do per-file cleanup
processing. For more information, refer to Section 7.1.5 [The BEGINFILE and ENDFILE
Special Patterns], page 151.

10.3.2 Rereading the Current File
Another request for a new built-in function was for a function that would make it possible
to reread the current file. The requesting user didn’t want to have to use getline (seeSection 4.10 [Explicit Input with getline], page 89) inside a loop.
However, as long as you are not in the END rule, it is quite easy to arrange to immediately
close the current input file and then start over with it from the top. For lack of a better
name, we’ll call the function rewind():# rewind.awk --- rewind the current file and start overfunction rewind(i){# shift remaining arguments up
for (i = ARGC; i > ARGIND; i--)
ARGV[i] = ARGV[i-1]# make sure gawk knows to keep going
ARGC++# make current file next to get done
ARGV[ARGIND+1] = FILENAME

Chapter 10: A Library of awk Functions 257# do itnextfile}
The rewind() function relies on the ARGIND variable (see Section 7.5.2 [Built-in Variables That Convey Information], page 165), which is specific to gawk. It also relies on the
nextfile keyword (see Section 7.4.9 [The nextfile Statement], page 161). Because of this,you should not call it from an ENDFILE rule. (This isn’t necessary anyway, because gawk
goes to the next file as soon as an ENDFILE rule finishes!)
You need to be careful calling rewind(). You can end up causing infinite recursion if
you don’t pay attention. Here is an example use:
$ cat data
a a
a b
a c
a d
a e
$ cat test.awk
a FNR == 3 && ! rewound {rewound = 1
arewind()aa }aa { print FILENAME, FNR, $0 }$ gawk -f rewind.awk -f test.awk dataa data 1 a
a data 2 b
a data 1 a
a data 2 b
a data 3 c
a data 4 d
a data 5 e

10.3.3 Checking for Readable Data files
Normally, if you give awk a data file that isn’t readable, it stops with a fatal error. There
are times when you might want to just ignore such files and keep going.8 You can do thisby prepending the following program to your awk program:# readable.awk --- library file to skip over unreadable files
BEGIN {for (i = 1; i < ARGC; i++) {8

The BEGINFILE special pattern (see Section 7.1.5 [The BEGINFILE and ENDFILE Special Patterns],
page 151) provides an alternative mechanism for dealing with files that can’t be opened. However, thecode here provides a portable solution.

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if (ARGV[i] ~ /^[a-zA-Z_][a-zA-Z0-9_]*=.*/ \|| ARGV[i] == "-" || ARGV[i] == "/dev/stdin")continue# assignment or standard input
else if ((getline junk < ARGV[i]) < 0) # unreadable
delete ARGV[i]elseclose(ARGV[i])}}This works, because the getline won’t be fatal. Removing the element from ARGV withdelete skips the file (because it’s no longer in the list). See also Section 7.5.3 [Using ARGC
and ARGV], page 173.
Because awk variable names only allow the English letters, the regular expression
check purposely does not use character classes such as ‘[:alpha:]’ and ‘[:alnum:]’ (seeSection 3.4 [Using Bracket Expressions], page 58).

10.3.4 Checking for Zero-Length FilesAll known awk implementations silently skip over zero-length files. This is a by-productof awk’s implicit read-a-record-and-match-against-the-rules loop: when awk tries to read arecord from an empty file, it immediately receives an end-of-file indication, closes the file,and proceeds on to the next command-line data file, without executing any user-level awk
program code.
Using gawk’s ARGIND variable (see Section 7.5 [Predefined Variables], page 163), it ispossible to detect when an empty data file has been skipped. Similar to the library file
presented in Section 10.3.1 [Noting Data file Boundaries], page 254, the following library
file calls a function named zerofile() that the user must provide. The arguments passed
are the file name and the position in ARGV where it was found:# zerofile.awk --- library file to process empty input files
BEGIN { Argind = 0 }ARGIND > Argind + 1 {for (Argind++; Argind < ARGIND; Argind++)zerofile(ARGV[Argind], Argind)}
ARGIND != Argind { Argind = ARGIND }END {if (ARGIND > Argind)for (Argind++; Argind <= ARGIND; Argind++)zerofile(ARGV[Argind], Argind)}
The user-level variable Argind allows the awk program to track its progress through
ARGV. Whenever the program detects that ARGIND is greater than ‘Argind + 1’, it means

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that one or more empty files were skipped. The action then calls zerofile() for each such
file, incrementing Argind along the way.
The ‘Argind != ARGIND’ rule simply keeps Argind up to date in the normal case.
Finally, the END rule catches the case of any empty files at the end of the command-line
arguments. Note that the test in the condition of the for loop uses the ‘<=’ operator, not‘<’.

10.3.5 Treating Assignments as File names
Occasionally, you might not want awk to process command-line variable assignments (seeSection 6.1.3.2 [Assigning Variables on the Command Line], page 125). In particular, if you
have a file name that contains an ‘=’ character, awk treats the file name as an assignmentand does not process it.
Some users have suggested an additional command-line option for gawk to disablecommand-line assignments. However, some simple programming with a library file does
the trick:# noassign.awk --- library file to avoid the need for a# special option that disables command-line assignmentsfunction disable_assigns(argc, argv,i){for (i = 1; i < argc; i++)if (argv[i] ~ /^[a-zA-Z_][a-zA-Z0-9_]*=.*/)
argv[i] = ("./" argv[i])}BEGIN {
if (No_command_assign)
disable_assigns(ARGC, ARGV)}
You then run your program this way:
awk -v No_command_assign=1 -f noassign.awk -f yourprog.awk *The function works by looping through the arguments. It prepends ‘./’ to any argumentthat matches the form of a variable assignment, turning that argument into a file name.
The use of No_command_assign allows you to disable command-line assignments at invocation time, by giving the variable a true value. When not set, it is initially zero (i.e.,
false), so the command-line arguments are left alone.

10.4 Processing Command-Line Options
Most utilities on POSIX-compatible systems take options on the command line that canbe used to change the way a program behaves. awk is an example of such a program (seeSection 2.2 [Command-Line Options], page 33). Often, options take arguments (i.e., datathat the program needs to correctly obey the command-line option). For example, awk’s -F
option requires a string to use as the field separator. The first occurrence on the commandline of either -- or a string that does not begin with ‘-’ ends the options.

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Modern Unix systems provide a C function named getopt() for processing commandline arguments. The programmer provides a string describing the one-letter options. If
an option requires an argument, it is followed in the string with a colon. getopt() isalso passed the count and values of the command-line arguments and is called in a loop.
getopt() processes the command-line arguments for option letters. Each time around theloop, it returns a single character representing the next option letter that it finds, or ‘?’ if
it finds an invalid option. When it returns −1, there are no options left on the commandline.
When using getopt(), options that do not take arguments can be grouped together.
Furthermore, options that take arguments require that the argument be present. The
argument can immediately follow the option letter, or it can be a separate command-line
argument.
Given a hypothetical program that takes three command-line options, -a, -b, and -c,
where -b requires an argument, all of the following are valid ways of invoking the program:prog -a -b foo -c data1 data2 data3
prog -ac -bfoo -- data1 data2 data3
prog -acbfoo data1 data2 data3
Notice that when the argument is grouped with its option, the rest of the argument is
considered to be the option’s argument. In this example, -acbfoo indicates that all of the-a, -b, and -c options were supplied, and that ‘foo’ is the argument to the -b option.
getopt() provides four external variables that the programmer can use:
optindThe index in the argument value array (argv) where the first nonoptioncommand-line argument can be found.

optarg

The string value of the argument to an option.

opterrUsually getopt() prints an error message when it finds an invalid option. Setting opterr to zero disables this feature. (An application might want to print
its own error message.)

optopt

The letter representing the command-line option.

The following C fragment shows how getopt() might process command-line arguments
for awk:
int
main(int argc, char *argv[]){...
/* print our own message */opterr = 0;
while ((c = getopt(argc, argv, "v:f:F:W:")) != -1) {switch (c) {case 'f':/* file */...
break;
case 'F':/* field separator */...

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break;
case 'v':/* variable assignment */...
break;
case 'W':/* extension */...
break;
case '?':
default:
usage();
break;}}...
}The GNU project’s version of the original Unix utilities popularized the use of longcommand line options. For example, --help in addition to -h. Arguments to long options
are either provided as separate command line arguments (‘--source 'program-text'’) or
separated from the option with an ‘=’ sign (‘--source='program-text'’).
As a side point, gawk actually uses the GNU getopt_long() function to process bothnormal and GNU-style long options (see Section 2.2 [Command-Line Options], page 33).
The abstraction provided by getopt() is very useful and is quite handy in awk programs
as well. Following is an awk version of getopt() that accepts both short and long options.
(Support for long options was supplied by Greg Minshall. We thank him.)
This function highlights one of the greatest weaknesses in awk, which is that it is very
poor at manipulating single characters. The function needs repeated calls to substr()
in order to access individual characters (see Section 9.1.4 [String-Manipulation Functions],
page 196).9The discussion that follows walks through the code a bit at a time:# getopt.awk --- Do C library getopt(3) function in awk#
Also supports long options.
# External variables:#
Optind -- index in ARGV of first nonoption argument#
Optarg -- string value of argument to current option#
Opterr -- if nonzero, print our own diagnostic#
Optopt -- current option letter# Returns:
#
-1
#
"?"
#


9

at end of options
for unrecognized option
a string representing the current option

This function was written before gawk acquired the ability to split strings into single characters using ""as the separator. We have left it alone, as using substr() is more portable.

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GAWK: Effective AWK Programming# Private Data:#_opti -- index in multiflag option, e.g., -abc
The function starts out with comments presenting a list of the global variables it uses,
what the return values are, what they mean, and any global variables that are “private” tothis library function. Such documentation is essential for any program, and particularly forlibrary functions.
The getopt() function first checks that it was indeed called with a string of options(the options parameter). If both options and longoptions have a zero length, getopt()
immediately returns −1:function getopt(argc, argv, options, longopts,
thisopt, i, j){
if (length(options) == 0 && length(longopts) == 0)
return -1# no options given
if (argv[Optind] == "--") { # all done
Optind++_opti = 0
return -1} else if (argv[Optind] !~ /^-[^:[:space:]]/) {_opti = 0
return -1}
The next thing to check for is the end of the options. A -- ends the command-line
options, as does any command-line argument that does not begin with a ‘-’ (unless it isan argument to a preceding option). Optind steps through the array of command-line
arguments; it retains its value across calls to getopt(), because it is a global variable.
The regular expression /^-[^:[:space:]/ checks for a ‘-’ followed by anything that is
not whitespace and not a colon. If the current command-line argument does not match thispattern, it is not an option, and it ends option processing. Now, we check to see if we areprocessing a short (single letter) option, or a long option (indicated by two dashes, e.g.,‘--filename’). If it is a short option, we continue on:if (argv[Optind] !~ /^--/) {# if this is a short option
if (_opti == 0)_opti = 2
thisopt = substr(argv[Optind], _opti, 1)
Optopt = thisopt
i = index(options, thisopt)
if (i == 0) {if (Opterr)
printf("%c -- invalid option\n", thisopt) > "/dev/stderr"if (_opti >= length(argv[Optind])) {Optind++_opti = 0
} else_opti++

Chapter 10: A Library of awk Functions 263

return "?"}The _opti variable tracks the position in the current command-line argument(argv[Optind]). If multiple options are grouped together with one ‘-’ (e.g., -abx), it isnecessary to return them to the user one at a time.
If _opti is equal to zero, it is set to two, which is the index in the string of the next
character to look at (we skip the ‘-’, which is at position one). The variable thisopt holds
the character, obtained with substr(). It is saved in Optopt for the main program to use.
If thisopt is not in the options string, then it is an invalid option. If Opterr is nonzero,
getopt() prints an error message on the standard error that is similar to the message fromthe C version of getopt().
Because the option is invalid, it is necessary to skip it and move on to the next option
character. If _opti is greater than or equal to the length of the current command-line
argument, it is necessary to move on to the next argument, so Optind is incremented and_opti is reset to zero. Otherwise, Optind is left alone and _opti is merely incremented.
In any case, because the option is invalid, getopt() returns "?". The main program can
examine Optopt if it needs to know what the invalid option letter actually is. Continuingon:
if (substr(options, i + 1, 1) == ":") {# get option argument
if (length(substr(argv[Optind], _opti + 1)) > 0)
Optarg = substr(argv[Optind], _opti + 1)
else
Optarg = argv[++Optind]_opti = 0
} elseOptarg = ""If the option requires an argument, the option letter is followed by a colon in theoptions string. If there are remaining characters in the current command-line argument(argv[Optind]), then the rest of that string is assigned to Optarg. Otherwise, the next
command-line argument is used (‘-xFOO’ versus ‘-x FOO’). In either case, _opti is reset
to zero, because there are no more characters left to examine in the current command-line
argument. Continuing:
if (_opti == 0 || _opti >= length(argv[Optind])) {Optind++_opti = 0
} else_opti++
return thisopt
Finally, for a short option, if _opti is either zero or greater than the length of thecurrent command-line argument, it means this element in argv is through being processed,
so Optind is incremented to point to the next element in argv. If neither condition is true,
then only _opti is incremented, so that the next option letter can be processed on the next
call to getopt().

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On the other hand, if the earlier test found that this was a long option, we take a different
branch:} else {j = index(argv[Optind], "=")
if (j > 0)
thisopt = substr(argv[Optind], 3, j - 3)
else
thisopt = substr(argv[Optind], 3)
Optopt = thisopt
First, we search this option for a possible embedded equal sign, as the specification of longoptions allows an argument to an option ‘--someopt’ to be specified as ‘--someopt=answer’
as well as ‘--someopt answer’.
i = match(longopts, "(^|,)" thisopt "($|[,:])")
if (i == 0) {if (Opterr)
printf("%s -- invalid option\n", thisopt) > "/dev/stderr"
Optind++return "?"}Next, we try to find the current option in longopts. The regular expression given to
match(), "(^|,)" thisopt "($|[,:])", matches this option at the beginning of longopts,
or at the beginning of a subsequent long option (the previous long option would have been
terminated by a comma), and, in any case, either at the end of the longopts string (‘$’), or
followed by a comma (separating this option from a subsequent option) or a colon (indicating
this long option takes an argument (‘[,:]’).
Using this regular expression, we check to see if the current option might possibly bein longopts (if longopts is not specified, this test will also fail). In case of an error, wepossibly print an error message and then return "?". Continuing on:
if (substr(longopts, i-1+RLENGTH, 1) == ":") {if (j > 0)
Optarg = substr(argv[Optind], j + 1)
else
Optarg = argv[++Optind]} elseOptarg = ""We now check to see if this option takes an argument and, if so, we set Optarg to thevalue of that argument (either a value after an equal sign specified on the command line,
immediately adjoining the long option string, or as the next argument on the commandline).
Optind++return thisopt}}

Chapter 10: A Library of awk Functions 265

We increase Optind (which we already increased once if a required argument was separated from its option by an equal sign), and return the long option (minus its leading
dashes).
The BEGIN rule initializes both Opterr and Optind to one. Opterr is set to one, because
the default behavior is for getopt() to print a diagnostic message upon seeing an invalid
option. Optind is set to one, because there’s no reason to look at the program name, whichis in ARGV[0]:BEGIN {
Opterr = 1
Optind = 1# default is to diagnose# skip ARGV[0]# test programif (_getopt_test) {_myshortopts = "ab:cd"_mylongopts = "longa,longb:,otherc,otherd"
while ((_go_c = getopt(ARGC, ARGV, _myshortopts, _mylongopts)) != -1)
printf("c = <%s>, Optarg = <%s>\n", _go_c, Optarg)
printf("non-option arguments:\n")for (; Optind < ARGC; Optind++)printf("\tARGV[%d] = <%s>\n", Optind, ARGV[Optind])}}
The rest of the BEGIN rule is a simple test program. Here are the results of some sample
runs of the test program:$ awk -f getopt.awk -v _getopt_test=1 -- -a -cbARG bax -x
a c = <a>, Optarg = <>a c = <c>, Optarg = <>a c = <b>, Optarg = <ARG>
a non-option arguments:
ARGV[3] = <bax>a
ARGV[4] = <-x>a$ awk -f getopt.awk -v _getopt_test=1 -- -a -x -- xyz abc
a c = <a>, Optarg = <>error x -- invalid optiona c = <?>, Optarg = <>a non-option arguments:
ARGV[4] = <xyz>a
ARGV[5] = <abc>a$ awk -f getopt.awk -v _getopt_test=1 -- -a \> --longa -b xx --longb=foo=bar --otherd --otherc arg1 arg2
a c = <a>, Optarg = <>a c = <longa>, Optarg = <>a c = <b>, Optarg = <xx>

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a c = <otherd>, Optarg = <>a c = <otherc>, Optarg = <>a non-option arguments:
ARGV[8] = <arg1>a
ARGV[9] = <arg2>a
In all the runs, the first -- terminates the arguments to awk, so that it does not try tointerpret the -a, etc., as its own options.
NOTE: After getopt() is through, user-level code must clear out all the elements of ARGV from 1 to Optind, so that awk does not try to process thecommand-line options as file names.
Using ‘#!’ with the -E option may help avoid conflicts between your program’s options and gawk’s options, as -E causes gawk to abandon processing of further options (seeSection 1.1.4 [Executable awk Programs], page 19, and see Section 2.2 [Command-Line
Options], page 33).
Several of the sample programs presented in Chapter 11 [Practical awk Programs],
page 279, use getopt() to process their arguments.

10.5 Reading the User Database
The PROCINFO array (see Section 7.5 [Predefined Variables], page 163) provides access tothe current user’s real and effective user and group ID numbers, and, if available, theuser’s supplementary group set. However, because these are numbers, they do not provide
very useful information to the average user. There needs to be some way to find the user
information associated with the user and group ID numbers. This section presents a suiteof functions for retrieving information from the user database. See Section 10.6 [Reading
the Group Database], page 270, for a similar suite that retrieves information from the group
database.
The POSIX standard does not define the file where user information is kept. Instead, it
provides the <pwd.h> header file and several C language subroutines for obtaining user information. The primary function is getpwent(), for “get password entry.” The “password”
comes from the original user database file, /etc/passwd, which stores user information
along with the encrypted passwords (hence the name).
Although an awk program could simply read /etc/passwd directly, this file may notcontain complete information about the system’s set of users.10 To be sure you are able
to produce a readable and complete version of the user database, it is necessary to write asmall C program that calls getpwent(). getpwent() is defined as returning a pointer toa struct passwd. Each time it is called, it returns the next entry in the database. When
there are no more entries, it returns NULL, the null pointer. When this happens, the C
program should call endpwent() to close the database. Following is pwcat, a C programthat “cats” the password database:/** pwcat.c*
10

It is often the case that password information is stored in a network database.

Chapter 10: A Library of awk Functions 267* Generate a printable version of the password database.
*/#include <stdio.h>#include <pwd.h>
int
main(int argc, char **argv){
struct passwd *p;
while ((p = getpwent()) != NULL)
printf("%s:%s:%ld:%ld:%s:%s:%s\n",p->pw_name, p->pw_passwd, (long) p->pw_uid,(long) p->pw_gid, p->pw_gecos, p->pw_dir, p->pw_shell);
endpwent();
return 0;}
If you don’t understand C, don’t worry about it. The output from pwcat is the user
database, in the traditional /etc/passwd format of colon-separated fields. The fields are:
Login name
The user’s login name.
Encrypted password
The user’s encrypted password. This may not be available on some systems.
User-ID

The user’s numeric user ID number. (On some systems, it’s a C long, and not
an int. Thus, we cast it to long for all cases.)Group-ID

The user’s numeric group ID number. (Similar comments about long versus
int apply here.)

Full name The user’s full name, and perhaps other information associated with the user.
Home directory
The user’s login (or “home”) directory (familiar to shell programmers as $HOME).
Login shell
The program that is run when the user logs in. This is usually a shell, such as
Bash.
A few lines representative of pwcat’s output are as follows:$ pwcata root:x:0:1:Operator:/:/bin/sha nobody:*:65534:65534::/:a daemon:*:1:1::/:a sys:*:2:2::/:/bin/csh
a bin:*:3:3::/bin:a arnold:xyzzy:2076:10:Arnold Robbins:/home/arnold:/bin/sha miriam:yxaay:112:10:Miriam Robbins:/home/miriam:/bin/sh

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With that introduction, following is a group of functions for getting user information.
There are several functions here, corresponding to the C functions of the same names:# passwd.awk --- access password file information
BEGIN {
# tailor this to suit your system_pw_awklib = "/usr/local/libexec/awk/"}
function _pw_init({
if (_pw_inited)
returnoldfs, oldrs, olddol0, pwcat, using_fw, using_fpat)

oldfs = FSoldrs = RS
olddol0 = $0
using_fw = (PROCINFO["FS"] == "FIELDWIDTHS")
using_fpat = (PROCINFO["FS"] == "FPAT")
FS = ":"
RS = "\n"pwcat = _pw_awklib "pwcat"
while ((pwcat | getline) > 0) {_pw_byname[$1] = $0
_pw_byuid[$3] = $0
_pw_bycount[++_pw_total] = $0}
close(pwcat)_pw_count = 0_pw_inited = 1
FS = oldfs
if (using_fw)
FIELDWIDTHS = FIELDWIDTHS
else if (using_fpat)
FPAT = FPAT
RS = oldrs$0 = olddol0}
The BEGIN rule sets a private variable to the directory where pwcat is stored.
Because it is used to help out an awk library routine, we have chosen to put it in/usr/local/libexec/awk; however, you might want it to be in a different directory on
your system.

Chapter 10: A Library of awk Functions 269

The function _pw_init() fills three copies of the user information into three associativearrays. The arrays are indexed by username (_pw_byname), by user ID number (_pw_byuid),
and by order of occurrence (_pw_bycount). The variable _pw_inited is used for efficiency,
as _pw_init() needs to be called only once.
Because this function uses getline to read information from pwcat, it first saves thevalues of FS, RS, and $0. It notes in the variable using_fw whether field splitting with
FIELDWIDTHS is in effect or not. Doing so is necessary, as these functions could be calledfrom anywhere within a user’s program, and the user may have his or her own way ofsplitting records and fields. This makes it possible to restore the correct field-splitting
mechanism later. The test can only be true for gawk. It is false if using FS or FPAT, or onsome other awk implementation.
The code that checks for using FPAT, using using_fpat and PROCINFO["FS"], is similar.
The main part of the function uses a loop to read database lines, split the lines into
fields, and then store the lines into each array as necessary. When the loop is done,_pw_init() cleans up by closing the pipeline, setting _pw_inited to one, and restoring
FS (and FIELDWIDTHS or FPAT if necessary), RS, and $0. The use of _pw_count is explained
shortly.
The getpwnam() function takes a username as a string argument. If that user is in the
database, it returns the appropriate line. Otherwise, it relies on the array reference to anonexistent element to create the element with the null string as its value:function getpwnam(name){_pw_init()
return _pw_byname[name]}
Similarly, the getpwuid() function takes a user ID number argument. If that user
number is in the database, it returns the appropriate line. Otherwise, it returns the null
string:function getpwuid(uid){_pw_init()
return _pw_byuid[uid]}The getpwent() function simply steps through the database, one entry at a time. It
uses _pw_count to track its current position in the _pw_bycount array:function getpwent(){_pw_init()
if (_pw_count < _pw_total)
return _pw_bycount[++_pw_count]return ""}The endpwent() function resets _pw_count to zero, so that subsequent calls to
getpwent() start over again:
function endpwent()

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A conscious design decision in this suite is that each subroutine calls _pw_init() toinitialize the database arrays. The overhead of running a separate process to generate theuser database, and the I/O to scan it, are only incurred if the user’s main program actuallycalls one of these functions. If this library file is loaded along with a user’s program, but noneof the routines are ever called, then there is no extra runtime overhead. (The alternative ismove the body of _pw_init() into a BEGIN rule, which always runs pwcat. This simplifies
the code but runs an extra process that may never be needed.)In turn, calling _pw_init() is not too expensive, because the _pw_inited variable keeps
the program from reading the data more than once. If you are worried about squeezing
every last cycle out of your awk program, the check of _pw_inited could be moved out of_pw_init() and duplicated in all the other functions. In practice, this is not necessary, as
most awk programs are I/O-bound, and such a change would clutter up the code.
The id program in Section 11.2.3 [Printing Out User Information], page 288, uses thesefunctions.

10.6 Reading the Group Database
Much of the discussion presented in Section 10.5 [Reading the User Database], page 266,
applies to the group database as well. Although there has traditionally been a well-known
file (/etc/group) in a well-known format, the POSIX standard only provides a set of C
library routines (<grp.h> and getgrent()) for accessing the information. Even though thisfile may exist, it may not have complete information. Therefore, as with the user database,
it is necessary to have a small C program that generates the group database as its output.
grcat, a C program that “cats” the group database, is as follows:/** grcat.c** Generate a printable version of the group database.
*/#include <stdio.h>#include <grp.h>intmain(int argc, char **argv){
struct group *g;
int i;
while ((g = getgrent()) != NULL) {printf("%s:%s:%ld:", g->gr_name, g->gr_passwd,(long) g->gr_gid);for (i = 0; g->gr_mem[i] != NULL; i++) {printf("%s", g->gr_mem[i]);Chapter 10: A Library of awk Functions 271

if (g->gr_mem[i+1] != NULL)
putchar(',');}
putchar('\n');}endgrent();
return 0;}
Each line in the group database represents one group. The fields are separated withcolons and represent the following information:
Group Name
The group’s name.
Group Password
The group’s encrypted password. In practice, this field is never used; it isusually empty or set to ‘*’.
Group ID Number
The group’s numeric group ID number; the association of name to number must
be unique within the file. (On some systems it’s a C long, and not an int.
Thus, we cast it to long for all cases.)Group Member List
A comma-separated list of usernames. These users are members of the group.
Modern Unix systems allow users to be members of several groups simultaneously. If your system does, then there are elements "group1" through "groupN"in PROCINFO for those group ID numbers. (Note that PROCINFO is a gawk extension; see Section 7.5 [Predefined Variables], page 163.)
Here is what running grcat might produce:$ grcata wheel:*:0:arnold
a nogroup:*:65534:
a daemon:*:1:
a kmem:*:2:
a staff:*:10:arnold,miriam,andya other:*:20:...
Here are the functions for obtaining information from the group database. There areseveral, modeled after the C library functions of the same names:# group.awk --- functions for dealing with the group file
BEGIN {
# Change to suit your system_gr_awklib = "/usr/local/libexec/awk/"}
function _gr_init(

oldfs, oldrs, olddol0, grcat,

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using_fw, using_fpat, n, a, i){if (_gr_inited)
returnoldfs = FSoldrs = RS
olddol0 = $0
using_fw = (PROCINFO["FS"] == "FIELDWIDTHS")
using_fpat = (PROCINFO["FS"] == "FPAT")
FS = ":"
RS = "\n"grcat = _gr_awklib "grcat"
while ((grcat | getline) > 0) {if ($1 in _gr_byname)_gr_byname[$1] = _gr_byname[$1] "," $4else_gr_byname[$1] = $0
if ($3 in _gr_bygid)_gr_bygid[$3] = _gr_bygid[$3] "," $4else_gr_bygid[$3] = $0
n = split($4, a, "[ \t]*,[ \t]*")for (i = 1; i <= n; i++)if (a[i] in _gr_groupsbyuser)_gr_groupsbyuser[a[i]] = _gr_groupsbyuser[a[i]] " " $1else
_gr_groupsbyuser[a[i]] = $1
_gr_bycount[++_gr_count] = $0}
close(grcat)_gr_count = 0_gr_inited++
FS = oldfs
if (using_fw)
FIELDWIDTHS = FIELDWIDTHS
else if (using_fpat)
FPAT = FPAT
RS = oldrs$0 = olddol0}
The BEGIN rule sets a private variable to the directory where grcat is stored.
Because it is used to help out an awk library routine, we have chosen to put it inChapter 10: A Library of awk Functions 273/usr/local/libexec/awk. You might want it to be in a different directory on yoursystem.
These routines follow the same general outline as the user database routines (seeSection 10.5 [Reading the User Database], page 266). The _gr_inited variable is used toensure that the database is scanned no more than once. The _gr_init() function firstsaves FS, RS, and $0, and then sets FS and RS to the correct values for scanning the group
information. It also takes care to note whether FIELDWIDTHS or FPAT is being used, and torestore the appropriate field-splitting mechanism.
The group information is stored in several associative arrays. The arrays are indexed
by group name (_gr_byname), by group ID number (_gr_bygid), and by positionin the database (_gr_bycount). There is an additional array indexed by username(_gr_groupsbyuser), which is a space-separated list of groups to which each user belongs.
Unlike in the user database, it is possible to have multiple records in the database forthe same group. This is common when a group has a large number of members. A pair ofsuch entries might look like the following:
tvpeople:*:101:johnny,jay,arsenio
tvpeople:*:101:david,conan,tom,joan
For this reason, _gr_init() looks to see if a group name or group ID number is alreadyseen. If so, the usernames are simply concatenated onto the previous list of users.11
Finally, _gr_init() closes the pipeline to grcat, restores FS (and FIELDWIDTHS or FPAT,
if necessary), RS, and $0, initializes _gr_count to zero (it is used later), and makes _gr_
inited nonzero.
The getgrnam() function takes a group name as its argument, and if that group exists,
it is returned. Otherwise, it relies on the array reference to a nonexistent element to create
the element with the null string as its value:function getgrnam(group){_gr_init()
return _gr_byname[group]}
The getgrgid() function is similar; it takes a numeric group ID and looks up theinformation associated with that group ID:
function getgrgid(gid){_gr_init()
return _gr_bygid[gid]}The getgruser() function does not have a C counterpart. It takes a username andreturns the list of groups that have the user as a member:function getgruser(user){_gr_init()
11

There is a subtle problem with the code just presented. Suppose that the first time there were no names.
This code adds the names with a leading comma. It also doesn’t check that there is a $4.

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return _gr_groupsbyuser[user]}
The getgrent() function steps through the database one entry at a time. It uses _gr_
count to track its position in the list:
function getgrent(){_gr_init()
if (++_gr_count in _gr_bycount)return _gr_bycount[_gr_count]return ""}The endgrent() function resets _gr_count to zero so that getgrent() can start overagain:
function endgrent(){_gr_count = 0}
As with the user database routines, each function calls _gr_init() to initialize thearrays. Doing so only incurs the extra overhead of running grcat if these functions areused (as opposed to moving the body of _gr_init() into a BEGIN rule).
Most of the work is in scanning the database and building the various associative arrays.
The functions that the user calls are themselves very simple, relying on awk’s associativearrays to do work.
The id program in Section 11.2.3 [Printing Out User Information], page 288, uses thesefunctions.

10.7 Traversing Arrays of Arrays
Section 8.6 [Arrays of Arrays], page 189, described how gawk provides arrays of arrays. In
particular, any element of an array may be either a scalar or another array. The isarray()function (see Section 9.1.8 [Getting Type Information], page 219) lets you distinguish anarray from a scalar. The following function, walk_array(), recursively traverses an array,
printing the element indices and values. You call it with the array and a string representing
the name of the array:function walk_array(arr, name,i){for (i in arr) {if (isarray(arr[i]))walk_array(arr[i], (name "[" i "]"))
else
printf("%s[%s] = %s\n", name, i, arr[i])}}It works by looping over each element of the array. If any given element is itself an array,the function calls itself recursively, passing the subarray and a new string representing theChapter 10: A Library of awk Functions 275

current index. Otherwise, the function simply prints the element’s name, index, and value.
Here is a main program to demonstrate:
BEGIN {
a[1] = 1a[2][1] = 21a[2][2] = 22a[3] = 3a[4][1][1] = 411
a[4][2] = 42
walk_array(a, "a")}When run, the program produces the following output:$ gawk -f walk_array.awk
a a[1] = 1
a a[2][1] = 21a a[2][2] = 22a a[3] = 3
a a[4][1][1] = 411
a a[4][2] = 42
The function just presented simply prints the name and value of each scalar array element. However, it is easy to generalize it, by passing in the name of a function to call when
walking an array. The modified function looks like this:function process_array(arr, name, process, do_arrays,
i, new_name){for (i in arr) {new_name = (name "[" i "]")
if (isarray(arr[i])) {if (do_arrays)@process(new_name, arr[i])process_array(arr[i], new_name, process, do_arrays)} else@process(new_name, arr[i])}}The arguments are as follows:arr

The array.

name

The name of the array (a string).

process

The name of the function to call.

do_arrays
If this is true, the function can handle elements that are subarrays.
If subarrays are to be processed, that is done before walking them further.

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When run with the following scaffolding, the function produces the same results as does
the earlier version of walk_array():
BEGIN {
a[1] = 1a[2][1] = 21a[2][2] = 22a[3] = 3a[4][1][1] = 411
a[4][2] = 42
process_array(a, "a", "do_print", 0)}function do_print(name, element){
printf "%s = %s\n", name, element}

10.8 Summary• Reading programs is an excellent way to learn Good Programming. The functions andprograms provided in this chapter and the next are intended to serve that purpose.
• When writing general-purpose library functions, put some thought into how to nameany global variables so that they won’t conflict with variables from a user’s program.
• The functions presented here fit into the following categories:
General problems
Number-to-string conversion, testing assertions, rounding, random number
generation, converting characters to numbers, joining strings, getting easily
usable time-of-day information, and reading a whole file in one shot
Managing data files
Noting data file boundaries, rereading the current file, checking for readable
files, checking for zero-length files, and treating assignments as file names
Processing command-line options
An awk version of the standard C getopt() function
Reading the user and group databases
Two sets of routines that parallel the C library versions
Traversing arrays of arrays
Two functions that traverse an array of arrays to any depth

10.9 Exercises
1. In Section 10.3.4 [Checking for Zero-Length Files], page 258, we presented thezerofile.awk program, which made use of gawk’s ARGIND variable. Can this problem
be solved without relying on ARGIND? If so, how?

Chapter 10: A Library of awk Functions 277

2. As a related challenge, revise that code to handle the case where an intervening value
in ARGV is a variable assignment.

Chapter 11: Practical awk Programs279

11 Practical awk Programs
Chapter 10 [A Library of awk Functions], page 241, presents the idea that reading programsin a language contributes to learning that language. This chapter continues that theme, presenting a potpourri of awk programs for your reading enjoyment. There are three sections.
The first describes how to run the programs presented in this chapter.
The second presents awk versions of several common POSIX utilities. These are programs
that you are hopefully already familiar with, and therefore whose problems are understood.
By reimplementing these programs in awk, you can focus on the awk-related aspects ofsolving the programming problems.
The third is a grab bag of interesting programs. These solve a number of different
data-manipulation and management problems. Many of the programs are short, whichemphasizes awk’s ability to do a lot in just a few lines of code.
Many of these programs use library functions presented in Chapter 10 [A Library of awk
Functions], page 241.

11.1 Running the Example Programs
To run a given program, you would typically do something like this:awk -f program -- options files
Here, program is the name of the awk program (such as cut.awk), options are any commandline options for the program that start with a ‘-’, and files are the actual data files.
If your system supports the ‘#!’ executable interpreter mechanism (see Section 1.1.4[Executable awk Programs], page 19), you can instead run your program directly:
cut.awk -c1-8 myfiles > results
If your awk is not gawk, you may instead need to use this:cut.awk -- -c1-8 myfiles > results

11.2 Reinventing Wheels for Fun and Profit
This section presents a number of POSIX utilities implemented in awk. Reinventing these
programs in awk is often enjoyable, because the algorithms can be very clearly expressed,
and the code is usually very concise and simple. This is true because awk does so much foryou.
It should be noted that these programs are not necessarily intended to replace the installed versions on your system. Nor may all of these programs be fully compliant withthe most recent POSIX standard. This is not a problem; their purpose is to illustrate awk
language programming for “real-world” tasks.
The programs are presented in alphabetical order.

11.2.1 Cutting Out Fields and Columns
The cut utility selects, or “cuts,” characters or fields from its standard input and sends
them to its standard output. Fields are separated by TABs by default, but you may supply
a command-line option to change the field delimiter (i.e., the field-separator character).
cut’s definition of fields is less general than awk’s.

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A common use of cut might be to pull out just the login names of logged-on users fromthe output of who. For example, the following pipeline generates a sorted, unique list of thelogged-on users:
who | cut -c1-8 | sort | uniq
The options for cut are:-c listUse list as the list of characters to cut out. Items within the list may beseparated by commas, and ranges of characters can be separated with dashes.
The list ‘1-8,15,22-35’ specifies characters 1 through 8, 15, and 22 through35.

-d delim

Use delim as the field-separator character instead of the TAB character.

-f listUse list as the list of fields to cut out.

-sSuppress printing of lines that do not contain the field delimiter.

The awk implementation of cut uses the getopt() library function (see Section 10.4
[Processing Command-Line Options], page 259) and the join() library function (seeSection 10.2.6 [Merging an Array into a String], page 249).
The current POSIX version of cut has options to cut fields based on both bytes and characters. This version does not attempt to implement those options, as awk works exclusively
in terms of characters.
The program begins with a comment describing the options, the library functions needed,
and a usage() function that prints out a usage message and exits. usage() is called if
invalid arguments are supplied:# cut.awk --- implement cut in awk# Options:#-c listCut characters#-f listCut fields#-d c
Field delimiter character##-s
Suppress lines without the delimiter## Requires getopt() and join() library functions
function usage(){print("usage: cut [-f list] [-d c] [-s] [files...]") > "/dev/stderr"
print("
cut [-c list] [files...]") > "/dev/stderr"
exit 1}
Next comes a BEGIN rule that parses the command-line options. It sets FS to a single
TAB character, because that is cut’s default field separator. The rule then sets the output
field separator to be the same as the input field separator. A loop using getopt() steps
through the command-line options. Exactly one of the variables by_fields or by_chars isChapter 11: Practical awk Programs

281

set to true, to indicate that processing should be done by fields or by characters, respectively.
When cutting by characters, the output field separator is set to the null string:
BEGIN {
FS = "\t"# default
OFS = FS
while ((c = getopt(ARGC, ARGV, "sf:c:d:")) != -1) {if (c == "f") {by_fields = 1
fieldlist = Optarg} else if (c == "c") {by_chars = 1
fieldlist = Optarg
OFS = ""
} else if (c == "d") {if (length(Optarg) > 1) {printf("cut: using first character of %s" \" for delimiter\n", Optarg) > "/dev/stderr"
Optarg = substr(Optarg, 1, 1)}
fs = FS = Optarg
OFS = FS
if (FS == " ")# defeat awk semantics
FS = "[ ]"} else if (c == "s")suppress = 1
else
usage()}# Clear out optionsfor (i = 1; i < Optind; i++)
ARGV[i] = ""The code must take special care when the field delimiter is a space. Using a single space(" ") for the value of FS is incorrect—awk would separate fields with runs of spaces, TABs,
and/or newlines, and we want them to be separated with individual spaces. To this end, wesave the original space character in the variable fs for later use; after setting FS to "[ ]"we can’t use it directly to see if the field delimiter character is in the string.
Also remember that after getopt() is through (as described in Section 10.4 [Processing
Command-Line Options], page 259), we have to clear out all the elements of ARGV from 1
to Optind, so that awk does not try to process the command-line options as file names.
After dealing with the command-line options, the program verifies that the options makesense. Only one or the other of -c and -f should be used, and both require a field list.
Then the program calls either set_fieldlist() or set_charlist() to pull apart the listof fields or characters:if (by_fields && by_chars)
usage()

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by_fields = 1# default
if (fieldlist == "") {print "cut: needs list for -c or -f" > "/dev/stderr"
exit 1}if (by_fields)
set_fieldlist()else
set_charlist()}
set_fieldlist() splits the field list apart at the commas into an array. Then, for eachelement of the array, it looks to see if the element is actually a range, and if so, splits itapart. The function checks the range to make sure that the first number is smaller than thesecond. Each number in the list is added to the flist array, which simply lists the fields
that will be printed. Normal field splitting is used. The program lets awk handle the job of
doing the field splitting:function set_fieldlist(n, m, i, j, k, f, g){
n = split(fieldlist, f, ",")j = 1
# index in flist
for (i = 1; i <= n; i++) {if (index(f[i], "-") != 0) { # a range
m = split(f[i], g, "-")
if (m != 2 || g[1] >= g[2]) {printf("cut: bad field list: %s\n",f[i]) > "/dev/stderr"
exit 1}for (k = g[1]; k <= g[2]; k++)flist[j++] = k} elseflist[j++] = f[i]}nfields = j - 1}
The set_charlist() function is more complicated than set_fieldlist(). The idea
here is to use gawk’s FIELDWIDTHS variable (see Section 4.6 [Reading Fixed-Width Data],
page 80), which describes constant-width input. When using a character list, that is exactly
what we have.
Setting up FIELDWIDTHS is more complicated than simply listing the fields that need to
be printed. We have to keep track of the fields to print and also the intervening charactersthat have to be skipped. For example, suppose you wanted characters 1 through 8, 15, andChapter 11: Practical awk Programs283

22 through 35. You would use ‘-c 1-8,15,22-35’. The necessary value for FIELDWIDTHS
is "8 6 1 6 14". This yields five fields, and the fields to print are $1, $3, and $5. Theintermediate fields are filler, which is stuff in between the desired data. flist lists thefields to print, and t tracks the complete field list, including filler fields:
function set_charlist(

field, i, j, f, g, n, m, t,
filler, last, len){field = 1
# count total fields
n = split(fieldlist, f, ",")j = 1
# index in flist
for (i = 1; i <= n; i++) {if (index(f[i], "-") != 0) { # range
m = split(f[i], g, "-")
if (m != 2 || g[1] >= g[2]) {printf("cut: bad character list: %s\n",f[i]) > "/dev/stderr"
exit 1}
len = g[2] - g[1] + 1
if (g[1] > 1) # compute length of filler
filler = g[1] - last - 1
else
filler = 0
if (filler)t[field++] = fillert[field++] = len # length of fieldlast = g[2]flist[j++] = field - 1} else {if (f[i] > 1)
filler = f[i] - last - 1
else
filler = 0
if (filler)t[field++] = fillert[field++] = 1
last = f[i]flist[j++] = field - 1}}
FIELDWIDTHS = join(t, 1, field - 1)
nfields = j - 1}
Next is the rule that processes the data. If the -s option is given, then suppress is true.
The first if statement makes sure that the input record does have the field separator. If

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cut is processing fields, suppress is true, and the field separator character is not in therecord, then the record is skipped.
If the record is valid, then gawk has split the data into fields, either using the characterin FS or using fixed-length fields and FIELDWIDTHS. The loop goes through the list of fields
that should be printed. The corresponding field is printed if it contains data. If the next
field also has data, then the separator character is written out between the fields:{
if (by_fields && suppress && index($0, fs) == 0)
next
for (i = 1; i <= nfields; i++) {if ($flist[i] != "") {printf "%s", $flist[i]if (i < nfields && $flist[i+1] != "")printf "%s", OFS}}
print ""}
This version of cut relies on gawk’s FIELDWIDTHS variable to do the character-based cutting. It is possible in other awk implementations to use substr() (see Section 9.1.4 [StringManipulation Functions], page 196), but it is also extremely painful. The FIELDWIDTHS
variable supplies an elegant solution to the problem of picking the input line apart bycharacters.

11.2.2 Searching for Regular Expressions in Files
The grep family of programs searches files for patterns. These programs have an unusual
history. Initially there was grep (Global Regular Expression Print), which used what are
now called Basic Regular Expressions (BREs). Later there was egrep (Extended grep)
which used what are now called Extended Regular Expressions (EREs). (These are almost
identical to those available in awk; see Chapter 3 [Regular Expressions], page 51). There
was also fgrep (Fast grep), which searched for matches of one more fixed strings.
POSIX chose to combine these three programs into one, simply named grep. On a
POSIX system, grep’s default behavior is to search using BREs. You use -E to specify theuse of EREs, and -F to specify searching for fixed strings.
In practice, systems continue to come with separate egrep and fgrep utilities, for backwards compatibility. This section provides an awk implementation of egrep, which supports
all of the POSIX-mandated options. You invoke it as follows:
egrep [options] 'pattern' files . . .
The pattern is a regular expression. In typical usage, the regular expression is quoted
to prevent the shell from expanding any of the special characters as file name wildcards.
Normally, egrep prints the lines that matched. If multiple file names are provided on thecommand line, each output line is preceded by the name of the file and a colon.
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Print a count of the lines that matched the pattern, instead of the lines themselves.

-c-e pattern

Use pattern as the regexp to match. The purpose of the -e option is to allow
patterns that start with a ‘-’.
-i

Ignore case distinctions in both the pattern and the input data.

-l

Only print (list) the names of the files that matched, not the lines that matched.

-q

Be quiet. No output is produced and the exit value indicates whether thepattern was matched.

-s

Be silent. Do not print error messages for files that could not be opened.

-v

Invert the sense of the test. egrep prints the lines that do not match the patternand exits successfully if the pattern is not matched.

-x

Match the entire input line in order to consider the match as having succeeded.

This version uses the getopt() library function (see Section 10.4 [Processing
Command-Line Options], page 259) and gawk’s BEGINFILE and ENDFILE special patterns(see Section 7.1.5 [The BEGINFILE and ENDFILE Special Patterns], page 151).
The program begins with descriptive comments and then a BEGIN rule that processes
the command-line arguments with getopt(). The -i (ignore case) option is particularly
easy with gawk; we just use the IGNORECASE predefined variable (see Section 7.5 [Predefined
Variables], page 163):# egrep.awk --- simulate egrep in awk## Options:#-ccount of lines#-eargument is pattern#-iignore case#-lprint filenames only#-n
add line number to output#-qquiet - use exit value#-ssilent - don't print errors#-v
invert test, success if no match#-x
the entire line must match## Requires getopt library function# Uses IGNORECASE, BEGINFILE and ENDFILE# Invoke using gawk -f egrep.awk -- options ...
BEGIN {
while ((c = getopt(ARGC, ARGV, "ce:ilnqsvx")) != -1) {if (c == "c")count_only++else if (c == "e")pattern = Optarg

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else if (c == "i")IGNORECASE = 1
else if (c == "l")filenames_only++else if (c == "n")line_numbers++
else if (c == "q")no_print++
else if (c == "s")no_errors++
else if (c == "v")invert++else if (c == "x")full_line++
else
usage()}
Note the comment about invocation: Because several of the options overlap with gawk’s, a-- is needed to tell gawk to stop looking for options.
Next comes the code that handles the egrep-specific behavior. egrep uses the first
nonoption on the command line if no pattern is supplied with -e. If the pattern is empty,that means no pattern was supplied, so it’s necessary to print an error message and exit.
The awk command-line arguments up to ARGV[Optind] are cleared, so that awk won’t try
to process them as files. If no files are specified, the standard input is used, and if multiple
files are specified, we make sure to note this so that the file names can precede the matchedlines in the output:
if (pattern == "")pattern = ARGV[Optind++]if (pattern == "")usage()for (i = 1; i < Optind; i++)
ARGV[i] = ""if (Optind >= ARGC) {ARGV[1] = "-"ARGC = 2} else if (ARGC - Optind > 1)
do_filenames++}
The BEGINFILE rule executes when each new file is processed. In this case, it is fairly
simple; it initializes a variable fcount to zero. fcount tracks how many lines in the current
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Here also is where we implement the -s option. We check if ERRNO has been set, and if-s was supplied. In that case, it’s necessary to move on to the next file. Otherwise gawk
would exit with an error:
BEGINFILE {fcount = 0
if (ERRNO && no_errors)
nextfile}
The ENDFILE rule executes after each file has been processed. It affects the output only
when the user wants a count of the number of lines that matched. no_print is true only
if the exit status is desired. count_only is true if line counts are desired. egrep therefore
only prints line counts if printing and counting are enabled. The output format must be
adjusted depending upon the number of files to process. Finally, fcount is added to total,so that we know the total number of lines that matched the pattern:
ENDFILE {if (! no_print && count_only) {if (do_filenames)
print file ":" fcountelse
print fcount}total += fcount}
The following rule does most of the work of matching lines. The variable matches istrue (non-zero) if the line matched the pattern. If the user specified that the entire line
must match (with -x), the code checks this condition by looking at the values of RSTART
and RLENGTH. If those indicate that the match is not over the full line, matches is set tozero (false).
If the user wants lines that did not match, we invert the sense of matches using the ‘!’
operator. We then increment fcount with the value of matches, which is either one or zero,depending upon a successful or unsuccessful match. If the line does not match, the next
statement just moves on to the next input line.
We make a number of additional tests, but only if we are not counting lines. First, if
the user only wants the exit status (no_print is true), then it is enough to know that one
line in this file matched, and we can skip on to the next file with nextfile. Similarly, if
we are only printing file names, we can print the file name, and then skip to the next filewith nextfile. Finally, each line is printed, with a leading file name, optional colon andline number, and the final colon if necessary:{
matches = match($0, pattern)
if (matches && full_line && (RSTART != 1 || RLENGTH != length()))
matches = 0
if (invert)

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matches = ! matches
fcount += matches# 1 or 0

if (! matches)next
if (! count_only) {if (no_print)
nextfile
if (filenames_only) {print FILENAME
nextfile}
if (do_filenames)
if (line_numbers)
print FILENAME ":" FNR ":" $0
else
print FILENAME ":" $0
else
print
}
}
The END rule takes care of producing the correct exit status. If there are no matches,
the exit status is one; otherwise, it is zero:
END {exit (total == 0)}
The usage() function prints a usage message in case of invalid options, and then exits:
function usage(){print("Usage:\tegrep [-cilnqsvx] [-e pat] [files ...]") > "/dev/stderr"
print("\tegrep [-cilnqsvx] pat [files ...]") > "/dev/stderr"
exit 1}11.2.3 Printing Out User Information
The id utility lists a user’s real and effective user ID numbers, real and effective group ID
numbers, and the user’s group set, if any. id only prints the effective user ID and group ID
if they are different from the real ones. If possible, id also supplies the corresponding userand group names. The output might look like this:$ id
a uid=1000(arnold) gid=1000(arnold) groups=1000(arnold),4(adm),7(lp),27(sudo)Chapter 11: Practical awk Programs

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This information is part of what is provided by gawk’s PROCINFO array (see Section 7.5[Predefined Variables], page 163). However, the id utility provides a more palatable output
than just individual numbers.
The POSIX version of id takes several options that give you control over the output’s
format, such as printing only real ids, or printing only numbers or only names. Additionally,
you can print the information for a specific user, instead of that of the current user.
Here is a version of POSIX id written in awk. It uses the getopt() library function(see Section 10.4 [Processing Command-Line Options], page 259), the user database libraryfunctions (see Section 10.5 [Reading the User Database], page 266), and the group databaselibrary functions (see Section 10.6 [Reading the Group Database], page 270) from Chapter 10[A Library of awk Functions], page 241.
The program is moderately straightforward. All the work is done in the BEGIN rule. It
starts with explanatory comments, a list of options, and then a usage() function:# id.awk --- implement id in awk## Requires user and group library functions and getopt# output is:# uid=12(foo) euid=34(bar) gid=3(baz) \#egid=5(blat) groups=9(nine),2(two),1(one)# Options:#-G Output all group ids as space separated numbers (ruid, euid, groups)#-g Output only the euid as a number#-n Output name instead of the numeric value (with -g/-G/-u)#-r Output ruid/rguid instead of effective id#-u Output only effective user id, as a number
function usage(){printf("Usage:\n" \"\tid [user]\n" \"\tid -G [-n] [user]\n" \"\tid -g [-nr] [user]\n" \"\tid -u [-nr] [user]\n") > "/dev/stderr"
exit 1}
The first step is to parse the options using getopt(), and to set various flag variables
according to the options given:
BEGIN {# parse args
while ((c = getopt(ARGC, ARGV, "Ggnru")) != -1) {if (c == "G")groupset_only++else if (c == "g")290

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egid_only++else if (c == "n")names_not_groups++
else if (c == "r")real_ids_only++else if (c == "u")euid_only++else
usage()}The next step is to check that no conflicting options were provided. -G and -r are
mutually exclusive. It is also not allowed to provide more than one user name on thecommand line:
if (groupset_only && real_ids_only)usage()
else if (ARGC - Optind > 1)
usage()The user and group ID numbers are obtained from PROCINFO for the current user, orfrom the user and password databases for a user supplied on the command line. In the latter
case, real_ids_only is set, since it’s not possible to print information about the effective
user and group IDs:
if (ARGC - Optind == 0) {# gather info for current user
uid = PROCINFO["uid"]
euid = PROCINFO["euid"]
gid = PROCINFO["gid"]
egid = PROCINFO["egid"]for (i = 1; ("group" i) in PROCINFO; i++)groupset[i] = PROCINFO["group" i]} else {fill_info_for_user(ARGV[ARGC-1])
real_ids_only++}
The test in the for loop is worth noting. Any supplementary groups in the PROCINFO
array have the indices "group1" through "groupN" for some N (i.e., the total number ofsupplementary groups). However, we don’t know in advance how many of these groups
there are.
This loop works by starting at one, concatenating the value with "group", and then using
in to see if that value is in the array (see Section 8.1.2 [Referring to an Array Element],
page 179). Eventually, i increments past the last group in the array and the loop exits.
The loop is also correct if there are no supplementary groups; then the condition is false
the first time it’s tested, and the loop body never executes.
Now, based on the options, we decide what information to print. For -G (print just thegroup set), we then select whether to print names or numbers. In either case, when done
we exit:Chapter 11: Practical awk Programs291

if (groupset_only) {if (names_not_groups) {for (i = 1; i in groupset; i++) {entry = getgrgid(groupset[i])name = get_first_field(entry)
printf("%s", name)if ((i + 1) in groupset)
printf(" ")}} else {for (i = 1; i in groupset; i++) {printf("%u", groupset[i])if ((i + 1) in groupset)
printf(" ")}}print ""
exit 0# final newline}
Otherwise, for -g (effective group ID only), we check if -r was also provided, in whichcase we use the real group ID. Then based on -n, we decide whether to print names or
numbers. Here too, when done, we exit:
else if (egid_only) {id = real_ids_only ? gid : egid
if (names_not_groups) {entry = getgrgid(id)name = get_first_field(entry)
printf("%s\n", name)} else {printf("%u\n", id)}
exit 0}
The get_first_field() function extracts the group name from the group database
entry for the given group ID.
Similar processing logic applies to -u (effective user ID only), combined with -r and -n:else if (euid_only) {id = real_ids_only ? uid : euid
if (names_not_groups) {entry = getpwuid(id)name = get_first_field(entry)
printf("%s\n", name)} else {printf("%u\n", id)

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exit 0}At this point, we haven’t exited yet, so we print the regular, default output, based either
on the current user’s information, or that of the user whose name was provided on thecommand line. We start with the real user ID:
printf("uid=%d", uid)
pw = getpwuid(uid)print_first_field(pw)
The print_first_field() function prints the user’s login name from the password file
entry, surrounded by parentheses. It is shown soon. Printing the effective user ID is next:
if (euid != uid && ! real_ids_only) {printf(" euid=%d", euid)
pw = getpwuid(euid)print_first_field(pw)}Similar logic applies to the real and effective group IDs:
printf(" gid=%d", gid)pw = getgrgid(gid)print_first_field(pw)
if (egid != gid && ! real_ids_only) {printf(" egid=%d", egid)
pw = getgrgid(egid)
print_first_field(pw)}Finally, we print the group set and the terminating newline:
for (i = 1; i in groupset; i++) {if (i == 1)
printf(" groups=")group = groupset[i]printf("%d", group)
pw = getgrgid(group)
print_first_field(pw)if ((i + 1) in groupset)
printf(",")}
print ""}The get_first_field() function extracts the first field from a password or group file
entry for use as a user or group name. Fields are separated by ‘:’ characters:function get_first_field(str, a){Chapter 11: Practical awk Programs

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if (str != "") {split(str, a, ":")
return a[1]}}This function is then used by print_first_field() to output the given name surrounded by parentheses:
function print_first_field(str){first = get_first_field(str)printf("(%s)", first)}
These two functions simply isolate out some code that is used repeatedly, making thewhole program shorter and cleaner. In particular, moving the check for the empty string
into get_first_field() saves several lines of code.
Finally, fill_info_for_user() fetches user, group, and group set information for theuser named on the command. The code is fairly straightforward, merely requiring that weexit if the given user doesn’t exist:
function fill_info_for_user(user,
pwent, fields, groupnames, grent, groups, i){
pwent = getpwnam(user)
if (pwent == "") {printf("id: '%s': no such user\n", user) > "/dev/stderr"
exit 1}
split(pwent, fields, ":")
uid = fields[3] + 0
gid = fields[4] + 0
Getting the group set is a little awkward. The library routine getgruser() returns alist of group names. These have to be gone through and turned back into group numbers,
so that the rest of the code will work as expected:
groupnames = getgruser(user)
split(groupnames, groups, " ")for (i = 1; i in groups; i++) {grent = getgrnam(groups[i])split(grent, fields, ":")
groupset[i] = fields[3] + 0}}

11.2.4 Splitting a Large File into Pieces
The split utility splits large text files into smaller pieces. The usage follows the POSIX
standard for split and is as follows:

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split [-l count] [-a suffix-len] [file [outname]]
split -b N [k|m]] [-a suffix-len] [file [outname]]
By default, the output files are named xaa, xab, and so on. Each file has 1,000 lines init, with the likely exception of the last file.
The split program has evolved over time, and the current POSIX version is more
complicated than the original Unix version. The options and what they do are as follows:-a suffix-len
Use suffix-len characters for the suffix. For example, if suffix-len is four, theoutput files would range from xaaaa to xzzzz.
-b N [k|m]]
Instead of each file containing a specified number of lines, each file should have(at most) N bytes. Supplying a trailing ‘k’ multiplies N by 1,024, yielding
kilobytes. Supplying a trailing ‘m’ multiplies N by 1,048,576 (1, 024 × 1, 024)
yielding megabytes. (This option is mutually exclusive with -l).
-l count

Each file should have at most count lines, instead of the default 1,000. (This
option is mutually exclusive with -b).

If supplied, file is the input file to read. Otherwise standard input is processed. If
supplied, outname is the leading prefix to use for file names, instead of ‘x’.
In order to use the -b option, gawk should be invoked with its -b option (see Section 2.2[Command-Line Options], page 33), or with the environment variable LC_ALL set to ‘C’, sothat each input byte is treated as a separate character.1
Here is an implementation of split in awk. It uses the getopt() function presented in
Section 10.4 [Processing Command-Line Options], page 259.
The program begins with a standard descriptive comment and then a usage() function
describing the options. The variable common keeps the function’s lines short so that theylook nice on the page:# split.awk --- do split in awk## Requires getopt() library function.
function usage(
common){common = "[-a suffix-len] [file [outname]]"printf("usage: split [-l count] %s\n", common) > "/dev/stderr"
printf("split [-b N[k|m]] %s\n", common) > "/dev/stderr"
exit 1}
Next, in a BEGIN rule we set the default values and parse the arguments. After that we
initialize the data structures used to cycle the suffix from ‘aa...’ to ‘zz...’. Finally we set
the name of the first output file:
BEGIN {
1

Using -b twice requires separating gawk’s options from those of the program. For example: ‘gawk -f
getopt.awk -f split.awk -b -- -b 42m large-file.txt split-’.

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295# Set defaults:
Suffix_length = 2
Line_count = 1000
Byte_count = 0
Outfile = "x"parse_arguments()
init_suffix_data()
Output = (Outfile compute_suffix())}
Parsing the arguments is straightforward. The program follows our convention (seeSection 10.1 [Naming Library Function Global Variables], page 242) of having important
global variables start with an uppercase letter:function parse_arguments(
i, c, l, modifier){while ((c = getopt(ARGC, ARGV, "a:b:l:")) != -1) {if (c == "a")Suffix_length = Optarg + 0
else if (c == "b") {Byte_count = Optarg + 0
Line_count = 0
l = length(Optarg)
modifier = substr(Optarg, l, 1)
if (modifier == "k")Byte_count *= 1024
else if (modifier == "m")Byte_count *= 1024 * 1024
} else if (c == "l") {Line_count = Optarg + 0
Byte_count = 0} elseusage()}# Clear out optionsfor (i = 1; i < Optind; i++)
ARGV[i] = ""# Check for filename
if (ARGV[Optind]) {Optind++# Check for different prefix

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if (ARGV[Optind]) {Outfile = ARGV[Optind]
ARGV[Optind] = ""if (++Optind < ARGC)
usage()}}}
Managing the file name suffix is interesting. Given a suffix of length three, say, the values
go from ‘aaa’, ‘aab’, ‘aac’ and so on, all the way to ‘zzx’, ‘zzy’, and finally ‘zzz’. There
are two important aspects to this:• We have to be able to easily generate these suffixes, and in particular easily handle“rolling over”; for example, going from ‘abz’ to ‘aca’.
• We have to tell when we’ve finished with the last file, so that if we still have more input
data we can print an error message and exit. The trick is to handle this after using
the last suffix, and not when the final suffix is created.
The computation is handled by compute_suffix(). This function is called every timea new file is opened.
The flow here is messy, because we want to generate ‘zzzz’ (say), and use it, and only
produce an error after all the file name suffixes have been used up. The logical steps are as
follows:
1. Generate the suffix, saving the value in result to return. To do this, the supplementaryarray Suffix_ind contains one element for each letter in the suffix. Each element ranges
from 1 to 26, acting as the index into a string containing all the lowercase letters of
the English alphabet. It is initialized by init_suffix_data(). result is built up one
letter at a time, using each substr().
2. Prepare the data structures for the next time compute_suffix() is called. To dothis, we loop over Suffix_ind, backwards. If the current element is less than 26, it’sincremented and the loop breaks (‘abq’ goes to ‘abr’). Otherwise, the element is reset
to one and we move down the list (‘abz’ to ‘aca’). Thus, the Suffix_ind array isalways “one step ahead” of the actual file name suffix to be returned.
3. Check if we’ve gone past the limit of possible file names. If Reached_last is true, print
a message and exit. Otherwise, check if Suffix_ind describes a suffix where all theletters are ‘z’. If that’s the case we’re about to return the final suffix. If so, we set
Reached_last to true so that the next call to compute_suffix() will cause a failure.
Physically, the steps in the function occur in the order 3, 1, 2:
function compute_suffix(
i, result, letters){# Logical step 3
if (Reached_last) {printf("split: too many files!\n") > "/dev/stderr"
exit 1} else if (on_last_file())

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Reached_last = 1

297# fail when wrapping after 'zzz'# Logical step 1
result = ""letters = "abcdefghijklmnopqrstuvwxyz"
for (i = 1; i <= Suffix_length; i++)result = result substr(letters, Suffix_ind[i], 1)# Logical step 2
for (i = Suffix_length; i >= 1; i--) {if (++Suffix_ind[i] > 26) {Suffix_ind[i] = 1} elsebreak}
return result}
The Suffix_ind array and Reached_last are initialized by init_suffix_data():function init_suffix_data( i){for (i = 1; i <= Suffix_length; i++)
Suffix_ind[i] = 1
Reached_last = 0}
The function on_last_file() returns true if Suffix_ind describes a suffix where all
the letters are ‘z’ by checking that all the elements in the array are equal to 26:
function on_last_file( i, on_last){
on_last = 1
for (i = 1; i <= Suffix_length; i++) {on_last = on_last && (Suffix_ind[i] == 26)}
return on_last}
The actual work of splitting the input file is done by the next two rules. Since splittingby line count and splitting by byte count are mutually exclusive, we simply use two separate
rules, one for when Line_count is greater than zero, and another for when Byte_count isgreater than zero.
The variable tcount counts how many lines have been processed so far. When it exceeds
Line_count, it’s time to close the previous file and switch to a new one:
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close(Output)
Output = (Outfile compute_suffix())
tcount = 1}
print > Output}
The rule for handling bytes is more complicated. Since lines most likely vary in length,
the Byte_count boundary may be hit in the middle of an input record. In that case, split
has to write enough of the first bytes of the input record to finish up Byte_count bytes,
close the file, open a new file, and write the rest of the record to the new file. The logic
here does all that:
Byte_count > 0 {# `+ 1' is for the final newline
if (tcount + length($0) + 1 > Byte_count) { # would overflow# compute leading bytes
leading_bytes = Byte_count - tcount# write leading bytes
printf("%s", substr($0, 1, leading_bytes)) > Output# close old file, open new fileclose(Output)
Output = (Outfile compute_suffix())# set up first bytes for new file
$0 = substr($0, leading_bytes + 1)
tcount = 0# trailing bytes}# write full record or trailing bytes
tcount += length($0) + 1
print > Output}
Finally, the END rule cleans up by closing the last output file:END {close(Output)}

11.2.5 Duplicating Output into Multiple Files
The tee program is known as a “pipe fitting.” tee copies its standard input to its standard
output and also duplicates it to the files named on the command line. Its usage is as follows:tee [-a] file . . .
The -a option tells tee to append to the named files, instead of truncating them andstarting over.

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The BEGIN rule first makes a copy of all the command-line arguments into an array
named copy. ARGV[0] is not needed, so it is not copied. tee cannot use ARGV directly,
because awk attempts to process each file name in ARGV as input data.
If the first argument is -a, then the flag variable append is set to true, and both ARGV[1]and copy[1] are deleted. If ARGC is less than two, then no file names were supplied andtee prints a usage message and exits. Finally, awk is forced to read the standard input by
setting ARGV[1] to "-" and ARGC to two:# tee.awk --- tee in awk## Copy standard input to all named output files.
# Append content if -a option is supplied.
#
BEGIN {for (i = 1; i < ARGC; i++)copy[i] = ARGV[i]if (ARGV[1] == "-a") {append = 1
delete ARGV[1]delete copy[1]ARGC-}
if (ARGC < 2) {print "usage: tee [-a] file ..." > "/dev/stderr"
exit 1}
ARGV[1] = "-"ARGC = 2}
The following single rule does all the work. Because there is no pattern, it is executedfor each line of input. The body of the rule simply prints the line into each file on thecommand line, and then to the standard output:{# moving the if outside the loop makes it run faster
if (append)for (i in copy)
print >> copy[i]elsefor (i in copy)
print > copy[i]print}
It is also possible to write the loop this way:for (i in copy)
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else
print > copy[i]This is more concise, but it is also less efficient. The ‘if’ is tested for each record and foreach output file. By duplicating the loop body, the ‘if’ is only tested once for each input
record. If there are N input records and M output files, the first method only executes N‘if’ statements, while the second executes N *M ‘if’ statements.
Finally, the END rule cleans up by closing all the output files:
END {for (i in copy)
close(copy[i])}11.2.6 Printing Nonduplicated Lines of Text
The uniq utility reads sorted lines of data on its standard input, and by default removes
duplicate lines. In other words, it only prints unique lines—hence the name. uniq has anumber of options. The usage is as follows:
uniq [-udc [-f n] [-s n]] [inputfile [outputfile]]
The options for uniq are:-dPrint only repeated (duplicated) lines.

-uPrint only nonrepeated (unique) lines.

-cCount lines. This option overrides -d and -u. Both repeated and nonrepeatedlines are counted.

-f nSkip n fields before comparing lines. The definition of fields is similar to awk’s
default: nonwhitespace characters separated by runs of spaces and/or TABs.

-s n

Skip n characters before comparing lines. Any fields specified with -f areskipped first.

inputfile
Data is read from the input file named on the command line, instead of fromthe standard input.
outputfile
The generated output is sent to the named output file, instead of to the standard
output.
Normally uniq behaves as if both the -d and -u options are provided.
uniq uses the getopt() library function (see Section 10.4 [Processing Command-Line
Options], page 259) and the join() library function (see Section 10.2.6 [Merging an Array
into a String], page 249).
The program begins with a usage() function and then a brief outline of the options andtheir meanings in comments:# uniq.awk --- do uniq in awk## Requires getopt() and join() library functions

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function usage(){print("Usage: uniq [-udc [-f fields] [-s chars]] " \"[ in [ out ]]") > "/dev/stderr"
exit 1
}
# -c
# -d
# -u
# -f n# -s ncount lines. overrides -d and -u
only repeated lines
only nonrepeated lines
skip n fields
skip n characters, skip fields first

The POSIX standard for uniq allows options to start with ‘+’ as well as with ‘-’. An initial BEGIN rule traverses the arguments changing any leading ‘+’ to ‘-’ so that the getopt()
function can parse the options:# As of 2020, '+' can be used as the option character in addition to '-'# Previously allowed use of -N to skip fields and +N to skip
# characters is no longer allowed, and not supported by this version.
BEGIN {# Convert + to - so getopt can handle thingsfor (i = 1; i < ARGC; i++) {first = substr(ARGV[i], 1, 1)
if (ARGV[i] == "--" || (first != "-" && first != "+"))
breakelse if (first == "+")
# Replace "+" with "-"ARGV[i] = "-" substr(ARGV[i], 2)}}The next BEGIN rule deals with the command-line arguments and options. If no options
are supplied, then the default is taken, to print both repeated and nonrepeated lines. The
output file, if provided, is assigned to outputfile. Early on, outputfile is initialized tothe standard output, /dev/stdout:
BEGIN {
count = 1
outputfile = "/dev/stdout"
opts = "udcf:s:"
while ((c = getopt(ARGC, ARGV, opts)) != -1) {if (c == "u")non_repeated_only++else if (c == "d")repeated_only++else if (c == "c")do_count++

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else if (c == "f")fcount = Optarg + 0
else if (c == "s")charcount = Optarg + 0
else
usage()}for (i = 1; i < Optind; i++)
ARGV[i] = ""if (repeated_only == 0 && non_repeated_only == 0)
repeated_only = non_repeated_only = 1
if (ARGC - Optind == 2) {outputfile = ARGV[ARGC - 1]
ARGV[ARGC - 1] = ""}}The following function, are_equal(), compares the current line, $0, to the previous line,
last. It handles skipping fields and characters. If no field count and no character countare specified, are_equal() returns one or zero depending upon the result of a simple string
comparison of last and $0.
Otherwise, things get more complicated. If fields have to be skipped, each line is broken
into an array using split() (see Section 9.1.4 [String-Manipulation Functions], page 196);
the desired fields are then joined back into a line using join(). The joined lines are stored
in clast and cline. If no fields are skipped, clast and cline are set to last and $0,
respectively. Finally, if characters are skipped, substr() is used to strip off the leading
charcount characters in clast and cline. The two strings are then compared and are_equal() returns the result:function are_equal(n, m, clast, cline, alast, aline){
if (fcount == 0 && charcount == 0)
return (last == $0)
if (fcount > 0) {n = split(last, alast)m = split($0, aline)
clast = join(alast, fcount+1, n)cline = join(aline, fcount+1, m)} else {clast = lastcline = $0}
if (charcount) {clast = substr(clast, charcount + 1)

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cline = substr(cline, charcount + 1)}
return (clast == cline)}
The following two rules are the body of the program. The first one is executed only forthe very first line of data. It sets last equal to $0, so that subsequent lines of text havesomething to be compared to.
The second rule does the work. The variable equal is one or zero, depending upon the
results of are_equal()’s comparison. If uniq is counting repeated lines, and the lines areequal, then it increments the count variable. Otherwise, it prints the line and resets count,because the two lines are not equal.
If uniq is not counting, and if the lines are equal, count is incremented. Nothing is
printed, as the point is to remove duplicates. Otherwise, if uniq is counting repeated lines
and more than one line is seen, or if uniq is counting nonrepeated lines and only one line
is seen, then the line is printed, and count is reset.
Finally, similar logic is used in the END rule to print the final line of input data:
NR == 1 {last = $0
next}{
equal = are_equal()if (do_count) {# overrides -d and -u
if (equal)
count++
else {
printf("%4d %s\n", count, last) > outputfile
last = $0
count = 1
# reset}next}
if (equal)
count++
else {
if ((repeated_only && count > 1) ||(non_repeated_only && count == 1))
print last > outputfile
last = $0
count = 1
}
}

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END {if (do_count)
printf("%4d %s\n", count, last) > outputfile
else if ((repeated_only && count > 1) ||(non_repeated_only && count == 1))
print last > outputfile
close(outputfile)}As a side note, this program does not follow our recommended convention of naming
global variables with a leading capital letter. Doing that would make the program a little
easier to follow.

11.2.7 Counting Things
The wc (word count) utility counts lines, words, characters and bytes in one or more input
files.

11.2.7.1 Modern Character Sets
In the early days of computing, single bytes were used for storing characters. The mostcommon character sets were ASCII and EBCDIC, which each provided all the English upperand lowercase letters, the 10 Hindu-Arabic numerals from 0 through 9, and a number ofother standard punctuation and control characters.
Today, the most popular character set in use is Unicode (of which ASCII is a puresubset). Unicode provides tens of thousands of unique characters (called code points) tocover most existing human languages (living and dead) and a number of nonhuman ones aswell (such as Klingon and J.R.R. Tolkien’s elvish languages).
To save space in files, Unicode code points are encoded, where each character takes
from one to four bytes in the file. UTF-8 is possibly the most popular of such multibyte
encodings.
The POSIX standard requires that awk function in terms of characters, not bytes.
Thus in gawk, length(), substr(), split(), match() and the other string functions (seeSection 9.1.4 [String-Manipulation Functions], page 196) all work in terms of characters in
the local character set, and not in terms of bytes. (Not all awk implementations do so,though).
There is no standard, built-in way to distinguish characters from bytes in an awk program. For an awk implementation of wc, which needs to make such a distinction, we will
have to use an external extension.

11.2.7.2 A Brief Introduction To Extensions
Loadable extensions are presented in full detail in Chapter 17 [Writing Extensions for gawk],
page 405. They provide a way to add functions to gawk which can call out to other facilities
written in C or C++.
For the purposes of wc.awk, it’s enough to know that the extension is loaded with the@load directive, and the additional function we will use is called mbs_length(). Thisfunction returns the number of bytes in a string, not the number of characters.

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The "mbs" extension comes from the gawkextlib project.
gawkextlib Project], page 467, for more information.

305

See Section 17.8 [The11.2.7.3 Code for wc.awk
The usage for wc is as follows:
wc [-lwcm] [files . . . ]If no files are specified on the command line, wc reads its standard input. If there are
multiple files, it also prints total counts for all the files. The options and their meanings
are as follows:-cCount only bytes. Once upon a time, the ‘c’ in this option stood for “characters.” But, as explained earlier, bytes and character are no longer synonymouswith each other.

-lCount only lines.

-mCount only characters.

-w

Count only words. A “word” is a contiguous sequence of nonwhitespace characters, separated by spaces and/or TABs. Luckily, this is the normal way awk
separates fields in its input data.

Implementing wc in awk is particularly elegant, because awk does a lot of the work forus; it splits lines into words (i.e., fields) and counts them, it counts lines (i.e., records), andit can easily tell us how long a line is in characters.
This program uses the getopt() library function (see Section 10.4 [Processing
Command-Line Options], page 259) and the file-transition functions (see Section 10.3.1[Noting Data file Boundaries], page 254).
This version has one notable difference from older versions of wc: it always prints thecounts in the order lines, words, characters and bytes. Older versions note the order of the-l, -w, and -c options on the command line, and print the counts in that order. POSIX
does not mandate this behavior, though.
The BEGIN rule does the argument processing. The variable print_total is true if morethan one file is named on the command line:# wc.awk --- count lines, words, characters, bytes# Options:#-lonly count lines#-w
only count words#-c
only count bytes#-monly count characters## Default is to count lines, words, bytes## Requires getopt() and file transition library functions# Requires mbs extension from gawkextlib@load "mbs"

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BEGIN {
# let getopt() print a message about# invalid options. we ignore themwhile ((c = getopt(ARGC, ARGV, "lwcm")) != -1) {if (c == "l")do_lines = 1
else if (c == "w")do_words = 1
else if (c == "c")do_bytes = 1
else if (c == "m")do_chars = 1}
for (i = 1; i < Optind; i++)
ARGV[i] = ""# if no options, do lines, words, bytes
if (! do_lines && ! do_words && ! do_chars && ! do_bytes)
do_lines = do_words = do_bytes = 1
print_total = (ARGC - i > 1)}
The beginfile() function is simple; it just resets the counts of lines, words, charactersand bytes to zero, and saves the current file name in fname:function beginfile(file){lines = words = chars = bytes = 0
fname = FILENAME}
The endfile() function adds the current file’s numbers to the running totals of lines,
words, and characters. It then prints out those numbers for the file that was just read. It
relies on beginfile() to reset the numbers for the following data file:
function endfile(file){tlines += lines
twords += words
tchars += chars
tbytes += bytes
if (do_lines)
printf "\t%d", lines
if (do_words)
printf "\t%d", words
if (do_chars)
printf "\t%d", chars
if (do_bytes)

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printf "\t%d", bytes
printf "\t%s\n", fname}
There is one rule that is executed for each line. It adds the length of the record, plus
one, to chars. Adding one plus the record length is needed because the newline character
separating records (the value of RS) is not part of the record itself, and thus not included
in its length. Similarly, it adds the length of the record in bytes, plus one, to bytes. Next,lines is incremented for each line read, and words is incremented by the value of NF, whichis the number of “words” on this line:# do per line{
chars += length($0) + 1# get newline
bytes += mbs_length($0) + 1
lines++
words += NF}
Finally, the END rule simply prints the totals for all the files:
END {
if (print_total) {if (do_lines)
printf "\t%d", tlines
if (do_words)
printf "\t%d", twords
if (do_chars)
printf "\t%d", tchars
if (do_bytes)
printf "\t%d", tbytes
print "\ttotal"}}

11.3 A Grab Bag of awk Programs
This section is a large “grab bag” of miscellaneous programs. We hope you find them bothinteresting and enjoyable.

11.3.1 Finding Duplicated Words in a Document
A common error when writing large amounts of prose is to accidentally duplicate words.
Typically you will see this in text as something like “the the program does the following. . . ”
When the text is online, often the duplicated words occur at the end of one line and at thethe beginning of another, making them very difficult to spot.
This program, dupword.awk, scans through a file one line at a time and looks for adjacentoccurrences of the same word. It also saves the last word on a line (in the variable prev)
for comparison with the first word on the next line.
The first two statements make sure that the line is all lowercase, so that, for example,“The” and “the” compare equal to each other. The next statement replaces nonalphanu-

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meric and nonwhitespace characters with spaces, so that punctuation does not affect thecomparison either. The characters are replaced with spaces so that formatting controls
don’t create nonsense words (e.g., the Texinfo ‘@code{NF}’ becomes ‘codeNF’ if punctuation is simply deleted). The record is then resplit into fields, yielding just the actual words
on the line, and ensuring that there are no empty fields.
If there are no fields left after removing all the punctuation, the current record is skipped.
Otherwise, the program loops through each word, comparing it to the previous one:# dupword.awk --- find duplicate words in text{
$0 = tolower($0)
gsub(/[^[:alnum:][:blank:]]/, " ");$0 = $0# re-split
if (NF == 0)
next
if ($1 == prev)
printf("%s:%d: duplicate %s\n",FILENAME, FNR, $1)
for (i = 2; i <= NF; i++)if ($i == $(i-1))
printf("%s:%d: duplicate %s\n",FILENAME, FNR, $i)prev = $NF}

11.3.2 An Alarm Clock Program
Nothing cures insomnia like a ringing alarm clock.
—Arnold Robbins
Sleep is for web developers.
—Erik Quanstrom
The following program is a simple “alarm clock” program. You give it a time of day andan optional message. At the specified time, it prints the message on the standard output.
In addition, you can give it the number of times to repeat the message as well as a delay
between repetitions.
This program uses the getlocaltime() function from Section 10.2.7 [Managing theTime of Day], page 249.
All the work is done in the BEGIN rule. The first part is argument checking and settingof defaults: the delay, the count, and the message to print. If the user supplied a message
without the ASCII BEL character (known as the “alert” character, "\a"), then it is added
to the message. (On many systems, printing the ASCII BEL generates an audible alert.
Thus, when the alarm goes off, the system calls attention to itself in case the user is notlooking at the computer.) Just for a change, this program uses a switch statement (seeSection 7.4.5 [The switch Statement], page 157), but the processing could be done with aseries of if-else statements instead. Here is the program:# alarm.awk --- set an alarm#Chapter 11: Practical awk Programs309# Requires getlocaltime() library function# usage: alarm time [ "message" [ count [ delay ] ] ]BEGIN {# Initial argument sanity checking
usage1 = "usage: alarm time ['message' [count [delay]]]"usage2 = sprintf("\t(%s) time ::= hh:mm", ARGV[1])if (ARGC < 2) {print usage1 > "/dev/stderr"
print usage2 > "/dev/stderr"
exit 1}switch (ARGC) {case 5:delay = ARGV[4] + 0# fall throughcase 4:
count = ARGV[3] + 0# fall throughcase 3:message = ARGV[2]breakdefault:
if (ARGV[1] !~ /[[:digit:]]?[[:digit:]]:[[:digit:]]{2}/) {print usage1 > "/dev/stderr"
print usage2 > "/dev/stderr"
exit 1}break}# set defaults for once we reach the desired time
if (delay == 0)
delay = 180# 3 minutes
if (count == 0)
count = 5
if (message == "")message = sprintf("\aIt is now %s!\a", ARGV[1])else if (index(message, "\a") == 0)
message = "\a" message "\a"The next section of code turns the alarm time into hours and minutes, converts it (ifnecessary) to a 24-hour clock, and then turns that time into a count of the seconds sincemidnight. Next it turns the current time into a count of seconds since midnight. The
difference between the two is how long to wait before setting off the alarm:# split up alarm time310

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split(ARGV[1], atime, ":")
hour = atime[1] + 0# force numericminute = atime[2] + 0 # force numeric# get current broken down time
getlocaltime(now)# if time given is 12-hour hours and it's after that# hour, e.g., `alarm 5:30' at 9 a.m. means 5:30 p.m.,# then add 12 to real hourif (hour < 12 && now["hour"] > hour)
hour += 12# set target time in seconds since midnighttarget = (hour * 60 * 60) + (minute * 60)# get current time in seconds since midnight
current = (now["hour"] * 60 * 60) + \(now["minute"] * 60) + now["second"]# how long to sleep for
naptime = target - current
if (naptime <= 0) {print "alarm: time is in the past!" > "/dev/stderr"
exit 1}
Finally, the program uses the system() function (see Section 9.1.5 [Input/Output Functions], page 208) to call the sleep utility. The sleep utility simply pauses for the given
number of seconds. If the exit status is not zero, the program assumes that sleep wasinterrupted and exits. If sleep exited with an OK status (zero), then the program prints
the message in a loop, again using sleep to delay for however many seconds are necessary:# zzzzzz..... go away if interrupted
if (system(sprintf("sleep %d", naptime)) != 0)
exit 1# time to notify!
command = sprintf("sleep %d", delay)for (i = 1; i <= count; i++) {print message# if sleep command interrupted, go away
if (system(command) != 0)
break}exit 0}Chapter 11: Practical awk Programs

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11.3.3 Transliterating Characters
The system tr utility transliterates characters. For example, it is often used to map uppercase letters into lowercase for further processing:
generate data | tr 'A-Z' 'a-z' | process data ...
tr requires two lists of characters.2 When processing the input, the first character inthe first list is replaced with the first character in the second list, the second character inthe first list is replaced with the second character in the second list, and so on. If there are
more characters in the “from” list than in the “to” list, the last character of the “to” list is
used for the remaining characters in the “from” list.
Once upon a time, a user proposed adding a transliteration function to gawk. The
following program was written to prove that character transliteration could be done witha user-level function. This program is not as complete as the system tr utility, but it does
most of the job.
The translate program was written long before gawk acquired the ability to split eachcharacter in a string into separate array elements. Thus, it makes repeated use of thesubstr(), index(), and gsub() built-in functions (see Section 9.1.4 [String-Manipulation
Functions], page 196). There are two functions. The first, stranslate(), takes threearguments:
fromA list of characters from which to translatetoA list of characters to which to translate

targetThe string on which to do the translation

Associative arrays make the translation part fairly easy. t_ar holds the “to” characters,
indexed by the “from” characters. Then a simple loop goes through from, one character at
a time. For each character in from, if the character appears in target, it is replaced withthe corresponding to character.
The translate() function calls stranslate(), using $0 as the target. The main program sets two global variables, FROM and TO, from the command line, and then changes
ARGV so that awk reads from the standard input.
Finally, the processing rule simply calls translate() for each record:# translate.awk --- do tr-like stuff# Bugs: does not handle things like tr A-Z a-z; it has# to be spelled out. However, if `to' is shorter than `from',# the last character in `to' is used for the rest of `from'.
function stranslate(from, to, target,

lf, lt, ltarget, t_ar, i, c,
result){lf = length(from)lt = length(to)ltarget = length(target)
2

On some older systems, including Solaris, the system version of tr may require that the lists be written as
range expressions enclosed in square brackets (‘[a-z]’) and quoted, to prevent the shell from attempting
a file name expansion. This is not a feature.

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GAWK: Effective AWK Programmingfor (i = 1; i <= lt; i++)t_ar[substr(from, i, 1)] = substr(to, i, 1)
if (lt < lf)
for (; i <= lf; i++)t_ar[substr(from, i, 1)] = substr(to, lt, 1)
for (i = 1; i <= ltarget; i++) {c = substr(target, i, 1)
if (c in t_ar)c = t_ar[c]result = result c}return result}
function translate(from, to){return $0 = stranslate(from, to, $0)}# main program
BEGIN {
if (ARGC < 3) {print "usage: translate from to" > "/dev/stderr"
exit}
FROM = ARGV[1]TO = ARGV[2]
ARGC = 2
ARGV[1] = "-"}{translate(FROM, TO)
print}
It is possible to do character transliteration in a user-level function, but it is not necessarily efficient, and we (the gawk developers) started to consider adding a built-in function.
However, shortly after writing this program, we learned that Brian Kernighan had added
the toupper() and tolower() functions to his awk (see Section 9.1.4 [String-Manipulation
Functions], page 196). These functions handle the vast majority of the cases where character transliteration is necessary, and so we chose to simply add those functions to gawk as
well and then leave well enough alone.
An obvious improvement to this program would be to set up the t_ar array only once,in a BEGIN rule. However, this assumes that the “from” and “to” lists will never change
throughout the lifetime of the program.

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Another obvious improvement is to enable the use of ranges, such as ‘a-z’, as allowedby the tr utility. Look at the code for cut.awk (see Section 11.2.1 [Cutting Out Fields andColumns], page 279) for inspiration.

11.3.4 Printing Mailing Labels
Here is a “real-world”3 program. This script reads lists of names and addresses and generates
mailing labels. Each page of labels has 20 labels on it, two across and 10 down. The
addresses are guaranteed to be no more than five lines of data. Each address is separated
from the next by a blank line.
The basic idea is to read 20 labels’ worth of data. Each line of each label is stored in
the line array. The single rule takes care of filling the line array and printing the page
when 20 labels have been read.
The BEGIN rule simply sets RS to the empty string, so that awk splits records at blanklines (see Section 4.1 [How Input Is Split into Records], page 65). It sets MAXLINES to 100,
because 100 is the maximum number of lines on the page (20 · 5 = 100).
Most of the work is done in the printpage() function. The label lines are stored
sequentially in the line array. But they have to print horizontally: line[1] next toline[6], line[2] next to line[7], and so on. Two loops accomplish this. The outer loop,
controlled by i, steps through every 10 lines of data; this is each row of labels. The inner
loop, controlled by j, goes through the lines within the row. As j goes from 0 to 4, ‘i+j’
is the jth line in the row, and ‘i+j+5’ is the entry next to it. The output ends up looking
something like this:line 1line 2line 3line 4line 5...

line 6line 7line 8line 9line 10The printf format string ‘%-41s’ left-aligns the data and prints it within a fixed-width
field.
As a final note, an extra blank line is printed at lines 21 and 61, to keep the output
lined up on the labels. This is dependent on the particular brand of labels in use when the
program was written. You will also note that there are two blank lines at the top and two
blank lines at the bottom.
The END rule arranges to flush the final page of labels; there may not have been an evenmultiple of 20 labels in the data:# labels.awk --- print mailing labels# Each label is 5 lines of data that may have blank lines.
# The label sheets have 2 blank lines at the top and 2 at# the bottom.
BEGIN
3{ RS = "" ; MAXLINES = 100 }“Real world” is defined as “a program actually used to get something done.”

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function printpage({if (Nlines <= 0)
returnprintf "\n\n"i, j)# headerfor (i = 1; i <= Nlines; i += 10) {if (i == 21 || i == 61)
print ""for (j = 0; j < 5; j++) {if (i + j > MAXLINES)
break
printf "%-41s %s\n", line[i+j], line[i+j+5]}
print ""}
printf "\n\n"# footerdelete line}
# main rule{
if (Count >= 20) {printpage()Count = 0
Nlines = 0}
n = split($0, a, "\n")for (i = 1; i <= n; i++)line[++Nlines] = a[i]for (; i <= 5; i++)line[++Nlines] = ""Count++}
END {printpage()}

11.3.5 Generating Word-Usage Counts
When working with large amounts of text, it can be interesting to know how often different
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might wish to find synonyms to substitute for words that appear too often. This subsection
develops a program for counting words and presenting the frequency information in a usefulformat.
At first glance, a program like this would seem to do the job:
# wordfreq-first-try.awk --- print list of word frequencies{
for (i = 1; i <= NF; i++)freq[$i]++}
END {for (word in freq)
printf "%s\t%d\n", word, freq[word]}
The program relies on awk’s default field-splitting mechanism to break each line up into“words” and uses an associative array named freq, indexed by each word, to count the
number of times the word occurs. In the END rule, it prints the counts.
This program has several problems that prevent it from being useful on real text files:• The awk language considers upper- and lowercase characters to be distinct. Therefore,“bartender” and “Bartender” are not treated as the same word. This is undesirable,
because words are capitalized if they begin sentences in normal text, and a frequency
analyzer should not be sensitive to capitalization.
• Words are detected using the awk convention that fields are separated just by whitespace. Other characters in the input (except newlines) don’t have any special meaning
to awk. This means that punctuation characters count as part of words.
• The output does not come out in any useful order. You’re more likely to be interested inwhich words occur most frequently or in having an alphabetized table of how frequently
each word occurs.
The first problem can be solved by using tolower() to remove case distinctions. The
second problem can be solved by using gsub() to remove punctuation characters. Finally,
we solve the third problem by using the system sort utility to process the output of the
awk script. Here is the new version of the program:# wordfreq.awk --- print list of word frequencies{$0 = tolower($0)# remove case distinctions# remove punctuationgsub(/[^[:alnum:]_[:blank:]]/, "", $0)
for (i = 1; i <= NF; i++)freq[$i]++}
END {for (word in freq)

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printf "%s\t%d\n", word, freq[word]}
The regexp /[^[:alnum:]_[:blank:]]/ might have been written /[[:punct:]]/, butthen underscores would also be removed, and we want to keep them.
Assuming we have saved this program in a file named wordfreq.awk, and that the data
is in file1, the following pipeline:
awk -f wordfreq.awk file1 | sort -k 2nr
produces a table of the words appearing in file1 in order of decreasing frequency.
The awk program suitably massages the data and produces a word frequency table, which
is not ordered. The awk script’s output is then sorted by the sort utility and printed on
the screen.
The options given to sort specify a sort that uses the second field of each input line
(skipping one field), that the sort keys should be treated as numeric quantities (otherwise‘15’ would come before ‘5’), and that the sorting should be done in descending (reverse)
order.
The sort could even be done from within the program, by changing the END action to:
END {sort = "sort -k 2nr"
for (word in freq)
printf "%s\t%d\n", word, freq[word] | sortclose(sort)}This way of sorting must be used on systems that do not have true pipes at the commandline (or batch-file) level. See the general operating system documentation for more information on how to use the sort program.

11.3.6 Removing Duplicates from Unsorted Text
The uniq program (see Section 11.2.6 [Printing Nonduplicated Lines of Text], page 300)
removes duplicate lines from sorted data.
Suppose, however, you need to remove duplicate lines from a data file but that you wantto preserve the order the lines are in. A good example of this might be a shell history file.
The history file keeps a copy of all the commands you have entered, and it is not unusual
to repeat a command several times in a row. Occasionally you might want to compact
the history by removing duplicate entries. Yet it is desirable to maintain the order of theoriginal commands.
This simple program does the job. It uses two arrays. The data array is indexed by
the text of each line. For each line, data[$0] is incremented. If a particular line has notbeen seen before, then data[$0] is zero. In this case, the text of the line is stored inlines[count]. Each element of lines is a unique command, and the indices of lines
indicate the order in which those lines are encountered. The END rule simply prints out thelines, in order:# histsort.awk --- compact a shell history file
# Thanks to Byron Rakitzis for the general idea

Chapter 11: Practical awk Programs317{
if (data[$0]++ == 0)lines[++count] = $0}
END {for (i = 1; i <= count; i++)print lines[i]}This program also provides a foundation for generating other useful information. For
example, using the following print statement in the END rule indicates how often a particularcommand is used:
print data[lines[i]], lines[i]This works because data[$0] is incremented each time a line is seen.
Rick van Rein offers the following one-liner to do the same job of removing duplicatesfrom unsorted text:
awk '{ if (! seen[$0]++) print }'This can be simplified even further, at the risk of becoming almost too obscure:awk '! seen[$0]++'This version uses the expression as a pattern, relying on awk’s default action of printing theline when the pattern is true.

11.3.7 Extracting Programs from Texinfo Source Files
Both this chapter and the previous chapter (Chapter 10 [A Library of awk Functions],
page 241) present a large number of awk programs. If you want to experiment with these
programs, it is tedious to type them in by hand. Here we present a program that can extract
parts of a Texinfo input file into separate files.
This book is written in Texinfo, the GNU Project’s document formatting language. A
single Texinfo source file can be used to produce both printed documentation, with TEX,
and online documentation. (Texinfo is fully documented in the book Texinfo—The GNU
Documentation Format, available from the Free Software Foundation, and also available
online.)For our purposes, it is enough to know three things about Texinfo input files:• The “at” symbol (‘@’) is special in Texinfo, much as the backslash (‘\’) is in C or awk.
Literal ‘@’ symbols are represented in Texinfo source files as ‘@@’.
• Comments start with either ‘@c’ or ‘@comment’. The file-extraction program works byusing special comments that start at the beginning of a line.
• Lines containing ‘@group’ and ‘@end group’ commands bracket example text thatshould not be split across a page boundary. (Unfortunately, TEX isn’t always smart
enough to do things exactly right, so we have to give it some help.)
The following program, extract.awk, reads through a Texinfo source file and does twothings, based on the special comments. Upon seeing ‘@c system ...’, it runs a command, by extracting the command text from the control line and passing it on to the318

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system() function (see Section 9.1.5 [Input/Output Functions], page 208). Upon seeing‘@c file filename’, each subsequent line is sent to the file filename, until ‘@c endfile’is encountered. The rules in extract.awk match either ‘@c’ or ‘@comment’ by letting the‘omment’ part be optional. Lines containing ‘@group’ and ‘@end group’ are simply removed.
extract.awk uses the join() library function (see Section 10.2.6 [Merging an Array intoa String], page 249).
The example programs in the online Texinfo source for GAWK: Effective AWK Programming (gawktexi.in) have all been bracketed inside ‘file’ and ‘endfile’ lines. The
gawk distribution uses a copy of extract.awk to extract the sample programs and install
many of them in a standard directory where gawk can find them. The Texinfo file looks
something like this:...
This program has a @code{BEGIN} rulethat prints a nice message:@example@c file examples/messages.awk
BEGIN @{ print "Don't panic!" @}@c endfile@end example
It also prints some final advice:@example@c file examples/messages.awk
END @{ print "Always avoid bored archaeologists!" @}@c endfile@end example...
extract.awk begins by setting IGNORECASE to one, so that mixed upper- and lowercase
letters in the directives won’t matter.
The first rule handles calling system(), checking that a command is given (NF is at least
three) and also checking that the command exits with a zero exit status, signifying OK:# extract.awk --- extract files and run programs from Texinfo files
BEGIN{ IGNORECASE = 1 }/^@c(omment)?[ \t]+system/ {if (NF < 3) {e = ("extract: " FILENAME ":" FNR)
e = (e ": badly formed `system' line")
print e > "/dev/stderr"next}$1 = ""$2 = ""

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stat = system($0)
if (stat != 0) {e = ("extract: " FILENAME ":" FNR)
e = (e ": warning: system returned " stat)
print e > "/dev/stderr"}}
The variable e is used so that the rule fits nicely on the page.
The second rule handles moving data into files. It verifies that a file name is given in the
directive. If the file named is not the current file, then the current file is closed. Keeping
the current file open until a new file is encountered allows the use of the ‘>’ redirection for
printing the contents, keeping open-file management simple.
The for loop does the work. It reads lines using getline (see Section 4.10 [Explicit
Input with getline], page 89). For an unexpected end-of-file, it calls the unexpected_eof()function. If the line is an “endfile” line, then it breaks out of the loop. If the line is an‘@group’ or ‘@end group’ line, then it ignores it and goes on to the next line. Similarly,
comments within examples are also ignored.
Most of the work is in the following few lines. If the line has no ‘@’ symbols, the program
can print it directly. Otherwise, each leading ‘@’ must be stripped off. To remove the ‘@’
symbols, the line is split into separate elements of the array a, using the split() function(see Section 9.1.4 [String-Manipulation Functions], page 196). The ‘@’ symbol is used as theseparator character. Each element of a that is empty indicates two successive ‘@’ symbols
in the original line. For each two empty elements (‘@@’ in the original file), we have to add
a single ‘@’ symbol back in.
When the processing of the array is finished, join() is called with the value of SUBSEP(see Section 8.5 [Multidimensional Arrays], page 188), to rejoin the pieces back into a single
line. That line is then printed to the output file:/^@c(omment)?[ \t]+file/ {if (NF != 3) {e = ("extract: " FILENAME ":" FNR ": badly formed `file' line")
print e > "/dev/stderr"
next}
if ($3 != curfile) {if (curfile != "")filelist[curfile] = 1# save to close later
curfile = $3}
for (;;) {if ((getline line) <= 0)
unexpected_eof()
if (line ~ /^@c(omment)?[ \t]+endfile/)
breakelse if (line ~ /^@(end[ \t]+)?group/)320

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continueelse if (line ~ /^@c(omment+)?[ \t]+/)continue
if (index(line, "@") == 0) {print line > curfile
continue}
n = split(line, a, "@")
# if a[1] == "", means leading @,# don't add one back in.
for (i = 2; i <= n; i++) {if (a[i] == "") { # was an @@a[i] = "@"if (a[i+1] == "")i++}}print join(a, 1, n, SUBSEP) > curfile}}
An important thing to note is the use of the ‘>’ redirection. Output done with ‘>’
only opens the file once; it stays open and subsequent output is appended to the file (seeSection 5.6 [Redirecting Output of print and printf], page 108). This makes it easy tomix program text and explanatory prose for the same sample source file (as has been done
here!) without any hassle. The file is only closed when a new data file name is encounteredor at the end of the input file.
When a new file name is encountered, instead of closing the file, the program saves thename of the current file in filelist. This makes it possible to interleave the code for morethan one file in the Texinfo input file. (Previous versions of this program did close the file.
But because of the ‘>’ redirection, a file whose parts were not all one after the other ended
up getting clobbered.) An END rule then closes all the open files when processing is finished:
END {close(curfile)for (f in filelist)
close(f)}# close the last one# close all the rest

Finally, the function unexpected_eof() prints an appropriate error message and thenexits:
function unexpected_eof(){
printf("extract: %s:%d: unexpected EOF or error\n",FILENAME, FNR) > "/dev/stderr"
exit 1}

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11.3.8 A Simple Stream Editor
The sed utility is a stream editor, a program that reads a stream of data, makes changes
to it, and passes it on. It is often used to make global changes to a large file or to a streamof data generated by a pipeline of commands. Although sed is a complicated program in
its own right, its most common use is to perform global substitutions in the middle of apipeline:command1 < orig.data | sed 's/old/new/g' | command2 > result
Here, ‘s/old/new/g’ tells sed to look for the regexp ‘old’ on each input line and globally
replace it with the text ‘new’ (i.e., all the occurrences on a line). This is similar to awk’s
gsub() function (see Section 9.1.4 [String-Manipulation Functions], page 196).
The following program, awksed.awk, accepts at least two command-line arguments: thepattern to look for and the text to replace it with. Any additional arguments are treatedas data file names to process. If none are provided, the standard input is used:# awksed.awk --- do s/foo/bar/g using just print#
Thanks to Michael Brennan for the idea
function usage(){print "usage: awksed pat repl [files...]" > "/dev/stderr"
exit 1}
BEGIN {# validate arguments
if (ARGC < 3)usage()
RS = ARGV[1]ORS = ARGV[2]# don't use arguments as files
ARGV[1] = ARGV[2] = ""}# look ma, no hands!
{
if (RT == "")printf "%s", $0
else
print
}
The program relies on gawk’s ability to have RS be a regexp, as well as on the setting of
RT to the actual text that terminates the record (see Section 4.1 [How Input Is Split into
Records], page 65).

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The idea is to have RS be the pattern to look for. gawk automatically sets $0 to the text
between matches of the pattern. This is text that we want to keep, unmodified. Then, bysetting ORS to the replacement text, a simple print statement outputs the text we want tokeep, followed by the replacement text.
There is one wrinkle to this scheme, which is what to do if the last record doesn’t end
with text that matches RS. Using a print statement unconditionally prints the replacement
text, which is not correct. However, if the file did not end in text that matches RS, RT is
set to the null string. In this case, we can print $0 using printf (see Section 5.5 [Using
printf Statements for Fancier Printing], page 102).
The BEGIN rule handles the setup, checking for the right number of arguments and callingusage() if there is a problem. Then it sets RS and ORS from the command-line arguments
and sets ARGV[1] and ARGV[2] to the null string, so that they are not treated as file names(see Section 7.5.3 [Using ARGC and ARGV], page 173).
The usage() function prints an error message and exits. Finally, the single rule handles
the printing scheme outlined earlier, using print or printf as appropriate, depending upon
the value of RT.

11.3.9 An Easy Way to Use Library Functions
In Section 2.7 [Including Other Files into Your Program], page 46, we saw how gawk provides
a built-in file-inclusion capability. However, this is a gawk extension. This section provides
the motivation for making file inclusion available for standard awk, and shows how to do it
using a combination of shell and awk programming.
Using library functions in awk can be very beneficial. It encourages code reuse and thewriting of general functions. Programs are smaller and therefore clearer. However, using
library functions is only easy when writing awk programs; it is painful when running them,requiring multiple -f options. If gawk is unavailable, then so too is the AWKPATH environment variable and the ability to put awk functions into a library directory (see Section 2.2[Command-Line Options], page 33). It would be nice to be able to write programs in thefollowing manner:# library functions@include getopt.awk@include join.awk...
# main program
BEGIN {
while ((c = getopt(ARGC, ARGV, "a:b:cde")) != -1)
...
...
}
The following program, igawk.sh, provides this service. It simulates gawk’s searchingof the AWKPATH variable and also allows nested includes (i.e., a file that is included with@include can contain further @include statements). igawk makes an effort to only include
files once, so that nested includes don’t accidentally include a library function twice.

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igawk should behave just like gawk externally. This means it should accept all of gawk’s
command-line arguments, including the ability to have multiple source files specified via -f
and the ability to mix command-line and library source files.
The program is written using the POSIX Shell (sh) command language.4 It works asfollows:
1. Loop through the arguments, saving anything that doesn’t represent awk source codefor later, when the expanded program is run.
2. For any arguments that do represent awk text, put the arguments into a shell variable
that will be expanded. There are two cases:
a. Literal text, provided with -e or --source. This text is just appended directly.
b. Source file names, provided with -f. We use a neat trick and append ‘@include
filename’ to the shell variable’s contents. Because the file-inclusion program works
the way gawk does, this gets the text of the file included in the program at thecorrect point.
3. Run an awk program (naturally) over the shell variable’s contents to expand @include
statements. The expanded program is placed in a second shell variable.
4. Run the expanded program with gawk and any other original command-line argumentsthat the user supplied (such as the data file names).
This program uses shell variables extensively: for storing command-line arguments andthe text of the awk program that will expand the user’s program, for the user’s original
program, and for the expanded program. Doing so removes some potential problems thatmight arise were we to use temporary files instead, at the cost of making the script somewhat
more complicated.
The initial part of the program turns on shell tracing if the first argument is ‘debug’.
The next part loops through all the command-line arguments. There are several casesof interest:--

This ends the arguments to igawk. Anything else should be passed on to theuser’s awk program without being evaluated.

-W

This indicates that the next option is specific to gawk. To make argument
processing easier, the -W is appended to the front of the remaining arguments
and the loop continues. (This is an sh programming trick. Don’t worry about
it if you are not familiar with sh.)-v, -F

These are saved and passed on to gawk.

-f, --file, --file=, -Wfile=
The file name is appended to the shell variable program with an @include
statement. The expr utility is used to remove the leading option part of theargument (e.g., ‘--file=’). (Typical sh usage would be to use the echo and sed
utilities to do this work. Unfortunately, some versions of echo evaluate escapesequences in their arguments, possibly mangling the program text. Using expr
avoids this problem.)
4

Fully explaining the sh language is beyond the scope of this book. We provide some minimal explanations,but see a good shell programming book if you wish to understand things in more depth.

324

GAWK: Effective AWK Programming--source, --source=, -Wsource=
The source text is appended to program.
--version, -Wversion
igawk prints its version number, runs ‘gawk --version’ to get the gawk version
information, and then exits.
If none of the -f, --file, -Wfile, --source, or -Wsource arguments are supplied, thenthe first nonoption argument should be the awk program. If there are no command-line
arguments left, igawk prints an error message and exits. Otherwise, the first argument
is appended to program. In any case, after the arguments have been processed, the shell
variable program contains the complete text of the original awk program.
The program is as follows:#! /bin/sh# igawk --- like gawk but do @include processing
if [ "$1" = debug ]thenset -xshiftfi# A literal newline, so that program text is formatted correctlyn=''# Initialize variables to empty
program=opts=
while [ $# -ne 0 ] # loop over arguments
docase $1 in
--)
shift
break ;;-W)

shift
# The ${x?'message here'} construct prints a# diagnostic if $x is the null string
set -- -W"${@?'missing operand'}"continue ;;-[vF])

opts="$opts $1 '${2?'missing operand'}'"shift ;;-[vF]*) opts="$opts '$1'" ;;-f)program="$program$n@include ${2?'missing operand'}"Chapter 11: Practical awk Programs

325

shift ;;
-f*)f=$(expr "$1" : '-f\(.*\)')
program="$program$n@include $f" ;;-[W-]file=*)f=$(expr "$1" : '-.file=\(.*\)')
program="$program$n@include $f" ;;-[W-]file)program="$program$n@include ${2?'missing operand'}"shift ;;-[W-]source=*)t=$(expr "$1" : '-.source=\(.*\)')
program="$program$n$t" ;;-[W-]source)
program="$program$n${2?'missing operand'}"shift ;;-[W-]version)echo igawk: version 3.0 1>&2
gawk --version
exit 0 ;;-[W-]*) opts="$opts '$1'" ;;
*)
esac
shift
done

break ;;

if [ -z "$program" ]thenprogram=${1?'missing program'}
shiftfi# At this point, `program' has the program.
The awk program to process @include directives is stored in the shell variable expand_prog. Doing this keeps the shell script readable. The awk program reads through the user’s
program, one line at a time, using getline (see Section 4.10 [Explicit Input with getline],
page 89). The input file names and @include statements are managed using a stack. As
each @include is encountered, the current file name is “pushed” onto the stack and the file
named in the @include directive becomes the current file name. As each file is finished, the326

GAWK: Effective AWK Programming

stack is “popped,” and the previous input file becomes the current input file again. The
process is started by making the original file the first one on the stack.
The pathto() function does the work of finding the full path to a file. It simulates
gawk’s behavior when searching the AWKPATH environment variable (see Section 2.5.1 [TheAWKPATH Environment Variable], page 42). If a file name has a ‘/’ in it, no path search isdone. Similarly, if the file name is "-", then that string is used as-is. Otherwise, the filename is concatenated with the name of each directory in the path, and an attempt is madeto open the generated file name. The only way to test if a file can be read in awk is to go
ahead and try to read it with getline; this is what pathto() does.5 If the file can be read,
it is closed and the file name is returned:
expand_prog='
function pathto(file,i, t, junk){if (index(file, "/") != 0)
return fileif (file == "-")
return filefor (i = 1; i <= ndirs; i++) {t = (pathlist[i] "/" file)if ((getline junk < t) > 0) {# found it
close(t)return t}}
return ""}The main program is contained inside one BEGIN rule. The first thing it does is set up
the pathlist array that pathto() uses. After splitting the path on ‘:’, null elements are
replaced with ".", which represents the current directory:
BEGIN {
path = ENVIRON["AWKPATH"]
ndirs = split(path, pathlist, ":")for (i = 1; i <= ndirs; i++) {if (pathlist[i] == "")pathlist[i] = "."}The stack is initialized with ARGV[1], which will be "/dev/stdin". The main loop
comes next. Input lines are read in succession. Lines that do not start with @include areprinted verbatim. If the line does start with @include, the file name is in $2. pathto() is5

On some very old versions of awk, the test ‘getline junk < t’ can loop forever if the file exists but isempty.

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called to generate the full path. If it cannot, then the program prints an error message andcontinues.
The next thing to check is if the file is included already. The processed array is indexedby the full file name of each included file and it tracks this information for us. If the file isseen again, a warning message is printed. Otherwise, the new file name is pushed onto thestack and processing continues.
Finally, when getline encounters the end of the input file, the file is closed and thestack is popped. When stackptr is less than zero, the program is done:stackptr = 0
input[stackptr] = ARGV[1] # ARGV[1] is first file}'for (; stackptr >= 0; stackptr--) {while ((getline < input[stackptr]) > 0) {if (tolower($1) != "@include") {print
continue}fpath = pathto($2)
if (fpath == "") {printf("igawk: %s:%d: cannot find %s\n",input[stackptr], FNR, $2) > "/dev/stderr"
continue}if (! (fpath in processed)) {processed[fpath] = input[stackptr]
input[++stackptr] = fpath # push onto stack} elseprint $2, "included in", input[stackptr],"already included in",processed[fpath] > "/dev/stderr"}close(input[stackptr])}# close quote ends `expand_prog' variable

processed_program=$(gawk -- "$expand_prog" /dev/stdin << EOF$program
EOF)
The shell construct ‘command << marker’ is called a here document. Everything in theshell script up to the marker is fed to command as input. The shell processes the contentsof the here document for variable and command substitution (and possibly other things aswell, depending upon the shell).
The shell construct ‘$(...)’ is called command substitution. The output of the command inside the parentheses is substituted into the command line. Because the result is328

GAWK: Effective AWK Programmingused in a variable assignment, it is saved as a single string, even if the results containwhitespace.
The expanded program is saved in the variable processed_program. It’s done in thesesteps:
1. Run gawk with the @include-processing program (the value of the expand_prog shell
variable) reading standard input.
2. Standard input is the contents of the user’s program, from the shell variable program.
Feed its contents to gawk via a here document.
3. Save the results of this processing in the shell variable processed_program by usingcommand substitution.
The last step is to call gawk with the expanded program, along with the original options
and command-line arguments that the user supplied:eval gawk $opts -- '"$processed_program"' '"$@"'The eval command is a shell construct that reruns the shell’s parsing process. This
keeps things properly quoted.
This version of igawk represents the fifth version of this program. There are four key
simplifications that make the program work better:• Using @include even for the files named with -f makes building the initial collected
awk program much simpler; all the @include processing can be done once.
• Not trying to save the line read with getline in the pathto() function when testingfor the file’s accessibility for use with the main program simplifies things considerably.
• Using a getline loop in the BEGIN rule does it all in one place. It is not necessary tocall out to a separate loop for processing nested @include statements.
• Instead of saving the expanded program in a temporary file, putting it in a shell variableavoids some potential security problems. This has the disadvantage that the script relies
upon more features of the sh language, making it harder to follow for those who aren’t
familiar with sh.
Also, this program illustrates that it is often worthwhile to combine sh and awk programming together. You can usually accomplish quite a lot, without having to resort to
low-level programming in C or C++, and it is frequently easier to do certain kinds of string
and argument manipulation using the shell than it is in awk.
Finally, igawk shows that it is not always necessary to add new features to a program;they can often be layered on top.6
Before gawk acquired its built-in @include mechanism, igawk and its manual page were
installed as part of the regular gawk installation (‘make install’). This is no longer done,
because it’s no longer necessary. But we’ve kept the program in this book for its educational
value.

11.3.10 Finding Anagrams from a Dictionary
An interesting programming challenge is to search for anagrams in a word list (such as/usr/share/dict/words on many GNU/Linux systems). One word is an anagram of another if both words contain the same letters (e.g., “babbling” and “blabbing”).
6

gawk does @include processing itself in order to support the use of awk programs as Web CGI scripts.

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Column 2, Problem C, of Jon Bentley’s Programming Pearls, Second Edition, presents
an elegant algorithm. The idea is to give words that are anagrams a common signature,sort all the words together by their signatures, and then print them. Dr. Bentley observes
that taking the letters in each word and sorting them produces those common signatures.
The following program uses arrays of arrays to bring together words with the same
signature and array sorting to print the words in sorted order:# anagram.awk --- An implementation of the anagram-finding algorithm#
from Jon Bentley's "Programming Pearls," 2nd edition.
#
Addison Wesley, 2000, ISBN 0-201-65788-0.
#
Column 2, Problem C, section 2.8, pp 18-20.
/'s$/{ next }# Skip possessives

The program starts with a header, and then a rule to skip possessives in the dictionary
file. The next rule builds up the data structure. The first dimension of the array is indexedby the signature; the second dimension is the word itself:{
key = word2key($1)
data[key][$1] = $1

# Build signature# Store word with signature}
The word2key() function creates the signature. It splits the word apart into individual
letters, sorts the letters, and then joins them back together:# word2key --- split word apart into letters, sort, and join back together
function word2key(word,{n = split(word, a, "")asort(a)a, i, n, result)for (i = 1; i <= n; i++)result = result a[i]return result}
Finally, the END rule traverses the array and prints out the anagram lists. It sends
the output to the system sort command because otherwise the anagrams would appear in
arbitrary order:
END {sort = "sort"for (key in data) {# Sort words with same key
nwords = asorti(data[key], words)
if (nwords == 1)
continue# And print. Minor glitch: trailing space at end of each line

330

GAWK: Effective AWK Programmingfor (j = 1; j <= nwords; j++)printf("%s ", words[j]) | sortprint "" | sort}close(sort)}
Here is some partial output when the program is run:$ gawk -f anagram.awk /usr/share/dict/words | grep '^b'...
babbled blabbed
babbler blabber brabble
babblers blabbers brabbles
babbling blabbing
babbly blabby
babel bablebabels beslab
babery yabber...

11.3.11 And Now for Something Completely Different
The following program was written by Davide Brini and is published on his website. It serves
as his signature in the Usenet group comp.lang.awk. He supplies the following copyrightterms:
Copyright c 2008 Davide Brini
Copying and distribution of the code published in this page, with or without
modification, are permitted in any medium without royalty provided the copyright notice and this notice are preserved.
Here is the program:awk 'BEGIN{O="~"~"~";o="=="=="==";o+=+o;x=O""O;while(X++<=x+o+o)c=c"%c";printf c,(x-O)*(x-O),x*(x-o)-o,x*(x-O)+x-O-o,+x*(x-O)-x+o,X*(o*o+O)+x-O,
X*(X-x)-o*o,(x+X)*o*o+o,x*(X-x)-O-O,x-O+(O+o+X+x)*(o+O),X*X-X*(x-O)-x+O,
O+X*(o*(o+O)+O),+x+O+X*o,x*(x-o),(o+X+x)*o*o-(x-O-O),O+(X-x)*(X+O),x-O}'
We leave it to you to determine what the program does. (If you are truly desperate tounderstand it, see Chris Johansen’s explanation, which is embedded in the Texinfo source
file for this book.)

11.4 Summary• The programs provided in this chapter continue on the theme that reading programs
is an excellent way to learn Good Programming.
• Using ‘#!’ to make awk programs directly runnable makes them easier to use. Otherwise, invoke the program using ‘awk -f ...’.
• Reimplementing standard POSIX programs in awk is a pleasant exercise; awk’s expressive power lets you write such programs in relatively few lines of code, yet they arefunctionally complete and usable.

Chapter 11: Practical awk Programs

331• One of standard awk’s weaknesses is working with individual characters. The ability
to use split() with the empty string as the separator can considerably simplify suchtasks.
• The examples here demonstrate the usefulness of the library functions from Chapter 10[A Library of awk Functions], page 241, for a number of real (if small) programs.
• Besides reinventing POSIX wheels, other programs solved a selection of interesting
problems, such as finding duplicate words in text, printing mailing labels, and finding
anagrams.

11.5 Exercises
1. Rewrite cut.awk (see Section 11.2.1 [Cutting Out Fields and Columns], page 279) using
split() with "" as the separator.
2. In Section 11.2.2 [Searching for Regular Expressions in Files], page 284, we mentionedthat ‘egrep -i’ could be simulated in versions of awk without IGNORECASE by using
tolower() on the line and the pattern. In a footnote there, we also mentioned that
this solution has a bug: the translated line is output, and not the original one. Fix thisproblem.
3. The POSIX version of id takes options that control which information is printed.
Modify the awk version (see Section 11.2.3 [Printing Out User Information], page 288)
to accept the same arguments and perform in the same way.
4. The split.awk program (see Section 11.2.4 [Splitting a Large File into Pieces],
page 293) assumes that letters are contiguous in the character set, which isn’t true for
EBCDIC systems. Fix this problem. (Hint: Consider a different way to work throughthe alphabet, without relying on ord() and chr().)
5. In uniq.awk (see Section 11.2.6 [Printing Nonduplicated Lines of Text], page 300, thelogic for choosing which lines to print represents a state machine, which is “a device
which can be in one of a set number of stable conditions depending on its previous
condition and on the present values of its inputs.”7 Brian Kernighan suggests that “analternative approach to state machines is to just read the input into an array, then use
indexing. It’s almost always easier code, and for most inputs where you would use this,just as fast.” Rewrite the logic to follow this suggestion.
6. Why can’t the wc.awk program (see Section 11.2.7 [Counting Things], page 304) justuse the value of FNR in endfile()? Hint: Examine the code in Section 10.3.1 [Noting
Data file Boundaries], page 254.
7. Manipulation of individual characters in the translate program (see Section 11.3.3[Transliterating Characters], page 311) is painful using standard awk functions. Given
that gawk can split strings into individual characters using "" as the separator, how
might you use this feature to simplify the program?
8. The extract.awk program (see Section 11.3.7 [Extracting Programs from Texinfo
Source Files], page 317) was written before gawk had the gensub() function. Use
it to simplify the code.
7

This definition is from https://www.lexico.com/en/definition/state_machine.

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9. Compare the performance of the awksed.awk program (see Section 11.3.8 [A Simple
Stream Editor], page 321) with the more straightforward:
BEGIN {
pat = ARGV[1]repl = ARGV[2]
ARGV[1] = ARGV[2] = ""}{ gsub(pat, repl); print }10. What are the advantages and disadvantages of awksed.awk versus the real sed utility?
11. In Section 11.3.9 [An Easy Way to Use Library Functions], page 322, we mentioned that
not trying to save the line read with getline in the pathto() function when testingfor the file’s accessibility for use with the main program simplifies things considerably.
What problem does this engender though?
12. As an additional example of the idea that it is not always necessary to add new features
to a program, consider the idea of having two files in a directory in the search path:default.awk
This file contains a set of default library functions, such as getopt() andassert().
site.awk

This file contains library functions that are specific to a site or installation;i.e., locally developed functions. Having a separate file allows default.awk
to change with new gawk releases, without requiring the system administrator to update it each time by adding the local functions.

One user suggested that gawk be modified to automatically read these files upon
startup. Instead, it would be very simple to modify igawk to do this. Since igawk
can process nested @include directives, default.awk could simply contain @include
statements for the desired library functions. Make this change.
13. Modify anagram.awk (see Section 11.3.10 [Finding Anagrams from a Dictionary],
page 328), to avoid the use of the external sort utility.

Part III:
Moving Beyond Standard awk with gawk

Chapter 12: Advanced Features of gawk 335

12 Advanced Features of gawk
Write documentation as if whoever reads it is a violent psychopath who knows
where you live.
—Steve English, as quoted by Peter Langston
This chapter discusses advanced features in gawk. It’s a bit of a “grab bag” of items
that are otherwise unrelated to each other. First, we look at a command-line option thatallows gawk to recognize nondecimal numbers in input data, not just in awk programs. Then,gawk’s special features for sorting arrays are presented. Next, two-way I/O, discussed briefly
in earlier parts of this book, is described in full detail, along with the basics of TCP/IP
networking. We then see how gawk can profile an awk program, making it possible to tune itfor performance. Next, we present an experimental feature that allows you to preserve thevalues of awk variables and arrays between runs of gawk. Finally, we discuss the philosophy
behind gawk’s extension mechanism.
Additional advanced features are discussed in separate chapters of their own:
• Chapter 13 [Internationalization with gawk], page 357, discusses how to internationalize
your awk programs, so that they can speak multiple national languages.
• Chapter 14 [Debugging awk Programs], page 367, describes gawk’s built-in commandline debugger for debugging awk programs.
• Chapter 16 [Arithmetic and Arbitrary-Precision Arithmetic with gawk], page 389, describes how you can use gawk to perform arbitrary-precision arithmetic.
• Chapter 17 [Writing Extensions for gawk], page 405, discusses the ability to dynamicallyadd new built-in functions to gawk.

12.1 Allowing Nondecimal Input Data
If you run gawk with the --non-decimal-data option, you can have nondecimal values inyour input data:$ echo 0123 123 0x123 |> gawk --non-decimal-data '{ printf "%d, %d, %d\n", $1, $2, $3 }'a 83, 123, 291
For this feature to work, write your program so that gawk treats your data as numeric:$ echo 0123 123 0x123 | gawk '{ print $1, $2, $3 }'a 0123 123 0x123
The print statement treats its expressions as strings. Although the fields can act as numbers when necessary, they are still strings, so print does not try to treat them numerically.
You need to add zero to a field to force it to be treated as a number. For example:$ echo 0123 123 0x123 | gawk --non-decimal-data '> { print $1, $2, $3>
print $1 + 0, $2 + 0, $3 + 0 }'a 0123 123 0x123
a 83 123 291
Because it is common to have decimal data with leading zeros, and because using thisfacility could lead to surprising results, the default is to leave it disabled. If you want it,
you must explicitly request it.

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CAUTION: Use of this option is not recommended. It can break old programs
very badly. Instead, use the strtonum() function to convert your data (seeSection 9.1.4 [String-Manipulation Functions], page 196). This makes yourprograms easier to write and easier to read, and leads to less surprising results.
This option may disappear in a future version of gawk.

12.2 Boolean Typed Values
Scalar values in awk are either numbers or strings. gawk also supports values of type regexp(see Section 6.1.2.2 [Strongly Typed Regexp Constants], page 123).
As described in Section 6.3.1 [True and False in awk], page 134, Boolean values in awk
don’t have a separate type: a value counts as “true” if it is nonzero or non-null, and as“false” otherwise.
When interchanging data with languages that do have a real Boolean type, using a standard format such as JSON or XML, the lack of a true Boolean type in awk is problematic.
(See, for example, the json extension provided by the gawkextlib project.)It’s easy to import Boolean data into awk, but then the fact that it was originally
Boolean is lost. Exporting data is even harder; there’s no way to indicate that a value isreally Boolean.
To solve this problem, gawk provides a function named mkbool(). It takes one argument,
which is any awk expression, and it returns a value of Boolean type.
The returned values are normal awk numeric values, with values of either one or zero,depending upon the truth value of the original expression passed in the call to mkbool().
The typeof() function (see Section 9.1.8 [Getting Type Information], page 219) returns"number|bool" for these values.
Thus Boolean-typed values are numbers as far as gawk is concerned, except that extension code can treat them as Booleans if desired.
While it would have been possible to add two new built-in variables of Boolean type
named TRUE and FALSE, doing so would undoubtedly have broken many existing awk programs. Instead, having a “generator” function that creates Boolean values gives flexibility,
without breaking as much existing code.

12.3 Controlling Array Traversal and Array Sorting
gawk lets you control the order in which a ‘for (indx in array)’ loop traverses an array.
In addition, two built-in functions, asort() and asorti(), let you sort arrays based on
the array values and indices, respectively. These two functions also provide control over thesorting criteria used to order the elements during sorting.

12.3.1 Controlling Array Traversal
By default, the order in which a ‘for (indx in array)’ loop scans an array is not defined;
it is generally based upon the internal implementation of arrays inside awk.
Often, though, it is desirable to be able to loop over the elements in a particular orderthat you, the programmer, choose. gawk lets you do this.

Chapter 12: Advanced Features of gawk 337

Section 8.1.6 [Using Predefined Array Scanning Orders with gawk], page 182, describes
how you can assign special, predefined values to PROCINFO["sorted_in"] in order to control
the order in which gawk traverses an array during a for loop.
In addition, the value of PROCINFO["sorted_in"] can be a function name.1 This lets you
traverse an array based on any custom criterion. The array elements are ordered according
to the return value of this function. The comparison function should be defined with atleast four arguments:
function comp_func(i1, v1, i2, v2){
compare elements 1 and 2 in some fashion
return < 0; 0; or > 0}
Here, i1 and i2 are the indices, and v1 and v2 are the corresponding values of thetwo elements being compared. Either v1 or v2, or both, can be arrays if the array being
traversed contains subarrays as values. (See Section 8.6 [Arrays of Arrays], page 189, formore information about subarrays.) The three possible return values are interpreted asfollows:
comp_func(i1, v1, i2, v2) < 0
Index i1 comes before index i2 during loop traversal.
comp_func(i1, v1, i2, v2) == 0
Indices i1 and i2 come together, but the relative order with respect to eachother is undefined.
comp_func(i1, v1, i2, v2) > 0
Index i1 comes after index i2 during loop traversal.
Our first comparison function can be used to scan an array in numerical order of theindices:function cmp_num_idx(i1, v1, i2, v2){# numerical index comparison, ascending orderreturn (i1 - i2)}
Our second function traverses an array based on the string order of the element values
rather than by indices:function cmp_str_val(i1, v1, i2, v2){# string value comparison, ascending order
v1 = v1 ""v2 = v2 ""if (v1 < v2)
return -1
return (v1 != v2)}
1

This is why the predefined sorting orders start with an ‘@’ character, which cannot be part of an identifier.

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The third comparison function makes all numbers, and numeric strings without any
leading or trailing spaces, come out first during loop traversal:
function cmp_num_str_val(i1, v1, i2, v2,
n1, n2){# numbers before string value comparison, ascending ordern1 = v1 + 0
n2 = v2 + 0
if (n1 == v1)
return (n2 == v2) ? (n1 - n2) : -1
else if (n2 == v2)
return 1
return (v1 < v2) ? -1 : (v1 != v2)}
Here is a main program to demonstrate how gawk behaves using each of the previous
functions:
BEGIN {
data["one"] = 10
data["two"] = 20
data[10] = "one"data[100] = 100
data[20] = "two"f[1] = "cmp_num_idx"
f[2] = "cmp_str_val"
f[3] = "cmp_num_str_val"
for (i = 1; i <= 3; i++) {printf("Sort function: %s\n", f[i])PROCINFO["sorted_in"] = f[i]for (j in data)
printf("\tdata[%s] = %s\n", j, data[j])print ""}}
Here are the results when the program is run:$ gawk -f compdemo.awk
Sort by numeric index
a Sort function: cmp_num_idx
data[two] = 20adata[one] = 10
Both strings are numerically zeroadata[10] = oneadata[20] = twoadata[100] = 100aaSort by element values as strings
a Sort function: cmp_str_val
data[one] = 10adata[100] = 100
String 100 is less than string 20adata[two] = 20aChapter 12: Advanced Features of gawk 339

data[10] = oneadata[20] = twoaaa Sort function: cmp_num_str_val
data[one] = 10adata[two] = 20adata[100] = 100adata[10] = oneadata[20] = twoaSort all numeric values before all strings

Consider sorting the entries of a GNU/Linux system password file according to loginname. The following program sorts records by a specific field position and can be used forthis purpose:# passwd-sort.awk --- simple program to sort by field position# field position is specified by the global variable POS
function cmp_field(i1, v1, i2, v2){# comparison by value, as string, and ascending orderreturn v1[POS] < v2[POS] ? -1 : (v1[POS] != v2[POS])}{
for (i = 1; i <= NF; i++)a[NR][i] = $i}
END {PROCINFO["sorted_in"] = "cmp_field"
if (POS < 1 || POS > NF)
POS = 1for (i in a) {for (j = 1; j <= NF; j++)printf("%s%c", a[i][j], j < NF ? ":" : "")print ""}}
The first field in each entry of the password file is the user’s login name, and the fields
are separated by colons. Each record defines a subarray, with each field as an element in
the subarray. Running the program produces the following output:$ gawk -v POS=1 -F: -f sort.awk /etc/passwd
a adm:x:3:4:adm:/var/adm:/sbin/nologin
a apache:x:48:48:Apache:/var/www:/sbin/nologin
a avahi:x:70:70:Avahi daemon:/:/sbin/nologin
...

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The comparison should normally always return the same value when given a specificpair of array elements as its arguments. If inconsistent results are returned, then the orderis undefined. This behavior can be exploited to introduce random order into otherwise
seemingly ordered data:
function cmp_randomize(i1, v1, i2, v2){# random order (caution: this may never terminate!)
return (2 - 4 * rand())}
As already mentioned, the order of the indices is arbitrary if two elements compare equal.
This is usually not a problem, but letting the tied elements come out in arbitrary ordercan be an issue, especially when comparing item values. The partial ordering of the equal
elements may change the next time the array is traversed, if other elements are added to or
removed from the array. One way to resolve ties when comparing elements with otherwise
equal values is to include the indices in the comparison rules. Note that doing this may makethe loop traversal less efficient, so consider it only if necessary. The following comparison
functions force a deterministic order, and are based on the fact that the (string) indices of
two elements are never equal:
function cmp_numeric(i1, v1, i2, v2){# numerical value (and index) comparison, descending orderreturn (v1 != v2) ? (v2 - v1) : (i2 - i1)}function cmp_string(i1, v1, i2, v2){# string value (and index) comparison, descending order
v1 = v1 i1
v2 = v2 i2
return (v1 > v2) ? -1 : (v1 != v2)}
A custom comparison function can often simplify ordered loop traversal, and the sky isreally the limit when it comes to designing such a function.
When string comparisons are made during a sort, either for element values where one
or both aren’t numbers, or for element indices handled as strings, the value of IGNORECASE(see Section 7.5 [Predefined Variables], page 163) controls whether the comparisons treat
corresponding upper- and lowercase letters as equivalent or distinct.
Another point to keep in mind is that in the case of subarrays, the element values canthemselves be arrays; a production comparison function should use the isarray() function(see Section 9.1.8 [Getting Type Information], page 219) to check for this, and choose adefined sorting order for subarrays.
All sorting based on PROCINFO["sorted_in"] is disabled in POSIX mode, because the
PROCINFO array is not special in that case.

Chapter 12: Advanced Features of gawk 341

As a side note, sorting the array indices before traversing the array has been reported to
add a 15% to 20% overhead to the execution time of awk programs. For this reason, sorted
array traversal is not the default.

12.3.2 Sorting Array Values and Indices with gawk
In most awk implementations, sorting an array requires writing a sort() function. This canbe educational for exploring different sorting algorithms, but usually that’s not the point of
the program. gawk provides the built-in asort() and asorti() functions (see Section 9.1.4[String-Manipulation Functions], page 196) for sorting arrays. For example:
populate the array datan = asort(data)for (i = 1; i <= n; i++)do something with data[i]After the call to asort(), the array data is indexed from 1 to some number n, the total
number of elements in data. (This count is asort()’s return value.) data[1] ≤ data[2]≤ data[3], and so on. The default comparison is based on the type of the elements (seeSection 6.3.2 [Variable Typing and Comparison Expressions], page 135). All numeric values
come before all string values, which in turn come before all subarrays.
An important side effect of calling asort() is that the array’s original indices are irrevocably lost. As this isn’t always desirable, asort() accepts a second argument:
populate the array sourcen = asort(source, dest)for (i = 1; i <= n; i++)do something with dest[i]In this case, gawk copies the source array into the dest array and then sorts dest,
destroying its indices. However, the source array is not affected.
Often, what’s needed is to sort on the values of the indices instead of the values of theelements. To do that, use the asorti() function. The interface and behavior are identicalto that of asort(), except that the index values are used for sorting and become the valuesof the result array:{ source[$0] = some_func($0) }END {n = asorti(source, dest)for (i = 1; i <= n; i++) {Work with sorted indices directly:
do something with dest[i]...
Access original array via sorted indices:
do something with source[dest[i]]}}So far, so good. Now it starts to get interesting. Both asort() and asorti() accept
a third string argument to control comparison of array elements. When we introduced

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asort() and asorti() in Section 9.1.4 [String-Manipulation Functions], page 196, we ignored this third argument; however, now is the time to describe how this argument affects
these two functions.
Basically, the third argument specifies how the array is to be sorted. There are two
possibilities. As with PROCINFO["sorted_in"], this argument may be one of the predefinednames that gawk provides (see Section 8.1.6 [Using Predefined Array Scanning Orders withgawk], page 182), or it may be the name of a user-defined function (see Section 12.3.1[Controlling Array Traversal], page 336).
In the latter case, the function can compare elements in any way it chooses, taking into
account just the indices, just the values, or both. This is extremely powerful.
Once the array is sorted, asort() takes the values in their final order and uses themto fill in the result array, whereas asorti() takes the indices in their final order and uses
them to fill in the result array.
NOTE: Copying array indices and elements isn’t expensive in terms of memory. Internally, gawk maintains reference counts to data. For example, whenasort() copies the first array to the second one, there is only one copy of theoriginal array elements’ data, even though both arrays use the values.
You may use the same array for both the first and second arguments to asort() andasorti(). Doing so only makes sense if you are also supplying the third argument, since
awk doesn’t provide a way to pass that third argument without also passing the first andsecond ones.
Because IGNORECASE affects string comparisons, the value of IGNORECASE also affects
sorting for both asort() and asorti(). Note also that the locale’s sorting order does notcome into play; comparisons are based on character values only.2
The following example demonstrates the use of a comparison function with asort().
The comparison function, case_fold_compare(), maps both values to lowercase in order
to compare them ignoring case.
# case_fold_compare --- compare as strings, ignoring casefunction case_fold_compare(i1, v1, i2, v2,{
l = tolower(v1)r = tolower(v2)l, r)if (l < r)return -1
else if (l == r)return 0
else
return 1}
And here is the test program for it:# Test program2

This is true because locale-based comparison occurs only when in POSIX-compatibility mode, and because asort() and asorti() are gawk extensions, they are not available in that case.

Chapter 12: Advanced Features of gawk 343

BEGIN {
Letters = "abcdefghijklmnopqrstuvwxyz" \"ABCDEFGHIJKLMNOPQRSTUVWXYZ"
split(Letters, data, "")asort(data, result, "case_fold_compare")j = length(result)for (i = 1; i <= j; i++) {printf("%s", result[i])if (i % (j/2) == 0)
printf("\n")else
printf(" ")}}When run, we get the following:$ gawk -f case_fold_compare.awk
a A a B b c C D d e E F f g G H h i I J j k K l L M m
a n N O o p P Q q r R S s t T u U V v w W X x y Y z Z
NOTE: “Under the hood,” gawk uses the C library qsort() function to manage
the sorting. qsort() can call itself recursively. This means that when you write
a comparison function, you should be careful to avoid the use of global variables
and arrays; use only local variables and arrays that you declare as additionalparameters to the comparison function. Otherwise, you are likely to cause
unintentional memory corruption in your global arrays and possibly cause gawk
itself to fail.

12.4 Two-Way Communications with Another Process
It is often useful to be able to send data to a separate program for processing and then readthe result. This can always be done with temporary files:# Write the data for processing
tempfile = ("mydata." PROCINFO["pid"])while (not done with data)
print data | ("subprogram > " tempfile)close("subprogram > " tempfile)# Read the results, remove tempfile when done
while ((getline newdata < tempfile) > 0)process newdata appropriately
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This works, but not elegantly. Among other things, it requires that the program be run ina directory that cannot be shared among users; for example, /tmp will not do, as anotheruser might happen to be using a temporary file with the same name.3
However, with gawk, it is possible to open a two-way pipe to another process. The second
process is termed a coprocess, as it runs in parallel with gawk. The two-way connection iscreated using the ‘|&’ operator (borrowed from the Korn shell, ksh):4
do {print data |& "subprogram""subprogram" |& getline results} while (data left to process)close("subprogram")
The first time an I/O operation is executed using the ‘|&’ operator, gawk creates a twoway pipeline to a child process that runs the other program. Output created with print
or printf is written to the program’s standard input, and output from the program’s
standard output can be read by the gawk program using getline. As is the case withprocesses started by ‘|’, the subprogram can be any program, or pipeline of programs, thatcan be started by the shell.
There are some cautionary items to be aware of:• As the code inside gawk currently stands, the coprocess’s standard error goes to thesame place that the parent gawk’s standard error goes. It is not possible to read thechild’s standard error separately.
• I/O buffering may be a problem. gawk automatically flushes all output down the pipe
to the coprocess. However, if the coprocess does not flush its output, gawk may hangwhen doing a getline in order to read the coprocess’s results. This could lead to asituation known as deadlock, where each process is waiting for the other one to dosomething.
It is possible to close just one end of the two-way pipe to a coprocess, by supplying asecond argument to the close() function of either "to" or "from" (see Section 5.9 [Closing
Input and Output Redirections], page 113). These strings tell gawk to close the end of thepipe that sends data to the coprocess or the end that reads from it, respectively.
This is particularly necessary in order to use the system sort utility as part of a coprocess; sort must read all of its input data before it can produce any output. The sort
program does not receive an end-of-file indication until gawk closes the write end of thepipe.
When you have finished writing data to the sort utility, you can close the "to" end of
the pipe, and then start reading sorted data via getline. For example:
BEGIN {
command = "LC_ALL=C sort"n = split("abcdefghijklmnopqrstuvwxyz", a, "")for (i = n; i > 0; i--)3

Michael Brennan suggests the use of rand() to generate unique file names. This is a valid point;
nevertheless, temporary files remain more difficult to use than two-way pipes.
4This is very different from the same operator in the C shell and in Bash.

Chapter 12: Advanced Features of gawk 345

print a[i] |& command
close(command, "to")while ((command |& getline line) > 0)
print "got", line
close(command)}
This program writes the letters of the alphabet in reverse order, one per line, down the
two-way pipe to sort. It then closes the write end of the pipe, so that sort receives an
end-of-file indication. This causes sort to sort the data and write the sorted data back to
the gawk program. Once all of the data has been read, gawk terminates the coprocess andexits.
As a side note, the assignment ‘LC_ALL=C’ in the sort command ensures traditional Unix
(ASCII) sorting from sort. This is not strictly necessary here, but it’s good to know how
to do this.
Be careful when closing the "from" end of a two-way pipe; in this case gawk waits forthe child process to exit, which may cause your program to hang. (Thus, this particularfeature is of much less use in practice than being able to close the "to" end.)
CAUTION: Normally, it is a fatal error to write to the "to" end of a two-way
pipe which has been closed, and it is also a fatal error to read from the "from"end of a two-way pipe that has been closed.
You may set PROCINFO["command", "NONFATAL"] to make such operations become nonfatal. If you do so, you then need to check ERRNO after each print,
printf, or getline. See Section 5.11 [Enabling Nonfatal Output], page 116,
for more information.
You may also use pseudo-ttys (ptys) for two-way communication instead of pipes, if yoursystem supports them. This is done on a per-command basis, by setting a special elementin the PROCINFO array (see Section 7.5.2 [Built-in Variables That Convey Information],
page 165), like so:command = "sort -nr"
PROCINFO[command, "pty"] = 1
print ... |& command...

# command, save in convenience variable# update PROCINFO# start two-way pipe

If your system does not have ptys, or if all the system’s ptys are in use, gawk automaticallyfalls back to using regular pipes.
Using ptys usually avoids the buffer deadlock issues described earlier, at some loss inperformance. This is because the tty driver buffers and sends data line-by-line. On systems
with the stdbuf (part of the GNU Coreutils package), you can use that program instead ofptys.
Note also that ptys are not fully transparent. Certain binary control codes, such Ctrl-d
for end-of-file, are interpreted by the tty driver and not passed through.
CAUTION: Finally, coprocesses open up the possibility of deadlock between
gawk and the program running in the coprocess. This can occur if you send
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blocked writing data with no one available to read what they’ve already written.
There is no workaround for deadlock; careful programming and knowledge ofthe behavior of the coprocess are required.
The following example, due to Andrew Schorr, demonstrates how using ptys can help
deal with buffering deadlocks.
Suppose gawk were unable to add numbers. You could use a coprocess to do it. Here’s
an exceedingly simple program written for that purpose:$ cat add.c#include <stdio.h>
int
main(void){
int x, y;while (scanf("%d %d", & x, & y) == 2)printf("%d\n", x + y);return 0;}$ cc -O add.c -o add
Compile the program
You could then write an exceedingly simple gawk program to add numbers by passing
them to the coprocess:$ echo 1 2 |> gawk -v cmd=./add '{ print |& cmd; cmd |& getline x; print x }'And it would deadlock, because add.c fails to call ‘setlinebuf(stdout)’. The add
program freezes.
Now try instead:$ echo 1 2 |> gawk -v cmd=add 'BEGIN { PROCINFO[cmd, "pty"] = 1 }>{ print |& cmd; cmd |& getline x; print x }'a 3
By using a pty, gawk fools the standard I/O library into thinking it has an interactive
session, so it defaults to line buffering. And now, magically, it works!

12.5 Using gawk for Network Programming
EMRED:A host is a host from coast to coast,and nobody talks to a host that’s close,
unless the host that isn’t close
is busy, hung, or dead.
—Mike O’Brien (aka Mr. Protocol)
In addition to being able to open a two-way pipeline to a coprocess on the same system(see Section 12.4 [Two-Way Communications with Another Process], page 343), it is possible
to make a two-way connection to another process on another system across an IP network
connection.

Chapter 12: Advanced Features of gawk 347

You can think of this as just a very long two-way pipeline to a coprocess. The way gawk
decides that you want to use TCP/IP networking is by recognizing special file names thatbegin with one of ‘/inet/’, ‘/inet4/’, or ‘/inet6/’.
The full syntax of the special file name is /net-type/protocol/local-port/remotehost/remote-port. The components are:net-type

Specifies the kind of Internet connection to make. Use ‘/inet4/’ to force IPv4,
and ‘/inet6/’ to force IPv6. Plain ‘/inet/’ (which used to be the only option)
uses the system default, most likely IPv4.

protocolThe protocol to use over IP. This must be either ‘tcp’, or ‘udp’, for a TCP or
UDP IP connection, respectively. TCP should be used for most applications.

local-port

The local TCP or UDP port number to use. Use a port number of ‘0’ when
you want the system to pick a port. This is what you should do when writinga TCP or UDP client. You may also use a well-known service name, such as‘smtp’ or ‘http’, in which case gawk attempts to determine the predefined port
number using the C getaddrinfo() function.

remote-host
The IP address or fully qualified domain name of the Internet host to which
you want to connect.
remote-port
The TCP or UDP port number to use on the given remote-host. Again, use ‘0’
if you don’t care, or else a well-known service name.
NOTE: Failure in opening a two-way socket will result in a nonfatal error being returned to the calling code. The value of ERRNO indicates the error (seeSection 7.5.2 [Built-in Variables That Convey Information], page 165).
Consider the following very simple example:
BEGIN {
Service = "/inet/tcp/0/localhost/daytime"
Service |& getline
print $0
close(Service)}
This program reads the current date and time from the local system’s TCP daytime
server. It then prints the results and closes the connection.
Because this topic is extensive, the use of gawk for TCP/IP programming is documented
separately. See TCP/IP Internetworking with gawk, which comes as part of the gawk
distribution, for a much more complete introduction and discussion, as well as extensive
examples.
NOTE: gawk can only open direct sockets. There is currently no way to accessservices available over Secure Socket Layer (SSL); this includes any web service
whose URL starts with ‘https://’.

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12.6 Profiling Your awk Programs
You may produce execution traces of your awk programs. This is done by passing the option--profile to gawk. When gawk has finished running, it creates a profile of your programin a file named awkprof.out. Because it is profiling, it also executes up to 45% slower thangawk normally does.
As shown in the following example, the --profile option can be used to change thename of the file where gawk will write the profile:gawk --profile=myprog.prof -f myprog.awk data1 data2
In the preceding example, gawk places the profile in myprog.prof instead of in awkprof.out.
Here is a sample session showing a simple awk program, its input data, and the resultsfrom running gawk with the --profile option. First, the awk program:
BEGIN { print "First BEGIN rule" }END { print "First END rule" }/foo/ {
print "matched /foo/, gosh"for (i = 1; i <= 3; i++)sing()}{
if (/foo/)
print "if is true"
else
print "else is true"}
BEGIN { print "Second BEGIN rule" }END { print "Second END rule" }function sing(dummy){
print "I gotta be me!"}
Following is the input data:
foo
bar
baz
foo
junk
Here is the awkprof.out that results from running the gawk profiler on this programand data (this example also illustrates that awk programmers sometimes get up very earlyin the morning to work):

Chapter 12: Advanced Features of gawk 349# gawk profile, created Mon Sep 29 05:16:21 2014
# BEGIN rule(s)BEGIN {
1

print "First BEGIN rule"}
BEGIN {

1

print "Second BEGIN rule"}# Rule(s)5266/foo/ { # 2
print "matched /foo/, gosh"for (i = 1; i <= 3; i++) {sing()
}
}

5
5
2
3
3

{
if (/foo/) { # 2
print "if is true"} else {print "else is true"}}# END rule(s)
END {

1

print "First END rule"
}
END {

1

print "Second END rule"}# Functions, listed alphabetically66function sing(dummy){
print "I gotta be me!"}

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This example illustrates many of the basic features of profiling output. They are as
follows:• The program is printed in the order BEGIN rules, BEGINFILE rules, pattern–action rules,
ENDFILE rules, END rules, and functions, listed alphabetically. Multiple BEGIN and END
rules retain their separate identities, as do multiple BEGINFILE and ENDFILE rules.
• Pattern–action rules have two counts. The first count, to the left of the rule, shows
how many times the rule’s pattern was tested. The second count, to the right of therule’s opening left brace in a comment, shows how many times the rule’s action was
executed. The difference between the two indicates how many times the rule’s patternevaluated to false.
• Similarly, the count for an if-else statement shows how many times the condition wastested. To the right of the opening left brace for the if’s body is a count showing how
many times the condition was true. The count for the else indicates how many times
the test failed.
• The count for a loop header (such as for or while) shows how many times the loop test
was executed. (Because of this, you can’t just look at the count on the first statement
in a rule to determine how many times the rule was executed. If the first statement isa loop, the count is misleading.)• For user-defined functions, the count next to the function keyword indicates how
many times the function was called. The counts next to the statements in the bodyshow how many times those statements were executed.
• The layout uses “K&R” style with TABs. Braces are used everywhere, even when the
body of an if, else, or loop is only a single statement.
• Parentheses are used only where needed, as indicated by the structure of the program
and the precedence rules. For example, ‘(3 + 5) * 4’ means add three and five, thenmultiply the total by four. However, ‘3 + 5 * 4’ has no parentheses, and means ‘3 + (5* 4)’. However, explicit parentheses in the source program are retained.
• Parentheses are used around the arguments to print and printf only when the print
or printf statement is followed by a redirection. Similarly, if the target of a redirection
isn’t a scalar, it gets parenthesized.
• gawk supplies leading comments in front of the BEGIN and END rules, the BEGINFILE
and ENDFILE rules, the pattern–action rules, and the functions.
• Functions are listed alphabetically. All functions in the awk namespace are listed first, inalphabetical order. Then come the functions in namespaces. The namespaces are listed
in alphabetical order, and the functions within each namespace are listed alphabetically.
The profiled version of your program may not look exactly like what you typed when you
wrote it. This is because gawk creates the profiled version by “pretty-printing” its internal
representation of the program. The advantage to this is that gawk can produce a standard
representation. Also, things such as:/foo/come out as:
/foo/
{
print
}

Chapter 12: Advanced Features of gawk 351

which is correct, but possibly unexpected. (If a program uses both ‘print $0’ and plain‘print’, that distinction is retained.)
Besides creating profiles when a program has completed, gawk can produce a profile
while it is running. This is useful if your awk program goes into an infinite loop and you
want to see what has been executed. To use this feature, run gawk with the --profile
option in the background:$ gawk --profile -f myprog &[1] 13992
The shell prints a job number and process ID number; in this case, 13992. Use the killcommand to send the USR1 signal to gawk:$ kill -USR1 13992
As usual, the profiled version of the program is written to awkprof.out, or to a different
file if one was specified with the --profile option.
Along with the regular profile, as shown earlier, the profile file includes a trace of any
active functions:# Function Call Stack:#3. baz#
2. bar
#
1. foo# -- main -You may send gawk the USR1 signal as many times as you like. Each time, the profileand function call trace are appended to the output profile file.
If you use the HUP signal instead of the USR1 signal, gawk produces the profile and thefunction call trace and then exits.
When gawk runs on MS-Windows systems, it uses the INT and QUIT signals for producing
the profile, and in the case of the INT signal, gawk exits. This is because these systems don’t
support the kill command, so the only signals you can deliver to a program are thosegenerated by the keyboard. The INT signal is generated by the Ctrl-c or Ctrl-BREAK key,
while the QUIT signal is generated by the Ctrl-\ key.
Finally, gawk also accepts another option, --pretty-print. When called this way, gawk
“pretty-prints” the program into awkprof.out, without any execution counts.
NOTE: Once upon a time, the --pretty-print option would also run yourprogram. This is no longer the case.
There is a significant difference between the output created when profiling, and thatcreated when pretty-printing. Pretty-printed output preserves the original comments thatwere in the program, although their placement may not correspond exactly to their original
locations in the source code. However, no comments should be lost. Also, gawk does thebest it can to preserve the distinction between comments at the end of a statement andcomments on lines by themselves. This isn’t always perfect, though.
However, as a deliberate design decision, profiling output omits the original program’scomments. This allows you to focus on the execution count data and helps you avoid thetemptation to use the profiler for pretty-printing.

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Additionally, pretty-printed output does not have the leading indentation that the profiling output does. This makes it easy to pretty-print your code once development is completed, and then use the result as the final version of your program.
Because the internal representation of your program is formatted to recreate an awk
program, profiling and pretty-printing automatically disable gawk’s default optimizations.
Profiling and pretty-printing also preserve the original format of numeric constants; if
you used an octal or hexadecimal value in your source code, it will appear that way in the
output.

12.7 Preserving Data Between Runs
Starting with version 5.2, gawk supports persistent memory. This experimental feature
stores the values of all of gawk’s variables, arrays and user-defined functions in a persistentheap, which resides in a file in the filesystem. When persistent memory is not in use (thenormal case), gawk’s data resides in ephemeral system memory.
Persistent memory is enabled on certain 64-bit systems supporting the mmap() andmunmap() system calls. gawk must be compiled as a non-PIE (Position Independent Executable) binary, since the persistent store ends up holding pointers to functions held within
the gawk executable. This also means that to use the persistent memory, you must use thesame gawk executable from run to run.
You can see if your version of gawk supports persistent memory like so:$ gawk --version
a GNU Awk 5.2.2, API 3.2, PMA Avon 8-g1, (GNU MPFR 4.1.0, GNU MP 6.2.1)
a Copyright (C) 1989, 1991-2023 Free Software Foundation.
...
If you see the ‘PMA’ with a version indicator, then it’s supported.
As of this writing, persistent memory has only been tested on GNU/Linux, Cygwin,
Solaris 2.11, Intel architecture macOS systems, FreeBSD 13.1 and OpenBSD 7.1. On allothers, persistent memory is disabled by default. You can force it to be enabled by exporting the shell variable REALLY_USE_PERSIST_MALLOC with a nonempty value before runningconfigure (see Section B.2.1 [Compiling gawk for Unix-Like Systems], page 495). If you
do so and all the tests pass, please let the maintainer know.
To use persistent memory, follow these steps:
1. Create a new, empty sparse file of the desired size. For example, four gigabytes. On a
GNU/Linux system, you can use the truncate utility:$ truncate -s 4G data.pma
2. It is recommended (but not required) to change the permissions on the file so that only
the owner can read and write it:$ chmod 0600 data.pma
3. Provide the path to the data file in the GAWK_PERSIST_FILE environment variable. Thisis best done by placing the value in the environment just for the run of gawk, like so:$ GAWK_PERSIST_FILE=data.pma gawk 'BEGIN { print ++i }'1Chapter 12: Advanced Features of gawk 353

4. Use the same data file in subsequent runs to use the preserved data values:$ GAWK_PERSIST_FILE=data.pma gawk 'BEGIN { print ++i }'2$ GAWK_PERSIST_FILE=data.pma gawk 'BEGIN { print ++i }'3
As shown, in subsequent runs using the same data file, the values of gawk’s variables
are preserved. However, gawk’s special variables, such as NR, are reset upon each run.
Only the variables defined by the program are preserved across runs.
Interestingly, the program that you execute need not be the same from run to run;the persistent store only maintains the values of variables, arrays, and user-defined functions, not the totality of gawk’s internal state. This lets you share data between unrelated
programs, eliminating the need for scripts to communicate via text files.
Terence Kelly, the author of the persistent memory allocator gawk uses, provides thefollowing advice about the backing file:
Regarding backing file size, I recommend making it far larger than all of thedata that will ever reside in it, assuming that the file system supports sparsefiles. The “pay only for what you use” aspect of sparse files ensures that theactual storage resource footprint of the backing file will meet the application’s
needs but will be as small as possible. If the file system does not support
sparse files, there’s a dilemma: Making the backing file too large is wasteful,but making it too small risks memory exhaustion, i.e., pma_malloc() returns
NULL. But persistent gawk should still work even without sparse files.
You can disable the use of the persistent memory allocator in gawk with the --disablepma option to the configure command at the time that you build gawk (see Section B.2[Compiling and Installing gawk on Unix-Like Systems], page 495).
You can set the PMA_VERBOSITY environment variable to a value between zero and three
to control how much debugging and error information the persistent memory allocator will
print. gawk sets the default to one. See the support/pma.c source code to understand what
the different verbosity levels are.
There are a few constraints on the use of persistent memory:• If you use MPFR mode (the -M option) on the first run of a program using persistentmemory, you must continue to use it on all subsequent runs. Similarly, if you don’t use-M on the first run, do not use it on any subsequent runs.
Mixing and matching MPFR mode and regular mode with the same backing file is notallowed. gawk detects such a situation and issues a fatal error message.
• The GNU/Linux CIFS filesystem is known to not work well with the PMA allocator.
Don’t use a backing file on a CIFS filesystem.
• If gawk is run by the root user, then persistent memory is not allowed. This is to avoid
the possibility of private data “leaking” into the backing file and being recovered laterby an attacker.
• Over time, the backing file will be filled with memory “leaked” by gawk as it runs. Most
notably this is the memory used to compile your program into an internal form before
running it, which happens each time, but there are other leakages as well. (For an extreme example of this, see this thread in the bug-gawk@gnu-org mailing list archives.)

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It is up to you to use ‘du -sh pmafile’ occasionally to monitor how full the file is, andarrange to dump any data you may need before the backing file becomes full.
Terence Kelly has provided a separate Persistent-Memory gawk User Manual document,
which is included in the gawk distribution. It is worth reading.
Here are additional articles and web links that provide more information about persistentmemory and why it’s useful in a scripting language like gawk.
https://web.eecs.umich.edu/~tpkelly/pma/
This is the canonical source for Terence Kelly’s Persistent Memory Allocator(PMA). The latest source code and user manual will always be available
at this location. Kelly may be reached directly at any of the following
email addresses:
tpkelly@acm.org, tpkelly@cs.princeton.edu, or
tpkelly@eecs.umich.edu.
Persistent Memory Allocation
Terence Kelly, Zi Fan Tan, Jianan Li, and Haris Volos, ACM Queue magazine,
Vol. 20 No. 2 (March/April 2022), PDF, HTML. This paper explains thedesign of the PMA allocator used in persistent gawk.
Persistent Scripting
Zi Fan Tan, Jianan Li, Haris Volos, and Terence Kelly, Non-Volatile Memory Workshop (NVMW) 2022, http://nvmw.ucsd.edu/program/. This paper
motivates and describes a research prototype of persistent gawk and presents
performance evaluations on Intel Optane non-volatile memory; note that theinterface differs slightly.
Persistent Memory Programming on Conventional Hardware
Terence Kelly, ACM Queue magazine Vol. 17 No. 4 (July/Aug 2019), PDF,
HTML. This paper describes simple techniques for persistent memory for
C/C++ code on conventional computers that lack non-volatile memory hardware.
Is Persistent Memory Persistent?
Terence Kelly, ACM Queue magazine Vol. 18 No. 2 (March/April 2020), PDF,
HTML. This paper describes a simple and robust testbed for testing software
against real power failures.
Crashproofing the Original NoSQL Key/Value Store
Terence Kelly, ACM Queue magazine Vol. 19 No. 4 (July/Aug 2021), PDF,
HTML. This paper describes a crash-tolerance feature added to GNU DBM’
(gdbm).
When Terence Kelly published his papers, his collaborators produced a prototype integration of PMA with gawk. That version used a (mandatory!) option --persist=file
to specify the file for storing the persistent heap. If this option is given to gawk, it produces a fatal error message instructing the user to use the GAWK_PERSIST_FILE environment
variable instead. Except for this paragraph, that option is otherwise undocumented.
The prototype only supported persistent data; it did not support persistent functions.

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As noted earlier, support for persistent memory is experimental. If it becomes burdensome,5 then the feature will be removed.

12.8 Builtin Features versus Extensions
As this and subsequent chapters show, gawk has a large number of extensions over standard
awk built-in to the program. These have developed over time. More recently, the focus has
moved to using the extension mechanism (see Chapter 17 [Writing Extensions for gawk],
page 405) for adding features. This section discusses the “guiding philosophy” behind whatshould be added to the interpreter as a built-in feature versus what should be done inextensions.
There are several goals:
1. Keep the language awk; it should not become unrecognizable, even if programs in it
will only run on gawk.
2. Keep the core from getting any larger unless absolutely necessary.
3. Add new functionality either in awk scripts (-f, @include) or in loadable extensions
written in C or C++ (-l, @load).
4. Extend the core interpreter only if some feature is:
A. Truly desirable.
B. Cannot be done via library files or loadable extensions.
C. Can be implemented without too much pain in the core.
Combining modules with awk files is a powerful technique. Some of the sample extensions
demonstrate this.
Loading extensions and library files should not be done automatically, because thenthere’s overhead that most users don’t want or need.

12.9 Summary• The --non-decimal-data option causes gawk to treat octal- and hexadecimal-looking
input data as octal and hexadecimal. This option should be used with caution or not
at all; use of strtonum() is preferable. Note that this option may disappear in a futureversion of gawk.
• You can take over complete control of sorting in ‘for (indx in array)’ array traversal
by setting PROCINFO["sorted_in"] to the name of a user-defined function that does
the comparison of array elements based on index and value.
• Similarly, you can supply the name of a user-defined comparison function as the thirdargument to either asort() or asorti() to control how those functions sort arrays. Or
you may provide one of the predefined control strings that work for PROCINFO["sorted_in"].
• You can use the ‘|&’ operator to create a two-way pipe to a coprocess. You readfrom the coprocess with getline and write to it with print or printf. Use close()to close off the coprocess completely, or optionally, close off one side of the two-way
communications.
5

Meaning, there are too many bug reports, or too many strange differences in behavior from when gawk
is run normally.

356

GAWK: Effective AWK Programming• By using special file names with the ‘|&’ operator, you can open a TCP/IP (or UDP/IP)
connection to remote hosts on the Internet. gawk supports both IPv4 and IPv6.
• You can generate statement count profiles of your program. This can help you determine which parts of your program may be taking the most time and let you tune themmore easily. Sending the USR1 signal while profiling causes gawk to dump the profileand keep going, including a function call stack.
• You can also just “pretty-print” the program.
• Persistent memory allows you to preserve the values of variables and arrays between
runs of gawk. This feature is currently experimental.
• New features should be developed using the extension mechanism if possible; theyshould be added to the core interpreter only as a last resort.

Chapter 13: Internationalization with gawk 357

13 Internationalization with gawk
Moon. . . Gorgeous. . . MEDITATION!
—Pretty Guardian Sailor Moon Eternal, The Movie
It probably sounded better in Japanese.
—Malka Robbins
Once upon a time, computer makers wrote software that worked only in English. Eventually, hardware and software vendors noticed that if their systems worked in the native
languages of non-English-speaking countries, they were able to sell more systems. As aresult, internationalization and localization of programs and software systems became acommon practice.
For many years, the ability to provide internationalization was largely restricted toprograms written in C and C++. This chapter describes the underlying library gawk uses
for internationalization, as well as how gawk makes internationalization features available atthe awk program level. Having internationalization available at the awk level gives softwaredevelopers additional flexibility—they are no longer forced to write in C or C++ wheninternationalization is a requirement.

13.1 Internationalization and Localization
Internationalization means writing (or modifying) a program once, in such a way that it
can use multiple languages without requiring further source code changes. Localizationmeans providing the data necessary for an internationalized program to work in a particular language. Most typically, these terms refer to features such as the language used forprinting error messages, the language used to read responses, and information related to
how numerical and monetary values are printed and read.

13.2 GNU gettext
gawk uses GNU gettext to provide its internationalization features. The facilities in GNUgettext focus on messages: strings printed by a program, either directly or via formattingwith printf or sprintf().1
When using GNU gettext, each application has its own text domain. This is a unique
name, such as ‘kpilot’ or ‘gawk’, that identifies the application. A complete application
may have multiple components—programs written in C or C++, as well as scripts written
in sh or awk. All of the components use the same text domain.
To make the discussion concrete, assume we’re writing an application named guide.
Internationalization consists of the following steps, in this order:
1. The programmer reviews the source for all of guide’s components and marks eachstring that is a candidate for translation. For example, "`-F': option required" isa good candidate for translation. A table with strings of option names is not (e.g.,
gawk’s --profile option should remain the same, no matter what the local language).
2. The programmer indicates the application’s text domain ("guide") to the gettext
library, by calling the textdomain() function.
1

For some operating systems, the gawk port doesn’t support GNU gettext. Therefore, these features are
not available if you are using one of those operating systems. Sorry.

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3. Messages from the application are extracted from the source code and collected into a
portable object template file (guide.pot), which lists the strings and their translations.
The translations are initially empty. The original (usually English) messages serve as
the key for lookup of the translations.
4. For each language with a translator, guide.pot is copied to a portable object file(.po) and translations are created and shipped with the application. For example,
there might be a fr.po for a French translation.
5. Each language’s .po file is converted into a binary message object (.gmo) file. A
message object file contains the original messages and their translations in a binary
format that allows fast lookup of translations at runtime.
6. When guide is built and installed, the binary translation files are installed in a standard
place.
7. For testing and development, it is possible to tell gettext to use .gmo files in a different
directory than the standard one by using the bindtextdomain() function.
8. At runtime, guide looks up each string via a call to gettext(). The returned stringis the translated string if available, or the original string if not.
9. If necessary, it is possible to access messages from a different text domain than the one
belonging to the application, without having to switch the application’s default text
domain back and forth.
In C (or C++), the string marking and dynamic translation lookup are accomplished bywrapping each string in a call to gettext():
printf("%s", gettext("Don't Panic!\n"));The tools that extract messages from source code pull out all strings enclosed in calls togettext().
The GNU gettext developers, recognizing that typing ‘gettext(...)’ over and overagain is both painful and ugly to look at, use the macro ‘_’ (an underscore) to make thingseasier:/* In the standard header file: */#define _(str) gettext(str)/* In the program text: */printf("%s", _("Don't Panic!\n"));This reduces the typing overhead to just three extra characters per string and is considerablyeasier to read as well.
There are locale categories for different types of locale-related information. The defined
locale categories that gettext knows about are:
LC_MESSAGESText messages. This is the default category for gettext operations, but it ispossible to supply a different one explicitly, if necessary. (It is almost nevernecessary to supply a different category.)
LC_COLLATE
Text-collation information (i.e., how different characters and/or groups of characters sort in a given language).

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LC_CTYPE

Character-type information (alphabetic, digit, upper- or lowercase, and so on)
as well as character encoding. This information is accessed via the POSIX
character classes in regular expressions, such as /[[:alnum:]]/ (see Section 3.4[Using Bracket Expressions], page 58).

LC_MONETARY
Monetary information, such as the currency symbol, and whether the symbol
goes before or after a number.
LC_NUMERIC
Numeric information, such as which characters to use for the decimal point andthe thousands separator.2
LC_TIME

Time- and date-related information, such as 12- or 24-hour clock, month printed
before or after the day in a date, local month abbreviations, and so on.

LC_ALLAll of the above. (Not too useful in the context of gettext.)

NOTE: As described in Section 6.6 [Where You Are Makes a Difference],
page 145, environment variables with the same name as the locale categories(LC_CTYPE, LC_ALL, etc.) influence gawk’s behavior (and that of otherutilities).
Normally, these variables also affect how the gettext library finds translations.
However, the LANGUAGE environment variable overrides the LC_xxx variables.
Many GNU/Linux systems may define this variable without your knowledge,
causing gawk to not find the correct translations. If this happens to you, look to
see if LANGUAGE is defined, and if so, use the shell’s unset command to removeit.
For testing translations of gawk itself, you can set the GAWK_LOCALE_DIR environment
variable. See the documentation for the C bindtextdomain() function and also see
Section 2.5.3 [Other Environment Variables], page 44.

13.3 Internationalizing awk Programs
gawk provides the following variables for internationalization:
TEXTDOMAIN
This variable indicates the application’s text domain. For compatibility with
GNU gettext, the default value is "messages".
_"your message here"
String constants marked with a leading underscore are candidates for translation at runtime. String constants without a leading underscore are not translated.
gawk provides the following functions for internationalization:
dcgettext(string [, domain [, category]])
Return the translation of string in text domain domain for locale category
category. The default value for domain is the current value of TEXTDOMAIN.
The default value for category is "LC_MESSAGES".
2

Americans use a comma every three decimal places and a period for the decimal point, while many
Europeans do exactly the opposite: 1,234.56 versus 1.234,56.

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If you supply a value for category, it must be a string equal to one of the known
locale categories described in the previous section. You must also supply a text
domain. Use TEXTDOMAIN if you want to use the current domain.
CAUTION: The order of arguments to the awk version of thedcgettext() function is purposely different from the order for the
C version. The awk version’s order was chosen to be simple and toallow for reasonable awk-style default arguments.
dcngettext(string1, string2, number [, domain [, category]])
Return the plural form used for number of the translation of string1 and string2
in text domain domain for locale category category. string1 is the English
singular variant of a message, and string2 is the English plural variant of thesame message. The default value for domain is the current value of TEXTDOMAIN.
The default value for category is "LC_MESSAGES".
The same remarks about argument order as for the dcgettext() function apply.
bindtextdomain(directory [, domain ])Change the directory in which gettext looks for .gmo files, in case they will
not or cannot be placed in the standard locations (e.g., during testing). Returnthe directory in which domain is “bound.”
The default domain is the value of TEXTDOMAIN. If directory is the null string(""), then bindtextdomain() returns the current binding for the given domain.
To use these facilities in your awk program, follow these steps:
1. Set the variable TEXTDOMAIN to the text domain of your program. This is best donein a BEGIN rule (see Section 7.1.4 [The BEGIN and END Special Patterns], page 150), or
it can also be done via the -v command-line option (see Section 2.2 [Command-Line
Options], page 33):
BEGIN {TEXTDOMAIN = "guide"...
}
2. Mark all translatable strings with a leading underscore (‘_’) character. It must be
adjacent to the opening quote of the string. For example:
print _"hello, world"x = _"you goofed"
printf(_"Number of users is %d\n", nusers)
3. If you are creating strings dynamically, you can still translate them, using thedcgettext() built-in function:3
if (groggy)
message = dcgettext("%d customers disturbing me\n", "adminprog")else
message = dcgettext("enjoying %d customers\n", "adminprog")
printf(message, ncustomers)
3

Thanks to Bruno Haible for this example.

Chapter 13: Internationalization with gawk 361

Here, the call to dcgettext() supplies a different text domain ("adminprog") in which
to find the message, but it uses the default "LC_MESSAGES" category.
The previous example only works if ncustomers is greater than one. This example
would be better done with dcngettext():
if (groggy)
message = dcngettext("%d customer disturbing me\n","%d customers disturbing me\n",ncustomers, "adminprog")else
message = dcngettext("enjoying %d customer\n","enjoying %d customers\n",ncustomers, "adminprog")
printf(message, ncustomers)
4. During development, you might want to put the .gmo file in a private directory fortesting. This is done with the bindtextdomain() built-in function:
BEGIN {TEXTDOMAIN = "guide"# our text domain
if (Testing) {# where to find our files
bindtextdomain("testdir")# joe is in charge of adminprog
bindtextdomain("../joe/testdir", "adminprog")}...
}
See Section 13.5 [A Simple Internationalization Example], page 364, for an example
program showing the steps to create and use translations from awk.

13.4 Translating awk ProgramsOnce a program’s translatable strings have been marked, they must be extracted to create
the initial .pot file. As part of translation, it is often helpful to rearrange the order inwhich arguments to printf are output.
gawk’s --gen-pot command-line option extracts the messages and is discussed next.
After that, printf’s ability to rearrange the order for printf arguments at runtime iscovered.

13.4.1 Extracting Marked Strings
Once your awk program is working, and all the strings have been marked and you’ve set(and perhaps bound) the text domain, it is time to produce translations. First, use the--gen-pot command-line option to create the initial .pot file:gawk --gen-pot -f guide.awk > guide.pot
When run with --gen-pot, gawk does not execute your program. Instead, it parses it
as usual and prints all marked strings to standard output in the format of a GNU gettext
Portable Object file. Also included in the output are any constant strings that appear as the362

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first argument to dcgettext() or as the first and second argument to dcngettext().4 Youshould distribute the generated .pot file with your awk program; translators will eventually
use it to provide you translations that you can also then distribute. See Section 13.5 [A
Simple Internationalization Example], page 364, for the full list of steps to go through tocreate and test translations for guide.

13.4.2 Rearranging printf Arguments
Format strings for printf and sprintf() (see Section 5.5 [Using printf Statements for
Fancier Printing], page 102) present a special problem for translation. Consider the following:5
printf(_"String `%s' has %d characters\n",string, length(string)))
A possible German translation for this might be:"%d Zeichen lang ist die Zeichenkette `%s'\n"
The problem should be obvious: the order of the format specifications is different fromthe original! Even though gettext() can return the translated string at runtime, it cannotchange the argument order in the call to printf.
To solve this problem, printf format specifiers may have an additional optional element,
which we call a positional specifier. For example:"%2$d Zeichen lang ist die Zeichenkette `%1$s'\n"Here, the positional specifier consists of an integer count, which indicates which argument
to use, and a ‘$’. Counts are one-based, and the format string itself is not included. Thus, inthe following example, ‘string’ is the first argument and ‘length(string)’ is the second:$ gawk 'BEGIN {>string = "Don\47t Panic">
printf "%2$d characters live in \"%1$s\"\n",>string, length(string)> }'a 11 characters live in "Don't Panic"
If present, positional specifiers come first in the format specification, before the flags,
the field width, and/or the precision.
Positional specifiers can be used with the dynamic field width and precision capability:$ gawk 'BEGIN {>printf("%*.*s\n", 10, 20, "hello")>
printf("%3$*2$.*1$s\n", 20, 10, "hello")
> }'
hello
a
hello
a
NOTE: When using ‘*’ with a positional specifier, the ‘*’ comes first, then the
integer position, and then the ‘$’. This is somewhat counterintuitive.
4
5

The xgettext utility that comes with GNU gettext can handle .awk files.
This example is borrowed from the GNU gettext manual.

Chapter 13: Internationalization with gawk 363

gawk does not allow you to mix regular format specifiers and those with positional
specifiers in the same string:$ gawk 'BEGIN { printf "%d %3$s\n", 1, 2, "hi" }'error gawk: cmd. line:1: fatal: must use `count$' on all formats or none
NOTE: There are some pathological cases that gawk may fail to diagnose. In
such cases, the output may not be what you expect. It’s still a bad idea to try
mixing them, even if gawk doesn’t detect it.
Although positional specifiers can be used directly in awk programs, their primary purpose is to help in producing correct translations of format strings into languages different
from the one in which the program is first written.

13.4.3 awk Portability Issues
gawk’s internationalization features were purposely chosen to have as little impact as possible on the portability of awk programs that use them to other versions of awk. Consider
this program:
BEGIN {TEXTDOMAIN = "guide"
if (Test_Guide)# set with -v
bindtextdomain("/test/guide/messages")
print _"don't panic!"}
As written, it won’t work on other versions of awk. However, it is actually almost portable,
requiring very little change:• Assignments to TEXTDOMAIN won’t have any effect, because TEXTDOMAIN is not special
in other awk implementations.
• Non-GNU versions of awk treat marked strings as the concatenation of a variable named_ with the string following it.6 Typically, the variable _ has the null string ("") as its
value, leaving the original string constant as the result.
• By defining “dummy” functions to replace dcgettext(), dcngettext(), andbindtextdomain(), the awk program can be made to run, but all the messages are
output in the original language. For example:
function bindtextdomain(dir, domain){
return dir}function dcgettext(string, domain, category){return string}
function dcngettext(string1, string2, number, domain, category){
6

This is good fodder for an “Obfuscated awk” contest.

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return (number == 1 ? string1 : string2)}• The use of positional specifications in printf or sprintf() is not portable. To support
gettext() at the C level, many systems’ C versions of sprintf() do support positional
specifiers. But it works only if enough arguments are supplied in the function call.
Many versions of awk pass printf formats and arguments unchanged to the underlying
C library version of sprintf(), but only one format and argument at a time. What
happens if a positional specification is used is anybody’s guess. However, because thepositional specifications are primarily for use in translated format strings, and because
non-GNU awks never retrieve the translated string, this should not be a problem inpractice.

13.5 A Simple Internationalization Example
Now let’s look at a step-by-step example of how to internationalize and localize a simple
awk program, using guide.awk as our original source:
BEGIN {TEXTDOMAIN = "guide"bindtextdomain(".") # for testing
print _"Don't Panic"
print _"The Answer Is", 42
print "Pardon me, Zaphod who?"}
Run ‘gawk --gen-pot’ to create the .pot file:$ gawk --gen-pot -f guide.awk > guide.pot
This produces:#: guide.awk:4
msgid "Don't Panic"
msgstr ""#: guide.awk:5
msgid "The Answer Is"
msgstr ""This original portable object template file is saved and reused for each language into
which the application is translated. The msgid is the original string and the msgstr is thetranslation.
NOTE: Strings not marked with a leading underscore do not appear in theguide.pot file.
Next, the messages must be translated. Here is a translation to a hypothetical dialectof English, called “Mellow”:7$ cp guide.pot guide-mellow.po
Add translations to guide-mellow.po ...
7

Perhaps it would be better if it were called “Hippy.” Ah, well.

Chapter 13: Internationalization with gawk 365

Following are the translations:#: guide.awk:4
msgid "Don't Panic"
msgstr "Hey man, relax!"#: guide.awk:5
msgid "The Answer Is"
msgstr "Like, the scoop is"
NOTE: The following instructions apply to GNU/Linux with the GNU C Library. Be aware that the actual steps may change over time, that the following
description may not be accurate for all GNU/Linux distributions, and thatthings may work entirely differently on other operating systems.
The next step is to make the directory to hold the binary message object file and thento create the guide.mo file. The directory has the form locale/LC_MESSAGES, where locale
is a locale name known to the C gettext routines.
How do we know which locale to use? It turns out that there are four different environment variables used by the C gettext routines. In order, they are $LANGUAGE, $LC_ALL,$LANG, and $LC_MESSAGES.8 Thus, we check the value of $LANGUAGE:$ echo $LANGUAGEa en_US.UTF-8
We next make the directories:$ mkdir en_US.UTF-8 en_US.UTF-8/LC_MESSAGESThe msgfmt utility converts the human-readable .po file into a machine-readable .mofile. By default, msgfmt creates a file named messages. This file must be renamed andplaced in the proper directory (using the -o option) so that gawk can find it:$ msgfmt guide-mellow.po -o en_US.UTF-8/LC_MESSAGES/guide.mo
Finally, we run the program to test it:$ gawk -f guide.awk
a Hey man, relax!
a Like, the scoop is 42
a Pardon me, Zaphod who?
If the three replacement functions for dcgettext(), dcngettext(), andbindtextdomain() (see Section 13.4.3 [awk Portability Issues], page 363) are in a file
named libintl.awk, then we can run guide.awk unchanged as follows:$ gawk --posix -f guide.awk -f libintl.awk
a Don't Panica The Answer Is 42
a Pardon me, Zaphod who?
8

Well, sort of. It seems that if $LC_ALL is set to ‘C’, then no translations are done. Go figure.

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13.6 gawk Can Speak Your Language
gawk itself has been internationalized using the GNU gettext package. (GNU gettext
is described in complete detail in GNU gettext utilities.) As of this writing, the latest
version of GNU gettext is version 0.19.8.1.
If a translation of gawk’s messages exists, then gawk produces usage messages, warnings,
and fatal errors in the local language.

13.7 Summary• Internationalization means writing a program such that it can use multiple languages
without requiring source code changes. Localization means providing the data necessary for an internationalized program to work in a particular language.
• gawk uses GNU gettext to let you internationalize and localize awk programs. A
program’s text domain identifies the program for grouping all messages and other data
together.
• You mark a program’s strings for translation by preceding them with an underscore.
Once that is done, the strings are extracted into a .pot file. This file is copied foreach language into a .po file, and the .po files are compiled into .gmo files for use atruntime.
• You can use positional specifications with sprintf() and printf to rearrange theplacement of argument values in formatted strings and output. This is useful for thetranslation of format control strings.
• The internationalization features have been designed so that they can be easily worked
around in a standard awk.
• gawk itself has been internationalized and ships with a number of translations for its
messages.

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367

14 Debugging awk Programs
It would be nice if computer programs worked perfectly the first time they were run, but
in real life, this rarely happens for programs of any complexity. Thus, most programming
languages have facilities available for “debugging” programs, and awk is no exception.
The gawk debugger is purposely modeled after the GNU Debugger (GDB) commandline debugger. If you are familiar with GDB, learning how to use gawk for debugging yourprograms is easy.

14.1 Introduction to the gawk Debugger
This section introduces debugging in general and begins the discussion of debugging in
gawk.

14.1.1 Debugging in General(If you have used debuggers in other languages, you may want to skip ahead to Section 14.1.3[awk Debugging], page 368.)
Of course, a debugging program cannot remove bugs for you, because it has no wayof knowing what you or your users consider a “bug” versus a “feature.” (Sometimes, we
humans have a hard time with this ourselves.) In that case, what can you expect from sucha tool? The answer to that depends on the language being debugged, but in general, you
can expect at least the following:• The ability to watch a program execute its instructions one by one, giving you, theprogrammer, the opportunity to think about what is happening on a time scale ofseconds, minutes, or hours, rather than the nanosecond time scale at which the code
usually runs.
• The opportunity to not only passively observe the operation of your program, but tocontrol it and try different paths of execution, without having to change your source
files.
• The chance to see the values of data in the program at any point in execution, and also
to change that data on the fly, to see how that affects what happens afterward. (This
often includes the ability to look at internal data structures besides the variables you
actually defined in your code.)• The ability to obtain additional information about your program’s state or even its
internal structure.
All of these tools provide a great amount of help in using your own skills and understanding of the goals of your program to find where it is going wrong (or, for that matter,to better comprehend a perfectly functional program that you or someone else wrote).

14.1.2 Debugging Concepts
Before diving in to the details, we need to introduce several important concepts that apply
to just about all debuggers. The following list defines terms used throughout the rest of
this chapter:
Stack frame
Programs generally call functions during the course of their execution. One
function can call another, or a function can call itself (recursion). You can

368

GAWK: Effective AWK Programmingview the chain of called functions (main program calls A, which calls B, whichcalls C), as a stack of executing functions: the currently running function is the
topmost one on the stack, and when it finishes (returns), the next one downthen becomes the active function. Such a stack is termed a call stack.
For each function on the call stack, the system maintains a data area that
contains the function’s parameters, local variables, and return value, as well as
any other “bookkeeping” information needed to manage the call stack. Thisdata area is termed a stack frame.
gawk also follows this model, and gives you access to the call stack and to eachstack frame. You can see the call stack, as well as from where each function on
the stack was invoked. Commands that print the call stack print information
about each stack frame (as detailed later on).
Breakpoint
During debugging, you often wish to let the program run until it reaches acertain point, and then continue execution from there one statement (or instruction) at a time. The way to do this is to set a breakpoint within the
program. A breakpoint is where the execution of the program should break off(stop), so that you can take over control of the program’s execution. You can
add and remove as many breakpoints as you like.
Watchpoint
A watchpoint is similar to a breakpoint. The difference is that breakpoints areoriented around the code: stop when a certain point in the code is reached.
A watchpoint, however, specifies that program execution should stop when adata value is changed. This is useful, as sometimes it happens that a variable
receives an erroneous value, and it’s hard to track down where this happens
just by looking at the code. By using a watchpoint, you can stop whenever avariable is assigned to, and usually find the errant code quite quickly.

14.1.3 awk Debugging
Debugging an awk program has some specific aspects that are not shared with programs
written in other languages.
First of all, the fact that awk programs usually take input line by line from a file or files
and operate on those lines using specific rules makes it especially useful to organize viewing
the execution of the program in terms of these rules. As we will see, each awk rule is treatedalmost like a function call, with its own specific block of instructions.
In addition, because awk is by design a very concise language, it is easy to lose sightof everything that is going on “inside” each line of awk code. The debugger provides theopportunity to look at the individual primitive instructions carried out by the higher-levelawk commands.1
1

The “primitive instructions” are defined by gawk itself; the debugger does not work at the level of machine
instructions.

Chapter 14: Debugging awk Programs369

14.2 Sample gawk Debugging Session
In order to illustrate the use of gawk as a debugger, let’s look at a sample debugging session.
We will use the awk implementation of the POSIX uniq command presented earlier (seeSection 11.2.6 [Printing Nonduplicated Lines of Text], page 300) as our example.

14.2.1 How to Start the Debugger
Starting the debugger is almost exactly like running gawk normally, except you have to passan additional option, --debug, or the corresponding short option, -D. The file(s) containing
the program and any supporting code are given on the command line as arguments to one
or more -f options. (gawk is not designed to debug command-line programs, only programs
contained in files.) In our case, we invoke the debugger like this:$ gawk -D -f getopt.awk -f join.awk -f uniq.awk -- -1 inputfile
where both getopt.awk and uniq.awk are in $AWKPATH. (Experienced users of GDB or
similar debuggers should note that this syntax is slightly different from what you are used
to. With the gawk debugger, you give the arguments for running the program in thecommand line to the debugger rather than as part of the run command at the debugger
prompt.) The -- ends gawk’s command line options. It’s not strictly necessary here, but it
is needed if an option to the awk program conflicts with a gawk option. The -1 is an option
to uniq.awk.
Instead of immediately running the program on inputfile, as gawk would ordinarily
do, the debugger merely loads all the program source files, compiles them internally, andthen gives us a prompt:
gawk>from which we can issue commands to the debugger. At this point, no code has been
executed.

14.2.2 Finding the Bug
Let’s say that we are having a problem using (a faulty version of) uniq.awk in “fieldskipping” mode, and it doesn’t seem to be catching lines which should be identical whenskipping the first field, such as:
awk is a wonderful program!
gawk is a wonderful program!
This could happen if we were thinking (C-like) of the fields in a record as being numbered
in a zero-based fashion, so instead of the lines:
clast = join(alast, fcount+1, n)cline = join(aline, fcount+1, m)
we wrote:
clast = join(alast, fcount, n)cline = join(aline, fcount, m)
The first thing we usually want to do when trying to investigate a problem like this is
to put a breakpoint in the program so that we can watch it at work and catch what it isdoing wrong. A reasonable spot for a breakpoint in uniq.awk is at the beginning of the370

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function are_equal(), which compares the current line with the previous one. To set the
breakpoint, use the b (breakpoint) command:
gawk> b are_equala Breakpoint 1 set at file `awklib/eg/prog/uniq.awk', line 63
The debugger tells us the file and line number where the breakpoint is. Now type ‘r’ or‘run’ and the program runs until it hits the breakpoint for the first time:gawk> ra Starting program:a Stopping in Rule ...
a Breakpoint 1, are_equal(n, m, clast, cline, alast, aline)at `awklib/eg/prog/uniq.awk':63
if (fcount == 0 && charcount == 0)
a 63
gawk>
Now we can look at what’s going on inside our program. First of all, let’s see how we
got to where we are. At the prompt, we type ‘bt’ (short for “backtrace”), and the debugger
responds with a listing of the current stack frames:
gawk> bt
a #0 are_equal(n, m, clast, cline, alast, aline)at `awklib/eg/prog/uniq.awk':68
a #1 in main() at `awklib/eg/prog/uniq.awk':88
This tells us that are_equal() was called by the main program at line 88 of uniq.awk.
(This is not a big surprise, because this is the only call to are_equal() in the program,but in more complex programs, knowing who called a function and with what parameters
can be the key to finding the source of the problem.)
Now that we’re in are_equal(), we can start looking at the values of some variables.
Let’s say we type ‘p n’ (p is short for “print”). We would expect to see the value of n, a
parameter to are_equal(). Actually, the debugger gives us:
gawk> p n
a n = untyped variable
In this case, n is an uninitialized local variable, because the function was called without
arguments (see Section 6.4 [Function Calls], page 142).
A more useful variable to display might be the current record:
gawk> p $0
a $0 = "gawk is a wonderful program!"
This might be a bit puzzling at first, as this is the second line of our test input. Let’s lookat NR:
gawk> p NR
a NR = 2
So we can see that are_equal() was only called for the second record of the file. Of course,
this is because our program contains a rule for ‘NR == 1’:
NR == 1 {last = $0
nextChapter 14: Debugging awk Programs

371}
OK, let’s just check that that rule worked correctly:
gawk> p lasta last = "awk is a wonderful program!"Everything we have done so far has verified that the program has worked as planned, up
to and including the call to are_equal(), so the problem must be inside this function. To
investigate further, we must begin “stepping through” the lines of are_equal(). We startby typing ‘n’ (for “next”):
gawk> nif (fcount > 0) {a 66
This tells us that gawk is now ready to execute line 66, which decides whether to givethe lines the special “field-skipping” treatment indicated by the -1 command-line option.
(Notice that we skipped from where we were before, at line 63, to here, because the conditionin line 63, ‘if (fcount == 0 && charcount == 0)’, was false.)
Continuing to step, we now get to the splitting of the current and last records:
gawk> nn = split(last, alast)
a 67
gawk> nm = split($0, aline)
a 68
At this point, we should be curious to see what our records were split into, so we try to
look:gawk> p n m alast aline
a n = 5a m = untyped variablea alast = array, 5 elementsa aline = untyped variable(The p command can take more than one argument, similar to awk’s print statement.)
This is kind of disappointing, though. All we found out is that there are five elements inalast; m and aline don’t have values because we are at line 68 but haven’t executed it yet.
This information is useful enough (we now know that none of the words were accidentallyleft out), but what if we want to see inside the array?
The first choice would be to use subscripts:
gawk> p alast[0]a "0" not in array `alast'Oops!
gawk> p alast[1]a alast["1"] = "awk"This would be kind of slow for a 100-member array, though, so gawk provides a shortcut(reminiscent of another language not to be mentioned):gawk> p @alasta alast["1"] = "awk"a alast["2"] = "is"a alast["3"] = "a"372

GAWK: Effective AWK Programminga alast["4"] = "wonderful"a alast["5"] = "program!"
It looks like we got this far OK. Let’s take another step or two:gawk> na 69
gawk> na 70

clast = join(alast, fcount, n)cline = join(aline, fcount, m)Well, here we are at our error (sorry to spoil the suspense). What we had in mind was
to join the fields starting from the second one to make the virtual record to compare, andif the first field were numbered zero, this would work. Let’s look at what we’ve got:
gawk> p cline clasta cline = "gawk is a wonderful program!"a clast = "awk is a wonderful program!"Hey, those look pretty familiar! They’re just our original, unaltered input records. A
little thinking (the human brain is still the best debugging tool), and we realize that wewere off by one!
We get out of the debugger:
gawk> qa The program is running. Exit anyway (y/n)? y
Then we get into an editor:
clast = join(alast, fcount+1, n)cline = join(aline, fcount+1, m)
and problem solved!

14.3 Main Debugger Commands
The gawk debugger command set can be divided into the following categories:• Breakpoint control• Execution control• Viewing and changing data
• Working with the stack• Getting information• Miscellaneous
Each of these are discussed in the following subsections. In the following descriptions,commands that may be abbreviated show the abbreviation on a second description line.
A debugger command name may also be truncated if that partial name is unambiguous.
The debugger has the built-in capability to automatically repeat the previous command
just by hitting Enter. This works for the commands list, next, nexti, step, stepi, andcontinue executed without any argument.

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14.3.1 Control of Breakpoints
As we saw earlier, the first thing you probably want to do in a debugging session is to getyour breakpoints set up, because your program will otherwise just run as if it was not under
the debugger. The commands for controlling breakpoints are:break [[filename:]n | function] ["expression"]b [[filename:]n | function] ["expression"]
Without any argument, set a breakpoint at the next instruction to be executedin the selected stack frame. Arguments can be one of the following:n

Set a breakpoint at line number n in the current source file.

filename:n
Set a breakpoint at line number n in source file filename.
functionSet a breakpoint at entry to (the first instruction of) function
function.

Each breakpoint is assigned a number that can be used to delete it from thebreakpoint list using the delete command.
With a breakpoint, you may also supply a condition. This is an awk expression(enclosed in double quotes) that the debugger evaluates whenever the breakpoint is reached. If the condition is true, then the debugger stops execution
and prompts for a command. Otherwise, it continues executing the program.
clear [[filename:]n | function]
Without any argument, delete any breakpoint at the next instruction to be
executed in the selected stack frame. If the program stops at a breakpoint,
this deletes that breakpoint so that the program does not stop at that location
again. Arguments can be one of the following:nDelete breakpoint(s) set at line number n in the current source file.

filename:nDelete breakpoint(s) set at line number n in source file filename.
function

Delete breakpoint(s) set at entry to function function.

condition n "expression"
Add a condition to existing breakpoint or watchpoint n. The condition is an awk
expression enclosed in double quotes that the debugger evaluates whenever the
breakpoint or watchpoint is reached. If the condition is true, then the debugger
stops execution and prompts for a command. Otherwise, the debugger continues
executing the program. If the condition expression is not specified, any existing
condition is removed (i.e., the breakpoint or watchpoint is made unconditional).
delete [n1 n2 . . . ] [n–m]d [n1 n2 . . . ] [n–m]Delete specified breakpoints or a range of breakpoints. Delete all defined breakpoints if no argument is supplied.
disable [n1 n2 . . . | n–m]
Disable specified breakpoints or a range of breakpoints. Without any argument,
disable all breakpoints.

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enable [del | once] [n1 n2 . . . ] [n–m]e [del | once] [n1 n2 . . . ] [n–m]Enable specified breakpoints or a range of breakpoints. Without any argument, enable all breakpoints. Optionally, you can specify how to enable thebreakpoints:
delEnable the breakpoints temporarily, then delete each one when the
program stops at it.

once

Enable the breakpoints temporarily, then disable each one when
the program stops at it.

ignore n countIgnore breakpoint number n the next count times it is hit.
tbreak [[filename:]n | function]t [[filename:]n | function]
Set a temporary breakpoint (enabled for only one stop). The arguments are
the same as for break.

14.3.2 Control of Execution
Now that your breakpoints are ready, you can start running the program and observing its
behavior. There are more commands for controlling execution of the program than we saw
in our earlier example:
commands [n]silent...
end
Set a list of commands to be executed upon stopping at a breakpoint or watchpoint. n is the breakpoint or watchpoint number. Without a number, the last
one set is used. The actual commands follow, starting on the next line, and terminated by the end command. If the command silent is in the list, the usual
messages about stopping at a breakpoint and the source line are not printed.
Any command in the list that resumes execution (e.g., continue) terminates
the list (an implicit end), and subsequent commands are ignored. For example:
gawk> commands> silent> printf "A silent breakpoint; i = %d\n", i> info locals> set i = 10
> continue
> end
gawk>
continue [count]c [count] Resume program execution. If continued from a breakpoint and count is specified, ignore the breakpoint at that location the next count times before stopping.
finishExecute until the selected stack frame returns. Print the returned value.

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375

next [count]n [count] Continue execution to the next source line, stepping over function calls. The
argument count controls how many times to repeat the action, as in step.
nexti [count]ni [count] Execute one (or count) instruction(s), stepping over function calls.
return [value]
Cancel execution of a function call. If value (either a string or a number) is
specified, it is used as the function’s return value. If used in a frame otherthan the innermost one (the currently executing function; i.e., frame number0), discard all inner frames in addition to the selected one, and the caller ofthat frame becomes the innermost frame.
runrStart/restart execution of the program. When restarting, the debugger retains
the current breakpoints, watchpoints, command history, automatic display variables, and debugger options.

step [count]s [count] Continue execution until control reaches a different source line in the current
stack frame, stepping inside any function called within the line. If the argument
count is supplied, steps that many times before stopping, unless it encountersa breakpoint or watchpoint.
stepi [count]si [count] Execute one (or count) instruction(s), stepping inside function calls. (For illustration of what is meant by an “instruction” in gawk, see the output shown
under dump in Section 14.3.6 [Miscellaneous Commands], page 379.)
until [[filename:]n | function]u [[filename:]n | function]
Without any argument, continue execution until a line past the current line in
the current stack frame is reached. With an argument, continue execution until
the specified location is reached, or the current stack frame returns.

14.3.3 Viewing and Changing Data
The commands for viewing and changing variables inside of gawk are:display [var | $n]
Add variable var (or field $n) to the display list. The value of the variable or
field is displayed each time the program stops. Each variable added to the list
is identified by a unique number:gawk> display x
a 10: x = 1
This displays the assigned item number, the variable name, and its current
value. If the display variable refers to a function parameter, it is silently deleted
from the list as soon as the execution reaches a context where no such variableof the given name exists. Without argument, display displays the current
values of items on the list.

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Evaluate awk statements in the context of the running program. You cando anything that an awk program would do: assign values to variables, callfunctions, and so on.
NOTE: You cannot use eval to execute a statement containing anyof the following: exit, getline, next, nextfile, or return.
eval param, . . .
awk statements
endThis form of eval is similar, but it allows you to define “local variables” thatexist in the context of the awk statements, instead of using variables or function
parameters defined by the program.
print var1[, var2 . . . ]p var1[, var2 . . . ]Print the value of a gawk variable or field. Fields must be referenced by constants:
gawk> print $3
This prints the third field in the input record (if the specified field does not exist,
it prints ‘Null field’). A variable can be an array element, with the subscripts
being constant string values. To print the contents of an array, prefix the nameof the array with the ‘@’ symbol:
gawk> print @a
This prints the indices and the corresponding values for all elements in the arraya.
printf format [, arg . . . ]Print formatted text. The format may include escape sequences, such as ‘\n’(see Section 3.2 [Escape Sequences], page 52). No newline is printed unless one
is specified.
set var=value
Assign a constant (number or string) value to an awk variable or field. Stringvalues must be enclosed between double quotes (". . . ").
You can also set special awk variables, such as FS, NF, NR, and so on.
watch var | $n ["expression"]w var | $n ["expression"]
Add variable var (or field $n) to the watch list. The debugger then stops
whenever the value of the variable or field changes. Each watched item isassigned a number that can be used to delete it from the watch list using theunwatch command.
With a watchpoint, you may also supply a condition. This is an awk expression(enclosed in double quotes) that the debugger evaluates whenever the watchpoint is reached. If the condition is true, then the debugger stops execution andprompts for a command. Otherwise, gawk continues executing the program.
undisplay [n]Remove item number n (or all items, if no argument) from the automatic display
list.

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377

unwatch [n]Remove item number n (or all items, if no argument) from the watch list.

14.3.4 Working with the Stack
Whenever you run a program that contains any function calls, gawk maintains a stack of
all of the function calls leading up to where the program is right now. You can see how you
got to where you are, and also move around in the stack to see what the state of things wasin the functions that called the one you are in. The commands for doing this are:backtrace [count]bt [count]where [count]
Print a backtrace of all function calls (stack frames), or innermost count frames
if count > 0. Print the outermost count frames if count < 0. The backtrace
displays the name and arguments to each function, the source file name, andthe line number. The alias where for backtrace is provided for longtime GDB
users who may be used to that command.
down [count]Move count (default 1) frames down the stack toward the innermost frame.
Then select and print the frame.
frame [n]f [n]Select and print stack frame n. Frame 0 is the currently executing, or innermost, frame (function call); frame 1 is the frame that called the innermost one.
The highest-numbered frame is the one for the main program. The printed information consists of the frame number, function and argument names, source
file, and the source line.

up [count] Move count (default 1) frames up the stack toward the outermost frame. Then
select and print the frame.

14.3.5 Obtaining Information About the Program and the
Debugger State
Besides looking at the values of variables, there is often a need to get other sorts of information about the state of your program and of the debugging environment itself. The gawk
debugger has one command that provides this information, appropriately called info. info
is used with one of a number of arguments that tell it exactly what you want to know:
info what
i what
The value for what should be one of the following:args

List arguments of the selected frame.

break

List all currently set breakpoints.

display

List all items in the automatic display list.

frame

Give a description of the selected stack frame.

functions
List all function definitions including source file names and line
numbers.

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localsList local variables of the selected frame.

sourcePrint the name of the current source file. Each time the program
stops, the current source file is the file containing the current
instruction. When the debugger first starts, the current source
file is the first file included via the -f option. The ‘list
filename:lineno’ command can be used at any time to change
the current source.

sources

List all program sources.

variables
List all global variables.
watch

List all items in the watch list.

Additional commands give you control over the debugger, the ability to save the debugger’s state, and the ability to run debugger commands from a file. The commands
are:option [name[=value]]o [name[=value]]
Without an argument, display the available debugger options and their current
values. ‘option name’ shows the current value of the named option. ‘option
name=value’ assigns a new value to the named option. The available options
are:history_size
Set the maximum number of lines to keep in the history file./.gawk_history. The default is 100.
listsize

Specify the number of lines that list prints. The default is 15.

outfileSend gawk output to a file; debugger output still goes to standard
output. An empty string ("") resets output to standard output.

promptChange the debugger prompt. The default is ‘gawk> ’.

save_history [on | off]
Save command history to file ./.gawk_history. The default is on.
save_options [on | off]
Save current options to file ./.gawkrc upon exit. The default is
on. Options are read back into the next session upon startup.
trace [on | off]
Turn instruction tracing on or off. The default is off.
save filename
Save the commands from the current session to the given file name, so thatthey can be replayed using the source command.
source filename
Run command(s) from a file; an error in any command does not terminate
execution of subsequent commands. Comments (lines starting with ‘#’) areChapter 14: Debugging awk Programs

379

allowed in a command file. Empty lines are ignored; they do not repeat thelast command. You can’t restart the program by having more than one runcommand in the file. Also, the list of commands may include additional sourcecommands; however, the gawk debugger will not source the same file more thanonce in order to avoid infinite recursion.
In addition to, or instead of, the source command, you can use the -D fileor --debug=file command-line options to execute commands from a file noninteractively (see Section 2.2 [Command-Line Options], page 33).

14.3.6 Miscellaneous Commands
There are a few more commands that do not fit into the previous categories, as follows:
dump [filename]Dump byte code of the program to standard output or to the file named infilename. This prints a representation of the internal instructions that gawk
executes to implement the awk commands in a program. This can be very
enlightening, as the following partial dump of Davide Brini’s obfuscated code
(see Section 11.3.11 [And Now for Something Completely Different], page 330)
demonstrates:
gawk> dump
# BEGINaaa [ 1:0xfcd340] Op_rulea [ 1:0xfcc240] Op_push_ia [ 1:0xfcc2a0] Op_push_ia [ 1:0xfcc280] Op_matcha [ 1:0xfcc1e0] Op_store_vara [ 1:0xfcc2e0] Op_push_ia [ 1:0xfcc340] Op_push_ia [ 1:0xfcc320] Op_equala [ 1:0xfcc200] Op_store_vara [ 1:0xfcc380] Op_pusha [ 1:0xfcc360] Op_plus_ia [ 1:0xfcc220] Op_push_lhs
a [ 1:0xfcc300] Op_assign_plus:0xfcc2c0] Op_popa [a [ 1:0xfcc400] Op_pusha [ 1:0xfcc420] Op_push_i:0xfcc4a0] Op_no_opa [a [ 1:0xfcc480] Op_push:0xfcc4c0] Op_concata [a [ 1:0xfcc3c0] Op_store_vara [ 1:0xfcc440] Op_push_lhs
a [ 1:0xfcc3a0] Op_postincrementa [ 1:0xfcc4e0] Op_pusha [ 1:0xfcc540] Op_pusha [ 1:0xfcc500] Op_plus
a [ 1:0xfcc580] Op_pusha [ 1:0xfcc560] Op_plus
a [ 1:0xfcc460] Op_leq:0xfcc5c0] Op_jmp_falsea [a [ 1:0xfcc600] Op_push_i:0xfcc660] Op_no_opa [a [ 1:0xfcc520] Op_assign_concat: [in_rule = BEGIN] [source_file = brini.awk]: "~" [MALLOC|STRING|STRCUR]: "~" [MALLOC|STRING|STRCUR]:: O: "==" [MALLOC|STRING|STRCUR]: "==" [MALLOC|STRING|STRCUR]
:
: o
: o
: 0 [MALLOC|NUMCUR|NUMBER]: o [do_reference = true]::: O: "" [MALLOC|STRING|STRCUR]:: O: [expr_count = 3] [concat_flag = 0]: x
: X [do_reference = true]
:
: x
: o
:
: o
:
:
: [target_jmp = 0xfcc5e0]: "%c" [MALLOC|STRING|STRCUR]
:
: c

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a [
...
a [
a [
a [
a [
a [
a [
a [
a [
gawk>

exit
help
h

:0xfcc620] Op_jmp: [target_jmp = 0xfcc440]

2:0xfcc5a0] Op_K_printf:0xfcc140] Op_no_op:0xfcc1c0] Op_atexit:0xfcc640] Op_stop:0xfcc180] Op_no_op:0xfcd150] Op_after_beginfile:0xfcc160] Op_no_op:0xfcc1a0] Op_after_endfile: [expr_count = 17] [redir_type = ""]
:
:
:
:
:
:
:

Exit the debugger. See the entry for ‘quit’, later in this list.
Print a list of all of the gawk debugger commands with a short summary of theirusage. ‘help command’ prints the information about the command command.

list [- | + | n | filename:n | n–m | function]l [- | + | n | filename:n | n–m | function]
Print the specified lines (default 15) from the current source file or the file
named filename. The possible arguments to list are as follows:- (Minus) Print lines before the lines last printed.
+

Print lines after the lines last printed. list without any argument
does the same thing.

nPrint lines centered around line number n.

n–mPrint lines from n to m.

filename:nPrint lines centered around line number n in source file filename.
This command may change the current source file.
functionquit
qPrint lines centered around the beginning of the function function.
This command may change the current source file.

Exit the debugger. Debugging is great fun, but sometimes we all have to tend
to other obligations in life, and sometimes we find the bug and are free to go on
to the next one! As we saw earlier, if you are running a program, the debugger
warns you when you type ‘q’ or ‘quit’, to make sure you really want to quit.

trace [on | off]
Turn on or off continuous printing of the instructions that are about to be
executed, along with the awk lines they implement. The default is off.
It is to be hoped that most of the “opcodes” in these instructions are fairly
self-explanatory, and using stepi and nexti while trace is on will make theminto familiar friends.

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381

14.4 Readline Support
If gawk is compiled with the GNU Readline library, you can take advantage of that library’s
command completion and history expansion features. The following types of completion
are available:
Command completion
Command names.
Source file name completion
Source file names. Relevant commands are break, clear, list, tbreak, anduntil.
Argument completion
Non-numeric arguments to a command. Relevant commands are enable andinfo.
Variable name completion
Global variable names, and function arguments in the current context if the
program is running. Relevant commands are display, print, set, and watch.

14.5 Limitations
We hope you find the gawk debugger useful and enjoyable to work with, but as with any
program, especially in its early releases, it still has some limitations. A few that it’s worth
being aware of are:
• At this point, the debugger does not give a detailed explanation of what you did wrong
when you type in something it doesn’t like. Rather, it just responds ‘syntax error’.
When you do figure out what your mistake was, though, you’ll feel like a real guru.
• If you perused the dump of opcodes in Section 14.3.6 [Miscellaneous Commands],
page 379, (or if you are already familiar with gawk internals), you will realize thatmuch of the internal manipulation of data in gawk, as in many interpreters, is done ona stack. Op_push, Op_pop, and the like are the “bread and butter” of most gawk code.
Unfortunately, as of now, the gawk debugger does not allow you to examine the stack’s
contents. That is, the intermediate results of expression evaluation are on the stack,but cannot be printed. Rather, only variables that are defined in the program canbe printed. Of course, a workaround for this is to use more explicit variables at the
debugging stage and then change back to obscure, perhaps more optimal code later.
• There is no way to look “inside” the process of compiling regular expressions to see
if you got it right. As an awk programmer, you are expected to know the meaning of/[^[:alnum:][:blank:]]/.
• The gawk debugger is designed to be used by running a program (with all its parameters) on the command line, as described in Section 14.2.1 [How to Start the Debugger],
page 369. There is no way (as of now) to attach or “break into” a running program.
This seems reasonable for a language that is used mainly for quickly executing, shortprograms.
• The gawk debugger only accepts source code supplied with the -f option. If you have ashell script that provides an awk program as a command line parameter, and you need

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to use the debugger, you can write the script to a temporary file, and use that as the
program, with the -f option. This might look like this:cat << \EOF > /tmp/script.$$...
Your program here
EOF
gawk -D -f /tmp/script.$$
rm /tmp/script.$$

14.6 Summary• Programs rarely work correctly the first time. Finding bugs is called debugging, anda program that helps you find bugs is a debugger. gawk has a built-in debugger thatworks very similarly to the GNU Debugger, GDB.
• Debuggers let you step through your program one statement at a time, examine andchange variable and array values, and do a number of other things that let you understand what your program is actually doing (as opposed to what it is supposed todo).
• Like most debuggers, the gawk debugger works in terms of stack frames, and lets you
set both breakpoints (stop at a point in the code) and watchpoints (stop when a data
value changes).
• The debugger command set is fairly complete, providing control over breakpoints, execution, viewing and changing data, working with the stack, getting information, andother tasks.
• If the GNU Readline library is available when gawk is compiled, it is used by the
debugger to provide command-line history and editing.
• Usually, the debugger does not affect the program being debugged, but occasionally it
can.

Chapter 15: Namespaces in gawk 383

15 Namespaces in gawk
This chapter describes a feature that is specific to gawk.
CAUTION: This feature described in this chapter is new. It is entirely possible,
and even likely, that there are dark corners (if not bugs) still lurking within theimplementation. If you find any such, please report them (See Section B.4[Reporting Problems and Bugs], page 505).

15.1 Standard awk’s Single Namespace
In standard awk, there is a single, global, namespace. This means that all function names
and global variable names must be unique. For example, two different awk source files
cannot both define a function named min(), or define the same identifier, used as a scalarin one and as an array in the other.
This situation is okay when programs are small, say a few hundred lines, or even a few
thousand, but it prevents the development of reusable libraries of awk functions, and can
inadvertently cause independently-developed library files to accidentally step on each other’s“private” global variables (see Section 10.1 [Naming Library Function Global Variables],page 242).
Most other programming languages solve this issue by providing some kind of namespacecontrol: a way to say “this function is in namespace xxx, and that function is in namespaceyyy.” (Of course, there is then still a single namespace for the namespaces, but the hopeis that there are much fewer namespaces in use by any given program, and thus much lesschance for collisions.) These facilities are sometimes referred to as packages or modules.
Starting with version 5.0, gawk provides a simple mechanism to put functions and global
variables into separate namespaces.

15.2 Qualified Names
A qualified name is an identifier that includes a namespace name, the namespace
separator ::, and a component name. For example, one might have a function named
posix::getpid(). Here, the namespace is posix and the function name within thenamespace (the component) is getpid(). The namespace and component names are
separated by a double-colon. Only one such separator is allowed in a qualified name.
NOTE: Unlike C++, the :: is not an operator. No spaces are allowed between
the namespace name, the ::, and the component name.
You must use qualified names from one namespace to access variables and functions inanother. This is especially important when using variable names to index the special SYMTAB
array (see Section 7.5.2 [Built-in Variables That Convey Information], page 165), and when
making indirect function calls (see Section 9.3 [Indirect Function Calls], page 231).

15.3 The Default Namespace
The default namespace, not surprisingly, is awk. All of the predefined awk and gawk variables
are in this namespace, and thus have qualified names like awk::ARGC, awk::NF, and so on.
Furthermore, even when you have changed the namespace for your current source file(see Section 15.4 [Changing The Namespace], page 384), gawk forces unqualified identifiers

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whose names are all uppercase letters to be in the awk namespace. This makes it possible
for you to easily reference gawk’s global variables from different namespaces. It also keeps
your code looking natural.

15.4 Changing The Namespace
In order to set the current namespace, use an @namespace directive at the top level of yourprogram:@namespace "passwd"
BEGIN { ... }...
After this directive, all simple non-completely-uppercase identifiers are placed into thepasswd namespace.
You can change the namespace multiple times within a single source file, although thisis likely to become confusing if you do it too much.
NOTE: Association of unqualified identifiers to a namespace is handled while
gawk parses your program, before it starts to run. There is no concept of a “current” namespace once your program starts executing. Be sure you understandthis.
Each source file for -i and -f starts out with an implicit ‘@namespace "awk"’. Similarly,
each chunk of command-line code supplied with -e has such an implicit initial statement(see Section 2.2 [Command-Line Options], page 33).
Files included with @include (see Section 2.7 [Including Other Files into Your Program],
page 46) “push” and “pop” the current namespace. That is, each @include saves the current
namespace and starts over with an implicit ‘@namespace "awk"’ which remains in effect until
an explicit @namespace directive is seen. When gawk finishes processing the included file,the saved namespace is restored and processing continues where it left off in the originalfile.
The use of @namespace has no influence upon the order of execution of BEGIN, BEGINFILE,
END, and ENDFILE rules.

15.5 Namespace and Component Naming Rules
A number of rules apply to the namespace and component names, as follows.
• It is a syntax error to use qualified names for function parameter names.
• It is a syntax error to use any standard awk reserved word (such as if or for), or thename of any standard built-in function (such as sin() or gsub()) as either part of aqualified name. Thus, the following produces a syntax error:@namespace "example"
function gsub(str, pat, result) { ... }• Outside the awk namespace, the names of the additional gawk built-in functions (suchas gensub() or strftime()) may be used as component names. The same set of names
may be used as namespace names, although this has the potential to be confusing.

Chapter 15: Namespaces in gawk 385• The additional gawk built-in functions may still be called from outside the awk namespace by qualifying them. For example, awk::systime(). Here is a somewhat silly
example demonstrating this rule and the previous one:
BEGIN {
print "in awk namespace, systime() =", systime()}@namespace "testing"function systime(){
print "in testing namespace, systime() =", awk::systime()}
BEGIN {
systime()}
When run, it produces output like this:$ gawk -f systime.awk
a in awk namespace, systime() = 1500488503
a in testing namespace, systime() = 1500488503• gawk pre-defined variable names may be used: NF::NR is valid, if possibly not all that
useful.

15.6 Internal Name Management
For backwards compatibility, all identifiers in the awk namespace are stored internally as
unadorned identifiers (that is, without a leading ‘awk::’). This is mainly relevant whenusing such identifiers as indices for SYMTAB, FUNCTAB, and PROCINFO["identifiers"] (seeSection 7.5.2 [Built-in Variables That Convey Information], page 165), and for use in indirectfunction calls (see Section 9.3 [Indirect Function Calls], page 231).
In program code, to refer to variables and functions in the awk namespace from anothernamespace, you must still use the ‘awk::’ prefix. For example:@namespace "awk"This is the default namespace
BEGIN {
Title = "My Report"}

Qualified name is awk::Title@namespace "report"

Now in report namespacefunction compute(){
print awk::Title...
}

This is really report::compute()
But would be SYMTAB["Title"]

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15.7 Namespace Example
The following example is a revised version of the suite of routines developed in Section 10.5
[Reading the User Database], page 266. See there for an explanation of how the code works.
The formulation here, due mainly to Andrew Schorr, is rather elegant. All of the implementation functions and variables are in the passwd namespace, whereas the main interfacefunctions are defined in the awk namespace.
# ns_passwd.awk --- access password file information@namespace "passwd"
BEGIN {
# tailor this to suit your system
Awklib = "/usr/local/libexec/awk/"}
function Init({
if (Inited)
returnoldfs, oldrs, olddol0, pwcat, using_fw, using_fpat)

oldfs = FSoldrs = RS
olddol0 = $0
using_fw = (PROCINFO["FS"] == "FIELDWIDTHS")
using_fpat = (PROCINFO["FS"] == "FPAT")
FS = ":"
RS = "\n"pwcat = Awklib "pwcat"
while ((pwcat | getline) > 0) {Byname[$1] = $0
Byuid[$3] = $0
Bycount[++Total] = $0}
close(pwcat)
Count = 0
Inited = 1
FS = oldfs
if (using_fw)
FIELDWIDTHS = FIELDWIDTHS
else if (using_fpat)
FPAT = FPAT
RS = oldrs$0 = olddol0}Chapter 15: Namespaces in gawk 387

function awk::getpwnam(name){Init()
return Byname[name]}function awk::getpwuid(uid){
Init()
return Byuid[uid]}function awk::getpwent(){
Init()
if (Count < Total)
return Bycount[++Count]return ""}function awk::endpwent(){
Count = 0}
As you can see, this version also follows the convention mentioned in Section 10.1 [Naming Library Function Global Variables], page 242, whereby global variable and function
names start with a capital letter.
Here is a simple test program. Since it’s in a separate file, unadorned identifiers aresought for in the awk namespace:
BEGIN {
while ((p = getpwent()) != "")print p}Here’s what happens when it’s run:$ gawk -f ns_passwd.awk -f testpasswd.awk
a root:x:0:0:root:/root:/bin/basha daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
a bin:x:2:2:bin:/bin:/usr/sbin/nologin
a sys:x:3:3:sys:/dev:/usr/sbin/nologin...

15.8 Namespaces and Other gawk Features
This section looks briefly at how the namespace facility interacts with other important gawk
features.

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The profiler and pretty-printer (see Section 12.6 [Profiling Your awk Programs], page 348)
have been enhanced to understand namespaces and the namespace naming rules presented
in Section 15.5 [Namespace and Component Naming Rules], page 384. In particular, theoutput groups functions in the same namespace together, and has @namespace directives infront of rules as necessary. This allows component names to be simple identifiers, insteadof using qualified identifiers everywhere.
Interaction with the debugger (see Section 14.1 [Introduction to the gawk Debugger],
page 367) has not had to change (at least as of this writing). Some of the internal byte
codes changed in order to accommodate namespaces, and the debugger’s dump command
was adjusted to match.
The extension API (see Chapter 17 [Writing Extensions for gawk], page 405) has always
allowed for placing functions into a different namespace, although this was not previously
implemented. However, the symbol lookup and symbol update routines did not have provision for including a namespace. That has now been corrected (see Section 17.4.11.1 [Variable
Access and Update by Name], page 429). See Section 17.7.4 [Enabling In-Place File Editing], page 461, for a nice example of an extension that leverages a namespace shared bycooperating awk and C code.

15.9 Summary• Standard awk provides a single namespace for all global identifiers (scalars, arrays, andfunctions). This is limiting when one wants to develop libraries of reusable functions
or function suites.
• gawk provides multiple namespaces by using qualified names: names consisting of anamespace name, a double colon, ::, and a component name. Namespace names
might still possibly conflict, but this is true of any language providing namespaces,
modules, or packages.
• The default namespace is awk. The rules for namespace and component names areprovided in Section 15.5 [Namespace and Component Naming Rules], page 384. The
rules are designed in such a way as to make namespace-aware code continue to lookand work naturally while still providing the necessary power and flexibility.
• Other parts of gawk have been extended as necessary to integrate namespaces smoothlywith their operation. This applies most notably to the profiler / pretty-printer (seeSection 12.6 [Profiling Your awk Programs], page 348) and to the extension facility (seeChapter 17 [Writing Extensions for gawk], page 405).
• Overall, the namespace facility was designed and implemented such that backwards
compatibility is paramount. Programs that don’t use namespaces should see absolutely
no difference in behavior when run by a namespace-capable version of gawk.

Chapter 16: Arithmetic and Arbitrary-Precision Arithmetic with gawk 389

16 Arithmetic and Arbitrary-Precision Arithmetic
with gawk
This chapter introduces some basic concepts relating to how computers do arithmetic anddefines some important terms. It then proceeds to describe floating-point arithmetic, which
is what awk uses for all its computations, including a discussion of arbitrary-precision
floating-point arithmetic, which is a feature available only in gawk. It continues on topresent arbitrary-precision integers, and concludes with a description of some points where
gawk and the POSIX standard are not quite in agreement.
NOTE: Most users of gawk can safely skip this chapter. But if you want to do
scientific calculations with gawk, this is the place to be.

16.1 A General Description of Computer Arithmetic
Until now, we have worked with data as either numbers or strings. Ultimately, however,computers represent everything in terms of binary digits, or bits. A decimal digit can takeon any of 10 values: zero through nine. A binary digit can take on any of two values, zeroor one. Using binary, computers (and computer software) can represent and manipulate
numerical and character data. In general, the more bits you can use to represent a particularthing, the greater the range of possible values it can take on.
Modern computers support at least two, and often more, ways to do arithmetic. Eachkind of arithmetic uses a different representation (organization of the bits) for the numbers.
The kinds of arithmetic that interest us are:Decimal arithmetic
This is the kind of arithmetic you learned in elementary school, using paper andpencil (and/or a calculator). In theory, numbers can have an arbitrary numberof digits on either side (or both sides) of the decimal point, and the results ofa computation are always exact.
Some modern systems can do decimal arithmetic in hardware, but usually you
need a special software library to provide access to these instructions. There
are also libraries that do decimal arithmetic entirely in software.
Despite the fact that some users expect gawk to be performing decimal arithmetic,1 it does not do so.
Integer arithmetic
In school, integer values were referred to as “whole” numbers—that is, numbers
without any fractional part, such as 1, 42, or −17. The advantage to integer
numbers is that they represent values exactly. The disadvantage is that theirrange is limited.
In computers, integer values come in two flavors: signed and unsigned. Signed
values may be negative or positive, whereas unsigned values are always greaterthan or equal to zero.
In computer systems, integer arithmetic is exact, but the possible range of values
is limited. Integer arithmetic is generally faster than floating-point arithmetic.
1

We don’t know why they expect this, but they do.

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Floating-point arithmetic
Floating-point numbers represent what were called in school “real” numbers(i.e., those that have a fractional part, such as 3.1415927). The advantage tofloating-point numbers is that they can represent a much larger range of values
than can integers. The disadvantage is that there are numbers that they cannot
represent exactly.
Modern systems support floating-point arithmetic in hardware, with a limited
range of values. There are software libraries that allow the use of arbitraryprecision floating-point calculations.
POSIX awk uses double-precision floating-point numbers, which can hold moredigits than single-precision floating-point numbers. gawk has facilities for performing arbitrary-precision floating-point arithmetic, which we describe in moredetail shortly.
Computers work with integer and floating-point values of different ranges. Integer values
are usually either 32 or 64 bits in size. Single-precision floating-point values occupy 32
bits, whereas double-precision floating-point values occupy 64 bits. (Quadruple-precision
floating point values also exist. They occupy 128 bits, but such numbers are not available
in awk.) Floating-point values are always signed. The possible ranges of values are shownin Table 16.1 and Table 16.2.
Representation
32-bit signed integer
32-bit unsigned integer
64-bit signed integer
64-bit unsigned integer

Minimum value−2,147,483,648
0−9,223,372,036,854,775,808
0Maximum value
2,147,483,647
4,294,967,295
9,223,372,036,854,775,807
18,446,744,073,709,551,615

Table 16.1: Value ranges for integer representations
Representation
Single-precision floating-point
Double-precision floating-point
Quadruple-precision floating-point

Minimum positive
nonzero value
1.175494 · 10−38
2.225074 · 10−308
3.362103 · 10−4932

Minimum
finite value−3.402823 · 1038
−1.797693 · 10308−1.189731 · 104932

Maximum
finite value
3.402823 · 1038
1.797693 · 10308
1.189731 · 104932

Table 16.2: Approximate value ranges for floating-point number representations

16.2 Other Stuff to Know
The rest of this chapter uses a number of terms. Here are some informal definitions thatshould help you work your way through the material here:
Accuracy

A floating-point calculation’s accuracy is how close it comes to the real (paper
and pencil) value.

Error

The difference between what the result of a computation “should be” and what
it actually is. It is best to minimize error as much as possible.

Exponent

The order of magnitude of a value; some number of bits in a floating-point value
store the exponent.

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Inf

A special value representing infinity. Operations involving another number andinfinity produce infinity.

NaN“Not a number.” A special value that results from attempting a calculationthat has no answer as a real number. See Section 16.4.1.4 [Floating Point
Values They Didn’t Talk About In School], page 395, for more information
about infinity and not-a-number values.

Normalized
How the significand (see later in this list) is usually stored. The value is adjusted
so that the first bit is one, and then that leading one is assumed instead ofphysically stored. This provides one extra bit of precision.
Precision

The number of bits used to represent a floating-point number. The more bits,
the more digits you can represent. Binary and decimal precisions are related
approximately, according to the formula:prec = 3.322 · dps
Here, prec denotes the binary precision (measured in bits) and dps (short for
decimal places) is the decimal digits.

Rounding mode
How numbers are rounded up or down when necessary. More details are provided later.
Significand
A floating-point value consists of the significand multiplied by 10 to the powerof the exponent. For example, in 1.2345e67, the significand is 1.2345.
Stability

From the Wikipedia article on numerical stability: “Calculations that can beproven not to magnify approximation errors are called numerically stable.”See the Wikipedia article on accuracy and precision for more information on some ofthose terms.
On modern systems, floating-point hardware uses the representation and operations
defined by the IEEE 754 standard. Three of the standard IEEE 754 types are 32-bit
single precision, 64-bit double precision, and 128-bit quadruple precision. The standard
also specifies extended precision formats to allow greater precisions and larger exponent
ranges. (awk uses only the 64-bit double-precision format.)Table 16.3 lists the precision and exponent field values for the basic IEEE 754 binary
formats.
Name

Total bits

Precision

SingleDoubleQuadruple

32
64
128

24
53
113

Minimum
exponent−126−1022−16382

Maximum
exponent+127+1023+16383

Table 16.3: Basic IEEE format values
NOTE: The precision numbers include the implied leading one that gives them
one extra bit of significand.

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16.3 Arbitrary-Precision Arithmetic Features in gawk
This section briefly describes arbitrary-precision arithmetic in gawk.

16.3.1 Arbitrary Precision Arithmetic is On Parole!
As of version 5.2, arbitrary precision arithmetic in gawk is “on parole.” The primary gawk
maintainer is no longer maintaining it. Fortunately, a volunteer from the development teamhas agreed to take it over.
This feature is on parole because its inclusion was a mistake. It has led to endless bug
reports, misuse of the feature and public abuse of the maintainer, for no real increased
value.
If the situation with support changes, the feature will be removed from gawk.
If you use this feature, you should consider finding a different toolset with which toaccomplish your goals.2

16.3.2 Arbitrary Precision Introduction
By default, gawk uses the double-precision floating-point values supplied by the hardwareof the system it runs on. However, if it was compiled to do so, and the -M command-line
option is supplied, gawk uses the GNU MPFR and GNU MP (GMP) libraries for arbitraryprecision arithmetic on numbers. You can see if MPFR support is available like so:$ gawk --version
a GNU Awk 5.2.1, API 3.2, PMA Avon 8-g1, (GNU MPFR 4.1.0, GNU MP 6.2.1)
a Copyright (C) 1989, 1991-2022 Free Software Foundation.
...
(You may see different version numbers than what’s shown here. That’s OK; what’s important is to see that GNU MPFR and GNU MP are listed in the output.)
Additionally, there are a few elements available in the PROCINFO array to provide information about the MPFR and GMP libraries (see Section 7.5.2 [Built-in Variables That
Convey Information], page 165).
The MPFR library provides precise control over precisions and rounding modes, andgives correctly rounded, reproducible, platform-independent results. With the -M commandline option, all floating-point arithmetic operators and numeric functions can yield results
to any desired precision level supported by MPFR.
Two predefined variables, PREC and ROUNDMODE, provide control over the working precision and the rounding mode. The precision and the rounding mode are set globally for every
operation to follow. See Section 16.4.4 [Setting the Precision], page 397, and Section 16.4.5[Setting the Rounding Mode], page 398, for more information.

16.4 Floating-Point Arithmetic: Caveat Emptor!
Math class is tough!
—Teen Talk Barbie, July 1992
2

Of course, you can always continue to use a version of gawk that still supports arbitrary precision
arithmetic. It simply will be unmaintained.

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This section provides a high-level overview of the issues involved when doing lots offloating-point arithmetic.3 The discussion applies to both hardware and arbitrary-precision
floating-point arithmetic.
CAUTION: The material here is purposely general. If you need to do serious
computer arithmetic, you should do some research first, and not rely just on
what we tell you.

16.4.1 Floating-Point Arithmetic Is Not Exact
Binary floating-point representations and arithmetic are inexact. Simple values like 0.1
cannot be precisely represented using binary floating-point numbers, and the limited precision of floating-point numbers means that slight changes in the order of operations or
the precision of intermediate storage can change the result. To make matters worse, with
arbitrary-precision floating-point arithmetic, you can set the precision before starting acomputation, but then you cannot be sure of the number of significant decimal places in
the final result.

16.4.1.1 Many Numbers Cannot Be Represented Exactly
So, before you start to write any code, you should think about what you really want andwhat’s really happening. Consider the two numbers in the following example:
x = 0.875# 1/2 + 1/4 + 1/8
y = 0.425
Unlike the number in y, the number stored in x is exactly representable in binary because
it can be written as a finite sum of one or more fractions whose denominators are all powersof two. When gawk reads a floating-point number from program source, it automaticallyrounds that number to whatever precision your machine supports. If you try to print thenumeric content of a variable using an output format string of "%.17g", it may not produce
the same number as you assigned to it:$ gawk 'BEGIN { x = 0.875; y = 0.425>
printf("%0.17g, %0.17g\n", x, y) }'a 0.875, 0.42499999999999999
Often the error is so small you do not even notice it, and if you do, you can always
specify how much precision you would like in your output. Usually this is a format string
like "%.15g", which, when used in the previous example, produces an output identical tothe input.

16.4.1.2 Be Careful Comparing Values
Because the underlying representation can be a little bit off from the exact value, comparing
floating-point values to see if they are exactly equal is generally a bad idea. Here is an
example where it does not work like you would expect:$ gawk 'BEGIN { print (0.1 + 12.2 == 12.3) }'a 0
3There is a very nice paper on floating-point arithmetic by David Goldberg, “What Every Computer
Scientist Should Know About Floating-Point Arithmetic,” ACM Computing Surveys 23, 1 (1991-03):
5-48. This is worth reading if you are interested in the details, but it does require a background incomputer science.

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The general wisdom when comparing floating-point values is to see if they are withinsome small range of each other (called a delta, or tolerance). You have to decide how smalla delta is important to you. Code to do this looks something like the following:delta = 0.00001
difference = abs(a - b)
if (difference < delta)# all okelse
# not ok

# for example# subtract the two values(We assume that you have a simple absolute value function named abs() defined elsewhere
in your program.) If you write a function to compare values with a delta, you should be
sure to use ‘difference < abs(delta)’ in case someone passes in a negative delta value.

16.4.1.3 Errors Accumulate
The loss of accuracy during a single computation with floating-point numbers usually isn’t
enough to worry about. However, if you compute a value that is the result of a sequence offloating-point operations, the error can accumulate and greatly affect the computation itself.
Here is an attempt to compute the value of π using one of its many series representations:
BEGIN {
x = 1.0 / sqrt(3.0)
n = 6
for (i = 1; i < 30; i++) {n = n * 2.0
x = (sqrt(x * x + 1) - 1) / x
printf("%.15f\n", n * x)}}
When run, the early errors propagate through later computations, causing the loop toterminate prematurely after attempting to divide by zero:$ gawk -f pi.awk
a 3.215390309173475
a 3.159659942097510
a 3.146086215131467
a 3.142714599645573...
a 3.224515243534819
a 2.791117213058638
a 0.000000000000000
error gawk: pi.awk:6: fatal: division by zero attempted
Here is an additional example where the inaccuracies in internal representations yieldan unexpected result:$ gawk 'BEGIN {>for (d = 1.1; d <= 1.5; d += 0.1)>
i++>
print i# loop five times (?)

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> }'
a 4

16.4.1.4 Floating Point Values They Didn’t Talk About In School
Both IEEE 754 floating-point hardware, and MPFR, support two kinds of values that you
probably didn’t learn about in school. The first is infinity, a special value, that can be either
negative or positive, and which is either smaller than any other value (negative infinity),or larger than any other value (positive infinity). When such values are generated, gawk
prints them as either ‘-inf’ or ‘+inf’, respectively. It accepts those strings as data input
and converts them to the proper floating-point values internally.
Infinity values of the same sign compare as equal to each other. Otherwise, operations(addition, subtraction, etc.) involving another number and infinity produce mathematicallyreasonable results.
The second kind of value is “not a number”, or NaN for short.4 This is a special valuethat results from attempting a calculation that has no answer as a real number. In such acase, programs can either receive a floating-point exception, or get NaN back as the result.
The IEEE 754 standard recommends that systems return NaN. Some examples:√sqrt(-1) The −1 makes sense in the range of complex numbers, but not in the rangeof real numbers, so the result is NaN.
log(-8)−8 is out of the domain of log(), so the result is NaN.

NaN values are strange. In particular, they cannot be compared with other floating
point values; any such comparison, except for “is not equal to”, returns false. NaN values
are so much unequal to other values that even comparing two identical NaN values with !=returns true!
NaN values can also be signed, although it depends upon the implementation as to whichsign you get for any operation that returns a NaN. For example, on some systems, sqrt(-1)
returns a negative NaN. On others, it returns a positive NaN.
When such values are generated, gawk prints them as either ‘-nan’ or ‘+nan’, respectively.
Here too, gawk accepts those strings as data input and converts them to the proper floatingpoint values internally.
If you want to dive more deeply into this topic, you can find test programs in C, awk
and Python in the directory awklib/eg/test-programs in the gawk distribution. These
programs enable comparison among programming languages as to how they handle NaN
and infinity values.

16.4.2 Getting the Accuracy You Need
Can arbitrary-precision arithmetic give exact results? There are no easy answers. The
standard rules of algebra often do not apply when using floating-point arithmetic. Amongother things, the distributive and associative laws do not hold completely, and order ofoperation may be important for your computation. Rounding error, cumulative precision
loss, and underflow are often troublesome.
When gawk tests the expressions ‘0.1 + 12.2’ and ‘12.3’ for equality using the machine
double-precision arithmetic, it decides that they are not equal! (See Section 16.4.1.2 [Be4Thanks to Michael Brennan for this description, which we have paraphrased, and for the examples.

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Careful Comparing Values], page 393.) You can get the result you want by increasing the
precision; 56 bits in this case does the job:$ gawk -M -v PREC=56 'BEGIN { print (0.1 + 12.2 == 12.3) }'a 1
If adding more bits is good, perhaps adding even more bits of precision is better? Hereis what happens if we use an even larger value of PREC:$ gawk -M -v PREC=201 'BEGIN { print (0.1 + 12.2 == 12.3) }'a 0This is not a bug in gawk or in the MPFR library. It is easy to forget that the finite
number of bits used to store the value is often just an approximation after proper rounding.
The test for equality succeeds if and only if all bits in the two operands are exactly thesame. Because this is not necessarily true after floating-point computations with a particularprecision and effective rounding mode, a straight test for equality may not work. Instead,compare the two numbers to see if they are within the desirable delta of each other.
In applications where 15 or fewer decimal places suffice, hardware double-precision arithmetic can be adequate, and is usually much faster. But you need to keep in mind that every
floating-point operation can suffer a new rounding error with catastrophic consequences, as
illustrated by our earlier attempt to compute the value of π. Extra precision can greatly
enhance the stability and the accuracy of your computation in such cases.
Additionally, you should understand that repeated addition is not necessarily equivalent
to multiplication in floating-point arithmetic. In the example in Section 16.4.1.3 [Errors
Accumulate], page 394:
$ gawk 'BEGIN {>for (d = 1.1; d <= 1.5; d += 0.1)>
i++>
print i> }'a 4# loop five times (?)

you may or may not succeed in getting the correct result by choosing an arbitrarily large
value for PREC. Reformulation of the problem at hand is often the correct approach in suchsituations.

16.4.3 Try a Few Extra Bits of Precision and Rounding
Instead of arbitrary-precision floating-point arithmetic, often all you need is an adjustmentof your logic or a different order for the operations in your calculation. The stability andthe accuracy of the computation of π in the earlier example can be enhanced by using thefollowing simple algebraic transformation:(sqrt(x * x + 1) - 1) / x ≡ x / (sqrt(x * x + 1) + 1)
After making this change, the program converges to π in under 30 iterations:$ gawk -f pi2.awk
a 3.215390309173473
a 3.159659942097501
a 3.146086215131436
a 3.142714599645370

Chapter 16: Arithmetic and Arbitrary-Precision Arithmetic with gawk 397

a 3.141873049979825...
a 3.141592653589797
a 3.141592653589797

16.4.4 Setting the Precision
gawk uses a global working precision; it does not keep track of the precision or accuracyof individual numbers. Performing an arithmetic operation or calling a built-in function
rounds the result to the current working precision. The default working precision is 53 bits,
which you can modify using the predefined variable PREC. You can also set the value toone of the predefined case-insensitive strings shown in Table 16.4, to emulate an IEEE 754
binary format.
PREC"half""single""double""quad""oct"

IEEE 754 binary format
16-bit half-precision
Basic 32-bit single precision
Basic 64-bit double precision
Basic 128-bit quadruple precision256-bit octuple precision

Table 16.4: Predefined precision strings for PREC
The following example illustrates the effects of changing precision on arithmetic operations:$ gawk -M -v PREC=100 'BEGIN { x = 1.0e-400; print x + 0>
PREC = "double"; print x + 0 }'1e-400aa 0
CAUTION: Be wary of floating-point constants! When reading a floating-point
constant from program source code, gawk uses the default precision (that of a
C double), unless overridden by an assignment to the special variable PREC on
the command line, to store it internally as an MPFR number. Changing the
precision using PREC in the program text does not change the precision of aconstant.
If you need to represent a floating-point constant at a higher precision than thedefault and cannot use a command-line assignment to PREC, you should either
specify the constant as a string, or as a rational number, whenever possible.
The following example illustrates the differences among various ways to print a
floating-point constant:$ gawk -M 'BEGIN { PREC = 113; printf("%0.25f\n", 0.1) }'a 0.1000000000000000055511151$ gawk -M -v PREC=113 'BEGIN { printf("%0.25f\n", 0.1) }'a 0.1000000000000000000000000$ gawk -M 'BEGIN { PREC = 113; printf("%0.25f\n", "0.1") }'a 0.1000000000000000000000000$ gawk -M 'BEGIN { PREC = 113; printf("%0.25f\n", 1/10) }'a 0.1000000000000000000000000

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16.4.5 Setting the Rounding Mode
The ROUNDMODE variable provides program-level control over the rounding mode. The correspondence between ROUNDMODE and the IEEE rounding modes is shown in Table 16.5.
Rounding mode
Round to nearest, ties to even
Round toward positive infinity
Round toward negative infinity
Round toward zeroRound away from zeroIEEE nameroundTiesToEvenroundTowardPositive
roundTowardNegativeroundTowardZero

ROUNDMODE"N" or "n""U" or "u""D" or "d""Z" or "z""A" or "a"Table 16.5: gawk rounding modes
ROUNDMODE has the default value "N", which selects the IEEE 754 rounding moderoundTiesToEven. In Table 16.5, the value "A" selects rounding away from zero. Thisis only available if your version of the MPFR library supports it; otherwise, setting
ROUNDMODE to "A" has no effect.
The default mode roundTiesToEven is the most preferred, but the least intuitive. Thismethod does the obvious thing for most values, by rounding them up or down to the nearest
digit. For example, rounding 1.132 to two digits yields 1.13, and rounding 1.157 yields 1.16.
However, when it comes to rounding a value that is exactly halfway between, things do
not work the way you probably learned in school. In this case, the number is rounded to
the nearest even digit. So rounding 0.125 to two digits rounds down to 0.12, but rounding
0.6875 to three digits rounds up to 0.688. You probably have already encountered thisrounding mode when using printf to format floating-point numbers. For example:
BEGIN {
x = -4.5
for (i = 1; i < 10; i++) {x += 1.0
printf("%4.1f => %2.0f\n", x, x)}}
produces the following output when run on the author’s system:5-3.5 => -4-2.5 => -2-1.5 => -2-0.5 => 0
0.5 => 0
1.5 => 2
2.5 => 2
3.5 => 4
4.5 => 4
The theory behind roundTiesToEven is that it more or less evenly distributes upwardand downward rounds of exact halves, which might cause any accumulating round-off error
5

It is possible for the output to be completely different if the C library in your system does not use theIEEE 754 even-rounding rule to round halfway cases for printf.

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to cancel itself out. This is the default rounding mode for IEEE 754 computing functions
and operators.
Rounding Modes and Conversion
It’s important to understand that, along with CONVFMT and OFMT, the rounding mode
affects how numbers are converted to strings. For example, consider the following program:
BEGIN {
pi = 3.1416
OFMT = "%.f"# Print value as integer
print pi
# ROUNDMODE = "N" by default.
ROUNDMODE = "U"# Now change ROUNDMODE
print pi}
Running this program produces this output:$ gawk -M -f roundmode.awk
a 3
a 4
The other rounding modes are rarely used.
Rounding toward positive infinity(roundTowardPositive) and toward negative infinity (roundTowardNegative) are often
used to implement interval arithmetic, where you adjust the rounding mode to calculate
upper and lower bounds for the range of output. The roundTowardZero mode can be used
for converting floating-point numbers to integers. When rounding away from zero, thenearest number with magnitude greater than or equal to the value is selected.
Some numerical analysts will tell you that your choice of rounding style has tremendous
impact on the final outcome, and advise you to wait until final output for any rounding.
Instead, you can often avoid round-off error problems by setting the precision initially tosome value sufficiently larger than the final desired precision, so that the accumulationof round-off error does not influence the outcome. If you suspect that results from yourcomputation are sensitive to accumulation of round-off error, look for a significant differencein output when you change the rounding mode to be sure.

16.5 Arbitrary-Precision Integer Arithmetic with gawk
When given the -M option, gawk performs all integer arithmetic using GMP arbitraryprecision integers. Any number that looks like an integer in a source or data file is stored
as an arbitrary-precision integer. The size of the integer is limited only by the available
32
memory. For example, the following computes 54 , the result of which is beyond the limitsof ordinary hardware double-precision floating-point values:$ gawk -M 'BEGIN {>x = 5^4^3^2>
print "number of digits =", length(x)>print substr(x, 1, 20), "...", substr(x, length(x) - 19, 20)> }'a number of digits = 183231
a 62060698786608744707 ... 92256259918212890625

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If instead you were to compute the same value using arbitrary-precision floating-pointvalues, the precision needed for correct output (using the formula prec = 3.322 · dps) would
be 3.322 · 183231, or 608693.
The result from an arithmetic operation with an integer and a floating-point value is a
floating-point value with a precision equal to the working precision. The following program
calculates the eighth term in Sylvester’s sequence6 using a recurrence:$ gawk -M 'BEGIN {>s = 2.0>for (i = 1; i <= 7; i++)>s = s * (s - 1) + 1>
print s> }'a 113423713055421845118910464
The output differs from the actual number, 113,423,713,055,421,844,361,000,443, because
the default precision of 53 bits is not enough to represent the floating-point results exactly.
You can either increase the precision (100 bits is enough in this case), or replace the floatingpoint constant ‘2.0’ with an integer, to perform all computations using integer arithmetic
to get the correct output.
Sometimes gawk must implicitly convert an arbitrary-precision integer into an arbitraryprecision floating-point value. This is primarily because the MPFR library does not always
provide the relevant interface to process arbitrary-precision integers or mixed-mode numbers
as needed by an operation or function. In such a case, the precision is set to the minimum
value necessary for exact conversion, and the working precision is not used for this purpose.
If this is not what you need or want, you can employ a subterfuge and convert the integer
to floating point first, like this:gawk -M 'BEGIN { n = 13; print (n + 0.0) % 2.0 }'You can avoid this issue altogether by specifying the number as a floating-point value
to begin with:
gawk -M 'BEGIN { n = 13.0; print n % 2.0 }'Note that for this particular example, it is likely best to just use the following:
gawk -M 'BEGIN { n = 13; print n % 2 }'When dividing two arbitrary precision integers with either ‘/’ or ‘%’, the result is typicallyan arbitrary precision floating point value (unless the denominator evenly divides into thenumerator).

16.6 How To Check If MPFR Is Available
Occasionally, you might like to be able to check if gawk was invoked with the -M option,
enabling arbitrary-precision arithmetic. You can do so with the following function, contributed by Andrew Schorr:# adequate_math_precision --- return true if we have enough bits
6

Weisstein, Eric W. Sylvester’s Sequence.
From MathWorld—A Wolfram Web Resource(http://mathworld.wolfram.com/SylvestersSequence.html).

Chapter 16: Arithmetic and Arbitrary-Precision Arithmetic with gawk 401

function adequate_math_precision(n){return (1 != (1+(1/(2^(n-1)))))}
Here is code that invokes the function in order to check if arbitrary-precision arithmetic
is available:
BEGIN {# How many bits of mantissa precision are required# for this program to function properly?
fpbits = 123# We hope that we were invoked with MPFR enabled. If so, the# following statement should configure calculations to our desired# precision.
PREC = fpbits
if (! adequate_math_precision(fpbits)) {print("Error: insufficient computation precision available.\n" \"Try again with the -M argument?") > "/dev/stderr"# Note: you may need to set a flag here to bail out of END rules
exit 1}}
Please be aware that exit will jump to the END rules, if present (see Section 7.4.10 [Theexit Statement], page 162).

16.7 Standards Versus Existing PracticeHistorically, awk has converted any nonnumeric-looking string to the numeric value zero,
when required. Furthermore, the original definition of the language and the original POSIX
standards specified that awk only understands decimal numbers (base 10), and not octal(base 8) or hexadecimal numbers (base 16).
Changes in the language of the 2001 and 2004 POSIX standards can be interpreted to
imply that awk should support additional features. These features are:• Interpretation of floating-point data values specified in hexadecimal notation (e.g.,
0xDEADBEEF). (Note: data values, not source code constants.)• Support for the special IEEE 754 floating-point values “not a number” (NaN), positive
infinity (“inf”), and negative infinity (“−inf”). In particular, the format for thesevalues is as specified by the ISO 1999 C standard, which ignores case and can allow
implementation-dependent additional characters after the ‘nan’ and allow either ‘inf’or ‘infinity’.
The first problem is that both of these are clear changes to historical practice:• The gawk maintainer feels that supporting hexadecimal floating-point values, in particular, is ugly, and was never intended by the original designers to be part of thelanguage.

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departure from historical practice.
The second problem is that the gawk maintainer feels that this interpretation of thestandard, which required a certain amount of “language lawyering” to arrive at in the firstplace, was not even intended by the standard developers. In other words, “We see how you
got where you are, but we don’t think that that’s where you want to be.”Recognizing these issues, but attempting to provide compatibility with the earlier versions of the standard, the 2008 POSIX standard added explicit wording to allow, but notrequire, that awk support hexadecimal floating-point values and special values for “not anumber” and infinity.
Although the gawk maintainer continues to feel that providing those features is inadvisable, nevertheless, on systems that support IEEE floating point, it seems reasonable toprovide some way to support NaN and infinity values. The solution implemented in gawk
is as follows:• With the --posix command-line option, gawk becomes “hands off.” String values are
passed directly to the system library’s strtod() function, and if it successfully returns
a numeric value, that is what’s used.7 By definition, the results are not portable acrossdifferent systems. They are also a little surprising:$ echo nanny | gawk --posix '{ print $1 + 0 }'a nan
$ echo 0xDeadBeef | gawk --posix '{ print $1 + 0 }'a 3735928559• Without --posix, gawk interprets the four string values ‘+inf’, ‘-inf’, ‘+nan’, and‘-nan’ specially, producing the corresponding special numeric values. The leading sign
acts a signal to gawk (and the user) that the value is really numeric. Hexadecimal
floating point is not supported (unless you also use --non-decimal-data, which is notrecommended). For example:$ echo nanny | gawk '{ print $1 + 0 }'a 0$ echo +nan | gawk '{ print $1 + 0 }'a +nan$ echo 0xDeadBeef | gawk '{ print $1 + 0 }'a 0gawk ignores case in the four special values. Thus, ‘+nan’ and ‘+NaN’ are the same.
Besides handling input, gawk also needs to print “correct” values on output when a value
is either NaN or infinity. Starting with version 4.2.2, for such values gawk prints one of thefour strings just described: ‘+inf’, ‘-inf’, ‘+nan’, or ‘-nan’. Similarly, in POSIX mode,
gawk prints the result of the system’s C printf() function using the %g format string forthe value, whatever that may be.
NOTE: The sign used for NaN values can vary! The result depends upon boththe underlying system architecture and the underlying library used to format
NaN values. In particular, it’s possible to get different results for the same7

You asked for it, you got it.

Chapter 16: Arithmetic and Arbitrary-Precision Arithmetic with gawk 403

function call depending upon whether or not gawk is running in MPFR mode(-M) or not. Caveat Emptor!

16.8 Summary• Most computer arithmetic is done using either integers or floating-point values. Standard awk uses double-precision floating-point values.
• In the early 1990s Barbie mistakenly said, “Math class is tough!” Although math isn’t
tough, floating-point arithmetic isn’t the same as pencil-and-paper math, and care must
be taken:− Not all numbers can be represented exactly.
− Comparing values should use a delta, instead of being done directly with ‘==’ and‘!=’.
− Errors accumulate.
− Operations are not always truly associative or distributive.
• Increasing the accuracy can help, but it is not a panacea.
• Often, increasing the accuracy and then rounding to the desired number of digits
produces reasonable results.
• Use -M (or --bignum) to enable MPFR arithmetic. Use PREC to set the precision inbits, and ROUNDMODE to set the IEEE 754 rounding mode.
• With -M, gawk performs arbitrary-precision integer arithmetic using the GMP library.
This is faster and more space-efficient than using MPFR for the same calculations.
• There are several areas with respect to floating-point numbers where gawk disagrees
with the POSIX standard. It pays to be aware of them.
• Overall, there is no need to be unduly suspicious about the results from floating-point
arithmetic. The lesson to remember is that floating-point arithmetic is always more
complex than arithmetic using pencil and paper. In order to take advantage of thepower of floating-point arithmetic, you need to know its limitations and work withinthem. For most casual use of floating-point arithmetic, you will often get the expected
result if you simply round the display of your final results to the correct number ofsignificant decimal digits.
• As general advice, avoid presenting numerical data in a manner that implies better
precision than is actually the case.

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17 Writing Extensions for gawk
It is possible to add new functions written in C or C++ to gawk using dynamically loaded
libraries. This facility is available on systems that support the C dlopen() and dlsym()
functions. This chapter describes how to create extensions using code written in C or C++.
If you don’t know anything about C programming, you can safely skip this chapter,
although you may wish to review the documentation on the extensions that come with
gawk (see Section 17.7 [The Sample Extensions in the gawk Distribution], page 457), andthe information on the gawkextlib project (see Section 17.8 [The gawkextlib Project],
page 467). The sample extensions are automatically built and installed when gawk is.
NOTE: When --sandbox is specified, extensions are disabled (see Section 2.2[Command-Line Options], page 33).

17.1 Introduction
An extension (sometimes called a plug-in) is a piece of external compiled code that gawk canload at runtime to provide additional functionality, over and above the built-in capabilities
described in the rest of this book.
Extensions are useful because they allow you (of course) to extend gawk’s functionality.
For example, they can provide access to system calls (such as chdir() to change directory)
and to other C library routines that could be of use. As with most software, “the sky is
the limit”; if you can imagine something that you might want to do and can write in C or
C++, you can write an extension to do it!
Extensions are written in C or C++, using the application programming interface (API)defined for this purpose by the gawk developers. The rest of this chapter explains the facilities that the API provides and how to use them, and presents a small example extension.
In addition, it documents the sample extensions included in the gawk distribution and describes the gawkextlib project. See Section C.5 [Extension API Design], page 522, for a
discussion of the extension mechanism goals and design.

17.2 Extension Licensing
Every dynamic extension must be distributed under a license that is compatible with the
GNU GPL (see [GNU General Public License], page 541).
In order for the extension to tell gawk that it is properly licensed, the extension must
define the global symbol plugin_is_GPL_compatible. If this symbol does not exist, gawk
emits a fatal error and exits when it tries to load your extension.
The declared type of the symbol should be int. It does not need to be in any allocated section, though. The code merely asserts that the symbol exists in the global scope.
Something like this is enough:int plugin_is_GPL_compatible;

17.3 How It Works at a High Level
Communication between gawk and an extension is two-way. First, when an extension isloaded, gawk passes it a pointer to a struct whose fields are function pointers. This isshown in Figure 17.1.

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API
Structdl_load(api_p, id);

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Figure 17.1: Loading the extension
The extension can call functions inside gawk through these function pointers, at runtime,
without needing (link-time) access to gawk’s symbols. One of these function pointers is toa function for “registering” new functions. This is shown in Figure 17.2.

register_ext_func({ "chdir", do_chdir, 1 });

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Figure 17.2: Registering a new function
In the other direction, the extension registers its new functions with gawk by passing
function pointers to the functions that provide the new feature (do_chdir(), for example).
gawk associates the function pointer with a name and can then call it, using a defined calling
convention. This is shown in Figure 17.3.

Chapter 17: Writing Extensions for gawk 407

BEGIN {
chdir("/path")
}

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Figure 17.3: Calling the new function
The do_xxx() function, in turn, then uses the function pointers in the API struct to
do its work, such as updating variables or arrays, printing messages, setting ERRNO, and soon.
Convenience macros make calling through the function pointers look like regular function
calls so that extension code is quite readable and understandable.
Although all of this sounds somewhat complicated, the result is that extension codeis quite straightforward to write and to read. You can see this in the sample extension
filefuncs.c (see Section 17.6 [Example: Some File Functions], page 447) and also in thetestext.c code for testing the APIs.
Some other bits and pieces:• The API provides access to gawk’s do_xxx values, reflecting command-line options, like
do_lint, do_profiling, and so on (see Section 17.4.14 [API Variables], page 442).
These are informational: an extension cannot affect their values inside gawk. In addition, attempting to assign to them produces a compile-time error.
• The API also provides major and minor version numbers, so that an extension cancheck if the gawk it is loaded with supports the facilities it was compiled with. (Version
mismatches “shouldn’t” happen, but we all know how that goes.) See Section 17.4.14.1[API Version Constants and Variables], page 443, for details.

17.4 API Description
C or C++ code for an extension must include the header file gawkapi.h, which declares thefunctions and defines the data types used to communicate with gawk. This (rather large)
section describes the API in detail.

17.4.1 Introduction
Access to facilities within gawk is achieved by calling through function pointers passed into
your extension.

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API function pointers are provided for the following kinds of operations:• Allocating, reallocating, and releasing memory.
• Registration functions. You may register:− Extension functions− Exit callbacks− A version string− Input parsers− Output wrappers− Two-way processors
All of these are discussed in detail later in this chapter.
• Printing fatal, warning, and “lint” warning messages.
• Updating ERRNO, or unsetting it.
• Accessing parameters, including converting an undefined parameter into an array.
• Symbol table access: retrieving a global variable, creating one, or changing one.
• Creating and releasing cached values; this provides an efficient way to use values for
multiple variables and can be a big performance win.
• Manipulating arrays:− Retrieving, adding, deleting, and modifying elements− Getting the count of elements in an array− Creating a new array− Clearing an array− Flattening an array for easy C-style looping over all its indices and elements• Accessing and manipulating redirections.
Some points about using the API:
• The following types, macros, and/or functions are referenced in gawkapi.h. For correctuse, you must therefore include the corresponding standard header file before including
gawkapi.h. The list of macros and related header files is shown in Table 17.1.
C entity
EOF
Values for errno
FILE
NULL
memcpy()memset()size_tstruct stat

Header file<stdio.h><errno.h><stdio.h><stddef.h><string.h><string.h><sys/types.h><sys/stat.h>Table 17.1: Standard header files needed by API
Due to portability concerns, especially to systems that are not fully standardscompliant, it is your responsibility to include the correct files in the correct way. Thisrequirement is necessary in order to keep gawkapi.h clean, instead of becoming a
portability hodge-podge as can be seen in some parts of the gawk source code.

Chapter 17: Writing Extensions for gawk 409• If your extension uses MPFR facilities, and you wish to receive such values from gawk
and/or pass such values to it, you must include the <mpfr.h> header before including<gawkapi.h>.
• The gawkapi.h file may be included more than once without ill effect. Doing so,
however, is poor coding practice.
• Although the API only uses ISO C 90 features, there is an exception; the “constructor”
functions use the inline keyword. If your compiler does not support this keyword, you
should either place ‘-Dinline=''’ on your command line or use the GNU Autotools
and include a config.h file in your extensions.
• All pointers filled in by gawk point to memory managed by gawk and should be treatedby the extension as read-only.
Memory for all strings passed into gawk from the extension must come from calling
one of gawk_malloc(), gawk_calloc(), or gawk_realloc(), and is managed by gawk
from then on.
Memory for MPFR/GMP values that come from gawk should also be treated as readonly. However, unlike strings, memory for MPFR/GMP values allocated by an extension and passed into gawk is copied by gawk; the extension should then free the values
itself to avoid memory leaks. This is discussed further in API Ownership of MPFR
and GMP Values.
• The API defines several simple structs that map values as seen from awk. A value canbe a double, a string, or an array (as in multidimensional arrays, or when creating a
new array).
String values maintain both pointer and length, because embedded nul characters areallowed.
NOTE: By intent, gawk maintains strings using the current multibyte encoding (as defined by LC_xxx environment variables) and not using wide
characters. This matches how gawk stores strings internally and also how
characters are likely to be input into and output from files.
NOTE: String values passed to an extension by gawk are always nulterminated. Thus it is safe to pass such string values to standard libraryand system routines. However, because gawk allows embedded nul characters in string data, before using the data as a regular C string, you should
check that the length for that string passed to the extension matches thereturn value of strlen() for it.
• When retrieving a value (such as a parameter or that of a global variable or arrayelement), the extension requests a specific type (number, string, scalar, value cookie,array, or “undefined”). When the request is “undefined,” the returned value will havethe real underlying type.
However, if the request and actual type don’t match, the access function returns “false”
and fills in the type of the actual value that is there, so that the extension can, e.g.,
print an error message (such as “scalar passed where array expected”).
You may call the API functions by using the function pointers directly, but the interface
is not so pretty. To make extension code look more like regular code, the gawkapi.h header
file defines several macros that you should use in your code. This section presents themacros as if they were functions.

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17.4.2 General-Purpose Data Types
I have a true love/hate relationship with unions.
—Arnold Robbins
That’s the thing about unions: the compiler will arrange things so they canaccommodate both love and hate.
—Chet Ramey
The extension API defines a number of simple types and structures for general-purpose
use. Additional, more specialized, data structures are introduced in subsequent sections,
together with the functions that use them.
The general-purpose types and structures are as follows:

typedef void *awk_ext_id_t;
A value of this type is received from gawk when an extension is loaded. Thatvalue must then be passed back to gawk as the first parameter of each API
function.

#define awk_const ...
This macro expands to ‘const’ when compiling an extension, and to nothing
when compiling gawk itself. This makes certain fields in the API data structures
unwritable from extension code, while allowing gawk to use them as it needs to.

typedef enum awk_bool {awk_false = 0,
awk_true} awk_bool_t;
A simple Boolean type.

typedef struct awk_string {char *str;/* data */size_t len;/* length thereof, in chars */} awk_string_t;
This represents a mutable string. gawk owns the memory pointed to if it supplied the value. Otherwise, it takes ownership of the memory pointed to. Suchmemory must come from calling one of the gawk_malloc(), gawk_calloc(),
or gawk_realloc() functions!
As mentioned earlier, strings are maintained using the current multibyte encoding.

Chapter 17: Writing Extensions for gawk 411

typedef enum {AWK_UNDEFINED,AWK_NUMBER,AWK_STRING,AWK_REGEX,AWK_STRNUM,
AWK_ARRAY,AWK_SCALAR,/* opaque access to a variable */AWK_VALUE_COOKIE,/* for updating a previously created value */AWK_BOOL} awk_valtype_t;
This enum indicates the type of a value. It is used in the following struct.
typedef struct awk_value {awk_valtype_t val_type;union {
awk_string_ts;awknum_tn;awk_array_ta;awk_scalar_t
scl;
awk_value_cookie_t vc;
awk_bool_tb;} u;} awk_value_t;
An “awk value.” The val_type member indicates what kind of value the union
holds, and each member is of the appropriate type.
#define str_valueu.s#define strnum_value
str_value#define regex_value
str_value#define num_value
u.n.d#define num_type
u.n.type#define num_ptr
u.n.ptr#define array_cookie
u.a#define scalar_cookie u.scl#define value_cookie
u.vc#define bool_valueu.b
Using these macros makes accessing the fields of the awk_value_t more readable.
enum AWK_NUMBER_TYPE {AWK_NUMBER_TYPE_DOUBLE,
AWK_NUMBER_TYPE_MPFR,AWK_NUMBER_TYPE_MPZ};
This enum is used in the following structure for defining the type of numericvalue that is being worked with. It is declared at the top level of the file so that
it works correctly for C++ as well as for C.

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typedef struct awk_number {double d;enum AWK_NUMBER_TYPE type;void *ptr;} awk_number_t;
This represents a numeric value. Internally, gawk stores every number as eithera C double, a GMP integer, or an MPFR arbitrary-precision floating-pointvalue. In order to allow extensions to also support GMP and MPFR values,
numeric values are passed in this structure.
The double-precision d element is always populated in data received from gawk.
In addition, by examining the type member, an extension can determine if the
ptr member is either a GMP integer (type mpz_ptr), or an MPFR floatingpoint value (type mpfr_ptr_t), and cast it appropriately.
CAUTION: Any MPFR or MPZ values that you create and pass
to gawk to save are copied. This means you are responsible torelease the storage once you’re done with it. See the sample intdiv
extension for some example code.
typedef void *awk_scalar_t;
Scalars can be represented as an opaque type. These values are obtained fromgawk and then passed back into it. This is discussed in a general fashion in the
text following this list, and in more detail in Section 17.4.11.2 [Variable Accessand Update by Cookie], page 430.
typedef void *awk_value_cookie_t;
A “value cookie” is an opaque type representing a cached value. This is also
discussed in a general fashion in the text following this list, and in more detailin Section 17.4.11.3 [Creating and Using Cached Values], page 432.
Scalar values in awk are numbers, strings, strnums, or typed regexps. The awk_value_t
struct represents values. The val_type member indicates what is in the union.
Representing numbers is easy—the API uses a C double. Strings require more work.
Because gawk allows embedded nul bytes in string values, a string must be represented asa pair containing a data pointer and length. This is the awk_string_t type.
A strnum (numeric string) value is represented as a string and consists of user input
data that appears to be numeric. When an extension creates a strnum value, the result is astring flagged as user input. Subsequent parsing by gawk then determines whether it looks
like a number and should be treated as a strnum, or as a regular string.
This is useful in cases where an extension function would like to do something comparable to the split() function which sets the strnum attribute on the array elements itcreates. For example, an extension that implements CSV splitting would want to use thisfeature. This is also useful for a function that retrieves a data item from a database. The
PostgreSQL PQgetvalue() function, for example, returns a string that may be numeric or
textual depending on the contents.
Typed regexp values (see Section 6.1.2.2 [Strongly Typed Regexp Constants], page 123)
are not of much use to extension functions. Extension functions can tell that they’ve received
them, and create them for scalar values. Otherwise, they can examine the text of the regexp
through regex_value.str and regex_value.len.

Chapter 17: Writing Extensions for gawk 413

Identifiers (i.e., the names of global variables) can be associated with either scalar values
or with arrays. In addition, gawk provides true arrays of arrays, where any given arrayelement can itself be an array. Discussion of arrays is delayed until Section 17.4.12 [Array
Manipulation], page 433.
The various macros listed earlier make it easier to use the elements of the union as if
they were fields in a struct; this is a common coding practice in C. Such code is easierto write and to read, but it remains your responsibility to make sure that the val_type
member correctly reflects the type of the value in the awk_value_t struct.
Conceptually, the first three members of the union (number, string, and array) are allthat is needed for working with awk values. However, because the API provides routines
for accessing and changing the value of a global scalar variable only by using the variable’s
name, there is a performance penalty: gawk must find the variable each time it is accessedand changed. This turns out to be a real issue, not just a theoretical one.
Thus, if you know that your extension will spend considerable time reading and/orchanging the value of one or more scalar variables, you can obtain a scalar cookie 1 object
for that variable, and then use the cookie for getting the variable’s value or for changing
the variable’s value. The awk_scalar_t type holds a scalar cookie, and the scalar_cookie
macro provides access to the value of that type in the awk_value_t struct. Given a scalarcookie, gawk can directly retrieve or modify the value, as required, without having to findit first.
The awk_value_cookie_t type and value_cookie macro are similar. If you know thatyou wish to use the same numeric or string value for one or more variables, you can create
the value once, retaining a value cookie for it, and then pass in that value cookie whenever
you wish to set the value of a variable. This saves storage space within the running gawk
process and reduces the time needed to create the value.

17.4.3 Memory Allocation Functions and Convenience Macros
The API provides a number of memory allocation functions for allocating memory that canbe passed to gawk, as well as a number of convenience macros. This subsection presents
them all as function prototypes, in the way that extension code would use them:void *gawk_malloc(size_t size);
Call the correct version of malloc() to allocate storage that may be passed to
gawk.
void *gawk_calloc(size_t nmemb, size_t size);
Call the correct version of calloc() to allocate storage that may be passed to
gawk.
void *gawk_realloc(void *ptr, size_t size);
Call the correct version of realloc() to allocate storage that may be passed
to gawk.
void gawk_free(void *ptr);
Call the correct version of free() to release storage that was allocated with
gawk_malloc(), gawk_calloc(), or gawk_realloc().
1

See the “cookie” entry in the Jargon file for a definition of cookie, and the “magic cookie” entry in the
Jargon file for a nice example. See also the entry for “Cookie” in the [Glossary], page 529.

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The API has to provide these functions because it is possible for an extension to becompiled and linked against a different version of the C library than was used for the gawk
executable.2 If gawk were to use its version of free() when the memory came from anunrelated version of malloc(), unexpected behavior would likely result.
Three convenience macros may be used for allocating storage from gawk_malloc(),
gawk_calloc, and gawk_realloc(). If the allocation fails, they cause gawk to exit with afatal error message. They should be used as if they were procedure calls that do not return
a value:#define emalloc(pointer, type, size, message) ...
The arguments to this macro are as follows:
pointer

The pointer variable to point at the allocated storage.

type

The type of the pointer variable. This is used to create a cast forthe call to gawk_malloc().

size

The total number of bytes to be allocated.

message

A message to be prefixed to the fatal error message. Typically thisis the name of the function using the macro.

For example, you might allocate a string value like so:
awk_value_t result;char *message;
const char greet[] = "Don't Panic!";
emalloc(message, char *, sizeof(greet), "myfunc");
strcpy(message, greet);make_malloced_string(message, strlen(message), & result);#define ezalloc(pointer, type, size, message) ...
This is like emalloc(), but it calls gawk_calloc() instead of gawk_malloc().
The arguments are the same as for the emalloc() macro, but this macro guarantees that the memory returned is initialized to zero.
#define erealloc(pointer, type, size, message) ...
This is like emalloc(), but it calls gawk_realloc() instead of gawk_malloc().
The arguments are the same as for the emalloc() macro.
Two additional functions allocate MPFR and GMP objects for use by extension functions
that need to create and then return such values.
NOTE: These functions are obsolete. Extension functions that need local
MPFR and GMP values should simply allocate them on the stack and clear
them, as any other code would.
The functions are:2

This is more common on MS-Windows systems, but it can happen on Unix-like systems as well.

Chapter 17: Writing Extensions for gawk 415

void *get_mpfr_ptr();
Allocate and initialize an MPFR object and return a pointer to it. If theallocation fails, gawk exits with a fatal “out of memory” error. If gawk wascompiled without MPFR support, calling this function causes a fatal error.
void *get_mpz_ptr();
Allocate and initialize a GMP object and return a pointer to it. If the allocation
fails, gawk exits with a fatal “out of memory” error. If gawk was compiled
without MPFR support, calling this function causes a fatal error.
Both of these functions return ‘void *’, since the gawkapi.h header file should not havedependency upon <mpfr.h> (and <gmp.h>, which is included from <mpfr.h>). The actual
return values are of types mpfr_ptr and mpz_ptr respectively, and you should cast thereturn values appropriately before assigning the results to variables of the correct types.
The memory allocated by these functions should be freed with gawk_free().

17.4.4 Constructor Functions
The API provides a number of constructor functions for creating string and numeric values,
as well as a number of convenience macros. This subsection presents them all as function
prototypes, in the way that extension code would use them:static inline awk_value_t *make_const_string(const char *string, size_t length, awk_value_t *result);
This function creates a string value in the awk_value_t variable pointed to by
result. It expects string to be a C string constant (or other string data),
and automatically creates a copy of the data for storage in result. It returns
result.
static inline awk_value_t *make_malloced_string(const char *string, size_t length, awk_value_t *result);
This function creates a string value in the awk_value_t variable pointed to by
result. It expects string to be a ‘char *’ value pointing to data previously
obtained from gawk_malloc(), gawk_calloc(), or gawk_realloc(). The idea
here is that the data is passed directly to gawk, which assumes responsibility
for it. It returns result.
static inline awk_value_t *make_null_string(awk_value_t *result);
This specialized function creates a null string (the “undefined” value) in theawk_value_t variable pointed to by result. It returns result.
static inline awk_value_t *make_number(double num, awk_value_t *result);
This function simply creates a numeric value in the awk_value_t variable
pointed to by result.
static inline awk_value_t *make_number_mpz(void *mpz, awk_value_t *result);
This function creates a GMP number value in result. The mpz must be froma call to get_mpz_ptr() (and thus be of real underlying type mpz_ptr).

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static inline awk_value_t *make_number_mpfr(void *mpfr, awk_value_t *result);
This function creates an MPFR number value in result. The mpfr must befrom a call to get_mpfr_ptr().
static inline awk_value_t *make_const_user_input(const char *string, size_t length, awk_value_t*result);
This function is identical to make_const_string(), but the string is flagged asuser input that should be treated as a strnum value if the contents of the string
are numeric.
static inline awk_value_t *make_malloced_user_input(const char *string, size_t length, awk_value_t*result);
This function is identical to make_malloced_string(), but the string is flagged
as user input that should be treated as a strnum value if the contents of thestring are numeric.
static inline awk_value_t *make_const_regex(const char *string, size_t length, awk_value_t *result);
This function creates a strongly typed regexp value by allocating a copy of thestring. string is the regular expression of length len.
static inline awk_value_t *make_malloced_regex(const char *string, size_t length, awk_value_t *result);
This function creates a strongly typed regexp value. string is the regularexpression of length len. It expects string to be a ‘char *’ value pointing
to data previously obtained from gawk_malloc(), gawk_calloc(), or gawk_
realloc().
static inline awk_value_t *make_bool(awk_bool_t boolval, awk_value_t *result);
This function creates a boolean value in the awk_value_t variable pointed toby result.

17.4.5 Managing MPFR and GMP Values
MPFR and GMP values are different from string values, where you can “take ownership”
of the value simply by assigning pointers. For example:
char *p = gawk_malloc(42);char *q = p;
p = NULL;p ‘‘owns” the memory
now q ‘‘owns” it

MPFR and GMP objects are indeed allocated on the stack or dynamically, but the
MPFR and GMP libraries treat these objects as values, the same way that you would passan int or a double by value. There is no way to “transfer ownership” of MPFR and GMP
objects.
The final results of an MPFR or GMP calculation should be passed back to gawk, byvalue, as you would a string or a double. gawk will take care of freeing the storage.

Chapter 17: Writing Extensions for gawk 417

Thus, code in an extension should look like this:
mpz_t part1, part2, answer;

declare local values

mpz_set_si(part1, 21);
do some computations
mpz_set_si(part2, 21);
mpz_add(answer, part1, part2);...
/* assume that result is a parameter of type (awk_value_t *). */make_number_mpz(answer, & result);
set it with final GMP value
mpz_clear(part1);
mpz_clear(part2);

release intermediate values

return result;value in answer managed by gawk

17.4.6 Registration Functions
This section describes the API functions for registering parts of your extension with gawk.

17.4.6.1 Registering An Extension Function
Extension functions are described by the following record:
typedef struct awk_ext_func {const char *name;awk_value_t *(*const function)(int num_actual_args,
awk_value_t *result,struct awk_ext_func *finfo);
const size_t max_expected_args;
const size_t min_required_args;
awk_bool_t suppress_lint;
void *data;/* opaque pointer to any extra state */} awk_ext_func_t;
The fields are:const char *name;
The name of the new function. awk-level code calls the function by this name.
This is a regular C string.
Function names must obey the rules for awk identifiers. That is, they must
begin with either an English letter or an underscore, which may be followed byany number of letters, digits, and underscores. Letter case in function names
is significant.
awk_value_t *(*const function)(int num_actual_args,
awk_value_t *result,struct awk_ext_func *finfo);
This is a pointer to the C function that provides the extension’s functionality. The function must fill in *result with either a number, a string, or aregexp. gawk takes ownership of any string memory. As mentioned earlier,418

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string memory must come from one of gawk_malloc(), gawk_calloc(), or
gawk_realloc().
The num_actual_args argument tells the C function how many actual parameters were passed from the calling awk code.
The finfo parameter is a pointer to the awk_ext_func_t for this function.
The called function may access data within it as desired, or not.
The function must return the value of result. This is for the convenience ofthe calling code inside gawk.
const size_t max_expected_args;
This is the maximum number of arguments the function expects to receive. If
called with more arguments than this, and if lint checking has been enabled,
then gawk prints a warning message. For more information, see the entry forsuppress_lint, later in this list.
const size_t min_required_args;
This is the minimum number of arguments the function expects to receive. If
called with fewer arguments, gawk prints a fatal error message and exits.
awk_bool_t suppress_lint;
This flag tells gawk not to print a lint message if lint checking has been enabled
and if more arguments were supplied in the call than expected. An extension
function can tell if gawk already printed at least one such message by checking
if ‘num_actual_args > finfo->max_expected_args’. If so, and the function
does not want more lint messages to be printed, it should set finfo->suppress_
lint to awk_true.
void *data;
This is an opaque pointer to any data that an extension function may wish
to have available when called. Passing the awk_ext_func_t structure to theextension function, and having this pointer available in it enable writing a single
C or C++ function that implements multiple awk-level extension functions.
Once you have a record representing your extension function, you register it with gawk
using this API function:
awk_bool_t add_ext_func(const char *name_space, awk_ext_func_t *func);
This function returns true upon success, false otherwise. The name_spaceparameter is the namespace in which to place the function (see Chapter 15[Namespaces in gawk], page 383). Use an empty string ("") or "awk" to place
the function in the default awk namespace. The func pointer is the address ofa struct representing your function, as just described.
gawk does not modify what func points to, but the extension function itself
receives this pointer and can modify what it points to, thus it is purposely not
declared to be const.
The combination of min_required_args, max_expected_args, and suppress_lint may
be confusing. Here is how you should set things up.

Chapter 17: Writing Extensions for gawk 419

Any number of arguments is valid
Set min_required_args and max_expected_args to zero and set suppress_
lint to awk_true.
A minimum number of arguments is required, no limit on maximum number of arguments
Set min_required_args to the minimum required. Set max_expected_args tozero and set suppress_lint to awk_true.
A minimum number of arguments is required, a maximum number is expected
Set min_required_args to the minimum required. Set max_expected_args to
the maximum expected. Set suppress_lint to awk_false.
A minimum number of arguments is required, and no more than a maximum is allowed
Set min_required_args to the minimum required. Set max_expected_args
to the maximum expected. Set suppress_lint to awk_false. In your extension function, check that num_actual_args does not exceed f->max_expected_
args. If it does, issue a fatal error message.

17.4.6.2 Registering An Exit Callback Function
An exit callback function is a function that gawk calls before it exits. Such functions areuseful if you have general “cleanup” tasks that should be performed in your extension (suchas closing database connections or other resource deallocations). You can register such afunction with gawk using the following function:
void awk_atexit(void (*funcp)(void *data, int exit_status),
void *arg0);
The parameters are:funcp

A pointer to the function to be called before gawk exits. The data
parameter will be the original value of arg0. The exit_statusparameter is the exit status value that gawk intends to pass to theexit() system call.

arg0

A pointer to private data that gawk saves in order to pass to thefunction pointed to by funcp.

Exit callback functions are called in last-in, first-out (LIFO) order—that is, in the reverse
order in which they are registered with gawk.

17.4.6.3 Registering An Extension Version String
You can register a version string that indicates the name and version of your extension with
gawk, as follows:
void register_ext_version(const char *version);
Register the string pointed to by version with gawk. Note that gawk does notcopy the version string, so it should not be changed.
gawk prints all registered extension version strings when it is invoked with the --version
option.

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17.4.6.4 Customized Input Parsers
By default, gawk reads text files as its input. It uses the value of RS to find the end of an input
record, and then uses FS (or FIELDWIDTHS or FPAT) to split it into fields (see Chapter 4[Reading Input Files], page 65). Additionally, it sets the value of RT (see Section 7.5[Predefined Variables], page 163).
If you want, you can provide your own custom input parser. An input parser’s job is toreturn a record to the gawk record-processing code, along with indicators for the value andlength of the data to be used for RT, if any.
To provide an input parser, you must first provide two functions (where XXX is a prefixname for your extension):
awk_bool_t XXX_can_take_file(const awk_input_buf_t *iobuf);
This function examines the information available in iobuf (which we discussshortly). Based on the information there, it decides if the input parser should
be used for this file. If so, it should return true. Otherwise, it should returnfalse. It should not change any state (variable values, etc.) within gawk.
awk_bool_t XXX_take_control_of(awk_input_buf_t *iobuf);
When gawk decides to hand control of the file over to the input parser, it calls
this function. This function in turn must fill in certain fields in the awk_input_
buf_t structure and ensure that certain conditions are true. It should thenreturn true. If an error of some kind occurs, it should not fill in any fields andshould return false; then gawk will not use the input parser. The details arepresented shortly.
Your extension should package these functions inside an awk_input_parser_t, whichlooks like this:typedef struct awk_input_parser {const char *name;/* name of parser */awk_bool_t (*can_take_file)(const awk_input_buf_t *iobuf);
awk_bool_t (*take_control_of)(awk_input_buf_t *iobuf);
awk_const struct awk_input_parser *awk_const next;/* for gawk */} awk_input_parser_t;
The fields are:const char *name;
The name of the input parser. This is a regular C string.
awk_bool_t (*can_take_file)(const awk_input_buf_t *iobuf);
A pointer to your XXX_can_take_file() function.
awk_bool_t (*take_control_of)(awk_input_buf_t *iobuf);
A pointer to your XXX_take_control_of() function.
awk_const struct input_parser *awk_const next;
This is for use by gawk; therefore it is marked awk_const so that the extension
cannot modify it.
The steps are as follows:
1. Create a static awk_input_parser_t variable and initialize it appropriately.

Chapter 17: Writing Extensions for gawk 421

2. When your extension is loaded, register your input parser with gawk using theregister_input_parser() API function (described next).
An awk_input_buf_t looks like this:typedef struct awk_input {const char *name;/* filename */int fd;/* file descriptor */#define INVALID_HANDLE (-1)
void *opaque;/* private data for input parsers */int (*get_record)(char **out, struct awk_input *iobuf,
int *errcode, char **rt_start, size_t *rt_len,
const awk_fieldwidth_info_t **field_width);ssize_t (*read_func)();void (*close_func)(struct awk_input *iobuf);
struct stat sbuf;/* stat buf */} awk_input_buf_t;
The fields can be divided into two categories: those for use (initially, at least) by XXX_
can_take_file(), and those for use by XXX_take_control_of(). The first group of fields
and their uses are as follows:
const char *name;
The name of the file.
int fd;A file descriptor for the file. gawk attempts to open the file for reading using
the open() system call. If it was able to open the file, then fd will not be equal
to INVALID_HANDLE. Otherwise, it will.
An extension can decide that it doesn’t want to use the open file descriptor
provided by gawk. In such a case it can close the file and set fd to INVALID_HANDLE, or it can leave it alone and keep it’s own file descriptor in private data
pointed to by the opaque pointer (see further in this list). In any case, if thefile descriptor is valid, it should not just overwrite the value with somethingelse; doing so would cause a resource leak.

struct stat sbuf;
If the file descriptor is valid, then gawk will have filled in this structure via a callto the fstat() system call. Otherwise, if the lstat() system call is available,
it will use that. If lstat() is not available, then it uses stat().
Getting the file’s information allows extensions to check the type of the file even
if it could not be opened. This occurs, for example, on Windows systems whentrying to use open() on a directory.
If gawk was not able to get the file information, then sbuf will be zeroed out.
In particular, extension code can check if ‘sbuf.st_mode == 0’. If that’s true,
then there is no information in sbuf.
The XXX_can_take_file() function should examine these fields and decide if the input
parser should be used for the file. The decision can be made based upon gawk state (thevalue of a variable defined previously by the extension and set by awk code), the name of
the file, whether or not the file descriptor is valid, the information in the struct stat, or
any combination of these factors.

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Once XXX_can_take_file() has returned true, and gawk has decided to use your input
parser, it calls XXX_take_control_of(). That function then fills either the get_record
field or the read_func field in the awk_input_buf_t. It must also ensure that fd is not set
to INVALID_HANDLE. The following list describes the fields that may be filled by XXX_take_control_of():void *opaque;This is used to hold any state information needed by the input parser for thisfile. It is “opaque” to gawk. The input parser is not required to use this pointer.
int (*get_record)(char **out,
struct awk_input *iobuf,
int *errcode,char **rt_start,
size_t *rt_len,
const awk_fieldwidth_info_t **field_width);
This function pointer should point to a function that creates the input records.
Said function is the core of the input parser. Its behavior is described in the
text following this list.
ssize_t (*read_func)(int, void *, size_t);
This function pointer should point to a function that has the same behavior as
the standard POSIX read() system call. It is an alternative to the get_record
pointer. Its behavior is also described in the text following this list.
void (*close_func)(struct awk_input *iobuf);
This function pointer should point to a function that does the “teardown.” Itshould release any resources allocated by XXX_take_control_of(). It may also
close the file. If it does so, it should set the fd field to INVALID_HANDLE.
If fd is still not INVALID_HANDLE after the call to this function, gawk calls theregular close() system call.
Having a “teardown” function is optional. If your input parser does not needit, do not set this field. Then, gawk calls the regular close() system call on
the file descriptor, so it should be valid.
The XXX_get_record() function does the work of creating input records. The parameters are as follows:char **outThis is a pointer to a char * variable that is set to point to the record. gawk
makes its own copy of the data, so your extension must manage this storage.
struct awk_input *iobuf
This is the awk_input_buf_t for the file. Two of its fields should be used byyour extension: fd for reading data, and opaque for managing any private state.
int *errcode
If an error occurs, *errcode should be set to an appropriate code from<errno.h>.

Chapter 17: Writing Extensions for gawk 423

char **rt_start
size_t *rt_len
If the concept of a “record terminator” makes sense, then *rt_start should be
set to point to the data to be used for RT, and *rt_len should be set to thelength of the data. Otherwise, *rt_len should be set to zero. Here too, gawk
makes its own copy of this data, so your extension must manage this storage.
const awk_fieldwidth_info_t **field_widthIf field_width is not NULL, then *field_width will be initialized to NULL, andthe function may set it to point to a structure supplying field width information
to override the default field parsing mechanism. Note that this structure will
not be copied by gawk; it must persist at least until the next call to get_recordor close_func. Note also that field_width is NULL when getline is assigning
the results to a variable, thus field parsing is not needed.
If the parser sets *field_width, then gawk uses this layout to parse the input
record, and the PROCINFO["FS"] value will be "API" while this record is active
in $0. The awk_fieldwidth_info_t data structure is described below.
The return value is the length of the buffer pointed to by *out, or EOF if end-of-file wasreached or an error occurred.
It is guaranteed that errcode is a valid pointer, so there is no need to test for a NULL
value. gawk sets *errcode to zero, so there is no need to set it unless an error occurs.
If an error does occur, the function should return EOF and set *errcode to a value greaterthan zero. In that case, if *errcode does not equal zero, gawk automatically updates the
ERRNO variable based on the value of *errcode. (In general, setting ‘*errcode = errno’
should do the right thing.)
As an alternative to supplying a function that returns an input record, you may instead
supply a function that simply reads bytes, and let gawk parse the data into records. If you
do so, the data should be returned in the multibyte encoding of the current locale. Sucha function should follow the same behavior as the read() system call, and you fill in theread_func pointer with its address in the awk_input_buf_t structure.
By default, gawk sets the read_func pointer to point to the read() system call. So yourextension need not set this field explicitly.
NOTE: You must choose one method or the other: either a function that returns
a record, or one that returns raw data. In particular, if you supply a function
to get a record, gawk will call it, and will never call the raw read function.
gawk ships with a sample extension that reads directories, returning records for eachentry in a directory (see Section 17.7.6 [Reading Directories], page 463). You may wish to
use that code as a guide for writing your own input parser.
When writing an input parser, you should think about (and document) how it is expected
to interact with awk code. You may want it to always be called, and to take effect as
appropriate (as the readdir extension does). Or you may want it to take effect based upon
the value of an awk variable, as the XML extension from the gawkextlib project does (seeSection 17.8 [The gawkextlib Project], page 467). In the latter case, code in a BEGINFILE
rule can look at FILENAME and ERRNO to decide whether or not to activate your input parser(see Section 7.1.5 [The BEGINFILE and ENDFILE Special Patterns], page 151).

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If you would like to override the default field parsing mechanism for a given record, thenyou must populate an awk_fieldwidth_info_t structure, which looks like this:typedef struct {awk_bool_t
use_chars; /* false ==> use bytes */size_tnf;/* number of fields in record (NF) */struct awk_field_info {size_t skip;/* amount to skip before field starts */size_t len;/* length of field */} fields[1];/* actual dimension should be nf */} awk_fieldwidth_info_t;
The fields are:awk_bool_t use_chars;
Set this to awk_true if the field lengths are specified in terms of potentiallymulti-byte characters, and set it to awk_false if the lengths are in terms ofbytes. Performance will be better if the values are supplied in terms of bytes.
size_t nf;
Set this to the number of fields in the input record, i.e. NF.
struct awk_field_info fields[nf];This is a variable-length array whose actual dimension should be nf. For eachfield, the skip element should be set to the number of characters or bytes, as
controlled by the use_chars flag, to skip before the start of this field. The lenelement provides the length of the field. The values in fields[0] provide theinformation for $1, and so on through the fields[nf-1] element containing
the information for $NF.
A convenience macro awk_fieldwidth_info_size(numfields) is provided to calculate the appropriate size of a variable-length awk_fieldwidth_info_t structure containing
numfields fields. This can be used as an argument to malloc() or in a union to allocate
space statically. Please refer to the readdir_test sample extension for an example.
You register your input parser with the following function:
void register_input_parser(awk_input_parser_t *input_parser);
Register the input parser pointed to by input_parser with gawk.

17.4.6.5 Customized Output Wrappers
An output wrapper is the mirror image of an input parser. It allows an extension to takeover the output to a file opened with the ‘>’ or ‘>>’ I/O redirection operators (see Section 5.6[Redirecting Output of print and printf], page 108).
The output wrapper is very similar to the input parser structure:
typedef struct awk_output_wrapper {const char *name;/* name of the wrapper */awk_bool_t (*can_take_file)(const awk_output_buf_t *outbuf);
awk_bool_t (*take_control_of)(awk_output_buf_t *outbuf);
awk_const struct awk_output_wrapper *awk_const next; /* for gawk */} awk_output_wrapper_t;Chapter 17: Writing Extensions for gawk 425

The members are as follows:
const char *name;
This is the name of the output wrapper.
awk_bool_t (*can_take_file)(const awk_output_buf_t *outbuf);
This points to a function that examines the information in the awk_output_
buf_t structure pointed to by outbuf. It should return true if the output
wrapper wants to take over the file, and false otherwise. It should not change
any state (variable values, etc.) within gawk.
awk_bool_t (*take_control_of)(awk_output_buf_t *outbuf);
The function pointed to by this field is called when gawk decides to let the output
wrapper take control of the file. It should fill in appropriate members of the
awk_output_buf_t structure, as described next, and return true if successful,
false otherwise.
awk_const struct output_wrapper *awk_const next;
This is for use by gawk; therefore it is marked awk_const so that the extension
cannot modify it.
The awk_output_buf_t structure looks like this:typedef struct awk_output_buf {const char *name;/* name of output file */const char *mode;/* mode argument to fopen */FILE *fp;/* stdio file pointer */awk_bool_t redirected; /* true if a wrapper is active */void *opaque;/* for use by output wrapper */size_t (*gawk_fwrite)(const void *buf, size_t size, size_t count,
FILE *fp, void *opaque);int (*gawk_fflush)(FILE *fp, void *opaque);int (*gawk_ferror)(FILE *fp, void *opaque);int (*gawk_fclose)(FILE *fp, void *opaque);} awk_output_buf_t;
Here too, your extension will define XXX_can_take_file() and XXX_take_control_of() functions that examine and update data members in the awk_output_buf_t. The
data members are as follows:
const char *name;
The name of the output file.
const char *mode;
The mode string (as would be used in the second argument to fopen()) withwhich the file was opened.
FILE *fp; The FILE pointer from <stdio.h>. gawk opens the file before attempting tofind an output wrapper.
awk_bool_t redirected;
This field must be set to true by the XXX_take_control_of() function.

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void *opaque;This pointer is opaque to gawk. The extension should use it to store a pointerto any private data associated with the file.
size_t (*gawk_fwrite)(const void *buf, size_t size, size_t count,
FILE *fp, void *opaque);int (*gawk_fflush)(FILE *fp, void *opaque);int (*gawk_ferror)(FILE *fp, void *opaque);int (*gawk_fclose)(FILE *fp, void *opaque);These pointers should be set to point to functions that perform the equivalentfunction as the <stdio.h> functions do, if appropriate. gawk uses these function pointers for all output. gawk initializes the pointers to point to internal“pass-through” functions that just call the regular <stdio.h> functions, so an
extension only needs to redefine those functions that are appropriate for what
it does.
The XXX_can_take_file() function should make a decision based upon the name andmode fields, and any additional state (such as awk variable values) that is appropriate. gawk
attempts to open the named file for writing. The fp member will be NULL only if it fails.
When gawk calls XXX_take_control_of(), that function should fill in the other fields
as appropriate, except for fp, which it should just use normally if it’s not NULL.
You register your output wrapper with the following function:
void register_output_wrapper(awk_output_wrapper_t *output_wrapper);Register the output wrapper pointed to by output_wrapper with gawk.

17.4.6.6 Customized Two-way Processors
A two-way processor combines an input parser and an output wrapper for two-way I/O withthe ‘|&’ operator (see Section 5.6 [Redirecting Output of print and printf], page 108).
It makes identical use of the awk_input_parser_t and awk_output_buf_t structures as
described earlier.
A two-way processor is represented by the following structure:
typedef struct awk_two_way_processor {const char *name;/* name of the two-way processor */awk_bool_t (*can_take_two_way)(const char *name);
awk_bool_t (*take_control_of)(const char *name,awk_input_buf_t *inbuf,
awk_output_buf_t *outbuf);
awk_const struct awk_two_way_processor *awk_const next; /* for gawk */} awk_two_way_processor_t;
The fields are as follows:
const char *name;
The name of the two-way processor.
awk_bool_t (*can_take_two_way)(const char *name);
The function pointed to by this field should return true if it wants to take over
two-way I/O for this file name. It should not change any state (variable values,etc.) within gawk.

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awk_bool_t (*take_control_of)(const char *name,awk_input_buf_t *inbuf,
awk_output_buf_t *outbuf);
The function pointed to by this field should fill in the awk_input_buf_t andawk_output_buf_t structures pointed to by inbuf and outbuf, respectively.
These structures were described earlier.
awk_const struct two_way_processor *awk_const next;
This is for use by gawk; therefore it is marked awk_const so that the extension
cannot modify it.
As with the input parser and output processor, you provide “yes I can take this” and“take over for this” functions, XXX_can_take_two_way() and XXX_take_control_of().
You register your two-way processor with the following function:
void register_two_way_processor(awk_two_way_processor_t *two_way_processor);
Register the two-way processor pointed to by two_way_processor with gawk.

17.4.7 Printing Messages
You can print different kinds of warning messages from your extension, as described here.
Note that for these functions, you must pass in the extension ID received from gawk whenthe extension was loaded:3
void fatal(awk_ext_id_t id, const char *format, ...);
Print a message and then cause gawk to exit immediately.
void nonfatal(awk_ext_id_t id, const char *format, ...);
Print a nonfatal error message.
void warning(awk_ext_id_t id, const char *format, ...);
Print a warning message.
void lintwarn(awk_ext_id_t id, const char *format, ...);
Print a “lint warning.” Normally this is the same as printing a warning message,
but if gawk was invoked with ‘--lint=fatal’, then lint warnings become fatal
error messages.
All of these functions are otherwise like the C printf() family of functions, where theformat parameter is a string with literal characters and formatting codes intermixed.

17.4.8 Updating ERRNO
The following functions allow you to update the ERRNO variable:
void update_ERRNO_int(int errno_val);
Set ERRNO to the string equivalent of the error code in errno_val. The value
should be one of the defined error codes in <errno.h>, and gawk turns it intoa (possibly translated) string using the C strerror() function.
3

Because the API uses only ISO C 90 features, it cannot make use of the ISO C 99 variadic macro feature
to hide that parameter. More’s the pity.

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void update_ERRNO_string(const char *string);
Set ERRNO directly to the string value of ERRNO. gawk makes a copy of the valueof string.
void unset_ERRNO(void);
Unset ERRNO.

17.4.9 Requesting Values
All of the functions that return values from gawk work in the same way. You pass inan awk_valtype_t value to indicate what kind of value you expect. If the actual value
matches what you requested, the function returns true and fills in the awk_value_t result.
Otherwise, the function returns false, and the val_type member indicates the type of theactual value. You may then print an error message or reissue the request for the actual
value type, as appropriate. This behavior is summarized in Table 17.2.

Type
Requested

StringStrnumNumber
Regex
Bool
Array
ScalarUndefined
Value
cookieStringStringfalse
Number
false
false
false
ScalarStringfalseof Actual
ValueBool
Strnum Type
NumberRegex
StringString String String
StrnumStrnum false
false
Number Number false
Numberfalse
false
Regex false
false
false
false
Bool
false
false
false
false
ScalarScalar Scalar Scalar
StrnumNumber Regex Bool
false
false
false
false

Array
false
false
false
false
false
Array
false
Array
false

Undefined
false
false
false
false
false
false
false
Undefined
false

Table 17.2: API value types returned

17.4.10 Accessing and Updating Parameters
Two functions give you access to the arguments (parameters) passed to your extension
function. They are:awk_bool_t get_argument(size_t count,awk_valtype_t wanted,
awk_value_t *result);
Fill in the awk_value_t structure pointed to by result with the countth argument. Return true if the actual type matches wanted, and false otherwise. In
the latter case, result->val_type indicates the actual type (see Table 17.2).
Counts are zero-based—the first argument is numbered zero, the second one,and so on. wanted indicates the type of value expected.
awk_bool_t set_argument(size_t count, awk_array_t array);
Convert a parameter that was undefined into an array; this provides call byreference for arrays. Return false if count is too big, or if the argument’s type
is not undefined. See Section 17.4.12 [Array Manipulation], page 433, for more
information on creating arrays.

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17.4.11 Symbol Table AccessTwo sets of routines provide access to global variables, and one set allows you to create andrelease cached values.

17.4.11.1 Variable Access and Update by Name
The following routines provide the ability to access and update global awk-level variables byname. In compiler terminology, identifiers of different kinds are termed symbols, thus the“sym” in the routines’ names. The data structure that stores information about symbols is
termed a symbol table. The functions are as follows:
awk_bool_t sym_lookup(const char *name,awk_valtype_t wanted,
awk_value_t *result);
Fill in the awk_value_t structure pointed to by result with the value of thevariable named by the string name, which is a regular C string. wanted indicates
the type of value expected. Return true if the actual type matches wanted, andfalse otherwise. In the latter case, result->val_type indicates the actual type(see Table 17.2).
awk_bool_t sym_lookup_ns(const char *name,const char *name_space,
awk_valtype_t wanted,
awk_value_t *result);
This is like sym_lookup(), but the name_space parameter allows you to specify
which namespace name is part of. name_space cannot be NULL. If it is "" or"awk", then name is searched for in the default awk namespace.
Note that namespace is a C++ keyword. For interoperability with C++, you
should avoid using that identifier in C code.
awk_bool_t sym_update(const char *name, awk_value_t *value);
Update the variable named by the string name, which is a regular C string.
The variable is added to gawk’s symbol table if it is not there. Return true if
everything worked, and false otherwise.
Changing types (scalar to array or vice versa) of an existing variable is notallowed, nor may this routine be used to update an array. This routine cannot
be used to update any of the predefined variables (such as ARGC or NF).
awk_bool_t sym_update_ns(const char *name_space, const char *name,awk_value_t *value);
This is like sym_update(), but the name_space parameter allows you to specify
which namespace name is part of. name_space cannot be NULL. If it is "" or"awk", then name is searched for in the default awk namespace.
An extension can look up the value of gawk’s special variables. However, with the
exception of the PROCINFO array, an extension cannot change any of those variables.
When searching for or updating variables outside the awk namespace (see Chapter 15[Namespaces in gawk], page 383), function and variable names must be simple identifiers.4
4

Allowing both namespace plus identifier and foo::bar would have been too confusing to document, andto code and test.

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In addition, namespace names and variable and function names must follow the rules given
in Section 15.5 [Namespace and Component Naming Rules], page 384.

17.4.11.2 Variable Access and Update by Cookie
A scalar cookie is an opaque handle that provides access to a global variable or array. It isan optimization that avoids looking up variables in gawk’s symbol table every time accessis needed. This was discussed earlier, in Section 17.4.2 [General-Purpose Data Types],
page 410.
The following functions let you work with scalar cookies:
awk_bool_t sym_lookup_scalar(awk_scalar_t cookie,
awk_valtype_t wanted,
awk_value_t *result);
Retrieve the current value of a scalar cookie. Once you have obtained a scalarcookie using sym_lookup(), you can use this function to get its value more
efficiently. Return false if the value cannot be retrieved.
awk_bool_t sym_update_scalar(awk_scalar_t cookie, awk_value_t *value);
Update the value associated with a scalar cookie. Return false if the new value
is not of type AWK_STRING, AWK_STRNUM, AWK_REGEX, or AWK_NUMBER. Here too,
the predefined variables may not be updated.
It is not obvious at first glance how to work with scalar cookies or what their raisond’^etre really is. In theory, the sym_lookup() and sym_update() routines are all you reallyneed to work with variables. For example, you might have code that looks up the value ofa variable, evaluates a condition, and then possibly changes the value of the variable basedon the result of that evaluation, like so:/*

do_magic --- do something really great */static awk_value_t *do_magic(int nargs, awk_value_t *result){awk_value_t value;
if (
sym_lookup("MAGIC_VAR", AWK_NUMBER, & value)&& some_condition(value.num_value)) {value.num_value += 42;
sym_update("MAGIC_VAR", & value);}
return make_number(0.0, result);}
This code looks (and is) simple and straightforward. So what’s the problem?
Well, consider what happens if awk-level code associated with your extension calls themagic() function (implemented in C by do_magic()), once per record, while processing
hundreds of thousands or millions of records. The MAGIC_VAR variable is looked up in thesymbol table once or twice per function call!

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The symbol table lookup is really pure overhead; it is considerably more efficient to geta cookie that represents the variable, and use that to get the variable’s value and update it
as needed.5
Thus, the way to use cookies is as follows. First, install your extension’s variable in gawk’s
symbol table using sym_update(), as usual. Then get a scalar cookie for the variable using
sym_lookup():static awk_scalar_t magic_var_cookie;/* cookie for MAGIC_VAR */static void
my_extension_init(){
awk_value_t value;/* install initial value */sym_update("MAGIC_VAR", make_number(42.0, & value));/* get the cookie */sym_lookup("MAGIC_VAR", AWK_SCALAR, & value);/* save the cookie */magic_var_cookie = value.scalar_cookie;...
}
Next, use the routines in this section for retrieving and updating the value through thecookie. Thus, do_magic() now becomes something like this:/*do_magic --- do something really great */static awk_value_t *do_magic(int nargs, awk_value_t *result){awk_value_t value;
if (
sym_lookup_scalar(magic_var_cookie, AWK_NUMBER, & value)&& some_condition(value.num_value)) {value.num_value += 42;
sym_update_scalar(magic_var_cookie, & value);}...
return make_number(0.0, result);}
NOTE: The previous code omitted error checking for presentation purposes.
Your extension code should be more robust and carefully check the return values
from the API functions.
5

The difference is measurable and quite real. Trust us.

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17.4.11.3 Creating and Using Cached Values
The routines in this section allow you to create and release cached values. Like scalarcookies, in theory, cached values are not necessary. You can create numbers and strings
using the functions in Section 17.4.4 [Constructor Functions], page 415. You can then assign
those values to variables using sym_update() or sym_update_scalar(), as you like.
However, you can understand the point of cached values if you remember that every string value’s storage must come from gawk_malloc(), gawk_calloc(), or gawk_
realloc(). If you have 20 variables, all of which have the same string value, you must
create 20 identical copies of the string.6
It is clearly more efficient, if possible, to create a value once, and then tell gawk to reuse
the value for multiple variables. That is what the routines in this section let you do. The
functions are as follows:
awk_bool_t create_value(awk_value_t *value, awk_value_cookie_t *result);
Create a cached string or numeric value from value for efficient later assignment. Only values of type AWK_NUMBER, AWK_REGEX, AWK_STRNUM, and AWK_STRING are allowed. Any other type is rejected. AWK_UNDEFINED could beallowed, but doing so would result in inferior performance.
awk_bool_t release_value(awk_value_cookie_t vc);
Release the memory associated with a value cookie obtained from create_value().
You use value cookies in a fashion similar to the way you use scalar cookies. In theextension initialization routine, you create the value cookie:
static awk_value_cookie_t answer_cookie;/* static value cookie */static void
my_extension_init(){
awk_value_t value;
char *long_string;size_t long_string_len;/* code from earlier */...
/* ... fill in long_string and long_string_len ... */make_malloced_string(long_string, long_string_len, & value);create_value(& value, & answer_cookie);/* create cookie */...
}Once the value is created, you can use it as the value of any number of variables:
static awk_value_t *do_magic(int nargs, awk_value_t *result)
6

Numeric values are clearly less problematic, requiring only a C double to store. But of course, GMP
and MPFR values do take up more memory.

Chapter 17: Writing Extensions for gawk 433{
awk_value_t new_value;...

/* as earlier */value.val_type = AWK_VALUE_COOKIE;
value.value_cookie = answer_cookie;
sym_update("VAR1", & value);
sym_update("VAR2", & value);...
sym_update("VAR100", & value);...
}
Using value cookies in this way saves considerable storage, as all of VAR1 through VAR100
share the same value.
You might be wondering, “Is this sharing problematic? What happens if awk code assigns
a new value to VAR1; are all the others changed too?”That’s a great question. The answer is that no, it’s not a problem. Internally, gawk
uses reference-counted strings. This means that many variables can share the same stringvalue, and gawk keeps track of the usage. When a variable’s value changes, gawk simply
decrements the reference count on the old value and updates the variable to use the newvalue.
Finally, as part of your cleanup action (see Section 17.4.6.2 [Registering An Exit Callback
Function], page 419) you should release any cached values that you created, using release_value().

17.4.12 Array Manipulation
The primary data structure7 in awk is the associative array (see Chapter 8 [Arrays in awk],
page 177). Extensions need to be able to manipulate awk arrays. The API provides anumber of data structures for working with arrays, functions for working with individual
elements, and functions for working with arrays as a whole. This includes the ability to“flatten” an array so that it is easy for C code to traverse every element in an array. The
array data structures integrate nicely with the data structures for values to make it easy toboth work with and create true arrays of arrays (see Section 17.4.2 [General-Purpose Data
Types], page 410).

17.4.12.1 Array Data Types
The data types associated with arrays are as follows:
typedef void *awk_array_t;
If you request the value of an array variable, you get back an awk_array_t
value. This value is opaque8 to the extension; it uniquely identifies the arraybut can only be used by passing it into API functions or receiving it from
7
8

OK, the only data structure.
It is also a “cookie,” but the gawk developers did not wish to overuse this term.

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API functions. This is very similar to way ‘FILE *’ values are used with the<stdio.h> library routines.
typedef struct awk_element {/* convenience linked list pointer, not used by gawk */struct awk_element *next;
enum {AWK_ELEMENT_DEFAULT = 0, /* set by gawk */AWK_ELEMENT_DELETE = 1/* set by extension */} flags;
awk_value_t index;
awk_value_t value;} awk_element_t;
The awk_element_t is a “flattened” array element. awk produces an array ofthese inside the awk_flat_array_t (see the next item). Individual elements
may be marked for deletion. New elements must be added individually, one ata time, using the separate API for that purpose. The fields are as follows:
struct awk_element *next;
This pointer is for the convenience of extension writers. It allows
an extension to create a linked list of new elements that can thenbe added to an array in a loop that traverses the list.
enum { ... } flags;
A set of flag values that convey information between the extension
and gawk. Currently there is only one: AWK_ELEMENT_DELETE. Setting it causes gawk to delete the element from the original arrayupon release of the flattened array.
index
value

The index and value of the element, respectively. All memory
pointed to by index and value belongs to gawk.

typedef struct awk_flat_array {awk_const void *awk_const opaque1;/* for use by gawk */awk_const void *awk_const opaque2;/* for use by gawk */awk_const size_t count;/* how many elements */awk_element_t elements[1]; /* will be extended */} awk_flat_array_t;
This is a flattened array. When an extension gets one of these from gawk, theelements array is of actual size count. The opaque1 and opaque2 pointers arefor use by gawk; therefore they are marked awk_const so that the extension
cannot modify them.

17.4.12.2 Array Functions
The following functions relate to individual array elements:
awk_bool_t get_element_count(awk_array_t a_cookie, size_t *count);
For the array represented by a_cookie, place in *count the number of elements
it contains. A subarray counts as a single element. Return false if there is anerror.

Chapter 17: Writing Extensions for gawk 435

awk_bool_t get_array_element(awk_array_t a_cookie,
const awk_value_t *const index,
awk_valtype_t wanted,
awk_value_t *result);
For the array represented by a_cookie, return in *result the value of theelement whose index is index. wanted specifies the type of value you wish to
retrieve. Return false if wanted does not match the actual type or if index is
not in the array (see Table 17.2).
The value for index can be numeric, in which case gawk converts it to a string.
Using nonintegral values is possible, but requires that you understand how
such values are converted to strings (see Section 6.1.4 [Conversion of Strings
and Numbers], page 126); thus, using integral values is safest.
As with all strings passed into gawk from an extension, the string value ofindex must come from gawk_malloc(), gawk_calloc(), or gawk_realloc(),
and gawk releases the storage.
awk_bool_t set_array_element(awk_array_t a_cookie,
const awk_value_t *const index,
const awk_value_t *const value);In the array represented by a_cookie, create or modify the element whose index
is given by index. The ARGV and ENVIRON arrays may not be changed, although
the PROCINFO array can be.
awk_bool_t set_array_element_by_elem(awk_array_t a_cookie,
awk_element_t element);
Like set_array_element(), but take the index and value from element. Thisis a convenience macro.
awk_bool_t del_array_element(awk_array_t a_cookie,
const awk_value_t* const index);
Remove the element with the given index from the array represented by a_cookie. Return true if the element was removed, or false if the element did notexist in the array.
The following functions relate to arrays as a whole:
awk_array_t create_array(void);
Create a new array to which elements may be added. See Section 17.4.12.4[How To Create and Populate Arrays], page 439, for a discussion of how tocreate a new array and add elements to it.
awk_bool_t clear_array(awk_array_t a_cookie);
Clear the array represented by a_cookie. Return false if there was some kindof problem, true otherwise. The array remains an array, but after calling thisfunction, it has no elements. This is equivalent to using the delete statement(see Section 8.4 [The delete Statement], page 186).
awk_bool_t destroy_array(awk_array_t a_cookie);
Clear the array represented by a_cookie and release the array allocated bycreate_array. Return false if there was some kind of problem, true otherwise.
The array will no longer exist and cannot be used again.

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awk_bool_t flatten_array_typed(awk_array_t a_cookie,
awk_flat_array_t **data,
awk_valtype_t index_type,awk_valtype_t value_type);
For the array represented by a_cookie, create an awk_flat_array_t structureand fill it in with indices and values of the requested types. Set the pointerwhose address is passed as data to point to this structure. Return true uponsuccess, or false otherwise. See Section 17.4.12.3 [Working With All The Elements of an Array], page 436, for a discussion of how to flatten an array andwork with it.
awk_bool_t flatten_array(awk_array_t a_cookie, awk_flat_array_t **data);
For the array represented by a_cookie, create an awk_flat_array_t structureand fill it in with AWK_STRING indices and AWK_UNDEFINED values. This is superseded by flatten_array_typed(). It is provided as a macro, and remains
for convenience and for source code compatibility with the previous version of
the API.
awk_bool_t release_flattened_array(awk_array_t a_cookie,
awk_flat_array_t *data);
When done with a flattened array, release the storage using this function. You
must pass in both the original array cookie and the address of the createdawk_flat_array_t structure. The function returns true upon success, false
otherwise.

17.4.12.3 Working With All The Elements of an Array
To flatten an array is to create a structure that represents the full array in a fashionthat makes it easy for C code to traverse the entire array. Some of the code inextension/testext.c does this, and also serves as a nice example showing how to use theAPIs.
We walk through that part of the code one step at a time. First, the gawk script thatdrives the test extension:@load "testext"
BEGIN {
n = split("blacky rusty sophie raincloud lucky", pets)
printf("pets has %d elements\n", length(pets))
ret = dump_array_and_delete("pets", "3")printf("dump_array_and_delete(pets) returned %d\n", ret)if ("3" in pets)
printf("dump_array_and_delete() did NOT remove index \"3\"!\n")else
printf("dump_array_and_delete() did remove index \"3\"!\n")print ""}
This code creates an array with split() (see Section 9.1.4 [String-Manipulation Functions],
page 196) and then calls dump_array_and_delete(). That function looks up the array
whose name is passed as the first argument, and deletes the element at the index passed inChapter 17: Writing Extensions for gawk 437

the second argument. The awk code then prints the return value and checks if the element
was indeed deleted. Here is the C code that implements dump_array_and_delete(). It
has been edited slightly for presentation.
The first part declares variables, sets up the default return value in result, and checks
that the function was called with the correct number of arguments:
static awk_value_t *dump_array_and_delete(int nargs, awk_value_t *result){awk_value_t value, value2, value3;
awk_flat_array_t *flat_array;size_t count;char *name;
int i;
assert(result != NULL);
make_number(0.0, result);
if (nargs != 2) {printf("dump_array_and_delete: nargs not right ""(%d should be 2)\n", nargs);
goto out;}
The function then proceeds in steps, as follows. First, retrieve the name of the array,passed as the first argument, followed by the array itself. If either operation fails, print anerror message and return:/* get argument named array as flat array and print it */if (get_argument(0, AWK_STRING, & value)) {name = value.str_value.str;
if (sym_lookup(name, AWK_ARRAY, & value2))
printf("dump_array_and_delete: sym_lookup of %s passed\n",name);
else {
printf("dump_array_and_delete: sym_lookup of %s failed\n",name);
goto out;}} else {printf("dump_array_and_delete: get_argument(0) failed\n");goto out;}
For testing purposes and to make sure that the C code sees the same number of elements
as the awk code, the second step is to get the count of elements in the array and print it:if (! get_element_count(value2.array_cookie, & count)) {printf("dump_array_and_delete: get_element_count failed\n");goto out;

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printf("dump_array_and_delete: incoming size is %lu\n",(unsigned long) count);
The third step is to actually flatten the array, and then to double-check that the countin the awk_flat_array_t is the same as the count just retrieved:if (! flatten_array_typed(value2.array_cookie, & flat_array,
AWK_STRING, AWK_UNDEFINED)) {printf("dump_array_and_delete: could not flatten array\n");goto out;}
if (flat_array->count != count) {printf("dump_array_and_delete: flat_array->count (%lu)"" != count (%lu)\n",(unsigned long) flat_array->count,(unsigned long) count);
goto out;}
The fourth step is to retrieve the index of the element to be deleted, which was passed
as the second argument. Remember that argument counts passed to get_argument() are
zero-based, and thus the second argument is numbered one:if (! get_argument(1, AWK_STRING, & value3)) {printf("dump_array_and_delete: get_argument(1) failed\n");goto out;}
The fifth step is where the “real work” is done. The function loops over every element in
the array, printing the index and element values. In addition, upon finding the element with
the index that is supposed to be deleted, the function sets the AWK_ELEMENT_DELETE bit
in the flags field of the element. When the array is released, gawk traverses the flattenedarray, and deletes any elements that have this flag bit set:for (i = 0; i < flat_array->count; i++) {printf("\t%s[\"%.*s\"] = %s\n",name,(int) flat_array->elements[i].index.str_value.len,
flat_array->elements[i].index.str_value.str,
valrep2str(& flat_array->elements[i].value));if (strcmp(value3.str_value.str,
flat_array->elements[i].index.str_value.str) == 0) {flat_array->elements[i].flags |= AWK_ELEMENT_DELETE;
printf("dump_array_and_delete: marking element \"%s\" ""for deletion\n",flat_array->elements[i].index.str_value.str);}

Chapter 17: Writing Extensions for gawk 439}
The sixth step is to release the flattened array. This tells gawk that the extension is nolonger using the array, and that it should delete any elements marked for deletion. gawk
also frees any storage that was allocated, so you should not use the pointer (flat_array inthis code) once you have called release_flattened_array():if (! release_flattened_array(value2.array_cookie, flat_array)) {printf("dump_array_and_delete: could not release flattened array\n");goto out;}
Finally, because everything was successful, the function sets the return value to success,and returns:
make_number(1.0, result);out:
return result;}
Here is the output from running this part of the test:
pets has 5 elements
dump_array_and_delete: sym_lookup of pets passed
dump_array_and_delete: incoming size is 5
pets["1"] = "blacky"
pets["2"] = "rusty"
pets["3"] = "sophie"
dump_array_and_delete: marking element "3" for deletion
pets["4"] = "raincloud"pets["5"] = "lucky"
dump_array_and_delete(pets) returned 1
dump_array_and_delete() did remove index "3"!

17.4.12.4 How To Create and Populate Arrays
Besides working with arrays created by awk code, you can create arrays and populate themas you see fit, and then awk code can access them and manipulate them.
There are two important points about creating arrays from extension code:• You must install a new array into gawk’s symbol table immediately upon creating it.
Once you have done so, you can then populate the array.
Similarly, if installing a new array as a subarray of an existing array, you must add the
new array to its parent before adding any elements to it.
Thus, the correct way to build an array is to work “top down.” Create the array, andimmediately install it in gawk’s symbol table using sym_update(), or install it as anelement in a previously existing array using set_array_element(). We show example
code shortly.
• Due to gawk internals, after using sym_update() to install an array into gawk, youhave to retrieve the array cookie from the value passed in to sym_update() before
doing anything else with it, like so:
awk_value_t val;

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awk_array_t new_array;new_array = create_array();
val.val_type = AWK_ARRAY;
val.array_cookie = new_array;/* install array in the symbol table */sym_update("array", & val);
new_array = val.array_cookie;/* YOU MUST DO THIS */If installing an array as a subarray, you must also retrieve the value of the array cookie
after the call to set_element().
The following C code is a simple test extension to create an array with two regularelements and with a subarray. The leading #include directives and boilerplate variabledeclarations (see Section 17.4.15 [Boilerplate Code], page 444) are omitted for brevity. The
first step is to create a new array and then install it in the symbol table:/* create_new_array --- create a named array */static void
create_new_array(){awk_array_t a_cookie;
awk_array_t subarray;awk_value_t index, value;
a_cookie = create_array();
value.val_type = AWK_ARRAY;
value.array_cookie = a_cookie;
if (! sym_update("new_array", & value))
printf("create_new_array: sym_update(\"new_array\") failed!\n");a_cookie = value.array_cookie;
Note how a_cookie is reset from the array_cookie field in the value structure.
The second step is to install two regular values into new_array:(void) make_const_string("hello", 5, & index);(void) make_const_string("world", 5, & value);if (! set_array_element(a_cookie, & index, & value)) {printf("fill_in_array: set_array_element failed\n");return;}(void) make_const_string("answer", 6, & index);(void) make_number(42.0, & value);if (! set_array_element(a_cookie, & index, & value)) {printf("fill_in_array: set_array_element failed\n");Chapter 17: Writing Extensions for gawk 441

return;}The third step is to create the subarray and install it:(void) make_const_string("subarray", 8, & index);
subarray = create_array();
value.val_type = AWK_ARRAY;
value.array_cookie = subarray;if (! set_array_element(a_cookie, & index, & value)) {printf("fill_in_array: set_array_element failed\n");return;}subarray = value.array_cookie;
The final step is to populate the subarray with its own element:(void) make_const_string("foo", 3, & index);(void) make_const_string("bar", 3, & value);if (! set_array_element(subarray, & index, & value)) {printf("fill_in_array: set_array_element failed\n");return;}}Here is a sample script that loads the extension and then dumps the array:@load "subarray"function dumparray(name, array,i){for (i in array)
if (isarray(array[i]))dumparray(name "[\"" i "\"]", array[i])elseprintf("%s[\"%s\"] = %s\n", name, i, array[i])}BEGIN {
dumparray("new_array", new_array);}Here is the result of running the script:$ AWKLIBPATH=$PWD gawk -f subarray.awk
a new_array["subarray"]["foo"] = bar
a new_array["hello"] = world
a new_array["answer"] = 42
(See Section 17.5 [How gawk Finds Extensions], page 447, for more information on the
AWKLIBPATH environment variable.)

17.4.13 Accessing and Manipulating Redirections
The following function allows extensions to access and manipulate redirections.

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awk_bool_t get_file(const char *name,size_t name_len,
const char *filetype,int fd,const awk_input_buf_t **ibufp,
const awk_output_buf_t **obufp);
Look up file name in gawk’s internal redirection table. If name is NULL or name_len is zero, return data for the currently open input file corresponding to
FILENAME. (This does not access the filetype argument, so that may be
undefined). If the file is not already open, attempt to open it. The filetype
argument must be zero-terminated and should be one of:">"A file opened for output.

">>"

A file opened for append.

"<"

A file opened for input.

"|>"

A pipe opened for output.

"|<"

A pipe opened for input.

"|&"

A two-way coprocess.

On error, return awk_false. Otherwise, return awk_true, and return additional information about the redirection in the ibufp and obufp pointers.
For input redirections, the *ibufp value should be non-NULL, and *obufp should
be NULL. For output redirections, the *obufp value should be non-NULL, and*ibufp should be NULL. For two-way coprocesses, both values should be nonNULL.
In the usual case, the extension is interested in (*ibufp)->fd and/or
fileno((*obufp)->fp). If the file is not already open, and the fd argument
is nonnegative, gawk will use that file descriptor instead of opening the file in
the usual way. If fd is nonnegative, but the file exists already, gawk ignores
fd and returns the existing file. It is the caller’s responsibility to notice thatneither the fd in the returned awk_input_buf_t nor the fd in the returned
awk_output_buf_t matches the requested value.
Note that supplying a file descriptor is currently not supported for pipes. However, supplying a file descriptor should work for input, output, append, andtwo-way (coprocess) sockets. If filetype is two-way, gawk assumes that it isa socket! Note that in the two-way case, the input and output file descriptors
may differ. To check for success, you must check whether either matches.
It is anticipated that this API function will be used to implement I/O multiplexing anda socket library.

17.4.14 API Variables
The API provides two sets of variables. The first provides information about the version of
the API (both with which the extension was compiled, and with which gawk was compiled).
The second provides information about how gawk was invoked.

Chapter 17: Writing Extensions for gawk 443

17.4.14.1 API Version Constants and Variables
The API provides both a “major” and a “minor” version number. The API versions areavailable at compile time as C preprocessor defines to support conditional compilation, andas enum constants to facilitate debugging:
API Version
Major
Minor

C Preprocessor Define
gawk_api_major_version
gawk_api_minor_version

enum constant
GAWK_API_MAJOR_VERSIONGAWK_API_MINOR_VERSION

Table 17.3: gawk API version constants
The minor version increases when new functions are added to the API. Such new functions are always added to the end of the API struct.
The major version increases (and the minor version is reset to zero) if any of the datatypes change size or member order, or if any of the existing functions change signature.
It could happen that an extension may be compiled against one version of the API but
loaded by a version of gawk using a different version. For this reason, the major and minor
API versions of the running gawk are included in the API struct as read-only constantintegers:
api->major_version
The major version of the running gawk.
api->minor_version
The minor version of the running gawk.
It is up to the extension to decide if there are API incompatibilities. Typically, a checklike this is enough:if (api->major_version != GAWK_API_MAJOR_VERSION|| api->minor_version < GAWK_API_MINOR_VERSION) {fprintf(stderr, "foo_extension: version mismatch with gawk!\n");fprintf(stderr, "\tmy version (%d, %d), gawk version (%d, %d)\n",GAWK_API_MAJOR_VERSION, GAWK_API_MINOR_VERSION,
api->major_version, api->minor_version);exit(1);}
Such code is included in the boilerplate dl_load_func() macro provided in gawkapi.h(discussed in Section 17.4.15 [Boilerplate Code], page 444).

17.4.14.2 GMP and MPFR Version Information
The API also includes information about the versions of GMP and MPFR with which therunning gawk was compiled (if any). They are included in the API struct as read-only
constant integers:api->gmp_major_version
The major version of the GMP library used to compile gawk.
api->gmp_minor_version
The minor version of the GMP library used to compile gawk.

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api->mpfr_major_version
The major version of the MPFR library used to compile gawk.
api->mpfr_minor_version
The minor version of the MPFR library used to compile gawk.
These fields are set to zero if gawk was compiled without MPFR support.
You can check if the versions of MPFR and GMP that you are using match those ofgawk with the following macro:
check_mpfr_version(extension)
The extension is the extension id passed to all the other macros and functions
defined in gawkapi.h. If you have not included the <mpfr.h> header file, thenthis macro will be defined to do nothing.
If you have included that file, then this macro compares the MPFR and GMP
major and minor versions against those of the library you are compiling against.
If your libraries are newer than gawk’s, it produces a fatal error message.
The dl_load_func() macro (see Section 17.4.15 [Boilerplate Code], page 444)
calls check_mpfr_version().

17.4.14.3 Informational Variables
The API provides access to several variables that describe whether the corresponding
command-line options were enabled when gawk was invoked. The variables are:do_csv

This variable is true if gawk was invoked with --csv option.

do_debugThis variable is true if gawk was invoked with --debug option.

do_lint

This variable is true if gawk was invoked with --lint option.

do_mpfr

This variable is true if gawk was invoked with --bignum option.

do_profile
This variable is true if gawk was invoked with --profile option.
do_sandbox
This variable is true if gawk was invoked with --sandbox option.
do_traditional
This variable is true if gawk was invoked with --traditional option.
The value of do_lint can change if awk code modifies the LINT predefined variable(see Section 7.5 [Predefined Variables], page 163). The others should not change duringexecution.

17.4.15 Boilerplate CodeAs mentioned earlier (see Section 17.3 [How It Works at a High Level], page 405), thefunction definitions as presented are really macros. To use these macros, your extension
must provide a small amount of boilerplate code (variables and functions) toward the topof your source file, using predefined names as described here. The boilerplate needed is also
provided in comments in the gawkapi.h header file:Chapter 17: Writing Extensions for gawk 445/* Boilerplate code: */int plugin_is_GPL_compatible;
static gawk_api_t *const api;
static awk_ext_id_t ext_id;
static const char *ext_version = NULL; /* or ... = "some string" */static awk_ext_func_t func_table[] = {{ "name", do_name, 1, 0, awk_false, NULL },/* ... */};/* EITHER: */static awk_bool_t (*init_func)(void) = NULL;/* OR: */static awk_bool_t
init_my_extension(void){...
}
static awk_bool_t (*init_func)(void) = init_my_extension;
dl_load_func(func_table, some_name, "name_space_in_quotes")
These variables and functions are as follows:
int plugin_is_GPL_compatible;
This asserts that the extension is compatible with the GNU GPL (see [GNUGeneral Public License], page 541). If your extension does not have this, gawk
will not load it (see Section 17.2 [Extension Licensing], page 405).
static gawk_api_t *const api;
This global static variable should be set to point to the gawk_api_t pointerthat gawk passes to your dl_load() function. This variable is used by all of
the macros.
static awk_ext_id_t ext_id;
This global static variable should be set to the awk_ext_id_t value that gawk
passes to your dl_load() function. This variable is used by all of the macros.
static const char *ext_version = NULL; /* or ... = "some string" */This global static variable should be set either to NULL, or to point to a string
giving the name and version of your extension.

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static awk_ext_func_t func_table[] = { ... };This is an array of one or more awk_ext_func_t structures, as described earlier(see Section 17.4.6.1 [Registering An Extension Function], page 417). It canthen be looped over for multiple calls to add_ext_func().
static awk_bool_t (*init_func)(void) = NULL;
OR
static awk_bool_t init_my_extension(void) { ... }static awk_bool_t (*init_func)(void) = init_my_extension;
If you need to do some initialization work, you should define a function that
does it (creates variables, opens files, etc.) and then define the init_func
pointer to point to your function. The function should return awk_false upon
failure, or awk_true if everything goes well.
If you don’t need to do any initialization, define the pointer and initialize it to
NULL.
dl_load_func(func_table, some_name, "name_space_in_quotes")
This macro expands to a dl_load() function that performs all the necessary
initializations.
The point of all the variables and arrays is to let the dl_load() function (from thedl_load_func() macro) do all the standard work. It does the following:
1. Check the API versions. If the extension major version does not match gawk’s, or if
the extension minor version is greater than gawk’s, it prints a fatal error message andexits.
2. Check the MPFR and GMP versions. If there is a mismatch, it prints a fatal error
message and exits.
3. Load the functions defined in func_table. If any of them fails to load, it prints awarning message but continues on.
4. If the init_func pointer is not NULL, call the function it points to. If it returns awk_
false, print a warning message.
5. If ext_version is not NULL, register the version string with gawk.

17.4.16 Changes From Version 1 of the API
The current API is not binary compatible with version 1 of the API. You will have torecompile your extensions in order to use them with the current version of gawk.
Fortunately, at the possible expense of some compile-time warnings, the API remains
source-code–compatible with the previous API. The major differences are the additional
members in the awk_ext_func_t structure, and the addition of the third argument to
the C implementation function (see Section 17.4.6.1 [Registering An Extension Function],
page 417).
Here is a list of individual features that changed from version 1 to version 2 of the API:
• Numeric values can now have MPFR/MPZ variants (see Section 17.4.2[General-Purpose Data Types], page 410).
• There are new string types: AWK_REGEX and AWK_STRNUM (see Section 17.4.2 [GeneralPurpose Data Types], page 410).

Chapter 17: Writing Extensions for gawk 447• The ezalloc() macro is new (see Section 17.4.3 [Memory Allocation Functions andConvenience Macros], page 413).
• The awk_ext_func_t structure changed. Instead of num_expected_args, it now has
max_expected and min_required (see Section 17.4.6.1 [Registering An Extension
Function], page 417).
• For get_record(), an input parser can now specify field widths (see Section 17.4.6.4[Customized Input Parsers], page 420).
• Extensions can now produce nonfatal error messages (see Section 17.4.7 [Printing Messages], page 427).
• When flattening an array, you can now specify the index and value types (seeSection 17.4.12.2 [Array Functions], page 434).
• The get_file() API is new (see Section 17.4.13 [Accessing and Manipulating Redirections], page 441).

17.5 How gawk Finds Extensions
Compiled extensions have to be installed in a directory where gawk can find them. If gawk
is configured and built in the default fashion, the directory in which to find extensions is/usr/local/lib/gawk. You can also specify a search path with a list of directories tosearch for compiled extensions. See Section 2.5.2 [The AWKLIBPATH Environment Variable],
page 43, for more information.

17.6 Example: Some File Functions
No matter where you go, there you are.
—Buckaroo Banzai
Two useful functions that are not in awk are chdir() (so that an awk program canchange its directory) and stat() (so that an awk program can gather information about afile). In order to illustrate the API in action, this section implements these functions forgawk in an extension.

17.6.1 Using chdir() and stat()
This section shows how to use the new functions at the awk level once they’ve been integrated
into the running gawk interpreter. Using chdir() is very straightforward. It takes one
argument, the new directory to change to:@load "filefuncs"...
newdir = "/home/arnold/funstuff"
ret = chdir(newdir)
if (ret < 0) {printf("could not change to %s: %s\n", newdir, ERRNO) > "/dev/stderr"
exit 1}...
The return value is negative if the chdir() failed, and ERRNO (see Section 7.5 [Predefined
Variables], page 163) is set to a string indicating the error.

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Using stat() is a bit more complicated. The C stat() function fills in a structurethat has a fair amount of information. The right way to model this in awk is to fill in an
associative array with the appropriate information:
file = "/home/arnold/.profile"
ret = stat(file, fdata)
if (ret < 0) {printf("could not stat %s: %s\n",file, ERRNO) > "/dev/stderr"
exit 1}
printf("size of %s is %d bytes\n", file, fdata["size"])
The stat() function always clears the data array, even if the stat() fails. It fills in thefollowing elements:"name"The name of the file that was stat()ed.

"dev"
"ino"

The file’s device and inode numbers, respectively.

"mode"

The file’s mode, as a numeric value. This includes both the file’s type and its
permissions.

"nlink"The number of hard links (directory entries) the file has.

"uid"
"gid"

The numeric user and group ID numbers of the file’s owner.

"size"The size in bytes of the file.

"blocks"

The number of disk blocks the file actually occupies. This may not be a functionof the file’s size if the file has holes.

"atime""mtime""ctime"

The file’s last access, modification, and inode update times, respectively.
These are numeric timestamps, suitable for formatting with strftime() (seeSection 9.1.6 [Time Functions], page 211).

"pmode"

The file’s “printable mode.” This is a string representation of the file’s type
and permissions, such as is produced by ‘ls -l’—for example, "drwxr-xr-x".

"type"A printable string representation of the file’s type. The value is one of thefollowing:"blockdev""chardev"
The file is a block or character device (“special file”).
"directory"
The file is a directory.
"fifo"

The file is a named pipe (also known as a FIFO).

"file"

The file is just a regular file.

Chapter 17: Writing Extensions for gawk 449"socket"The file is an AF_UNIX (“Unix domain”) socket in the filesystem.

"symlink"
The file is a symbolic link.
"devbsize"
The size of a block for the element indexed by "blocks". This information isderived from either the DEV_BSIZE constant defined in <sys/param.h> on most
systems, or the S_BLKSIZE constant in <sys/stat.h> on BSD systems. For
some other systems, a priori knowledge is used to provide a value. Where novalue can be determined, it defaults to 512.
Several additional elements may be present, depending upon the operating system andthe type of the file. You can test for them in your awk program by using the in operator(see Section 8.1.2 [Referring to an Array Element], page 179):"blksize"
The preferred block size for I/O to the file. This field is not present on all
POSIX-like systems in the C stat structure.
"linkval"
If the file is a symbolic link, this element is the name of the file the link points
to (i.e., the value of the link).
"rdev""major""minor"If the file is a block or character device file, then these values represent thenumeric device number and the major and minor components of that number,respectively.

17.6.2 C Code for chdir() and stat()
Here is the C code for these extensions.9
The file includes a number of standard header files, and then includes the gawkapi.h
header file, which provides the API definitions. Those are followed by the necessary variable declarations to make use of the API macros and boilerplate code (see Section 17.4.15[Boilerplate Code], page 444):#ifdef HAVE_CONFIG_H#include <config.h>#endif#include <stdio.h>#include <assert.h>#include <errno.h>#include <stdlib.h>#include <string.h>#include <unistd.h>9This version is edited slightly for presentation. See extension/filefuncs.c in the gawk distribution forthe complete version.

450

GAWK: Effective AWK Programming#include <sys/types.h>#include <sys/stat.h>#include "gawkapi.h"#include "gettext.h"#define _(msgid) gettext(msgid)#define N_(msgid) msgid#include "gawkfts.h"#include "stack.h"static const gawk_api_t *api;/* for convenience macros to work */static awk_ext_id_t ext_id;
static awk_bool_t init_filefuncs(void);static awk_bool_t (*init_func)(void) = init_filefuncs;
static const char *ext_version = "filefuncs extension: version 1.0";
int plugin_is_GPL_compatible;
By convention, for an awk function foo(), the C function that implements it is called
do_foo(). The function should have two arguments. The first is an int, usually called
nargs, that represents the number of actual arguments for the function. The second is apointer to an awk_value_t structure, usually named result:/*do_chdir --- provide dynamically loaded chdir() function for gawk */static awk_value_t *do_chdir(int nargs, awk_value_t *result, struct awk_ext_func *unused){
awk_value_t newdir;
int ret = -1;
assert(result != NULL);The newdir variable represents the new directory to change to, which is retrieved with
get_argument(). Note that the first argument is numbered zero.
If the argument is retrieved successfully, the function calls the chdir() system call.
Otherwise, if the chdir() fails, it updates ERRNO:
if (get_argument(0, AWK_STRING, & newdir)) {ret = chdir(newdir.str_value.str);
if (ret < 0)update_ERRNO_int(errno);}
Finally, the function returns the return value to the awk level:
return make_number(ret, result);}Chapter 17: Writing Extensions for gawk 451

The stat() extension is more involved. First comes a function that turns a numericmode into a printable representation (e.g., octal 0644 becomes ‘-rw-r--r--’). This isomitted here for brevity:/* format_mode --- turn a stat mode field into something readable */static char *format_mode(unsigned long fmode){...
}
Next comes a function for reading symbolic links, which is also omitted here for brevity:/* read_symlink --- read a symbolic link into an allocated buffer.
... */
static char *read_symlink(const char *fname, size_t bufsize, ssize_t *linksize){...
}
Two helper functions simplify entering values in the array that will contain the result of
the stat():/* array_set --- set an array element */static voidarray_set(awk_array_t array, const char *sub, awk_value_t *value){
awk_value_t index;
set_array_element(array,make_const_string(sub, strlen(sub), & index),
value);}/* array_set_numeric --- set an array element with a number */static voidarray_set_numeric(awk_array_t array, const char *sub, double num){awk_value_t tmp;array_set(array, sub, make_number(num, & tmp));}
The following function does most of the work to fill in the awk_array_t result arraywith values obtained from a valid struct stat. This work is done in a separate function

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to support the stat() function for gawk and also to support the fts() extension, which isincluded in the same file but whose code is not shown here (see Section 17.7.1 [File-Related
Functions], page 457).
The first part of the function is variable declarations, including a table to map file types
to strings:/* fill_stat_array --- do the work to fill an array with stat info */static int
fill_stat_array(const char *name, awk_array_t array, struct stat *sbuf){
char *pmode;/* printable mode */const char *type = "unknown";
awk_value_t tmp;
static struct ftype_map {unsigned int mask;
const char *type;} ftype_map[] = {{ S_IFREG, "file" },{ S_IFBLK, "blockdev" },{ S_IFCHR, "chardev" },{ S_IFDIR, "directory" },#ifdef S_IFSOCK{ S_IFSOCK, "socket" },#endif#ifdef S_IFIFO{ S_IFIFO, "fifo" },#endif#ifdef S_IFLNK{ S_IFLNK, "symlink" },#endif#ifdef S_IFDOOR /* Solaris weirdness */{ S_IFDOOR, "door" },#endif};int j, k;
The destination array is cleared, and then code fills in various elements based on values
in the struct stat:/* empty out the array */clear_array(array);/* fill in the array */array_set(array, "name", make_const_string(name, strlen(name),& tmp));
array_set_numeric(array, "dev", sbuf->st_dev);
array_set_numeric(array, "ino", sbuf->st_ino);array_set_numeric(array, "mode", sbuf->st_mode);Chapter 17: Writing Extensions for gawk 453

array_set_numeric(array, "nlink", sbuf->st_nlink);array_set_numeric(array, "uid", sbuf->st_uid);array_set_numeric(array, "gid", sbuf->st_gid);array_set_numeric(array, "size", sbuf->st_size);array_set_numeric(array, "blocks", sbuf->st_blocks);
array_set_numeric(array, "atime", sbuf->st_atime);
array_set_numeric(array, "mtime", sbuf->st_mtime);
array_set_numeric(array, "ctime", sbuf->st_ctime);/* for block and character devices, add rdev,
major and minor numbers */if (S_ISBLK(sbuf->st_mode) || S_ISCHR(sbuf->st_mode)) {array_set_numeric(array, "rdev", sbuf->st_rdev);
array_set_numeric(array, "major", major(sbuf->st_rdev));
array_set_numeric(array, "minor", minor(sbuf->st_rdev));}
The latter part of the function makes selective additions to the destination array, depending
upon the availability of certain members and/or the type of the file. It then returns zero,for success:#ifdef HAVE_STRUCT_STAT_ST_BLKSIZE
array_set_numeric(array, "blksize", sbuf->st_blksize);#endifpmode = format_mode(sbuf->st_mode);array_set(array, "pmode", make_const_string(pmode, strlen(pmode),& tmp));/* for symbolic links, add a linkval field */if (S_ISLNK(sbuf->st_mode)) {char *buf;
ssize_t linksize;if ((buf = read_symlink(name, sbuf->st_size,& linksize)) != NULL)
array_set(array, "linkval",
make_malloced_string(buf, linksize, & tmp));else
warning(ext_id, _("stat: unable to read symbolic link `%s'"),name);}/* add a type field */type = "unknown";/* shouldn't happen */for (j = 0, k = sizeof(ftype_map)/sizeof(ftype_map[0]); j < k; j++) {if ((sbuf->st_mode & S_IFMT) == ftype_map[j].mask) {type = ftype_map[j].type;454

GAWK: Effective AWK Programmingbreak;}}
array_set(array, "type", make_const_string(type, strlen(type), & tmp));
return 0;}
The third argument to stat() was not discussed previously. This argument is optional.
If present, it causes do_stat() to use the stat() system call instead of the lstat() systemcall. This is done by using a function pointer: statfunc. statfunc is initialized to point tolstat() (instead of stat()) to get the file information, in case the file is a symbolic link.
However, if the third argument is included, statfunc is set to point to stat(), instead.
Here is the do_stat() function, which starts with variable declarations and argument
checking:/* do_stat --- provide a stat() function for gawk */static awk_value_t *do_stat(int nargs, awk_value_t *result, struct awk_ext_func *unused){
awk_value_t file_param, array_param;char *name;awk_array_t array;
int ret;struct stat sbuf;/* default is lstat() */int (*statfunc)(const char *path, struct stat *sbuf) = lstat;assert(result != NULL);
Then comes the actual work. First, the function gets the arguments. Next, it gets theinformation for the file. If the called function (lstat() or stat()) returns an error, thecode sets ERRNO and returns:/* file is first arg, array to hold results is second */if (! get_argument(0, AWK_STRING, & file_param)|| ! get_argument(1, AWK_ARRAY, & array_param)) {warning(ext_id, _("stat: bad parameters"));return make_number(-1, result);}
if (nargs == 3) {statfunc = stat;}name = file_param.str_value.str;
array = array_param.array_cookie;Chapter 17: Writing Extensions for gawk 455/* always empty out the array */clear_array(array);/* stat the file; if error, set ERRNO and return */ret = statfunc(name, & sbuf);
if (ret < 0) {update_ERRNO_int(errno);
return make_number(ret, result);}
The tedious work is done by fill_stat_array(), shown earlier. When done, the function returns the result from fill_stat_array():
ret = fill_stat_array(name, array, & sbuf);
return make_number(ret, result);}
Finally, it’s necessary to provide the “glue” that loads the new function(s) into gawk.
The filefuncs extension also provides an fts() function, which we omit here (seeSection 17.7.1 [File-Related Functions], page 457). For its sake, there is an initialization
function:/* init_filefuncs --- initialization routine */static awk_bool_t
init_filefuncs(void){...
}
We are almost done. We need an array of awk_ext_func_t structures for loading eachfunction into gawk:
static awk_ext_func_t func_table[] = {{ "chdir", do_chdir, 1, 1, awk_false, NULL },{ "stat", do_stat, 3, 2, awk_false, NULL },...
};Each extension must have a routine named dl_load() to load everything that needs tobe loaded. It is simplest to use the dl_load_func() macro in gawkapi.h:/* define the dl_load() function using the boilerplate macro */dl_load_func(func_table, filefuncs, "")And that’s it!

17.6.3 Integrating the Extensions
Now that the code is written, it must be possible to add it at runtime to the running gawk
interpreter. First, the code must be compiled. Assuming that the functions are in a file456

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named filefuncs.c, and idir is the location of the gawkapi.h header file, the following
steps10 create a GNU/Linux shared library:$ gcc -fPIC -shared -DHAVE_CONFIG_H -c -O -g -Iidir filefuncs.c$ gcc -o filefuncs.so -shared filefuncs.oOnce the library exists, it is loaded by using the @load keyword:# file testff.awk@load "filefuncs"
BEGIN {
"pwd" | getline curdir
close("pwd")
chdir("/tmp")
system("pwd")
chdir(curdir)# save current directory# test it# go backprint "Info for testff.awk"
ret = stat("testff.awk", data)
print "ret =", retfor (i in data)
printf "data[\"%s\"] = %s\n", i, data[i]print "testff.awk modified:",strftime("%m %d %Y %H:%M:%S", data["mtime"])print "\nInfo for JUNK"
ret = stat("JUNK", data)
print "ret =", retfor (i in data)
printf "data[\"%s\"] = %s\n", i, data[i]print "JUNK modified:", strftime("%m %d %Y %H:%M:%S", data["mtime"])}
The AWKLIBPATH environment variable tells gawk where to find extensions (seeSection 17.5 [How gawk Finds Extensions], page 447). We set it to the current directoryand run the program:$ AWKLIBPATH=$PWD gawk -f testff.awk
a /tmpa Info for testff.awk
a ret = 0
a data["blksize"] = 4096
a data["devbsize"] = 512a data["mtime"] = 1412004710
a data["mode"] = 33204
10

In practice, you would probably want to use the GNU Autotools (Automake, Autoconf, Libtool, andgettext) to configure and build your libraries. Instructions for doing so are beyond the scope of thisbook. See Section 17.8 [The gawkextlib Project], page 467, for Internet links to the tools.

Chapter 17: Writing Extensions for gawk 457

a
a
a
a
a
a
a
a
a
a
a
a
a
a
a
a
a

data["type"] = filedata["dev"] = 2053
data["gid"] = 1000
data["ino"] = 10358899
data["ctime"] = 1412004710
data["blocks"] = 8
data["nlink"] = 1
data["name"] = testff.awk
data["atime"] = 1412004716
data["pmode"] = -rw-rw-r-data["size"] = 666
data["uid"] = 1000
testff.awk modified: 09 29 2014 18:31:50
Info for JUNK
ret = -1
JUNK modified: 01 01 1970 02:00:00

17.7 The Sample Extensions in the gawk Distribution
This section provides a brief overview of the sample extensions that come in the gawk
distribution. Some of them are intended for production use (e.g., the filefuncs, readdir,
and inplace extensions). Others mainly provide example code that shows how to use theextension API.

17.7.1 File-Related Functions
The filefuncs extension provides three different functions, as follows. The usage is:@load "filefuncs"
This is how you load the extension.
result = chdir("/some/directory")
The chdir() function is a direct hook to the chdir() system call to change thecurrent directory. It returns zero upon success or a value less than zero uponerror. In the latter case, it updates ERRNO.
result = stat("/some/path", statdata [, follow])
The stat() function provides a hook into the stat() system call. It returns
zero upon success or a value less than zero upon error. In the latter case, it
updates ERRNO.
By default, it uses the lstat() system call. However, if passed a third argument, it uses stat() instead.
In all cases, it clears the statdata array. When the call is successful, stat() fills
the statdata array with information retrieved from the filesystem, as follows:
Subscript
Field in struct stat
File type"name"
The file name
All
"dev"
st_devAll"ino"
st_inoAll

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st_modest_nlink
st_uidst_gidst_size
st_atime
st_mtime
st_ctime
st_rdev
st_major
st_minorst_blksize
A human-readable version of themode value, like that printed by ls(for example, "-rwxr-xr-x")
The value of the symbolic link
The type of the file as a string—oneof "file", "blockdev", "chardev","directory", "socket", "fifo","symlink", "door", or "unknown"(not all systems support all file types)

All
All
All
All
All
All
All
All
Device files
Device files
Device files
All
All

Symboliclinks
Allflags = or(FTS_PHYSICAL, ...)
result = fts(pathlist, flags, filedata)
Walk the file trees provided in pathlist and fill in the filedata array, as
described next. flags is the bitwise OR of several predefined values, also
described in a moment. Return zero if there were no errors, otherwise return−1.
The fts() function provides a hook to the C library fts() routines for traversing filehierarchies. Instead of returning data about one file at a time in a stream, it fills in a
multidimensional array with data about each file and directory encountered in the requested
hierarchies.
The arguments are as follows:
pathlist

An array of file names. The element values are used; the index values are
ignored.

flags

This should be the bitwise OR of one or more of the following predefined constant flag values. At least one of FTS_LOGICAL or FTS_PHYSICAL must be provided; otherwise fts() returns an error value and sets ERRNO. The flags are:
FTS_LOGICAL
Do a “logical” file traversal, where the information returned for a
symbolic link refers to the linked-to file, and not to the symbolic
link itself. This flag is mutually exclusive with FTS_PHYSICAL.

Chapter 17: Writing Extensions for gawk 459

FTS_PHYSICAL
Do a “physical” file traversal, where the information returned for a
symbolic link refers to the symbolic link itself. This flag is mutually
exclusive with FTS_LOGICAL.
FTS_NOCHDIR
As a performance optimization, the C library fts() routines change
directory as they traverse a file hierarchy. This flag disables that
optimization.
FTS_COMFOLLOW
Immediately follow a symbolic link named in pathlist, whether or
not FTS_LOGICAL is set.
FTS_SEEDOTBy default, the C library fts() routines do not return entries for. (dot) and .. (dot-dot). This option causes entries for dot-dot toalso be included. (The extension always includes an entry for dot;
more on this in a moment.)
FTS_XDEV
filedata

During a traversal, do not cross onto a different mounted filesystem.

The filedata array holds the results. fts() first clears it. Then it creates anelement in filedata for every element in pathlist. The index is the name ofthe directory or file given in pathlist. The element for this index is itself anarray. There are two cases:
The path is a fileIn this case, the array contains two or three elements:"path"

The full path to this file, starting from the “root” thatwas given in the pathlist array.

"stat"

This element is itself an array, containing the same information as provided by the stat() function describedearlier for its statdata argument. The element may
not be present if the stat() system call for the file
failed.

"error"If some kind of error was encountered, the array will
also contain an element named "error", which is astring describing the error.

The path is a directory
In this case, the array contains one element for each entry in the
directory. If an entry is a file, that element is the same as forfiles, just described. If the entry is a directory, that element is(recursively) an array describing the subdirectory. If FTS_SEEDOT
was provided in the flags, then there will also be an element named"..". This element will be an array containing the data as provided
by stat().

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In addition, there will be an element whose index is ".". Thiselement is an array containing the same two or three elements as
for a file: "path", "stat", and "error".
The fts() function returns zero if there were no errors. Otherwise, it returns −1.
NOTE: The fts() extension does not exactly mimic the interface of the C library fts() routines, choosing instead to provide an interface that is based on
associative arrays, which is more comfortable to use from an awk program. This
includes the lack of a comparison function, because gawk already provides powerful array sorting facilities. Although an fts_read()-like interface could havebeen provided, this felt less natural than simply creating a multidimensional
array to represent the file hierarchy and its information.
See test/fts.awk in the gawk distribution for an example use of the fts() extension
function.

17.7.2 Interface to fnmatch()
This extension provides an interface to the C library fnmatch() function. The usage is:@load "fnmatch"This is how you load the extension.
result = fnmatch(pattern, string, flags)
The return value is zero on success, FNM_NOMATCH if the string did not match
the pattern, or a different nonzero value if an error occurred.
In addition to the fnmatch() function, the fnmatch extension adds one constant (FNM_
NOMATCH), and an array of flag values named FNM.
The arguments to fnmatch() are:pattern

The file name wildcard to match

string

The file name string

flag

Either zero, or the bitwise OR of one or more of the flags in the FNM array

The flags are as follows:
Array element
FNM["CASEFOLD"]FNM["FILE_NAME"]
FNM["LEADING_DIR"]FNM["NOESCAPE"]
FNM["PATHNAME"]
FNM["PERIOD"]

Corresponding flag defined by fnmatch()
FNM_CASEFOLDFNM_FILE_NAME
FNM_LEADING_DIR
FNM_NOESCAPE
FNM_PATHNAMEFNM_PERIOD

Here is an example:@load "fnmatch"...
flags = or(FNM["PERIOD"], FNM["NOESCAPE"])if (fnmatch("*.a", "foo.c", flags) == FNM_NOMATCH)
print "no match"

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17.7.3 Interface to fork(), wait(), and waitpid()
The fork extension adds three functions, as follows:@load "fork"
This is how you load the extension.
pid = fork()
This function creates a new process. The return value is zero in the child andthe process ID number of the child in the parent, or −1 upon error. In thelatter case, ERRNO indicates the problem. In the child, PROCINFO["pid"] andPROCINFO["ppid"] are updated to reflect the correct values.
ret = waitpid(pid)This function takes a numeric argument, which is the process ID to wait for.
The return value is that of the waitpid() system call.
ret = wait()
This function waits for the first child to die. The return value is that of thewait() system call.
There is no corresponding exec() function.
Here is an example:@load "fork"...
if ((pid = fork()) == 0)
print "hello from the child"else
print "hello from the parent"

17.7.4 Enabling In-Place File Editing
The inplace extension emulates GNU sed’s -i option, which performs “in-place” editingof each input file. It uses the bundled inplace.awk include file to invoke the extension
properly. This extension makes use of the namespace facility to place all the variables andfunctions in the inplace namespace (see Chapter 15 [Namespaces in gawk], page 383):

462

GAWK: Effective AWK Programming# inplace --- load and invoke the inplace extension.
@load "inplace"# Please set inplace::suffix to make a backup copy. For example, you may# want to set inplace::suffix to .bak on the command line or in a BEGIN rule.
# Before there were namespaces in gawk, this extension used
# INPLACE_SUFFIX as the variable for making backup copies. We allow this# too, so that any code that used the previous version continues to work.
# By default, each filename on the command line will be edited inplace.
# But you can selectively disable this by adding an inplace::enable=0 argument# prior to files that you do not want to process this way. You can then# reenable it later on the commandline by putting inplace::enable=1 before files# that you wish to be subject to inplace editing.
# N.B. We call inplace::end() in the BEGINFILE and END rules so that any# actions in an ENDFILE rule will be redirected as expected.
@namespace "inplace"
BEGIN {
enable = 1}# enabled by default

BEGINFILE {sfx = (suffix ? suffix : awk::INPLACE_SUFFIX)
if (filename != "")end(filename, sfx)
if (enable)
begin(filename = FILENAME, sfx)
else
filename = ""}END {if (filename != "")end(filename, (suffix ? suffix : awk::INPLACE_SUFFIX))}
For each regular file that is processed, the extension redirects standard output to atemporary file configured to have the same owner and permissions as the original. After the
file has been processed, the extension restores standard output to its original destination.
If inplace::suffix is not an empty string, the original file is linked to a backup file namecreated by appending that suffix. Finally, the temporary file is renamed to the original filename.

Chapter 17: Writing Extensions for gawk 463

Note that the use of this feature can be controlled by placing ‘inplace::enable=0’ on
the command-line prior to listing files that should not be processed this way. You canreenable inplace editing by adding an ‘inplace::enable=1’ argument prior to files thatshould be subject to inplace editing.
The inplace::filename variable serves to keep track of the current file name so as to
not invoke inplace::end() before processing the first file.
If any error occurs, the extension issues a fatal error to terminate processing immediately
without damaging the original file.
Here are some simple examples:$ gawk -i inplace '{ gsub(/foo/, "bar") }; { print }' file1 file2 file3
To keep a backup copy of the original files, try this:$ gawk -i inplace -v inplace::suffix=.bak '{ gsub(/foo/, "bar") }> { print }' file1 file2 file3
Please note that, while the extension does attempt to preserve ownership and permissions, it makes no attempt to copy the ACLs from the original file.
If the program dies prematurely, as might happen if an unhandled signal is received, atemporary file may be left behind.

17.7.5 Character and Numeric values: ord() and chr()
The ordchr extension adds two functions, named ord() and chr(), as follows:@load "ordchr"
This is how you load the extension.
number = ord(string)
Return the numeric value of the first character in string.
char = chr(number)
Return a string whose first character is that represented by number.
These functions are inspired by the Pascal language functions of the same name. Hereis an example:@load "ordchr"...
printf("The numeric value of 'A' is %d\n", ord("A"))printf("The string value of 65 is %s\n", chr(65))

17.7.6 Reading Directories
The readdir extension adds an input parser for directories. The usage is as follows:@load "readdir"
When this extension is in use, instead of skipping directories named on the commandline (or with getline), they are read, with each entry returned as a record.
The record consists of three fields separated by forward slash characters. The first two
are the inode number and the file name, and the third field is a single letter indicating thetype of the file. The letters and their corresponding file types are shown in Table 17.4.

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Letter
b
c
d
f
l
p
s

File type
Block device
Character device
Directory
Regular file
Symbolic link
Named pipe (FIFO)
SocketTable 17.4: File types returned by the readdir extension
On systems where the directory entry contains the file type, the third field is filled infrom that information. On systems without the file type information, the extension fallsback to calling the stat() system call in order to provide the information. Thus the third
field should never be ‘u’ (for “unknown”).
Normally, when reading directories, you should set FS equal to "/". However, you may
instead chose to create PROCINFO["readdir_override"] (with any value). If this element
exists when the directory is opened, then the extension automatically sets the fields in eachrecord for you.
By default, if a directory cannot be opened (due to permission problems, for example),
gawk will exit. As with regular files, this situation can be handled using a BEGINFILE rulethat checks ERRNO and prints an error or otherwise handles the problem.
Here is an example:@load "readdir"...
BEGIN { FS = "/" }{ print "file name is", $2 }17.7.7 Reversing Output
The revoutput extension adds a simple output wrapper that reverses the characters in each
output line. Its main purpose is to show how to write an output wrapper, although it may
be mildly amusing for the unwary. Here is an example:@load "revoutput"
BEGIN {REVOUT = 1
print "don't panic" > "/dev/stdout"}
The output from this program is ‘cinap t'nod’.

17.7.8 Two-Way I/O Example
The revtwoway extension adds a simple two-way processor that reverses the characters ineach line sent to it for reading back by the awk program. Its main purpose is to show how towrite a two-way processor, although it may also be mildly amusing. The following example
shows how to use it:@load "revtwoway"Chapter 17: Writing Extensions for gawk 465

BEGIN {
cmd = "/magic/mirror"
print "don't panic" |& cmd
cmd |& getline result
print resultclose(cmd)}The output from this program also is: ‘cinap t'nod’.

17.7.9 Dumping and Restoring an Array
The rwarray extension adds four functions, named writea(), reada(), writeall() andreadall(), as follows:@load "rwarray"
This is how you load the extension.
ret = writea(file, array)
This function takes a string argument, which is the name of the file to whichto dump the array, and the array itself as the second argument. writea()
understands arrays of arrays. It returns one on success, or zero upon failure.
ret = reada(file, array)
reada() is the inverse of writea(); it reads the file named as its first argument,
filling in the array named as the second argument. It clears the array first. Heretoo, the return value is one on success, or zero upon failure.
ret = writeall(file)
This function takes a string argument, which is the name of the file to which todump the state of all variables. Calling this function is completely equivalent tocalling writea(file, SYMTAB). It returns one on success, or zero upon failure
ret = readall(file)
This function takes a string argument, which is the name of the file from which
to read the contents of various global variables. For each variable in the file,the data is loaded unless the variable has already been assigned a value or used
as an array. In that case, the data for that variable in the file is ignored. It
returns one on success, or zero upon failure.
The array created by reada() is identical to that written by writea() in the sensethat the contents are the same. However, due to implementation issues, the array traversal
order of the re-created array is likely to be different from that of the original array. As
array traversal order in awk is by default undefined, this is (technically) not a problem. If
you need to guarantee a particular traversal order, use the array sorting features in gawk to
do so (see Section 12.3 [Controlling Array Traversal and Array Sorting], page 336).
The file contains binary data. All integral values are written in network byte order.
However, double-precision floating-point values are written as native binary data. Thus,
arrays containing only string data can theoretically be dumped on systems with one byteorder and restored on systems with a different one, but this has not been tried.

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Note that the writeall() and readall() functions provide a mechanism for maintaining persistent state across repeated invocations of a program. If, for example, a program
calculates some statistics based on the data in a series of files, it could save state using
writeall() after processing N files, and then reload the state using readall() when the
N+1st file arrives to update the results.
Here is an example:@load "rwarray"...
ret = writea("arraydump.bin", array)...
ret = reada("arraydump.bin", array)...
ret = writeall("globalstate.bin")...
ret = readall("globalstate.bin")

17.7.10 Reading an Entire File
The readfile extension adds a single function named readfile(), and an input parser:@load "readfile"
This is how you load the extension.
result = readfile("/some/path")
The argument is the name of the file to read. The return value is a string
containing the entire contents of the requested file. Upon error, the function
returns the empty string and sets ERRNO.
BEGIN { PROCINFO["readfile"] = 1 }In addition, the extension adds an input parser that is activated if
PROCINFO["readfile"] exists. When activated, each input file is returned inits entirety as $0. RT is set to the null string.
Here is an example:@load "readfile"...
contents = readfile("/path/to/file");if (contents == "" && ERRNO != "") {print("problem reading file", ERRNO) > "/dev/stderr"...
}

17.7.11 Extension Time Functions
The time extension adds three functions, named gettimeofday() sleep(), andstrptime(), as follows:@load "time"
This is how you load the extension.

Chapter 17: Writing Extensions for gawk 467

the_time = gettimeofday()Return the time in seconds that has elapsed since 1970-01-01 UTC as a floatingpoint value. If the time is unavailable on this platform, return −1 and set ERRNO.
The returned time should have sub-second precision, but the actual precision
may vary based on the platform. If the standard C gettimeofday() systemcall is available on this platform, then it simply returns the value. Otherwise,
if on MS-Windows, it tries to use GetSystemTimeAsFileTime().
result = sleep(seconds)
Attempt to sleep for seconds seconds. If seconds is negative, or the attempt tosleep fails, return −1 and set ERRNO. Otherwise, return zero after sleeping forthe indicated amount of time. Note that seconds may be a floating-point (nonintegral) value. Implementation details: depending on platform availability,
this function tries to use nanosleep() or select() to implement the delay.
timeval = strptime(string, format)
This function takes two arguments, a string representing a date and time, and aformat string describing the data in the string. It calls the C library strptime()function with the given values. If the parsing succeeds, the results are passed
to the C library mktime() function, and its result is returned, expressing thetime in seconds since the epoch in the current local timezone, regardless of any
timezone specified in the string arguments. (This is the same as gawk’s built-in
systime() function.) Otherwise it returns −1 upon error. In the latter case,
Note that the underlying strptime() C library routine apparently ignores any
time zone indication in the date string, producing values relative to the current
time zone.

17.7.12 API Tests
The testext extension exercises parts of the extension API that are not tested by the othersamples. The extension/testext.c file contains both the C code for the extension andawk test code inside C comments that run the tests. The testing framework extracts theawk code and runs the tests. See the source file for more information.

17.8 The gawkextlib Project
The gawkextlib project provides a number of gawk extensions, including one for processing
XML files. This is the evolution of the original xgawk (XML gawk) project.
There are a number of extensions. Some of the more interesting ones are:• abort extension. It allows you to exit immediately from your awk program without
running the END rules.
• json extension. This serializes a multidimensional array into a JSON string, and candeserialize a JSON string into a gawk array. This extension is interesting since it is
written in C++ instead of C.
• MPFR library extension. This provides access to a number of MPFR functions thatgawk’s native MPFR support does not.
• Select extension. It provides functionality based on the select() system call.
• XML parser extension, using the Expat XML parsing library

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You can check out the code for the gawkextlib project using the Git distributed sourcecode control system. The command is as follows:
git clone git://git.code.sf.net/p/gawkextlib/code gawkextlib-code
You will need to have the RapidJson JSON parser library installed in order to build anduse the json extension.
You will need to have the Expat XML parser library installed in order to build and use
the XML extension.
In addition, you must have the GNU Autotools installed (Autoconf, Automake, Libtool,
and GNU gettext).
The simple recipe for building and testing gawkextlib is as follows. First, build andinstall gawk:
cd .../path/to/gawk/code./configure --prefix=/tmp/newgawk
Install in /tmp/newgawk for now
make && make check
Build and check that all is OK
make install
Install gawk
Next, go to https://sourceforge.net/projects/gawkextlib/files to download
gawkextlib and any extensions that you would like to build. The README file at that site
explains how to build the code. If you installed gawk in a non-standard location, you willneed to specify ‘./configure --with-gawk=/path/to/gawk’ to find it. You may need
to use the sudo utility to install both gawk and gawkextlib, depending upon how yoursystem works.
If you write an extension that you wish to share with other gawk users, consider doing
so through the gawkextlib project. See the project’s website for more information.

17.9 Summary• You can write extensions (sometimes called plug-ins) for gawk in C or C++ using theapplication programming interface (API) defined by the gawk developers.
• Extensions must have a license compatible with the GNU General Public License(GPL), and they must assert that fact by declaring a variable named plugin_is_
GPL_compatible.
• Communication between gawk and an extension is two-way. gawk passes a struct tothe extension that contains various data fields and function pointers. The extension
can then call into gawk via the supplied function pointers to accomplish certain tasks.
• One of these tasks is to “register” the name and implementation of new awk-level
functions with gawk. The implementation takes the form of a C function pointer witha defined signature. By convention, implementation functions are named do_XXXX()
for some awk-level function XXXX().
• The API is defined in a header file named gawkapi.h. You must include a number ofstandard header files before including it in your source file.
• API function pointers are provided for the following kinds of operations:• Allocating, reallocating, and releasing memory• Registration functions (you may register extension functions, exit callbacks, a version string, input parsers, output wrappers, and two-way processors)

Chapter 17: Writing Extensions for gawk 469• Printing fatal, nonfatal, warning, and “lint” warning messages• Updating ERRNO, or unsetting it• Accessing parameters, including converting an undefined parameter into an array• Symbol table access (retrieving a global variable, creating one, or changing one)
• Creating and releasing cached values; this provides an efficient way to use values
for multiple variables and can be a big performance win• Manipulating arrays (retrieving, adding, deleting, and modifying elements; getting
the count of elements in an array; creating a new array; clearing an array; andflattening an array for easy C-style looping over all its indices and elements)• The API defines a number of standard data types for representing awk values, arrayelements, and arrays.
• The API provides convenience functions for constructing values. It also provides memory management functions to ensure compatibility between memory allocated by gawk
and memory allocated by an extension.
• All memory passed from gawk to an extension must be treated as read-only by theextension.
• All memory passed from an extension to gawk must come from the API’s memory
allocation functions. gawk takes responsibility for the memory and releases it whenappropriate.
• The API provides information about the running version of gawk so that an extension
can make sure it is compatible with the gawk that loaded it.
• It is easiest to start a new extension by copying the boilerplate code described in thischapter. Macros in the gawkapi.h header file make this easier to do.
• The gawk distribution includes a number of small but useful sample extensions. The
gawkextlib project includes several more (larger) extensions. If you wish to write an
extension and contribute it to the community of gawk users, the gawkextlib projectis the place to do so.

17.10 Exercises
1. Add functions to implement system calls such as chown(), chmod(), and umask() to thefile operations extension presented in Section 17.6.2 [C Code for chdir() and stat()],page 449.
2. Write an input parser that prints a prompt if the input is a from a “terminal” device.
You can use the isatty() function to tell if the input file is a terminal. (Hint: thisfunction is usually expensive to call; try to call it just once.) The content of the prompt
should come from a variable settable by awk-level code. You can write the prompt tostandard error. However, for best results, open a new file descriptor (or file pointer)on /dev/tty and print the prompt there, in case standard error has been redirected.
Why is standard error a better choice than standard output for writing the prompt?
Which reading mechanism should you replace, the one to get a record, or the one toread raw bytes?
3. Write a wrapper script that provides an interface similar to ‘sed -i’ for the “inplace”
extension presented in Section 17.7.4 [Enabling In-Place File Editing], page 461.

Part IV:
AppendicesAppendix A: The Evolution of the awk Language 473

Appendix A The Evolution of the awk Language
This book describes the GNU implementation of awk, which follows the POSIX specification.
Many longtime awk users learned awk programming with the original awk implementation in
Version 7 Unix. (This implementation was the basis for awk in Berkeley Unix, through 4.3Reno. Subsequent versions of Berkeley Unix, and, for a while, some systems derived from
4.4BSD-Lite, used various versions of gawk for their awk.) This chapter briefly describes
the evolution of the awk language, with cross-references to other parts of the book where
you can find more information.

A.1 Major Changes Between V7 and SVR3.1
The awk language evolved considerably between the release of Version 7 Unix (1978) andthe new version that was first made generally available in System V Release 3.1 (1987).
This section summarizes the changes, with cross-references to further details:• The requirement for ‘;’ to separate rules on a line (see Section 1.6 [awk Statements
Versus Lines], page 28)• User-defined functions and the return statement (see Section 9.2 [User-Defined Functions], page 221)
• The delete statement (see Section 8.4 [The delete Statement], page 186)
• The do-while statement (see Section 7.4.3 [The do-while Statement], page 156)
• The built-in functions atan2(), cos(), sin(), rand(), and srand() (see Section 9.1.3[Numeric Functions], page 194)
• The built-in functions gsub(), sub(), and match() (see Section 9.1.4 [StringManipulation Functions], page 196)
• The built-in functions close() and system() (see Section 9.1.5 [Input/Output Functions], page 208)
• The ARGC, ARGV, FNR, RLENGTH, RSTART, and SUBSEP predefined variables (seeSection 7.5 [Predefined Variables], page 163)
• Assignable $0 (see Section 4.4 [Changing the Contents of a Field], page 71)• The conditional expression using the ternary operator ‘?:’ (see Section 6.3.4 [Conditional Expressions], page 141)
• The expression ‘indx in array’ outside of for statements (see Section 8.1.2 [Referring
to an Array Element], page 179)
• The exponentiation operator ‘^’ (see Section 6.2.1 [Arithmetic Operators], page 128)
and its assignment operator form ‘^=’ (see Section 6.2.3 [Assignment Expressions],
page 130)• C-compatible operator precedence, which breaks some old awk programs (see Section 6.5[Operator Precedence (How Operators Nest)], page 144)
• Regexps as the value of FS (see Section 4.5 [Specifying How Fields Are Separated],
page 73) and as the third argument to the split() function (see Section 9.1.4 [StringManipulation Functions], page 196), rather than using only the first character of FS• Dynamic regexps as operands of the ‘~’ and ‘!~’ operators (see Section 3.6 [Using
Dynamic Regexps], page 60)

474

GAWK: Effective AWK Programming• The escape sequences ‘\b’, ‘\f’, and ‘\r’ (see Section 3.2 [Escape Sequences], page 52)• Redirection of input for the getline function (see Section 4.10 [Explicit Input withgetline], page 89)• Multiple BEGIN and END rules (see Section 7.1.4 [The BEGIN and END Special Patterns],
page 150)• Multidimensional arrays (see Section 8.5 [Multidimensional Arrays], page 188)

A.2 Changes Between SVR3.1 and SVR4The System V Release 4 (1989) version of Unix awk added these features (some of which
originated in gawk):• The ENVIRON array (see Section 7.5 [Predefined Variables], page 163)
• Multiple -f options on the command line (see Section 2.2 [Command-Line Options],
page 33)• The -v option for assigning variables before program execution begins (see Section 2.2[Command-Line Options], page 33)• The -- signal for terminating command-line options• The ‘\a’, ‘\v’, and ‘\x’ escape sequences (see Section 3.2 [Escape Sequences], page 52)• A defined return value for the srand() built-in function (see Section 9.1.3 [Numeric
Functions], page 194)
• The toupper() and tolower() built-in string functions for case translation (seeSection 9.1.4 [String-Manipulation Functions], page 196)
• A cleaner specification for the ‘%c’ format-control letter in the printf function (seeSection 5.5.2 [Format-Control Letters], page 103)
• The ability to dynamically pass the field width and precision ("%*.*d") in the argument
list of printf and sprintf() (see Section 5.5.2 [Format-Control Letters], page 103)
• The use of regexp constants, such as /foo/, as expressions, where they are equivalent
to using the matching operator, as in ‘$0 ~ /foo/’ (see Section 6.1.2 [Using Regular
Expression Constants], page 121)
• Processing of escape sequences inside command-line variable assignments (seeSection 6.1.3.2 [Assigning Variables on the Command Line], page 125)

A.3 Changes Between SVR4 and POSIX awk
The POSIX Command Language and Utilities standard for awk (1992) introduced the following changes into the language:• The use of -W for implementation-specific options (see Section 2.2 [Command-Line
Options], page 33)• The use of CONVFMT for controlling the conversion of numbers to strings (seeSection 6.1.4 [Conversion of Strings and Numbers], page 126)
• The concept of a numeric string and tighter comparison rules to go with it (seeSection 6.3.2 [Variable Typing and Comparison Expressions], page 135)
• The use of predefined variables as function parameter names is forbidden (seeSection 9.2.1 [Function Definition Syntax], page 221)

Appendix A: The Evolution of the awk Language 475• More complete documentation of many of the previously undocumented features of thelanguage
In 2012, a number of extensions that had been commonly available for many years were
finally added to POSIX. They are:
• The fflush() built-in function for flushing buffered output (see Section 9.1.5 [Input/Output Functions], page 208)
• The nextfile statement (see Section 7.4.9 [The nextfile Statement], page 161)
• The ability to delete all of an array at once with ‘delete array’ (see Section 8.4 [Thedelete Statement], page 186)
See Section A.7 [Common Extensions Summary], page 486, for a list of common extensions not permitted by the POSIX standard.
The 2018 POSIX standard can be found online at https://pubs.opengroup.org/
onlinepubs/9699919799/.

A.4 Extensions in Brian Kernighan’s awk
Brian Kernighan has made his version available via his home page (see Section B.5 [Other
Freely Available awk Implementations], page 510).
This section describes common extensions that originally appeared in his version of awk:
• The ‘**’ and ‘**=’ operators (see Section 6.2.1 [Arithmetic Operators], page 128, andSection 6.2.3 [Assignment Expressions], page 130)
• The use of func as an abbreviation for function (see Section 9.2.1 [Function Definition
Syntax], page 221)
• The fflush() built-in function for flushing buffered output (see Section 9.1.5 [Input/Output Functions], page 208)
See Section A.7 [Common Extensions Summary], page 486, for a full list of the extensions
available in his awk.

A.5 Extensions in gawk Not in POSIX awk
The GNU implementation, gawk, adds a large number of features. They can all be disabled with either the --traditional or --posix options (see Section 2.2 [Command-Line
Options], page 33).
A number of features have come and gone over the years. This section summarizes the
additional features over POSIX awk that are in the current version of gawk.
• Additional predefined variables:− The ARGIND, BINMODE, ERRNO, FIELDWIDTHS, FPAT, IGNORECASE, LINT, PROCINFO,
RT, and TEXTDOMAIN variables (see Section 7.5 [Predefined Variables], page 163)
• Special files in I/O redirections:− The /dev/stdin, /dev/stdout, /dev/stderr, and /dev/fd/N special file names(see Section 5.8 [Special File names in gawk], page 112)− The /inet, /inet4, and /inet6 special files for TCP/IP networking using ‘|&’ tospecify which version of the IP protocol to use (see Section 12.5 [Using gawk for
Network Programming], page 346)

476

GAWK: Effective AWK Programming• Changes and/or additions to the language:− The ‘\x’ escape sequence (see Section 3.2 [Escape Sequences], page 52)− Full support for both POSIX and GNU regexps (see Chapter 3 [Regular Expressions], page 51)− The ability for FS and for the third argument to split() to be null strings (seeSection 4.5.3 [Making Each Character a Separate Field], page 75)− The ability for RS to be a regexp (see Section 4.1 [How Input Is Split into Records],
page 65)− The ability to use octal and hexadecimal constants in awk program source code
(see Section 6.1.1.2 [Octal and Hexadecimal Numbers], page 120)− The ‘|&’ operator for two-way I/O to a coprocess (see Section 12.4 [Two-Way
Communications with Another Process], page 343)
− Indirect function calls (see Section 9.3 [Indirect Function Calls], page 231)
− Directories on the command line produce a warning and are skipped (seeSection 4.13 [Directories on the Command Line], page 97)− Output with print and printf need not be fatal (see Section 5.11 [EnablingNonfatal Output], page 116)
• New keywords:− The BEGINFILE and ENDFILE special patterns (see Section 7.1.5 [The BEGINFILE
and ENDFILE Special Patterns], page 151)
− The switch statement (see Section 7.4.5 [The switch Statement], page 157)
• Changes to standard awk functions:− The optional second argument to close() that allows closing one end of a twoway pipe to a coprocess (see Section 12.4 [Two-Way Communications with Another
Process], page 343)
− POSIX compliance for gsub() and sub() with --posix− The length() function accepts an array argument and returns the number of
elements in the array (see Section 9.1.4 [String-Manipulation Functions], page 196)
− The optional third argument to the match() function for capturing text-matchingsubexpressions within a regexp (see Section 9.1.4 [String-Manipulation Functions],
page 196)− Positional specifiers in printf formats for making translations easier (seeSection 13.4.2 [Rearranging printf Arguments], page 362)
− The split() function’s additional optional fourth argument, which is an arrayto hold the text of the field separators (see Section 9.1.4 [String-Manipulation
Functions], page 196)
• Additional functions only in gawk:− The gensub(), patsplit(), and strtonum() functions for more powerful text
manipulation (see Section 9.1.4 [String-Manipulation Functions], page 196)
− The asort() and asorti() functions for sorting arrays (see Section 12.3 [Controlling Array Traversal and Array Sorting], page 336)

Appendix A: The Evolution of the awk Language 477− The mktime(), systime(), and strftime() functions for working with timestamps (see Section 9.1.6 [Time Functions], page 211)
− The and(), compl(), lshift(), or(), rshift(), and xor() functions for bit
manipulation (see Section 9.1.7 [Bit-Manipulation Functions], page 216)
− The isarray() function to check if a variable is an array or not (see Section 9.1.8[Getting Type Information], page 219)
− The bindtextdomain(), dcgettext(), and dcngettext() functions for internationalization (see Section 13.3 [Internationalizing awk Programs], page 359)
• Changes and/or additions in the command-line options:− The AWKPATH environment variable for specifying a path search for the -fcommand-line option (see Section 2.2 [Command-Line Options], page 33)− The AWKLIBPATH environment variable for specifying a path search for the -lcommand-line option (see Section 2.2 [Command-Line Options], page 33)− The -b, -c, -C, -d, -D, -e, -E, -g, -h, -i, -l, -L, -M, -n, -N, -o, -O, -p,-P, -r, -s, -S, -t, and -V short options. Also, the ability to use GNU-stylelong-named options that start with --, and the --assign, --bignum,--characters-as-bytes, --copyright, --debug, --dump-variables, --exec,--field-separator, --file, --gen-pot, --help, --include, --lint,--lint-old, --load, --non-decimal-data, --optimize, --no-optimize,--posix, --pretty-print, --profile, --re-interval, --sandbox, --source,--traditional, --use-lc-numeric, and --version long options (see Section 2.2[Command-Line Options], page 33).
• Support for the following obsolete systems was removed from the code and the documentation for gawk version 4.0:
− Amiga− Atari− BeOS− Cray− MIPS RiscOS− MS-DOS with the Microsoft Compiler− MS-Windows with the Microsoft Compiler− NeXT− SunOS 3.x, Sun 386 (Road Runner)− Tandem (non-POSIX)− Prestandard VAX C compiler for VAX/VMS− GCC for Alpha has not been tested for a while.
• Support for the following obsolete system was removed from the code for gawk version
4.1:− Ultrix• Support for the following systems was removed from the code for gawk version 4.2:
− MirBSD

478

GAWK: Effective AWK Programming− GNU/Linux on Alpha• Support for the following systems was removed from the code for gawk version 5.2:
− OS/2− DJGPP− VAX/VMS

A.6 History of gawk Features
This section describes the features in gawk over and above those in POSIX awk, in the order
they were added to gawk.
Version 2.10 of gawk introduced the following features:• The AWKPATH environment variable for specifying a path search for the -f command-line
option (see Section 2.2 [Command-Line Options], page 33).
• The IGNORECASE variable and its effects (see Section 3.8 [Case Sensitivity in Matching],
page 63).
• The /dev/stdin, /dev/stdout, /dev/stderr and /dev/fd/N special file names (seeSection 5.8 [Special File names in gawk], page 112).
Version 2.13 of gawk introduced the following features:• The FIELDWIDTHS variable and its effects (see Section 4.6 [Reading Fixed-Width Data],
page 80).
• The systime() and strftime() built-in functions for obtaining and printing timestamps (see Section 9.1.6 [Time Functions], page 211).
• Additional command-line options (see Section 2.2 [Command-Line Options], page 33):
− The -W lint option to provide error and portability checking for both the sourcecode and at runtime.
− The -W compat option to turn off the GNU extensions.
− The -W posix option for full POSIX compliance.
Version 2.14 of gawk introduced the following feature:• The next file statement for skipping to the next data file (see Section 7.4.9 [Thenextfile Statement], page 161).
Version 2.15 of gawk introduced the following features:• New variables (see Section 7.5 [Predefined Variables], page 163):− ARGIND, which tracks the movement of FILENAME through ARGV.
− ERRNO, which contains the system error message when getline returns −1 orclose() fails.
• The /dev/pid, /dev/ppid, /dev/pgrpid, and /dev/user special file names. These
have since been removed.
• The ability to delete all of an array at once with ‘delete array’ (see Section 8.4 [Thedelete Statement], page 186).
• Command-line option changes (see Section 2.2 [Command-Line Options], page 33):
− The ability to use GNU-style long-named options that start with --.

Appendix A: The Evolution of the awk Language 479− The --source option for mixing command-line and library-file source code.
Version 3.0 of gawk introduced the following features:• New or changed variables:− IGNORECASE changed, now applying to string comparison as well as regexp operations (see Section 3.8 [Case Sensitivity in Matching], page 63).
− RT, which contains the input text that matched RS (see Section 4.1 [How Input Is
Split into Records], page 65).
• Full support for both POSIX and GNU regexps (see Chapter 3 [Regular Expressions],
page 51).
• The gensub() function for more powerful text manipulation (see Section 9.1.4 [StringManipulation Functions], page 196).
• The strftime() function acquired a default time format, allowing it to be called withno arguments (see Section 9.1.6 [Time Functions], page 211).
• The ability for FS and for the third argument to split() to be null strings (seeSection 4.5.3 [Making Each Character a Separate Field], page 75).
• The ability for RS to be a regexp (see Section 4.1 [How Input Is Split into Records],
page 65).
• The next file statement became nextfile (see Section 7.4.9 [The nextfile Statement], page 161).
• The fflush() function from BWK awk (then at Bell Laboratories; see Section 9.1.5[Input/Output Functions], page 208).
• New command-line options:− The --lint-old option to warn about constructs that are not available in theoriginal Version 7 Unix version of awk (see Section A.1 [Major Changes Between
V7 and SVR3.1], page 473).
− The -m option from BWK awk. (Brian was still at Bell Laboratories at the time.)
This was later removed from both his awk and from gawk.
− The --re-interval option to provide interval expressions in regexps (seeSection 3.3 [Regular Expression Operators], page 54).
− The --traditional option was added as a better name for --compat (seeSection 2.2 [Command-Line Options], page 33).
• The use of GNU Autoconf to control the configuration process (see Section B.2.1 [Compiling gawk for Unix-Like Systems], page 495).
• Amiga support. This has since been removed.
Version 3.1 of gawk introduced the following features:• New variables (see Section 7.5 [Predefined Variables], page 163):− BINMODE, for non-POSIX systems, which allows binary I/O for input and/or output
files (see Section B.3.1.3 [Using gawk on PC Operating Systems], page 499).
− LINT, which dynamically controls lint warnings.
− PROCINFO, an array for providing process-related information.
− TEXTDOMAIN, for setting an application’s internationalization text domain (seeChapter 13 [Internationalization with gawk], page 357).

480

GAWK: Effective AWK Programming• The ability to use octal and hexadecimal constants in awk program source code (seeSection 6.1.1.2 [Octal and Hexadecimal Numbers], page 120).
• The ‘|&’ operator for two-way I/O to a coprocess (see Section 12.4 [Two-Way Communications with Another Process], page 343).
• The /inet special files for TCP/IP networking using ‘|&’ (see Section 12.5 [Using gawk
for Network Programming], page 346).
• The optional second argument to close() that allows closing one end of a two-way
pipe to a coprocess (see Section 12.4 [Two-Way Communications with Another Process],
page 343).
• The optional third argument to the match() function for capturing text-matchingsubexpressions within a regexp (see Section 9.1.4 [String-Manipulation Functions],
page 196).
• Positional specifiers in printf formats for making translations easier (see Section 13.4.2[Rearranging printf Arguments], page 362).
• A number of new built-in functions:− The asort() and asorti() functions for sorting arrays (see Section 12.3 [Controlling Array Traversal and Array Sorting], page 336).
− The bindtextdomain(), dcgettext() and dcngettext() functions for internationalization (see Section 13.3 [Internationalizing awk Programs], page 359).
− The extension() function and the ability to add new built-in functions dynamically. This has seen removed. It was replaced by the new extension mechanism.
See Chapter 17 [Writing Extensions for gawk], page 405.
− The mktime() function for creating timestamps (see Section 9.1.6 [Time Functions], page 211).
− The and(), or(), xor(), compl(), lshift(), rshift(), and strtonum() functions (see Section 9.1.7 [Bit-Manipulation Functions], page 216).
• The support for ‘next file’ as two words was removed completely (see Section 7.4.9[The nextfile Statement], page 161).
• Additional command-line options (see Section 2.2 [Command-Line Options], page 33):
− The --dump-variables option to print a list of all global variables.
− The --exec option, for use in CGI scripts.
− The --gen-po command-line option and the use of a leading underscore to markstrings that should be translated (see Section 13.4.1 [Extracting Marked Strings],
page 361).
− The --non-decimal-data option to allow non-decimal input data (see Section 12.1
[Allowing Nondecimal Input Data], page 335).
− The --profile option and pgawk, the profiling version of gawk, for producing
execution profiles of awk programs (see Section 12.6 [Profiling Your awk Programs],
page 348).
− The --use-lc-numeric option to force gawk to use the locale’s decimal point
for parsing input data (see Section 6.1.4 [Conversion of Strings and Numbers],
page 126).

Appendix A: The Evolution of the awk Language 481• The use of GNU Automake to help in standardizing the configuration process (seeSection B.2.1 [Compiling gawk for Unix-Like Systems], page 495).
• The use of GNU gettext for gawk’s own message output (see Section 13.6 [gawk Can
Speak Your Language], page 366).
• BeOS support. This was later removed.
• Tandem support. This was later removed.
• The Atari port became officially unsupported and was later removed entirely.
• The source code changed to use ISO C standard-style function definitions.
• POSIX compliance for sub() and gsub() (see Section 9.1.4.1 [More about ‘\’ and ‘&’with sub(), gsub(), and gensub()], page 205).
• The length() function was extended to accept an array argument and return thenumber of elements in the array (see Section 9.1.4 [String-Manipulation Functions],
page 196).
• The strftime() function acquired a third argument to enable printing times as UTC(see Section 9.1.6 [Time Functions], page 211).
Version 4.0 of gawk introduced the following features:• Variable additions:− FPAT, which allows you to specify a regexp that matches the fields, instead ofmatching the field separator (see Section 4.7 [Defining Fields by Content], page 83).
− If PROCINFO["sorted_in"] exists, ‘for (iggy in foo)’ loops sort the indices before looping over them. The value of this element provides control over how theindices are sorted before the loop traversal starts (see Section 8.1.6 [Using Predefined Array Scanning Orders with gawk], page 182).
− PROCINFO["strftime"], which holds the default format for strftime() (seeSection 9.1.6 [Time Functions], page 211).
• The special files /dev/pid, /dev/ppid, /dev/pgrpid and /dev/user were removed.
• Support for IPv6 was added via the /inet6 special file. /inet4 forces IPv4 and /inetchooses the system default, which is probably IPv4 (see Section 12.5 [Using gawk for
Network Programming], page 346).
• The use of ‘\s’ and ‘\S’ escape sequences in regular expressions (see Section 3.7 [gawkSpecific Regexp Operators], page 61).
• Interval expressions became part of default regular expressions (see Section 3.3 [Regular
Expression Operators], page 54).
• POSIX character classes work even with --traditional (see Section 3.3 [Regular
Expression Operators], page 54).
• break and continue became invalid outside a loop, even with --traditional (seeSection 7.4.6 [The break Statement], page 159, and also see Section 7.4.7 [The continue
Statement], page 160).
• fflush(), nextfile, and ‘delete array’ are allowed if --posix or --traditional,
since they are all now part of POSIX.
• An optional third argument to asort() and asorti(), specifying how to sort (seeSection 9.1.4 [String-Manipulation Functions], page 196).

482

GAWK: Effective AWK Programming• The behavior of fflush() changed to match BWK awk and for POSIX; now both
‘fflush()’ and ‘fflush("")’ flush all open output redirections (see Section 9.1.5 [Input/Output Functions], page 208).
• The isarray() function which distinguishes if an item is an array or not, to makeit possible to traverse arrays of arrays (see Section 9.1.8 [Getting Type Information],
page 219).
• The patsplit() function which gives the same capability as FPAT, for splitting (seeSection 9.1.4 [String-Manipulation Functions], page 196).
• An optional fourth argument to the split() function, which is an array to hold thevalues of the separators (see Section 9.1.4 [String-Manipulation Functions], page 196).
• Arrays of arrays (see Section 8.6 [Arrays of Arrays], page 189).
• The BEGINFILE and ENDFILE special patterns (see Section 7.1.5 [The BEGINFILE andENDFILE Special Patterns], page 151).
• Indirect function calls (see Section 9.3 [Indirect Function Calls], page 231).
• switch / case are enabled by default (see Section 7.4.5 [The switch Statement],
page 157).
• Command-line option changes (see Section 2.2 [Command-Line Options], page 33):
− The -b and --characters-as-bytes options which prevent gawk from treating
input as a multibyte string.
− The redundant --compat, --copyleft, and --usage long options were removed.
− The --gen-po option was finally renamed to the correct --gen-pot.
− The --sandbox option which disables certain features.
− All long options acquired corresponding short options, for use in ‘#!’ scripts.
• Directories named on the command line now produce a warning, not a fatal error, unless--posix or --traditional are used (see Section 4.13 [Directories on the Command
Line], page 97).
• The gawk internals were rewritten, bringing the dgawk debugger and possibly improved
performance (see Chapter 14 [Debugging awk Programs], page 367).
• Per the GNU Coding Standards, dynamic extensions must now define a global symbol indicating that they are GPL-compatible (see Section 17.2 [Extension Licensing],
page 405).
• In POSIX mode, string comparisons use strcoll() / wcscoll() (see Section 6.3.2.3[String Comparison Based on Locale Collating Order], page 139).
• The option for raw sockets was removed, since it was never implemented (seeSection 12.5 [Using gawk for Network Programming], page 346).
• Ranges of the form ‘[d-h]’ are treated as if they were in the C locale, no matter whatkind of regexp is being used, and even if --posix (see Section A.8 [Regexp Ranges andLocales: A Long Sad Story], page 486).
• Support was removed for the following systems:− Atari− Amiga− BeOS

Appendix A: The Evolution of the awk Language 483− Cray− MIPS RiscOS− MS-DOS with the Microsoft Compiler− MS-Windows with the Microsoft Compiler− NeXT− SunOS 3.x, Sun 386 (Road Runner)− Tandem (non-POSIX)− Prestandard VAX C compiler for VAX/VMS
Version 4.1 of gawk introduced the following features:• Three new arrays: SYMTAB, FUNCTAB, and PROCINFO["identifiers"] (see Section 7.5.2[Built-in Variables That Convey Information], page 165).
• The three executables gawk, pgawk, and dgawk, were merged into one, named just gawk.
As a result the command-line options changed.
• Command-line option changes (see Section 2.2 [Command-Line Options], page 33):
− The -D option invokes the debugger.
− The -i and --include options load awk library files.
− The -l and --load options load compiled dynamic extensions.
− The -M and --bignum options enable MPFR.
− The -o option only does pretty-printing.
− The -p option is used for profiling.
− The -R option was removed.
• Support for high precision arithmetic with MPFR (see Chapter 16 [Arithmetic andArbitrary-Precision Arithmetic with gawk], page 389).
• The and(), or() and xor() functions changed to allow any number of arguments, witha minimum of two (see Section 9.1.7 [Bit-Manipulation Functions], page 216).
• The dynamic extension interface was completely redone (see Chapter 17 [Writing Extensions for gawk], page 405).
• Redirected getline became allowed inside BEGINFILE and ENDFILE (see Section 7.1.5[The BEGINFILE and ENDFILE Special Patterns], page 151).
• Support for nonfatal I/O (see Section 5.11 [Enabling Nonfatal Output], page 116).
• The where command was added to the debugger (see Section 14.3.4 [Working with the
Stack], page 377).
• Support for Ultrix was removed.
Version 4.2 of gawk introduced the following changes:• Changes to ENVIRON are reflected into gawk’s environment and that of programs that
it runs. See Section 7.5.2 [Built-in Variables That Convey Information], page 165.
• FIELDWIDTHS was enhanced to allow skipping characters before assigning a value to a
field (see Section 4.7 [Defining Fields by Content], page 83).
• The PROCINFO["argv"] array. See Section 7.5.2 [Built-in Variables That Convey Information], page 165.

484

GAWK: Effective AWK Programming• The maximum number of hexadecimal digits in ‘\x’ escapes is now two. See Section 3.2[Escape Sequences], page 52.
• Strongly typed regexp constants of the form ‘@/.../’ (see Section 6.1.2.2 [Strongly
Typed Regexp Constants], page 123).
• The bitwise functions changed, making negative arguments into a fatal error (seeSection 9.1.7 [Bit-Manipulation Functions], page 216).
• The mktime() function now accepts an optional second argument (see Section 9.1.6[Time Functions], page 211).
• The typeof() function (see Section 9.1.8 [Getting Type Information], page 219).
• Optimizations are enabled by default. Use -s / --no-optimize to disable optimizations.
• For many years, POSIX specified that default field splitting only allowed spaces andtabs to separate fields, and this was how gawk behaved with --posix. As of 2013, thestandard restored historical behavior, and now default field splitting with --posix also
allows newlines to separate fields.
• Nonfatal output with print and printf. See Section 5.11 [Enabling Nonfatal Output],
page 116.
• Retryable I/O via PROCINFO[input-file, "RETRY"]; (see Section 4.12 [Retrying
Reads After Certain Input Errors], page 97).
• Changes to the pretty-printer (see Section 12.6 [Profiling Your awk Programs],
page 348):− The --pretty-print option no longer runs the awk program too.
− Comments in the source program are preserved and placed into the output file.
− Explicit parentheses for expressions in the input are preserved in the generated
output.
• Improvements to the extension API (see Chapter 17 [Writing Extensions for gawk],
page 405):− The get_file() function to access open redirections.
− The nonfatal() function for generating nonfatal error messages.
− Support for GMP and MPFR values.
− Input parsers can now override the default field parsing mechanism by specifying
explicit locations.
• Shell startup files are supplied with the distribution and installed by ‘make install’(see Section B.2.2 [Shell Startup Files], page 496).
• The igawk program and its manual page are no longer installed when gawk is built.
See Section 11.3.9 [An Easy Way to Use Library Functions], page 322.
• Support for MirBSD was removed.
• Support for GNU/Linux on Alpha was removed.
Version 5.0 added the following features:• The PROCINFO["platform"] array element, which allows you to write code that takes
the operating system / platform into account.

Appendix A: The Evolution of the awk Language 485

Version 5.1 was created to release gawk with a correct major version number for the API.
This was overlooked for version 5.0, unfortunately. It added the following features:• The index for this manual was completely reworked.
• Support was added for MSYS2.
• asort() and asorti() were changed to allow FUNCTAB and SYMTAB as the first argument if a second destination array is supplied (see Section 9.1.4 [String-Manipulation
Functions], page 196).
• The -I/--trace options were added to print a trace of the byte codes as they execute(see Section 2.2 [Command-Line Options], page 33).
• $0 and the fields are now cleared before starting a BEGINFILE rule (see Section 7.1.5[The BEGINFILE and ENDFILE Special Patterns], page 151).
• Several example programs in the manual were updated to their modern POSIX equivalents.
• The “no effect” lint warnings from --lint were fixed up and now behave more sanely(see Section 2.2 [Command-Line Options], page 33).
• Handling of Infinity and NaN values were improved. See Section 16.2 [Other Stuff
to Know], page 390, and also see Section 16.7 [Standards Versus Existing Practice],
page 401.
Version 5.2 added the following features:• The mkbool() built-in function (see Section 9.1.2 [Generating Boolean Values],
page 194).
• Interval expressions in regular expressions are enabled by default (see Section 3.3.2[Some Notes On Interval Expressions], page 57).
• Support for the FNV1-A hash algorithm for its hash function (see Section 2.5.3 [Other
Environment Variables], page 44).
• The gawkbug script for reporting bugs (see Section B.4.2 [Submitting Bug Reports],
page 506).
• Terence Kelly’s persistent memory allocator (PMA) was added, allowing the use of
persistent data on certain systems (see Section 12.7 [Preserving Data Between Runs],
page 352).
• PROCINFO["pma"] exists if the PMA allocator is compiled in (see Section 7.5.2 [Built-in
Variables That Convey Information], page 165).
Version 5.3 added the following features:• Comma separated value (CSV) field splitting and the --csv command-line option (seeSection 4.5.4 [Working With Comma Separated Value Files], page 76).
• PROCINFO["CSV"] exists if gawk was invoked with --csv (see Section 7.5.2 [Built-in
Variables That Convey Information], page 165).
• The do_csv API information variable (see Section 17.4.14.3 [Informational Variables],page 444).
• The ability to make gawk buffer output to pipes (see Section 5.10 [Speeding Up Pipe
Output], page 116).
• The ‘\u’ escape sequence (see Section 3.2 [Escape Sequences], page 52).

486

GAWK: Effective AWK Programming• The need for GNU libsigsegv was removed from gawk. The value-add was never verymuch and it caused problems in some environments.

A.7 Common Extensions Summary
The following table summarizes the common extensions supported by gawk, Brian
Kernighan’s awk, and mawk, the three most widely used freely available versions of awk (seeSection B.5 [Other Freely Available awk Implementations], page 510).
Feature** and **= operators‘\x’ escape sequence‘\u’ escape sequence/dev/stdin special file
/dev/stdout special file
/dev/stderr special file
BINMODE variable
CSV support
FS as null string
delete without subscript
fflush() function
func keyword
length() of an arraynextfile statement
RS as regexp
Time-related functions

BWK awk
X
X
X
X
X
X
X
X
X
X
X
X
X
X

mawk
X
X
X
X
X
X
X
X
X
X
X
X

gawk
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X

Now standard

X
X

X

A.8 Regexp Ranges and Locales: A Long Sad Story
This section describes the confusing history of ranges within regular expressions and theirinteractions with locales, and how this affected different versions of gawk.
The original Unix tools that worked with regular expressions defined character ranges(such as ‘[a-z]’) to match any character between the first character in the range and thelast character in the range, inclusive. Ordering was based on the numeric value of eachcharacter in the machine’s native character set. Thus, on ASCII-based systems, ‘[a-z]’
matched all the lowercase letters, and only the lowercase letters, as the numeric values forthe letters from ‘a’ through ‘z’ were contiguous. (On an EBCDIC system, the range ‘[a-z]’
includes additional nonalphabetic characters as well.)Almost all introductory Unix literature explained range expressions as working in thisfashion, and in particular, would teach that the “correct” way to match lowercase letterswas with ‘[a-z]’, and that ‘[A-Z]’ was the “correct” way to match uppercase letters. And
indeed, this was true.1
The 1992 POSIX standard introduced the idea of locales (see Section 6.6 [Where You
Are Makes a Difference], page 145). Because many locales include other letters besides
the plain 26 letters of the English alphabet, the POSIX standard added character classes
1

And Life was good.

Appendix A: The Evolution of the awk Language 487(see Section 3.4 [Using Bracket Expressions], page 58) as a way to match different kinds ofcharacters besides the traditional ones in the ASCII character set.
However, the standard changed the interpretation of range expressions. In the "C" and"POSIX" locales, a range expression like ‘[a-dx-z]’ is still equivalent to ‘[abcdxyz]’, as in
ASCII. But outside those locales, the ordering was defined to be based on collation order.
What does that mean? In many locales, ‘A’ and ‘a’ are both less than ‘B’. In other words,
these locales sort characters in dictionary order, and ‘[a-dx-z]’ is typically not equivalentto ‘[abcdxyz]’; instead, it might be equivalent to ‘[ABCXYabcdxyz]’, for example.
This point needs to be emphasized: much literature teaches that you should use ‘[a-z]’
to match a lowercase character. But on systems with non-ASCII locales, this also matches
all of the uppercase characters except ‘A’ or ‘Z’! This was a continuous cause of confusion,even well into the twenty-first century.
To demonstrate these issues, the following example uses the sub() function, which does
text replacement (see Section 9.1.4 [String-Manipulation Functions], page 196). Here, theintent is to remove trailing uppercase characters:$ echo something1234abc | gawk-3.1.8 '{ sub("[A-Z]*$", ""); print }'a something1234a
This output is unexpected, as the ‘bc’ at the end of ‘something1234abc’ should not normally
match ‘[A-Z]*’. This result is due to the locale setting (and thus you may not see it on
your system).
Similar considerations apply to other ranges. For example, ‘["-/]’ is perfectly valid in
ASCII, but is not valid in many Unicode locales, such as en_US.UTF-8.
Early versions of gawk used regexp matching code that was not locale-aware, so ranges
had their traditional interpretation.
When gawk switched to using locale-aware regexp matchers, the problems began; especially as both GNU/Linux and commercial Unix vendors started implementing non-ASCII
locales, and making them the default. Perhaps the most frequently asked question became
something like, “Why does ‘[A-Z]’ match lowercase letters?!?”This situation existed for close to 10 years, if not more, and the gawk maintainer grew
weary of trying to explain that gawk was being nicely standards-compliant, and that theissue was in the user’s locale. During the development of version 4.0, he modified gawk
to always treat ranges in the original, pre-POSIX fashion, unless --posix was used (seeSection 2.2 [Command-Line Options], page 33).2
Fortunately, shortly before the final release of gawk 4.0, the maintainer learned that the2008 standard had changed the definition of ranges, such that outside the "C" and "POSIX"
locales, the meaning of range expressions was undefined.3
By using this lovely technical term, the standard gives license to implementers to implement ranges in whatever way they choose. The gawk maintainer chose to apply the
pre-POSIX meaning both with the default regexp matching and when --traditional or--posix are used. In all cases gawk remains POSIX-compliant.
2

And thus was born the Campaign for Rational Range Interpretation (or RRI). A number of GNU
tools have already implemented this change, or will soon. Thanks to Karl Berry for coining the phrase“Rational Range Interpretation.”3
See the standard and its rationale.

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A.9 Major Contributors to gawk
Always give credit where credit is due.
—Anonymous
This section names the major contributors to gawk and/or this book, in approximate
chronological order:• Dr. Alfred V. Aho, Dr. Peter J. Weinberger, and Dr. Brian W. Kernighan, all of Bell
Laboratories, designed and implemented Unix awk, from which gawk gets the majorityof its feature set.
• Paul Rubin did the initial design and implementation in 1986, and wrote the first draft(around 40 pages) of this book.
• Jay Fenlason finished the initial implementation.
• Diane Close revised the first draft of this book, bringing it to around 90 pages.
• Richard Stallman helped finish the implementation and the initial draft of this book.
He is also the founder of the FSF and the GNU Project.
• John Woods contributed parts of the code (mostly fixes) in the initial version of gawk.
• In 1988, David Trueman took over primary maintenance of gawk, making it compatible
with “new” awk, and greatly improving its performance.
• Conrad Kwok, Scott Garfinkle, and Kent Williams did the initial ports to MS-DOS
with various versions of MSC.
• Pat Rankin provided the VMS port and its documentation.
• Hal Peterson provided help in porting gawk to Cray systems. (This is no longer supported.)• Kai Uwe Rommel provided the initial port to OS/2 and its documentation.
• Michal Jaegermann provided the port to Atari systems and its documentation. (Thisport is no longer supported.) He continues to provide portability checking, and has
done a lot of work to make sure gawk works on non-32-bit systems.
• Fred Fish provided the port to Amiga systems and its documentation. (With Fred’s
sad passing, this is no longer supported.)• Scott Deifik formerly maintained the MS-DOS port using DJGPP.
• Eli Zaretskii currently maintains the MS-Windows port using MinGW.
• Juan Grigera provided a port to Windows32 systems. (This is no longer supported.)• For many years, Dr. Darrel Hankerson acted as coordinator for the various ports to different PC platforms and created binary distributions for various PC operating systems.
He was also instrumental in keeping the documentation up to date for the various PC
platforms.
• Christos Zoulas provided the extension() built-in function for dynamically adding
new functions. (This was obsoleted at gawk 4.1.)• Jürgen Kahrs contributed the initial version of the TCP/IP networking code and documentation, and motivated the inclusion of the ‘|&’ operator.
• Stephen Davies provided the initial port to Tandem systems and its documentation.
(However, this is no longer supported.) He was also instrumental in the initial work to
integrate the byte-code internals into the gawk code base. Additionally, he did most of
the work enabling the pretty-printer to preserve and output comments.

Appendix A: The Evolution of the awk Language 489• Matthew Woehlke provided improvements for Tandem’s POSIX-compliant systems.
• Martin Brown provided the port to BeOS and its documentation. (This is no longer
supported.)• Arno Peters did the initial work to convert gawk to use GNU Automake and GNUgettext.
• Alan J. Broder provided the initial version of the asort() function as well as the codefor the optional third argument to the match() function.
• Andreas Buening updated the gawk port for OS/2.
• Isamu Hasegawa, of IBM in Japan, contributed support for multibyte characters.
• Michael Benzinger contributed the initial code for switch statements.
• Patrick T.J. McPhee contributed the code for dynamic loading in Windows32 environments. (This is no longer supported.)• Anders Wallin helped keep the VMS port going for several years.
• Assaf Gordon contributed the initial code to implement the --sandbox option.
• John Haque made the following contributions:− The modifications to convert gawk into a byte-code interpreter, including the debugger− The addition of true arrays of arrays− The additional modifications for support of arbitrary-precision arithmetic− The initial text of Chapter 16 [Arithmetic and Arbitrary-Precision Arithmetic withgawk], page 389,
− The work to merge the three versions of gawk into one, for the 4.1 release− Improved array internals for arrays indexed by integers− The improved array sorting features were also driven by John, together with Pat
Rankin• Panos Papadopoulos contributed the original text for Section 2.7 [Including Other Files
into Your Program], page 46.
• Efraim Yawitz contributed the original text for Chapter 14 [Debugging awk Programs],
page 367.
• The development of the extension API first released with gawk 4.1 was driven primarily
by Arnold Robbins and Andrew Schorr, with notable contributions from the rest of thedevelopment team.
• John Malmberg contributed significant improvements to the OpenVMS port and therelated documentation.
• Antonio Giovanni Colombo rewrote a number of examples in the early chapters thatwere severely dated, for which I am incredibly grateful. He also provided and maintains
the Italian translation.
• Marco Curreli, together with Antonio Colombo, translated this book into Italian. It is
included in the gawk distribution.
• Juan Manuel Guerrero took over maintenance of the DJGPP port.
• “Jannick” provided support for MSYS2.

490

GAWK: Effective AWK Programming• Arnold Robbins has been working on gawk since 1988, at first helping David Trueman,
and as the primary maintainer since around 1994.

A.10 Summary• The awk language has evolved over time. The first release was with V7 Unix, circa 1978.
In 1987, for System V Release 3.1, major additions, including user-defined functions,were made to the language. Additional changes were made for System V Release 4,
in 1989. Since then, further minor changes have happened under the auspices of the
POSIX standard.
• Brian Kernighan’s awk provides a small number of extensions that are implemented incommon with other versions of awk.
• gawk provides a large number of extensions over POSIX awk. They can be disabledwith either the --traditional or --posix options.
• The interaction of POSIX locales and regexp matching in gawk has been confusing
over the years. Today, gawk implements Rational Range Interpretation, where rangesof the form ‘[a-z]’ match only the characters numerically between ‘a’ through ‘z’ inthe machine’s native character set. Usually this is ASCII, but it can be EBCDIC on
IBM S/390 systems.
• Many people have contributed to gawk development over the years. We hope that thelist provided in this chapter is complete and gives the appropriate credit where credit
is due.

Appendix B: Installing gawk 491

Appendix B Installing gawk
This appendix provides instructions for installing gawk on the various platforms that aresupported by the developers. The primary developer supports GNU/Linux (and Unix),
whereas the other ports are contributed. See Section B.4 [Reporting Problems and Bugs],
page 505, for the email addresses of the people who maintain the respective ports.

B.1 The gawk Distribution
This section describes how to get the gawk distribution, how to extract it, and then what
is in the various files and subdirectories.

B.1.1 Getting the gawk Distribution
There are two ways to get GNU software:• Copy it from someone else who already has it.
• Retrieve gawk from the Internet host ftp.gnu.org, in the directory /gnu/gawk. Bothanonymous ftp and http access are supported. If you have the wget program, you can
use a command like the following:
wget https://ftp.gnu.org/gnu/gawk/gawk-5.3.0.tar.gz
The GNU software archive is mirrored around the world. The up-to-date list of mirror
sites is available from the main FSF website. Try to use one of the mirrors; they will beless busy, and you can usually find one closer to your site.
You may also retrieve the gawk source code from the official Git repository; for more
information see Section C.2.1 [Accessing The gawk Git Repository], page 515.

B.1.2 Extracting the Distribution
gawk is distributed as several tar files compressed with different compression programs:
gzip, bzip2, and xz. For simplicity, the rest of these instructions assume you are using theone compressed with the GNU Gzip program (gzip).
Once you have the distribution (e.g., gawk-5.3.0.tar.gz), use gzip to expand the fileand then use tar to extract it. You can use the following pipeline to produce the gawk
distribution:
gzip -d -c gawk-5.3.0.tar.gz | tar -xvpf On a system with GNU tar, you can let tar do the decompression for you:tar -xvpzf gawk-5.3.0.tar.gz
Extracting the archive creates a directory named gawk-5.3.0 in the current directory.
The distribution file name is of the form gawk-V.R.P.tar.gz. The V represents the
major version of gawk, the R represents the current release of version V, and the P representsa patch level, meaning that minor bugs have been fixed in the release. The current patchlevel is 0, but when retrieving distributions, you should get the version with the highest
version, release, and patch level. (Note, however, that patch levels greater than or equal to
60 denote “beta” or nonproduction software; you might not want to retrieve such a version
unless you don’t mind experimenting.) If you are not on a Unix or GNU/Linux system,
you need to make other arrangements for getting and extracting the gawk distribution. You
should consult a local expert.

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B.1.3 Contents of the gawk Distribution
The gawk distribution has a number of C source files, documentation files, subdirectories,
and files related to the configuration process (see Section B.2 [Compiling and Installing
gawk on Unix-Like Systems], page 495), as well as several subdirectories related to different
non-Unix operating systems:
Various ‘.c’, ‘.y’, and ‘.h’ files
These files contain the actual gawk source code.
support/*C header and source files for routines that gawk uses, but that are not partof its core functionality. For example, argument parsing, regular expression
matching, and random number generating routines are all kept here.
ABOUT-NLS
A file containing information about GNU gettext and translations.
AUTHORSA file with some information about the authorship of gawk. It exists only tosatisfy the pedants at the Free Software Foundation.

READMEREADME_d/README.*
Descriptive files: README for gawk under Unix and the rest for the various
hardware and software combinations.
INSTALL

A file providing an overview of the configuration and installation process.

ChangeLog
A detailed list of source code changes as bugs are fixed or improvements made.
There are similar files in all of the subdirectories.
ChangeLog.0
ChangeLog.1
Older lists of source code changes. There are similar files in all of the subdirectories.
NEWS

A list of changes to gawk since the last release or patch. There may be similar
files in other subdirectories.

NEWS.0NEWS.1

Older lists of changes to gawk. There may be similar files in other subdirectories.

COPYINGThe GNU General Public License.

POSIX.STD
A description of behaviors in the POSIX standard for awk that are left undefined, or where gawk may not comply fully, as well as a list of things that the
POSIX standard should describe but does not.
doc/awkforai.txt
Pointers to the original draft of a short article describing why gawk is a goodlanguage for artificial intelligence (AI) programming.
doc/bc_notes
A brief description of gawk’s “byte code” internals.

Appendix B: Installing gawk 493

doc/README.card
doc/ad.block
doc/awkcard.in
doc/cardfonts
doc/colors
doc/macros
doc/no.colors
doc/setter.outline
The troff source for a five-color awk reference card. A modern version of troff
such as GNU troff (groff) is needed to produce the color version. See the file
README.card for instructions if you have an older troff.
doc/gawk.1
The troff source for a manual page describing gawk. This is distributed forthe convenience of Unix users.
doc/gawktexi.in
doc/sidebar.awk
The Texinfo source file for this book.
It should be processed by
doc/sidebar.awk before processing with texi2dvi or texi2pdf to produce
a printed document, and with makeinfo to produce an Info or HTML file.
The Makefile takes care of this processing and produces printable output via
texi2dvi or texi2pdf.
doc/gawk.texi
The file produced after processing gawktexi.in with sidebar.awk.
doc/gawk.info
The generated Info file for this book.
doc/gawkinet.texi
The Texinfo source file for TCP/IP Internetworking with gawk. It should beprocessed with TEX (via texi2dvi or texi2pdf) to produce a printed document
and with makeinfo to produce an Info or HTML file.
doc/gawkinet.info
The generated Info file for TCP/IP Internetworking with gawk.
doc/gawkworkflow.texi
The Texinfo source file for Participating in gawk Development. It should beprocessed with TEX (via texi2dvi or texi2pdf) to produce a printed document
and with makeinfo to produce an Info or HTML file.
doc/gawkworkflow.info
The generated Info file for Participating in gawk Development.
doc/pm-gawk.texi
The Texinfo source file for Persistent-Memory gawk User Manual. It should beprocessed with TEX (via texi2dvi or texi2pdf) to produce a printed document
and with makeinfo to produce an Info or HTML file.
doc/pm-gawk.info
The generated Info file for Persistent-Memory gawk User Manual.

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doc/pm-gawk.1
The troff source for a manual page describing the persistent memory features
presented in Section 12.7 [Preserving Data Between Runs], page 352.
doc/igawk.1
The troff source for a manual page describing the igawk program presented in
Section 11.3.9 [An Easy Way to Use Library Functions], page 322. (Since gawk
can do its own @include processing, neither igawk nor igawk.1 are installed.)
doc/it/*Files for the Italian translation of this book, produced and contributed byAntonio Colombo and Marco Curreli.

doc/Makefile.in
The input file used during the configuration process to generate the actual
Makefile for creating the documentation.
Makefile.am*/Makefile.am
Files used by the GNU Automake software for generating the Makefile.in files
used by Autoconf and configure.
Makefile.in
aclocal.m4
build-aux/*
configh.in
configure.ac
configurecustom.hmissing_d/*m4/*These files and subdirectories are used when configuring and compiling gawk forvarious Unix systems. Most of them are explained in Section B.2 [Compilingand Installing gawk on Unix-Like Systems], page 495. The rest are there to
support the main infrastructure.
po/*

The po library contains message translations.

awklib/extract.awk
awklib/Makefile.amawklib/Makefile.inawklib/eg/*
The awklib directory contains a copy of extract.awk (see Section 11.3.7 [Extracting Programs from Texinfo Source Files], page 317), which can be used to
extract the sample programs from the Texinfo source file for this book. It also
contains a Makefile.in file, which configure uses to generate a Makefile.
Makefile.am is used by GNU Automake to create Makefile.in. The libraryfunctions from Chapter 10 [A Library of awk Functions], page 241, are included
as ready-to-use files in the gawk distribution. They are installed as part ofthe installation process. The rest of the programs in this book are available inappropriate subdirectories of awklib/eg.

Appendix B: Installing gawk 495

extension/*
The source code, manual pages, and infrastructure files for the sample extensions included with gawk. See Chapter 17 [Writing Extensions for gawk],
page 405, for more information.
extras/*Additional non-essential files. Currently, this directory contains some shell
startup files to be installed in /etc/profile.d to aid in manipulating theAWKPATH and AWKLIBPATH environment variables. See Section B.2.2 [Shell
Startup Files], page 496, for more information.

posix/*

Files needed for building gawk on POSIX-compliant systems.

pc/*

Files needed for building gawk under MS-Windows (see Section B.3.1 [Installation on MS-Windows], page 499, for details).

vms/*

Files needed for building gawk under OpenVMS (see Section B.3.2 [Compilingand Installing gawk on OpenVMS], page 501, for details).

test/*A test suite for gawk. You can use ‘make check’ from the top-level gawk directory to run your version of gawk against the test suite. If gawk successfully
passes ‘make check’, then you can be confident of a successful port.

B.2 Compiling and Installing gawk on Unix-Like Systems
Usually, you can compile and install gawk by typing only two commands. However, if you
use an unusual system, you may need to configure gawk for your system yourself.

B.2.1 Compiling gawk for Unix-Like Systems
The normal installation steps should work on all modern commercial Unix-derived systems,
GNU/Linux, BSD-based systems, and the Cygwin environment for MS-Windows.
After you have extracted the gawk distribution, cd to gawk-5.3.0. As with most GNU
software, you configure gawk for your system by running the configure program. This program is a Bourne shell script that is generated automatically using GNU Autoconf. (The
Autoconf software is described fully in Autoconf—Generating Automatic Configuration
Scripts, which can be found online at the Free Software Foundation’s website.)
To configure gawk, simply run configure:sh ./configure
This produces a Makefile and config.h tailored to your system. The config.h filedescribes various facts about your system. You might want to edit the Makefile to change
the CFLAGS variable, which controls the command-line options that are passed to the Ccompiler (such as optimization levels or compiling for debugging).
Alternatively, you can add your own values for most make variables on the command
line, such as CC and CFLAGS, when running configure:
CC=cc CFLAGS=-g sh ./configure
See the file INSTALL in the gawk distribution for all the details.
After you have run configure and possibly edited the Makefile, type:
make

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Shortly thereafter, you should have an executable version of gawk. That’s all there is to it!
To verify that gawk is working properly, run ‘make check’. All of the tests should succeed.
If these steps do not work, or if any of the tests fail, check the files in the README_d directory
to see if you’ve found a known problem. If the failure is not described there, send in a bug
report (see Section B.4 [Reporting Problems and Bugs], page 505).
Of course, once you’ve built gawk, it is likely that you will wish to install it. To do so,
you need to run the command ‘make install’, as a user with the appropriate permissions.
How to do this varies by system, but on many systems you can use the sudo command to
do so. The command then becomes ‘sudo make install’. It is likely that you will be asked
for your password, and you will have to have been set up previously as a user who is allowed
to run the sudo command.

B.2.1.1 Building With MPFR
Use of the MPFR library with gawk is an optional feature: if you have the MPFR and GMP
libraries already installed when you configure and build gawk, gawk automatically will beable to use them.
You can install these libraries from source code by fetching them from the GNU distribution site at ftp.gnu.org.
Most modern systems provide package managers which save you the trouble of building
from source. They fetch and install the library header files and binaries for you. You willneed to research how to do this for your particular system.

B.2.2 Shell Startup Files
The distribution contains shell startup files gawk.sh and gawk.csh, containing functions
to aid in manipulating the AWKPATH and AWKLIBPATH environment variables. On a Fedora
GNU/Linux system, these files should be installed in /etc/profile.d; on other platforms,
the appropriate location may be different.
gawkpath_default
Reset the AWKPATH environment variable to its default value.
gawkpath_prepend
Add the argument to the front of the AWKPATH environment variable.
gawkpath_append
Add the argument to the end of the AWKPATH environment variable.
gawklibpath_default
Reset the AWKLIBPATH environment variable to its default value.
gawklibpath_prepend
Add the argument to the front of the AWKLIBPATH environment variable.
gawklibpath_append
Add the argument to the end of the AWKLIBPATH environment variable.

B.2.3 Additional Configuration Options
There are several additional options you may use on the configure command line whencompiling gawk from scratch, including:Appendix B: Installing gawk 497--disable-extensions
Disable the extension mechanism within gawk. With this option, it is notpossible to use dynamic extensions. This also disables configuring and building
the sample extensions in the extension directory.
This option may be useful for cross-compiling. The default action is to dynamically check if the extensions can be configured and compiled.
--disable-lint
Disable all lint checking within gawk. The --lint and --lint-old options(see Section 2.2 [Command-Line Options], page 33) are accepted, but silently
do nothing. Similarly, setting the LINT variable (see Section 7.5.1 [Built-in Variables That Control awk], page 163) has no effect on the running awk program.
When used with the GNU Compiler Collection’s (GCC’s) automatic dead-codeelimination, this option cuts almost 23K bytes off the size of the gawk executableon GNU/Linux x86 64 systems. Results on other systems and with other compilers are likely to vary. Using this option may bring you some slight performance improvement.
CAUTION: Using this option will cause some of the tests in thetest suite to fail. This option may be removed at a later date.
--disable-mpfr
Skip checking for the MPFR and GMP libraries. This is useful mainly for thedevelopers, to make sure nothing breaks if MPFR support is not available.
--disable-nls
Disable all message-translation facilities. This is usually not desirable, but it
may bring you some slight performance improvement.
--enable-versioned-extension-dir
Use a versioned directory for extensions. The directory name will include themajor and minor API versions in it. This makes it possible to keep extensions
for different API versions on the same system without their conflicting with one
another.
Use the command ‘./configure --help’ to see the full list of options supplied byconfigure.

B.2.4 The Configuration ProcessThis section is of interest only if you know something about using the C language andUnix-like operating systems.
The source code for gawk generally attempts to adhere to formal standards wherever
possible. This means that gawk uses library routines that are specified by the ISO C
standard and by the POSIX operating system interface standard. The gawk source code
requires using an ISO C compiler (the 1999 standard).
Many Unix systems do not support all of either the ISO or the POSIX standards. The
missing_d subdirectory in the gawk distribution contains replacement versions of thosefunctions that are most likely to be missing.
The config.h file that configure creates contains definitions that describe features ofthe particular operating system where you are attempting to compile gawk. The three things

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described by this file are: what header files are available, so that they can be correctly included, what (supposedly) standard functions are actually available in your C libraries, andvarious miscellaneous facts about your operating system. For example, there may not be anst_blksize element in the stat structure. In this case, ‘HAVE_STRUCT_STAT_ST_BLKSIZE’is undefined.
It is possible for your C compiler to lie to configure. It may do so by not exiting withan error when a library function is not available. To get around this, edit the custom.hfile. Use an ‘#ifdef’ that is appropriate for your system, and either #define any constants
that configure should have defined but didn’t, or #undef any constants that configure
defined and should not have. The custom.h file is automatically included by the config.h
file.
It is also possible that the configure program generated by Autoconf will not work on
your system in some other fashion. If you do have a problem, the configure.ac file isthe input for Autoconf. You may be able to change this file and generate a new versionof configure that works on your system (see Section B.4 [Reporting Problems and Bugs],
page 505, for information on how to report problems in configuring gawk). The samemechanism may be used to send in updates to configure.ac and/or custom.h.

B.2.5 Compiling from Git
Building gawk directly from the development source control repository is possible, but notrecommended for everyday users, as the code may not be as stable as released versions are.
If you really do want to do that, here are the steps:
git clone https://git.savannah.gnu.org/r/gawk.git
cd gawk./bootstrap.sh && ./configure && make && make check

B.2.6 Building the Documentation
The generated Info documentation is included in the distribution tar files and in the Git
source code repository; you should not need to rebuild it. However, if it needs to be done,
simply running make will do it, assuming that you have a recent enough version of makeinfo
installed.
If you wish to build the PDF version of the manuals, you will need to have TEX installed,
and possibly additional packages that provide the necessary fonts and tools, such as dvi2pdf
and ps2pdf. You will also need GNU Troff (groff) installed in order to format the referencecard and the manual page (see Section B.1.3 [Contents of the gawk Distribution], page 492).
Managing this process is beyond the scope of this book.
Assuming you have all you need, then the following commands produce the PDF versionsof the documentation:
cd docmake pdf
This creates PDF versions of all three Texinfo documents included in the distribution, as
well as of the manual page and the reference card.
Similarly, if you have a recent enough version of makeinfo, you can make the HTMLversion of the manuals with:
cd docAppendix B: Installing gawk 499

make html
This creates HTML versions of all three Texinfo documents included in the distribution.

B.3 Installation on Other Operating Systems
This section describes how to install gawk on various non-Unix systems.

B.3.1 Installation on MS-Windows
This section covers installation and usage of gawk on Intel architecture machines running any
version of MS-Windows. In this section, the term “Windows32” refers to any of Microsoft
Windows 95/98/ME/NT/2000/XP/Vista/7/8/10/11.
See also the README_d/README.pc file in the distribution.

B.3.1.1 Installing a Prepared Distribution for MS-Windows
Systems
The only supported binary distribution for MS-Windows systems is that provided by Eli
Zaretskii’s “ezwinports” project. Install the compiled gawk from there. Note that to run
that port, you need to have the libgcc_s_dw2-1.dll file installed on your system. This
file is part of the GCC distribution, and should reside either in the same directory where
you install gawk.exe or somewhere on your system’s Path. You can download this filefrom the MinGW site; look under the “MinGW.org Compiler Collection (GCC)” for the
LibGCC-1.DLL download.

B.3.1.2 Compiling gawk for PC Operating Systems
gawk can be compiled for Windows32 using MinGW (Windows32). The file README_
d/README.pc in the gawk distribution contains additional notes, and pc/Makefile contains
important information on compilation options.
To build gawk for Windows32, copy the files in the pc directory (except for ChangeLog)
to the directory with the rest of the gawk sources, then invoke make with the appropriate
target name as an argument to build gawk. The Makefile copied from the pc directory
contains a configuration section with comments and may need to be edited in order to workwith your make utility.
The Makefile supports a number of targets for building various Windows32 versions. A
list of targets is printed if the make command is given without a target. As an example, tobuild a native MS-Windows binary of gawk using the MinGW tools, type ‘make mingw32’.

B.3.1.3 Using gawk on PC Operating Systems
Information in this section applies to the MinGW port of gawk. See Section B.3.1.4 [Using
gawk In The Cygwin Environment], page 501, for information about the Cygwin port.
Under MS-Windows, the MinGW environment supports both the ‘|&’ operator and
TCP/IP networking (see Section 12.5 [Using gawk for Network Programming], page 346).
The MS-Windows version of gawk searches for program files as described in Section 2.5.1[The AWKPATH Environment Variable], page 42. However, semicolons (rather than colons)
separate elements in the AWKPATH variable. If AWKPATH is not set or is empty, then thedefault search path is ‘.;c:/lib/awk;c:/gnu/lib/awk’.

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Under MS-Windows, gawk (and many other text programs) silently translates end-ofline ‘\r\n’ to ‘\n’ on input and ‘\n’ to ‘\r\n’ on output. A special BINMODE variable (c.e.)allows control over these translations and is interpreted as follows:• If BINMODE is "r" or one, then binary mode is set on read (i.e., no translations on
reads).
• If BINMODE is "w" or two, then binary mode is set on write (i.e., no translations on
writes).
• If BINMODE is "rw" or "wr" or three, binary mode is set for both read and write.
• BINMODE=non-null-string is the same as ‘BINMODE=3’ (i.e., no translations on reads
or writes). However, gawk issues a warning message if the string is not one of "rw" or"wr".
The modes for standard input and standard output are set one time only (after the commandline is read, but before processing any of the awk program). Setting BINMODE for standard
input or standard output is accomplished by using an appropriate ‘-v BINMODE=N’ option
on the command line. BINMODE is set at the time a file or pipe is opened and cannot bechanged midstream.
On POSIX-compatible systems, this variable’s value has no effect. Thus, if you think
your program will run on multiple different systems and that you may need to use BINMODE,
you should simply set it (in the program or on the command line) unconditionally, and notworry about the operating system on which your program is running.
The name BINMODE was chosen to match mawk (see Section B.5 [Other Freely Available
awk Implementations], page 510). mawk and gawk handle BINMODE similarly; however, mawk
adds a ‘-W BINMODE=N’ option and an environment variable that can set BINMODE, RS, and
ORS. The files binmode[1-3].awk (under gnu/lib/awk in some of the prepared binary
distributions) have been chosen to match mawk’s ‘-W BINMODE=N’ option. These can bechanged or discarded; in particular, the setting of RS giving the fewest “surprises” is open
to debate. mawk uses ‘RS = "\r\n"’ if binary mode is set on read, which is appropriate forfiles with the MS-DOS-style end-of-line.
To illustrate, the following examples set binary mode on writes for standard output andother files, and set ORS as the “usual” MS-DOS-style end-of-line:
gawk -v BINMODE=2 -v ORS="\r\n" ...
or:gawk -v BINMODE=w -f binmode2.awk ...
These give the same result as the ‘-W BINMODE=2’ option in mawk. The following changes therecord separator to "\r\n" and sets binary mode on reads, but does not affect the modeon standard input:
gawk -v RS="\r\n" -e "BEGIN { BINMODE = 1 }" ...
or:gawk -f binmode1.awk ...
With proper quoting, in the first example the setting of RS can be moved into the BEGIN
rule.

Appendix B: Installing gawk 501

B.3.1.4 Using gawk In The Cygwin Environment
gawk can be built and used “out of the box” under MS-Windows if you are using the
Cygwin environment. This environment provides an excellent simulation of GNU/Linux,
using Bash, GCC, GNU Make, and other GNU programs. Compilation and installation for
Cygwin is the same as for a Unix system:tar -xvpzf gawk-5.3.0.tar.gz
cd gawk-5.3.0./configure
make && make check
When compared to GNU/Linux on the same system, the ‘configure’ step on Cygwin
takes considerably longer. However, it does finish, and then the ‘make’ proceeds as usual.
You may also install gawk using the regular Cygwin installer. In general Cygwin supplies
the latest released version.
Recent versions of Cygwin open all files in binary mode. This means that you shoulduse ‘RS = "\r?\n"’ in order to be able to handle standard MS-Windows text files withcarriage-return plus line-feed line endings.
The Cygwin environment supports both the ‘|&’ operator and TCP/IP networking (seeSection 12.5 [Using gawk for Network Programming], page 346).

B.3.1.5 Using gawk In The MSYS Environment
In the MSYS environment under MS-Windows, gawk automatically uses binary mode forreading and writing files. Thus, there is no need to use the BINMODE variable.
This can cause problems with other Unix-like components that have been ported to
MS-Windows that expect gawk to do automatic translation of "\r\n", because it won’t.
Under MSYS2, compilation using the standard ‘./configure && make’ recipe works “outof the box.”

B.3.2 Compiling and Installing gawk on OpenVMS
This subsection describes how to compile and install gawk under OpenVMS.

B.3.2.1 Compiling gawk on OpenVMS
To compile gawk under OpenVMS, there is a DCL command procedure that issues all thenecessary CC and LINK commands. There is also a Makefile for use with the MMS and MMK
utilities. From the source directory, use either:$ @[.vms]vmsbuild.com
or:$ MMS/DESCRIPTION=[.vms]descrip.mms gawkor:
$ MMK/DESCRIPTION=[.vms]descrip.mms gawk
Note that the vmsbuild.com method of building is no longer being maintained and isplanned to be removed in the future.
MMK is an open source, free, near-clone of MMS and can better handle ODS-5 volumes
with upper- and lowercase file names. MMK is available from https://github.com/
endlesssoftware/mmk.

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With ODS-5 volumes and extended parsing enabled, the case of the target parametermay need to be exact.
gawk has been tested using these VMS Software, Inc. Community editions:• HP C V7.3-010 on OpenVMS Alpha V8.4-2L1.
• HP C V7.3-020 on OpenVMS IA64 V8.4-2L3.1
Due to HPE cancelling the Hobbyist licensing program, no more testing is being doneon older releases of OpenVMS.
See Section B.3.2.5 [The OpenVMS GNV Project], page 505, for information on building
gawk as a PCSI kit that is compatible with the GNV product.

B.3.2.2 Compiling gawk Dynamic Extensions on OpenVMS
The extensions that have been ported to OpenVMS can be built using one of the following
commands:$ MMS/DESCRIPTION=[.vms]descrip.mms extensions
or:$ MMK/DESCRIPTION=[.vms]descrip.mms extensions
gawk uses AWKLIBPATH as either an environment variable or a logical name to find the
dynamic extensions.
Dynamic extensions need to be compiled with the same compiler options for floatingpoint, pointer size, and symbol name handling as were used to compile gawk itself. Alphaand Itanium should use IEEE floating point. The pointer size is 32 bits, and the symbolname handling should be exact case with CRC shortening for symbols longer than 32 bits.
/name=(as_is,short)/float=ieee/ieee_mode=denorm_resultsCompile-time macros need to be defined before the first OpenVMS-supplied header fileis included, as follows:#if (__CRTL_VER >= 70200000)#define _LARGEFILE 1#endif#ifdef __CRTL_VER#if __CRTL_VER >= 80200000#define _USE_STD_STAT 1#endif#endifIf you are writing your own extensions to run on OpenVMS, you must supply thesedefinitions yourself. The config.h file created when building gawk on OpenVMS does thisfor you; if instead you use that file or a similar one, then you must remember to include it
before any OpenVMS-supplied header files.
1

The IA64 architecture is also known as “Itanium.”Appendix B: Installing gawk 503

B.3.2.3 Installing gawk on OpenVMS
To use gawk, all you need is a “foreign” command, which is a DCL symbol whose value begins
with a dollar sign. For example:$ GAWK :== $disk1:[gnubin]gawk
Substitute the actual location of gawk.exe for ‘$disk1:[gnubin]’. The symbol should beplaced in the login.com of any user who wants to run gawk, so that it is defined every time
the user logs on. Alternatively, the symbol may be placed in the system-wide sylogin.com
procedure, which allows all users to run gawk.
If your gawk was installed by a PCSI kit into the GNV$GNU: directory tree, the program
will be known as GNV$GNU:[bin]gnv$gawk.exe and the help file will be GNV$GNU:[vms_help]gawk.hlp.
The PCSI kit also installs a GNV$GNU:[vms_bin]gawk_verb.cld file that can be used to
add gawk and awk as DCL commands.
For just the current process you can use:
$ set command gnv$gnu:[vms_bin]gawk_verb.cld
Or the system manager can use GNV$GNU:[vms_bin]gawk_verb.cld to add the gawk
and awk commands to the system-wide ‘DCLTABLES’.
The DCL syntax is documented in the gawk.hlp file.
Optionally, the gawk.hlp entry can be loaded into an OpenVMS help library:$ LIBRARY/HELP sys$help:helplib [.vms]gawk.hlp(You may want to substitute a site-specific help library rather than the standard OpenVMS
library ‘HELPLIB’.) After loading the help text, the command:$ HELP GAWK
provides information about both the gawk implementation and the awk programming language.
The logical name ‘AWK_LIBRARY’ can designate a default location for awk program files.
For the -f option, if the specified file name has no device or directory path information init, gawk looks in the current directory first, then in the directory specified by the translationof ‘AWK_LIBRARY’ if the file is not found. If, after searching in both directories, the file still
is not found, gawk appends the suffix ‘.awk’ to the file name and retries the file search. If‘AWK_LIBRARY’ has no definition, a default value of ‘SYS$LIBRARY:’ is used for it.

B.3.2.4 Running gawk on OpenVMS
Command-line parsing and quoting conventions are significantly different on OpenVMS, so
examples in this book or from other sources often need minor changes. They are minor
though, and all awk programs should run correctly.
Here are a couple of trivial tests:$ gawk -- "BEGIN {print ""Hello, World!""}"$ gawk -"W" version! could also be -"W version" or "-W version"
Note that uppercase and mixed-case text must be quoted.

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The OpenVMS port of gawk includes a DCL-style interface in addition to the original
shell-style interface (see the help entry for details). One side effect of dual commandline parsing is that if there is only a single parameter (as in the quoted program string),
the command becomes ambiguous. To work around this, the normally optional -- flag isrequired to force Unix-style parsing rather than DCL parsing. If any other dash-type options(or multiple parameters such as data files to process) are present, there is no ambiguity and-- can be omitted.
The exit value is a Unix-style value and is encoded into an OpenVMS exit status value
when the program exits.
The OpenVMS severity bits will be set based on the exit value. A failure is indicatedby 1, and OpenVMS sets the ERROR status. A fatal error is indicated by 2, and OpenVMS
sets the FATAL status. All other values will have the SUCCESS status. The exit value is
encoded to comply with OpenVMS coding standards and will have the C_FACILITY_NO of
0x350000 with the constant 0xA000 added to the number shifted over by 3 bits to makeroom for the severity codes.
To extract the actual gawk exit code from the OpenVMS status, use:
unix_status = (vms_status .and. %x7f8) / 8
A C program that uses exec() to call gawk will get the original Unix-style exit value.
OpenVMS reports time values in GMT unless one of the SYS$TIMEZONE_RULE or TZ
logical names is set.
The default search path, when looking for awk program files specified by the -f option,is "SYS$DISK:[],AWK_LIBRARY:". The logical name AWKPATH can be used to override thisdefault. The format of AWKPATH is a comma-separated list of directory specifications. Whendefining it, the value should be quoted so that it retains a single translation and not a
multitranslation RMS searchlist.
This restriction also applies to running gawk under GNV, as redirection is always to a
DCL command.
If you are redirecting data to an OpenVMS command or utility, the current implementation requires setting up an OpenVMS foreign command that runs a command file before
invoking gawk. (This restriction may be removed in a future release of gawk on OpenVMS.)
Without this command file, the input data will also appear prepended to the output
data.
This also allows simulating POSIX commands that are not found on OpenVMS or theuse of GNV utilities.
The example below is for gawk redirecting data to the OpenVMS sort command.
$ sort = "@device:[dir]vms_gawk_sort.com"
The command file needs to be of the format in the example below.
The first line inhibits the passed input data from also showing up in the output. It must
be in the format in the example.
The next line creates a foreign command that overrides the outer foreign command which
prevents an infinite recursion of command files.
The next to the last command redirects sys$input to be sys$command, in order to pickup the data that is being redirected to the command.

Appendix B: Installing gawk 505

The last line runs the actual command. It must be the last command as the data
redirected from gawk will be read when the command file ends.
$!'f$verify(0,0)'$ sort := sort$ define/user sys$input sys$command:$ sort sys$input: sys$output:

B.3.2.5 The OpenVMS GNV Project
The OpenVMS GNV package provides a build environment similar to POSIX with ports
of a collection of open source tools. The gawk found in the GNV base kit is an older port.
Currently, the GNV project is being reorganized to supply individual PCSI packages foreach component. See https://sourceforge.net/p/gnv/wiki/InstallingGNVPackages/.
The normal build procedure for gawk produces a program that is suitable for use withGNV.
The file vms/gawk_build_steps.txt in the distribution documents the procedure forbuilding an OpenVMS PCSI kit that is compatible with GNV.

B.4 Reporting Problems and Bugs
There is nothing more dangerous than a bored archaeologist.
—Douglas Adams, The Hitchhiker’s Guide to the Galaxy
If you have problems with gawk or think that you have found a bug, report it to thedevelopers; we cannot promise to do anything, but we might well want to fix it.

B.4.1 Defining What Is and What Is Not A Bug
Before talking about reporting bugs, let’s define what is a bug, and what is not.
A bug is:• When gawk behaves differently from what’s described in the POSIX standard, and thatdifference is not mentioned in this book as being done on purpose.
• When gawk behaves differently from what’s described in this book.
• When gawk behaves differently from other awk implementations in particular circumstances, and that behavior cannot be attributed to an additional feature in gawk.
• Something that is obviously wrong, such as a core dump.
• When this book is unclear or ambiguous about a particular feature’s behavior.
The following things are not bugs, and should not be reported to the bug mailing list.
You can ask about them on the “help” mailing list (see Section B.4.5 [Where To Send Nonbug Questions], page 509), but don’t be surprised if you get an answer of the form “that’s
how gawk behaves and it isn’t going to change.” Here’s the list:• Missing features, for any definition of feature. For example, additional built-in arithmetic functions, or additional ways to split fields or records, or anything else.
The number of features that gawk does not have is by definition infinite. It cannot
be all things to all people. In short, just because gawk doesn’t do what you think itshould, it’s not necessarily a bug.

506

GAWK: Effective AWK Programming• Behaviors that are defined by the POSIX standard and/or for historical compatibility
with Unix awk. Even if you happen to dislike those behaviors, they’re not going tochange: changing them would break millions of existing awk programs.
• Behaviors that differ from how it’s done in other languages. awk and gawk stand on
their own and do not have to follow the crowd. This is particularly true when therequested behavior change would break backwards compatibility.
This applies also to differences in behavior between gawk and other language compilers
and interpreters, such as wishes for more detailed descriptions of what the problem iswhen a syntax error is encountered.
• Documentation issues of the form “the manual doesn’t tell me how to do XYZ.” Themanual is not a cookbook to solve every little problem you may have. Its purpose is toteach you how to solve your problems on your own.
• General questions and discussion about awk programming or why gawk behaves theway it does. For that use the “help” mailing list: see Section B.4.5 [Where To Send
Non-bug Questions], page 509.
For more information, see Fork My Code, Please!—An Open Letter To Those of You
Who Are Unhappy, by Arnold Robbins and Chet Ramey.
A Note About Fuzzers
In recent years, people have been running “fuzzers” to generate invalid awk programsin order to find and report (so-called) bugs in gawk.
In general, such reports are not of much practical use. The programs they create are
not realistic and the bugs found are generally from some kind of memory corruption thatis fatal anyway.
So, if you want to run a fuzzer against gawk and report the results, you may do so, but
be aware that such reports don’t carry the same weight as reports of real bugs do.

B.4.2 Submitting Bug Reports
Before reporting a bug, make sure you have really found a genuine bug.
Here are the steps for submitting a bug report. Following them will make both your life
and the lives of the maintainers much easier.
1. Make sure that what you want to report is appropriate. See Section B.4.1 [Defining
What Is and What Is Not A Bug], page 505. If it’s not, you are wasting your time andours.
2. Verify that you have the latest version of gawk. Many bugs (usually subtle ones) are
fixed at each release, and if yours is out-of-date, the problem may already have beensolved.
3. Please see if setting the environment variable LC_ALL to LC_ALL=C causes things tobehave as you expect. If so, it’s a locale issue, and may or may not really be a bug.
4. Carefully reread the documentation and see if it says you can do what you’re trying
to do. If it’s not clear whether you should be able to do something or not, report that
too; it’s a bug in the documentation!

Appendix B: Installing gawk 507

5. Before reporting a bug or trying to fix it yourself, try to isolate it to the smallest
possible awk program and input data file that reproduce the problem.
6. Use the gawkbug program to submit the bug report. This program sets up a bug reporttemplate and opens it in your editor. You then need to edit it appropriately to include:• The program and data file.
• The exact results gawk gave you. Also say what you expected to occur; this helps
us decide whether the problem is really in the documentation.
• A fix if you have one.
7. Do not send screenshots. Instead, use copy/paste to send text, or send files.
8. Please be sure to send all mail in plain text, not (or not exclusively) in HTML.
9. All email must be in English. This is the only language understood in common by all
the maintainers.
The gawkbug program sends email to bug-gawk@gnu.org.
The gawk maintainers subscribe to this address, and thus they will receive your bug
report. Do not send mail to the maintainers directly; the bug reporting address is preferredbecause the email list is archived at the GNU Project.
If you are using OpenVMS or the MinGW build of gawk, the gawkbug script won’t beavailable. Please send the previously listed information directly in an email to the bug list.
Please send any test program or data files as attachments, instead of inline in the email, toavoid their being mangled by various mail systems.
NOTE: Many distributions of GNU/Linux and the various BSD-based operating systems have their own bug reporting systems. If you report a bug using
your distribution’s bug reporting system, you should also send a copy to buggawk@gnu.org.
This is for two reasons. First, although some distributions forward bug reports“upstream” to the GNU mailing list, many don’t, so there is a good chancethat the gawk maintainers won’t even see the bug report! Second, mail to the
GNU list is archived, and having everything at the GNU Project keeps things
self-contained and not dependent on other organizations.
Please note: We ask that you follow the GNU Kind Communication Guidelines in yourcorrespondence on the list (as well as off of it).

B.4.3 Please Don’t Post Bug Reports to USENET
I gave up on Usenet a couple of years ago and haven’t really looked back. It’s
like sports talk radio—you feel smarter for not having read it.
—Chet Ramey
Please do not try to report bugs in gawk by posting to the Usenet/Internet newsgroupcomp.lang.awk. Although some of the gawk developers occasionally read this news group,
the primary gawk maintainer no longer does. Thus it’s virtually guaranteed that he will not
see your posting.
If you really don’t care about the previous paragraph and continue to post bug reports
in comp.lang.awk, then understand that you’re not reporting bugs, you’re just whining.

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Similarly, posting bug reports or questions in web forums (such as Stack Overflow) may
get you an answer, but it won’t be from the gawk maintainers, who do not spend their timein web forums. The steps described here are the only officially recognized way for reporting
bugs. Really.

B.4.4 What To Do If You Think There Is A Performance Issue
If you think that gawk is too slow at doing a particular task, you should investigate before
sending in a bug report. Here are the steps to follow:
1. Run gawk with the --profile option (see Section 2.2 [Command-Line Options],
page 33) to see what your program is doing. It may be that you have written it in
an inefficient manner. For example, you may be doing something for every recordthat could be done just once, for every file. (Use a BEGINFILE rule; see Section 7.1.5[The BEGINFILE and ENDFILE Special Patterns], page 151.) Or you may be doing
something for every file that only needs to be done once per run of the program. (Usea BEGIN rule; see Section 7.1.4 [The BEGIN and END Special Patterns], page 150.)
2. If profiling at the awk level doesn’t help, then you will need to compile gawk itself for
profiling at the C language level.
To do that, start with the latest released version of gawk. Unpack the source code in
a new directory, and configure it:$ tar -xpzvf gawk-X.Y.Z.tar.gz
Output omitted
a ...
$ cd gawk-X.Y.Z$ ./configure
Output omitteda ...
3. Edit the files Makefile and support/Makefile. Change every instance of -O2 or -O
to -pg. This causes gawk to be compiled for profiling.
4. Compile the program by running the make command:$ makeOutput omitteda ...
5. Run the freshly compiled gawk on a real program, using real data. Using an artificial
program to try to time one particular feature of gawk is useless; real awk programs
generally spend most of their time doing I/O, not computing. If you want to provethat something is slow, it must be done using a real program and real data.
Use a data file that is large enough for the statistical profiling to measure where gawk
spends its time. It should be at least 100 megabytes in size.
$ ./gawk -f realprogram.awk realdata > /dev/null
6. When done, you should have a file in the current directory named gmon.out. Run thecommand ‘gprof gawk gmon.out > gprof.out’.
7. Submit a bug report explaining what you think is slow. Include the gprof.out filewith it.
Preferably, you should also submit the program and the data, or else indicate where to
get the data if the file is large.
8. If you have not submitted your program and data, be prepared to apply patches andrerun the profiling in order to see if the patches were effective.

Appendix B: Installing gawk 509

If you are incapable or unwilling to do the steps listed above, then you will just have tolive with gawk as it is.

B.4.5 Where To Send Non-bug Questions
If you have questions related to awk programming, or why gawk behaves a certain way, or
any other awk- or gawk-related issue, please do not send it to the bug reporting address.
As of July, 2021, there is a separate mailing list for this purpose: help-gawk@gnu.org.
Anything that is not a bug report should be sent to that list.
NOTE: If you disregard these directions and send non-bug mails to the bug list,
you will be told to use the help list. After two such requests you will be silently
blacklisted from the bug list.
Please note: As with the bug list, we ask that you follow the GNU Kind Communication
Guidelines in your correspondence on the help list (as well as off of it).
If you wish to the subscribe to the list, in order to help out others, or to learn fromothers, here are instructions, courtesy of Bob Proulx:
Subscribe by email
Send an email message to help-gawk-request@gnu.org with “subscribe” inthe body of the message. The subject does not matter and is not used.
Subscribe by web form
To use the web interface visit the list information page. Use the subscribe form
to fill out your email address and submit using the Subscribe button.
Reply to the confirmation message
In both cases then reply to the confirmation message that is sent to your address
in reply.
Bob mentions that you may also use email for subscribing and unsubscribing. For example:$ echo help | mailx -s request help-gawk-request@gnu.org$ echo subscribe | mailx -s request help-gawk-request@gnu.org$ echo unsubscribe | mailx -s request help-gawk-request@gnu.org

B.4.6 Reporting Problems with Non-Unix Ports
If you find bugs in one of the non-Unix ports of gawk, send an email to the bug list, with acopy to the person who maintains that port. The maintainers are named in the following
list, as well as in the README file in the gawk distribution. Information in the README fileshould be considered authoritative if it conflicts with this book.
The people maintaining the various gawk ports are:Unix and POSIX systems
Arnold Robbins, arnold@skeeve.com
MS-Windows with MinGWEli Zaretskii, eliz@gnu.org

OpenVMSJohn Malmberg, wb8tyw@qsl.net

z/OS (OS/390)

Daniel Richard G. skunk@iSKUNK.ORG

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If your bug is also reproducible under Unix, send a copy of your report to the buggawk@gnu.org email list as well.

B.5 Other Freely Available awk Implementations
It’s kind of fun to put comments like this in your awk code:// Do C++ comments work? answer: yes! of course—Michael Brennan
There are a number of other freely available awk implementations. This section briefly
describes where to get them:
Unix awk

Brian Kernighan, one of the original designers of Unix awk, has made his implementation of awk freely available. You can retrieve it from GitHub:
git clone https://github.com/onetrueawk/awk bwkawk
This command creates a copy of the Git repository in a directory named bwkawk.
If you omit the last argument from the git command line, the repository copy
is created in a directory named awk.
This version requires an ISO C (1990 standard) compiler; the C compiler fromGCC (the GNU Compiler Collection) works quite nicely.
To build it, review the settings in the makefile, and then just run make. Notethat the result of compilation is named a.out; you will have to rename it tosomething reasonable.
See Section A.7 [Common Extensions Summary], page 486, for a list of extensions in this awk that are not in POSIX awk.
In 2023, Brian Kernighan, along with Al Aho and Peter Weinberger, publisheda second edition of their book on awk. Professor Kernighan also maintains
a companion web site for the book. A copy of all the book’s programs areavailable there for download.
As a side note, Dan Bornstein has created a Git repository tracking all theversions of BWK awk that he could find. It’s available at https://github.
com/danfuzz/one-true-awk.

mawk

Michael Brennan wrote an independent implementation of awk, called mawk. It
is available under the GPL (see [GNU General Public License], page 541), just
as gawk is.
The original distribution site for the mawk source code no longer has it. A copy
is available at http://www.skeeve.com/gawk/mawk1.3.3.tar.gz.
In 2009, Thomas Dickey took on mawk maintenance. Basic information isavailable on the project’s web page. The download URL is http://
invisible-island.net/datafiles/release/mawk.tar.gz.
Once you have it, gunzip may be used to decompress this file. Installation issimilar to gawk’s (see Section B.2 [Compiling and Installing gawk on Unix-Like
Systems], page 495).
See Section A.7 [Common Extensions Summary], page 486, for a list of extensions in mawk that are not in POSIX awk.

Appendix B: Installing gawk 511

mawk 2.0

In 2016, Michael Brennan resumed mawk development. His development snapshots are available via Git from the project’s GitHub page.

awka

Written by Andrew Sumner, awka translates awk programs into C, compiles
them, and links them with a library of functions that provide the core awk
functionality. It also has a number of extensions.
Both the awk translator and the library are released under the GPL.
To get awka, go to https://sourceforge.net/projects/awka.
The project seems to be frozen; no new code changes have been made sinceapproximately 2001.

Revive Awka
This project, available at https://github.com/noyesno/awka, intends to fix
bugs in awka and add more features.
pawk

Nelson H.F. Beebe at the University of Utah has modified BWK awk to provide
timing and profiling information. It is different from gawk with the --profile
option (see Section 12.6 [Profiling Your awk Programs], page 348) in that it
uses CPU-based profiling, not line-count profiling. You may find it at either
ftp://ftp.math.utah.edu/pub/pawk/pawk-20030606.tar.gz or http://
www.math.utah.edu/pub/pawk/pawk-20030606.tar.gz.

BusyBox awk
BusyBox is a GPL-licensed program providing small versions of many applications within a single executable. It is aimed at embedded systems. It includes a full implementation of POSIX awk. When building it, be careful not
to do ‘make install’ as it will overwrite copies of other applications in your/usr/local/bin. For more information, see the project’s home page.
The OpenSolaris POSIX awk
The versions of awk in /usr/xpg4/bin and /usr/xpg6/bin on Solaris are moreor less POSIX-compliant. They are based on the awk from Mortice Kern
Systems for PCs. We were able to make this code compile and work under
GNU/Linux with 1–2 hours of work. Making it more generally portable (using
GNU Autoconf and/or Automake) would take more work, and this has notbeen done, at least to our knowledge.
The source code used to be available from the OpenSolaris website. However, that project was ended and the website shut down. Fortunately, theIllumos project makes this implementation available. You can view the files one
at a time from https://github.com/joyent/illumos-joyent/blob/master/
usr/src/cmd/awk_xpg4.
frawk

This is a language for writing short programs. “To a first approximation, it
is an implementation of the AWK language; many common awk programs produce equivalent output when passed to frawk.” However, it has a number ofimportant additional features. The code is available at https://github.com/
ezrosent/frawk.

goawk

This is an awk interpreter written in the Go programming language. It implements POSIX awk, with a few minor extensions. Source code is available

512

GAWK: Effective AWK Programmingfrom https://github.com/benhoyt/goawk. The author wrote a nice article
describing the implementation.
AWKgo

This is an awk to Go translator. It was written by the author of goawk. (Seethe previous entry in this list.) Source code is available from https://github.
com/benhoyt/goawk/tree/master/awkgo. The author’s article about it is athttps://benhoyt.com/writings/awkgo/.

jawk

This is an interpreter for awk written in Java. It claims to be a full interpreter,
although because it uses Java facilities for I/O and for regexp matching, thelanguage it supports is different from POSIX awk. More information is available
on the project’s home page.

Hoijui’s jawk
This project, available at https://github.com/hoijui/Jawk, is another awk
interpreter written in Java. It uses modern Java build tools.
Libmawk

This is an embeddable awk interpreter derived from mawk. For more information, see http://repo.hu/projects/libmawk/.

Mircea Neacsu’s Embeddable awk
Mircea Neacsu has created an embeddable awk interpreter, based on BWK awk.
It’s available at https://github.com/neacsum/awk.
pawkThis is a Python module that claims to bring awk-like features to Python. See
https://github.com/alecthomas/pawk for more information. (This is not
related to Nelson Beebe’s modified version of BWK awk, described earlier.)

awkcc

This is an early adaptation of Unix awk that translates awk into C code. It
was done by J. Christopher Ramming at Bell Labs, circa 1988. It’s available
at https://github.com/nokia/awkcc. Bringing this up to date would be aninteresting software engineering exercise.

QSE awk

This is an embeddable awk interpreter. For more information, see https://code.google.com/p/qse/.

QTawk

This is an independent implementation of awk distributed under the GPL. It
has a large number of extensions over standard awk and may not be 100%syntactically compatible with it. See http://www.quiktrim.org/QTawk.html
for more information, including the manual. The download link there is outof date; see http://www.quiktrim.org/#AdditionalResources for the latest
download link.
The project may also be frozen; no new code changes have been made sinceapproximately 2014.

cppawkQuoting from the web page, “cppawk is a tiny shell script that is used like awk.
It invokes the C preprocessor (GNU cpp) on the Awk code and calls Awk on the
result.” This program may be of use if the way gawk’s @include facility works
doesn’t suit your needs. For more information, see https://www.kylheku.
com/cgit/cppawk/.

Other versions
See also the “Versions and implementations” section of the Wikipedia article
on awk for information on additional versions.

Appendix B: Installing gawk 513

An interesting collection of library functions is available at https://github.com/
e36freak/awk-libs.
An interesting collection of gawk extensions is available https://github.com/su8/
gawk-extensions.

B.6 Summary• The gawk distribution is available from the GNU Project’s main distribution site,
ftp.gnu.org. The canonical build recipe is:wget https://ftp.gnu.org/gnu/gawk/gawk-5.3.0.tar.gz
tar -xvpzf gawk-5.3.0.tar.gz
cd gawk-5.3.0./configure && make && make check
NOTE: Because of the ‘https://’ URL, you may have to supply the --nocheck-certificate option to wget to download the file.
• gawk may be built on non-POSIX systems as well. The currently supported systems are
MS-Windows using MSYS, MSYS2, MinGW, and Cygwin, and OpenVMS. Instructions
for each system are included in this appendix.
• Bug reports should be sent via email to bug-gawk@gnu.org. Bug reports should be
in English and should include the version of gawk, how it was compiled, and a shortprogram and data file that demonstrate the problem.
• Non-bug emails should be sent to help-gawk@gnu.org. Repeatedly sending non-bug
emails to the bug list will get you blacklisted from it.
• There are a number of other freely available awk implementations. Many are POSIXcompliant; others are less so.

Appendix C: Implementation Notes

515

Appendix C Implementation Notes
This appendix contains information mainly of interest to implementers and maintainers ofgawk. Everything in it applies specifically to gawk and not to other implementations.

C.1 Downward Compatibility and Debugging
See Section A.5 [Extensions in gawk Not in POSIX awk], page 475, for a summary of the
GNU extensions to the awk language and program. All of these features can be turned off
by invoking gawk with the --traditional option or with the --posix option.
If gawk is compiled for debugging with ‘-DDEBUG’, then there is one more option available
on the command line:-Y--parsedebug
Print out the parse stack information as the program is being parsed.
This option is intended only for serious gawk developers and not for the casual user.
It probably has not even been compiled into your version of gawk, since it slows down
execution.

C.2 Making Additions to gawk
If you find that you want to enhance gawk in a significant fashion, you are perfectly free todo so. That is the point of having free software; the source code is available and you arefree to change it as you want (see [GNU General Public License], page 541).
This section discusses the ways you might want to change gawk as well as any considerations you should bear in mind.

C.2.1 Accessing The gawk Git Repository
As gawk is Free Software, the source code is always available. Section B.1 [The gawk
Distribution], page 491, describes how to get and build the formal, released versions ofgawk.
However, if you want to modify gawk and contribute back your changes, you will probably wish to work with the development version. To do so, you will need to access the gawk
source code repository. The code is maintained using the Git distributed version control
system. You will need to install it if your system doesn’t have it. Once you have done so,use the command:
git clone git://git.savannah.gnu.org/gawk.git
This clones the gawk repository. If you are behind a firewall that does not allow you to use
the Git native protocol, you can still access the repository using:git clone https://git.savannah.gnu.org/r/gawk.git(Using the https URL is considered to be more secure.)
Once you have made changes, you can use ‘git diff’ to produce a patch, and send thatto the gawk maintainer; see Section B.4 [Reporting Problems and Bugs], page 505, for how
to do that.

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Once upon a time there was Git–CVS gateway for use by people who could not install
Git. However, this gateway no longer works, so you may have better luck using a more
modern version control system like Bazaar, that has a Git plug-in for working with Git
repositories.

C.2.2 Adding New Features
You are free to add any new features you like to gawk. However, if you want your changes
to be incorporated into the gawk distribution, there are several steps that you need to takein order to make it possible to include them:
1. Discuss the proposed new feature with the gawk maintainer. The bug list may be used
for this. Even if I don’t wish to include your feature, be aware that you are still free
to add it and distribute your own “fork” of gawk.
2. Before building the new feature into gawk itself, consider writing it as an extension (seeChapter 17 [Writing Extensions for gawk], page 405). If that’s not possible, continue
with the rest of the steps in this list.
3. Be prepared to sign the appropriate paperwork. In order for the FSF to distribute
your changes, you must either place those changes in the public domain and submit
a signed statement to that effect, or assign the copyright in your changes to the FSF.
Both of these actions are easy to do and many people have done so already. If you
have questions, please contact me (see Section B.4 [Reporting Problems and Bugs],
page 505), or assign@gnu.org.
4. Get the latest version. It is much easier for me to integrate changes if they are relative
to the most recent distributed version of gawk, or better yet, relative to the latest codein the Git repository. If your version of gawk is very old, I may not be able to integrate
your changes at all. (See Section B.1.1 [Getting the gawk Distribution], page 491, for
information on getting the latest version of gawk.)
5. Follow the GNU Coding Standards. This document describes how GNU software should
be written. If you haven’t read it, please do so, preferably before starting to modify
gawk. (The GNU Coding Standards are available from the GNU Project’s website.
Texinfo, Info, and DVI versions are also available.)
6. Use the gawk coding style. The C code for gawk follows the instructions in the GNU
Coding Standards, with minor exceptions. The code is formatted using the traditional“K&R” style, particularly as regards to the placement of braces and the use of TABs.
In brief, the coding rules for gawk are as follows:• Use ANSI/ISO style (prototype) function headers when defining functions.
• Put the name of the function at the beginning of its own line.
• Use ‘#elif’ instead of nesting ‘#if’ inside ‘#else’.
• Put the return type of the function, even if it is int, on the line above the linewith the name and arguments of the function.
• Put spaces around parentheses used in control structures (if, while, for, do, andswitch).
• Do not parenthesize the expression used with return.
• Do not put spaces in front of parentheses used in function calls.
• Put spaces around all C operators and after commas in function calls.

Appendix C: Implementation Notes

517• Do not use the comma operator to produce multiple side effects, except in forloop initialization and increment parts, and in macro bodies.
• Use real TABs for indenting, not spaces.
• Use the “K&R” brace layout style.
• Use comparisons against NULL and '\0' in the conditions of if, while, and forstatements, as well as in the cases of switch statements, instead of just the plain
pointer or character value.
• Do not, under any circumstances, use the ‘-1 == foo’ or ‘0 >= bar’ style of comparison expressions. I have known about it for decades, and I understand whysome people like it. Nonetheless, I abhor it with a passion, and code that uses itwill never be accepted.
• Use true and false for bool values, the NULL symbolic constant for pointer values,
and the character constant '\0' where appropriate, instead of 1 and 0.
• Provide one-line descriptive comments for each function.
• Do not use the alloca() function for allocating memory off the stack. Its use
causes more portability trouble than is worth the minor benefit of not having tofree the storage. Instead, use malloc() and free().
• Do not use comparisons of the form ‘! strcmp(a, b)’ or similar. As Henry Spencer
once said, “strcmp() is not a boolean!” Instead, use ‘strcmp(a, b) == 0’.
• If adding new bit flag values, use explicit hexadecimal constants (0x001, 0x002,
0x004, and so on) instead of shifting one left by successive amounts (‘(1<<0)’,‘(1<<1)’, and so on).
NOTE: If I have to reformat your code to follow the coding style used ingawk, I may not bother to integrate your changes at all.
7. Update the documentation. Along with your new code, please supply new sections
and/or chapters for this book. If at all possible, please use real Texinfo, instead of justsupplying unformatted ASCII text (although even that is better than no documentation at all). Conventions to be followed in GAWK: Effective AWK Programming areprovided after the ‘@bye’ at the end of the Texinfo source file. If possible, please update
the man page as well.
You will also have to sign paperwork for your documentation changes.
8. Submit changes as unified diffs. Use ‘diff -u -r -N’ to compare the original gawk
source tree with your version. I recommend using the GNU version of diff, or best ofall, ‘git diff’ or ‘git format-patch’. Send the output produced by diff to me when
you submit your changes. (See Section B.4 [Reporting Problems and Bugs], page 505,
for the electronic mail information.)
Using this format makes it easy for me to apply your changes to the master version of
the gawk source code (using patch). If I have to apply the changes manually, using atext editor, I may not do so, particularly if there are lots of changes.
9. Include an entry for the ChangeLog file with your submission. This helps further
minimize the amount of work I have to do, making it easier for me to accept patches.
It is simplest if you just make this part of your diff.

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Although this sounds like a lot of work, please remember that while you may write the
new code, I have to maintain it and support it. If it isn’t possible for me to do that with aminimum of extra work, then I probably will not.

C.2.3 Porting gawk to a New Operating System
If you want to port gawk to a new operating system, there are several steps:
1. Follow the guidelines in the previous section concerning coding style, submission ofdiffs, and so on.
2. Be prepared to sign the appropriate paperwork. In order for the FSF to distribute
your code, you must either place your code in the public domain and submit a signed
statement to that effect, or assign the copyright in your code to the FSF. Both of theseactions are easy to do and many people have done so already. If you have questions,please contact me, or gnu@gnu.org.
3. When doing a port, bear in mind that your code must coexist peacefully with the restof gawk and the other ports. Avoid gratuitous changes to the system-independent partsof the code. If at all possible, avoid sprinkling ‘#ifdef’s just for your port throughout
the code.
If the changes needed for a particular system affect too much of the code, I probably
will not accept them. In such a case, you can, of course, distribute your changes on
your own, as long as you comply with the GPL (see [GNU General Public License],
page 541).
4. A number of the files that come with gawk are maintained by other people. Thus, you
should not change them unless it is for a very good reason; i.e., changes are not outof the question, but changes to these files are scrutinized extra carefully. These are all
the files in the support directory within the gawk distribution. See there.
5. A number of other files are provided by the GNU Autotools (Autoconf, Automake,and GNU gettext). You should not change them either, unless it is for a very goodreason. The files are ABOUT-NLS, config.guess, config.rpath, config.sub, depcomp,
INSTALL, install-sh, missing, mkinstalldirs, and ylwrap.
6. Be willing to continue to maintain the port. Non-Unix operating systems are supported
by volunteers who maintain the code needed to compile and run gawk on their systems.
If no-one volunteers to maintain a port, it becomes unsupported and it may be necessary
to remove it from the distribution.
7. Supply an appropriate gawkmisc.??? file. Each port has its own gawkmisc.??? that
implements certain operating system specific functions. This is cleaner than a plethoraof ‘#ifdef’s scattered throughout the code. The gawkmisc.c in the main source directory includes the appropriate gawkmisc.??? file from each subdirectory. Be sure to
update it as well.
Each port’s gawkmisc.??? file has a suffix reminiscent of the machine or operating
system for the port—for example, pc/gawkmisc.pc and vms/gawkmisc.vms. The useof separate suffixes, instead of plain gawkmisc.c, makes it possible to move files froma port’s subdirectory into the main subdirectory, without accidentally destroying thereal gawkmisc.c file. (Currently, this is only an issue for the PC operating system
ports.)

Appendix C: Implementation Notes

519

8. Supply a Makefile as well as any other C source and header files that are necessary foryour operating system. All your code should be in a separate subdirectory, with a namethat is the same as, or reminiscent of, either your operating system or the computer
system. If possible, try to structure things so that it is not necessary to move files outof the subdirectory into the main source directory. If that is not possible, then be sure
to avoid using names for your files that duplicate the names of files in the main source
directory.
9. Update the documentation. Please write a section (or sections) for this book describing
the installation and compilation steps needed to compile and/or install gawk for yoursystem.
Following these steps makes it much easier to integrate your changes into gawk and havethem coexist happily with other operating systems’ code that is already there.
In the code that you supply and maintain, feel free to use a coding style and brace layoutthat suits your taste.

C.2.4 Why Generated Files Are Kept In Git
If you look at the gawk source in the Git repository, you will notice that it includes files
that are automatically generated by GNU infrastructure tools, such as Makefile.in from
Automake and even configure from Autoconf.
This is different from many Free Software projects that do not store the derived files,
because that keeps the repository less cluttered, and it is easier to see the substantive
changes when comparing versions and trying to understand what changed between commits.
However, there are several reasons why the gawk maintainer likes to have everything in
the repository.
First, because it is then easy to reproduce any given version completely, without relying
upon the availability of (older, likely obsolete, and maybe even impossible to find) othertools.
As an extreme example, if you ever even think about trying to compile, oh, say, the V7
awk, you will discover that not only do you have to bootstrap the V7 yacc to do so, but you
also need the V7 lex. And the latter is pretty much impossible to bring up on a modern
GNU/Linux system.1(Or, let’s say gawk 1.2 required bison whatever-it-was in 1989 and that there was noawkgram.c file in the repository. Is there a guarantee that we could find that bison version?
Or that it would build?)
If the repository has all the generated files, then it’s easy to just check them out andbuild. (Or easier, depending upon how far back we go.)And that brings us to the second (and stronger) reason why all the files really need to
be in Git. It boils down to who do you cater to—the gawk developer(s), or the user whojust wants to check out a version and try it out?
The gawk maintainer wants it to be possible for any interested awk user in the world tojust clone the repository, check out the branch of interest and build it, without their having
1

We tried. It was painful.

520

GAWK: Effective AWK Programmingto have the correct version(s) of the autotools.2 That is the point of the bootstrap.sh file.
It touches the various other files in the right order such that
# The canonical incantation for building GNU software:./bootstrap.sh && ./configure && makewill just work.
This is extremely important for the master and gawk-X.Y-stable branches.
Further, the gawk maintainer would argue that it’s also important for the gawk developers. When he tried to check out the xgawk branch3 to build it, he couldn’t. (No ltmain.shfile, and he had no idea how to create it, and that was not the only problem.)
He felt extremely frustrated. With respect to that branch, the maintainer is no different
than Jane User who wants to try to build gawk-4.1-stable or master from the repository.
Thus, the maintainer thinks that it’s not just important, but critical, that for any given
branch, the above incantation just works.
A third reason to have all the files is that without them, using ‘git bisect’ to try to findthe commit that introduced a bug is exceedingly difficult. The maintainer tried to do thaton another project that requires running bootstrapping scripts just to create configureand so on; it was really painful. When the repository is self-contained, using git bisect
in it is very easy.
What are some of the consequences and/or actions to take?
1. We don’t mind that there are differing files in the different branches as a result ofdifferent versions of the autotools.
A. It’s the maintainer’s job to merge them and he will deal with it.
B. He is really good at ‘git diff x y > /tmp/diff1 ; gvim /tmp/diff1’ to remove
the diffs that aren’t of interest in order to review code.
2. It would certainly help if everyone used the same versions of the GNU tools as he does,
which in general are the latest released versions of Automake, Autoconf, bison, GNU
gettext, and Libtool.
Installing from source is quite easy. It’s how the maintainer worked for years (and stillworks). He had /usr/local/bin at the front of his PATH and just did:wget https://ftp.gnu.org/gnu/package/package-x.y.z.tar.gz
tar -xpzvf package-x.y.z.tar.gz
cd package-x.y.z./configure && make && make checkmake install# as root
NOTE: Because of the ‘https://’ URL, you may have to supply the --nocheck-certificate option to wget to download the file.
2

There is one GNU program that is (in our opinion) severely difficult to bootstrap from the Git repository.
For example, on the author’s old (but still working) PowerPC Macintosh with macOS 10.5, it wasnecessary to bootstrap a ton of software, starting with Git itself, in order to try to work with the latest
code. It’s not pleasant, and especially on older systems, it’s a big waste of time.

Starting with the latest tarball was no picnic either. The maintainers had dropped .gz and .bz2 files
and only distribute .tar.xz files. It was necessary to bootstrap xz first!
3
A branch (since removed) created by one of the other developers that did not include the generated files.

Appendix C: Implementation Notes

521

Most of the above was originally written by the maintainer to other gawk developers. It
raised the objection from one of the developers “. . . that anybody pulling down the sourcefrom Git is not an end user.”
However, this is not true. There are “power awk users” who can build gawk (using themagic incantation shown previously) but who can’t program in C. Thus, the major branches
should be kept buildable all the time.
It was then suggested that there be a cron job to create nightly tarballs of “the source.”
Here, the problem is that there are source trees, corresponding to the various branches! So,nightly tarballs aren’t the answer, especially as the repository can go for weeks without
significant change being introduced.
Fortunately, the Git server can meet this need. For any given branch named branchname,use:
wget https://git.savannah.gnu.org/cgit/gawk.git/snapshot/gawk-branchname.tar.gz
to retrieve a snapshot of the given branch.

C.3 Probable Future Extensions
AWK is a language similar to PERL, only considerably more elegant.
—Arnold Robbins
Hey!
—Larry Wall
The TODO file in the master branch of the gawk Git repository lists possible futureenhancements. Some of these relate to the source code, and others to possible new features.
Please see that file for the list. See Section C.2 [Making Additions to gawk], page 515, if
you are interested in tackling any of the projects listed there.

C.4 Some Limitations of the Implementation
This following table describes limits of gawk on a Unix-like system (although it is variable
even then). Other systems may have different limits.
Item
Characters in a character class
Length of input record in bytes
Length of output record
Length of source line
Number of fields in a record
Number of file redirections
Number of input records in one fileNumber of input records total
Number of pipe redirections
Numeric values
Size of a field in bytes
Size of a literal string in bytes
Size of a printf string in bytes

Limit2^(number of bits per byte)
ULONG_MAX
Unlimited
Unlimited
ULONG_MAX
Unlimited
MAX_LONG
MAX_LONG
min(number of processes per user, number of open
files)
Double-precision floating point (if not using MPFR)ULONG_MAXULONG_MAXULONG_MAX522

GAWK: Effective AWK Programming

C.5 Extension API Design
This section documents the design of the extension API, including a discussion of some ofthe history and problems that needed to be solved.
The first version of extensions for gawk was developed in the mid-1990s and released
with gawk 3.1 in the late 1990s. The basic mechanisms and design remained unchanged for
close to 15 years, until 2012.
The old extension mechanism used data types and functions from gawk itself, with a“clever hack” to install extension functions.
gawk included some sample extensions, of which a few were really useful. However, it
was clear from the outset that the extension mechanism was bolted onto the side and was
not really well thought out.

C.5.1 Problems With The Old Mechanism
The old extension mechanism had several problems:• It depended heavily upon gawk internals. Any time the NODE structure4 changed,
an extension would have to be recompiled. Furthermore, to really write extensions
required understanding something about gawk’s internal functions. There was somedocumentation in this book, but it was quite minimal.
• Being able to call into gawk from an extension required linker facilities that are common
on Unix-derived systems but that did not work on MS-Windows systems; users wanting
extensions on MS-Windows had to statically link them into gawk, even though MSWindows supports dynamic loading of shared objects.
• The API would change occasionally as gawk changed; no compatibility between versions
was ever offered or planned for.
Despite the drawbacks, the xgawk project developers forked gawk and developed several
significant extensions. They also enhanced gawk’s facilities relating to file inclusion andshared object access.
A new API was desired for a long time, but only in 2012 did the gawk maintainer and thexgawk developers finally start working on it together. More information about the xgawk
project is provided in Section 17.8 [The gawkextlib Project], page 467.

C.5.2 Goals For A New Mechanism
Some goals for the new API were:• The API should be independent of gawk internals. Changes in gawk internals should
not be visible to the writer of an extension function.
• The API should provide binary compatibility across gawk releases as long as the API
itself does not change.
• The API should enable extensions written in C or C++ to have roughly the same
“appearance” to awk-level code as awk functions do. This means that extensions should
have:− The ability to access function parameters.
4

A critical central data structure inside gawk.

Appendix C: Implementation Notes

523− The ability to turn an undefined parameter into an array (call by reference).
− The ability to create, access and update global variables.
− Easy access to all the elements of an array at once (“array flattening”) in order toloop over all the element in an easy fashion for C code.
− The ability to create arrays (including gawk’s true arrays of arrays).
Some additional important goals were:• The API should use only features in ISO C 90, so that extensions can be written using
the widest range of C and C++ compilers. The header should include the appropriate‘#ifdef __cplusplus’ and ‘extern "C"’ magic so that a C++ compiler could be used.
(If using C++, the runtime system has to be smart enough to call any constructors anddestructors, as gawk is a C program. As of this writing, this has not been tested.)• The API mechanism should not require access to gawk’s symbols5 by the compiletime or dynamic linker, in order to enable creation of extensions that also work on
MS-Windows.
During development, it became clear that there were other features that should beavailable to extensions, which were also subsequently provided:• Extensions should have the ability to hook into gawk’s I/O redirection mechanism. In
particular, the xgawk developers provided a so-called “open hook” to take over reading
records. During development, this was generalized to allow extensions to hook into
input processing, output processing, and two-way I/O.
• An extension should be able to provide a “call back” function to perform cleanup
actions when gawk exits.
• An extension should be able to provide a version string so that gawk’s --version option
can provide information about extensions as well.
The requirement to avoid access to gawk’s symbols is, at first glance, a difficult one tomeet.
One design, apparently used by Perl and Ruby and maybe others, would be to make themainline gawk code into a library, with the gawk utility a small C main() function linked
against the library.
This seemed like the tail wagging the dog, complicating build and installation and making
a simple copy of the gawk executable from one system to another (or one place to anotheron the same system!) into a chancy operation.
Pat Rankin suggested the solution that was adopted. See Section 17.3 [How It Works at
a High Level], page 405, for the details.

C.5.3 Other Design Decisions
As an arbitrary design decision, extensions can read the values of predefined variables andarrays (such as ARGV and FS), but cannot change them, with the exception of PROCINFO.
The reason for this is to prevent an extension function from affecting the flow of an awk
program outside its control. While a real awk function can do what it likes, that is at the5

The symbols are the variables and functions defined inside gawk. Access to these symbols by code
external to gawk loaded dynamically at runtime is problematic on MS-Windows.

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discretion of the programmer. An extension function should provide a service or make a C
API available for use within awk, and not mess with FS or ARGC and ARGV.
In addition, it becomes easy to start down a slippery slope. How much access to gawk
facilities do extensions need? Do they need getline? What about calling gsub() orcompiling regular expressions? What about calling into awk functions? (That would be
messy.)In order to avoid these issues, the gawk developers chose to start with the simplest, mostbasic features that are still truly useful.
Another decision is that although gawk provides nice things like MPFR, and arrays
indexed internally by integers, these features are not being brought out to the API inorder to keep things simple and close to traditional awk semantics. (In fact, arrays indexedinternally by integers are so transparent that they aren’t even documented!)
Additionally, all functions in the API check that their pointer input parameters are not
NULL. If they are, they return an error. (It is a good idea for extension code to verify thatpointers received from gawk are not NULL. Such a thing should not happen, but the gawk
developers are only human, and they have been known to occasionally make mistakes.)With time, the API will undoubtedly evolve; the gawk developers expect this to be driven
by user needs. For now, the current API seems to provide a minimal yet powerful set offeatures for creating extensions.

C.5.4 Room For Future Growth
The API can later be expanded, in at least the following way:
• gawk passes an “extension id” into the extension when it first loads the extension. The
extension then passes this id back to gawk with each function call. This mechanism
allows gawk to identify the extension calling into it, should it need to know.
Of course, as of this writing, no decisions have been made with respect to the above.

C.6 Summary• gawk’s extensions can be disabled with either the --traditional option or with the--posix option. The --parsedebug option is available if gawk is compiled with‘-DDEBUG’.
• The source code for gawk is maintained in a publicly accessible Git repository. Anyone
may check it out and view the source.
• Contributions to gawk are welcome. Following the steps outlined in this chapter will
make it easier to integrate your contributions into the code base. This applies both to
new feature contributions and to ports to additional operating systems.
• gawk has some limits—generally those that are imposed by the machine architecture.
• The extension API design was intended to solve a number of problems with the previous
extension mechanism, enable features needed by the xgawk project, and provide binary
compatibility going forward.
• The previous extension mechanism is no longer supported and was removed from thecode base with the 4.2 release.

Appendix D: Basic Programming Concepts 525

Appendix D Basic Programming Concepts
This appendix attempts to define some of the basic concepts and terms that are usedthroughout the rest of this book. As this book is specifically about awk, and not about
computer programming in general, the coverage here is by necessity fairly cursory andsimplistic. (If you need more background, there are many other introductory texts that you
should refer to instead.)

D.1 What a Program DoesAt the most basic level, the job of a program is to process some input data and produce
results. See Figure D.1.

Data

ProgramResults

Figure D.1: General Program Flow
The “program” in the figure can be either a compiled program1 (such as ls), or it may
be interpreted. In the latter case, a machine-executable program such as awk reads yourprogram, and then uses the instructions in your program to process the data.
When you write a program, it usually consists of the following, very basic set of steps,
as shown in Figure D.2:

Initialization

More
Data
?

No

Clean Up

Yes

Process

Figure D.2: Basic Program Steps
1

Compiled programs are typically written in lower-level languages such as C, C++, or Ada, and thentranslated, or compiled, into a form that the computer can execute directly.

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Initialization
These are the things you do before actually starting to process data, such as
checking arguments, initializing any data you need to work with, and so on.
This step corresponds to awk’s BEGIN rule (see Section 7.1.4 [The BEGIN and
END Special Patterns], page 150).
If you were baking a cake, this might consist of laying out all the mixing bowls
and the baking pan, and making sure you have all the ingredients that you
need.
Processing This is where the actual work is done. Your program reads data, one logicalchunk at a time, and processes it as appropriate.
In most programming languages, you have to manually manage the readingof data, checking to see if there is more each time you read a chunk. awk’s
pattern-action paradigm (see Chapter 1 [Getting Started with awk], page 17)
handles the mechanics of this for you.
In baking a cake, the processing corresponds to the actual labor: breaking eggs,
mixing the flour, water, and other ingredients, and then putting the cake into
the oven.
Clean Up

Once you’ve processed all the data, you may have things you need to do before
exiting. This step corresponds to awk’s END rule (see Section 7.1.4 [The BEGIN
and END Special Patterns], page 150).
After the cake comes out of the oven, you still have to wrap it in plastic wrap
to keep anyone from tasting it, as well as wash the mixing bowls and utensils.

An algorithm is a detailed set of instructions necessary to accomplish a task, or process
data. It is much the same as a recipe for baking a cake. Programs implement algorithms.
Often, it is up to you to design the algorithm and implement it, simultaneously.
The “logical chunks” we talked about previously are called records, similar to the records
a company keeps on employees, a school keeps for students, or a doctor keeps for patients.
Each record has many component parts, such as first and last names, date of birth, address,
and so on. The component parts are referred to as the fields of the record.
The act of reading data is termed input, and that of generating results, not too surprisingly, is termed output. They are often referred to together as “input/output,” and evenmore often, as “I/O” for short. (You will also see “input” and “output” used as verbs.)
awk manages the reading of data for you, as well as the breaking it up into records andfields. Your program’s job is to tell awk what to do with the data. You do this by describing
patterns in the data to look for, and actions to execute when those patterns are seen. This
data-driven nature of awk programs usually makes them both easier to write and easier toread.

D.2 Data Values in a Computer
In a program, you keep track of information and values in things called variables. A variable
is just a name for a given value, such as first_name, last_name, address, and so on. awk
has several predefined variables, and it has special names to refer to the current input record
and the fields of the record. You may also group multiple associated values under one name,as an array.

Appendix D: Basic Programming Concepts 527

Data, particularly in awk, consists of either numeric values, such as 42 or 3.1415927,
or string values. String values are essentially anything that’s not a number, such as aname. Strings are sometimes referred to as character data, since they store the individual
characters that comprise them. Individual variables, as well as numeric and string variables,
are referred to as scalar values. Groups of values, such as arrays, are not scalars.
Section 16.1 [A General Description of Computer Arithmetic], page 389, provided abasic introduction to numeric types (integer and floating-point) and how they are used
in a computer. Please review that information, including a number of caveats that were
presented.
While you are probably used to the idea of a number without a value (i.e., zero), it takes
a bit more getting used to the idea of zero-length character data. Nevertheless, such a thing
exists. It is called the null string. The null string is character data that has no value. In
other words, it is empty. It is written in awk programs like this: "".
Humans are used to working in decimal; i.e., base 10. In base 10, numbers go from 0 to
9, and then “roll over” into the next column. (Remember grade school? 42 = 4 × 10 + 2.)
There are other number bases though. Computers commonly use base 2 or binary, base8 or octal, and base 16 or hexadecimal. In binary, each column represents two times thevalue in the column to its right. Each column may contain either a 0 or a 1. Thus, binary
1010 represents (1 × 8) + (0 × 4) + (1 × 2) + (0 × 1), or decimal 10. Octal and hexadecimal
are discussed more in Section 6.1.1.2 [Octal and Hexadecimal Numbers], page 120.
At the very lowest level, computers store values as groups of binary digits, or bits.
Modern computers group bits into groups of eight, called bytes. Advanced applications
sometimes have to manipulate bits directly, and gawk provides functions for doing so.
Programs are written in programming languages. Hundreds, if not thousands, of programming languages exist. One of the most popular is the C programming language. The
C language had a very strong influence on the design of the awk language.
There have been several versions of C. The first is often referred to as “K&R” C, after
the initials of Brian Kernighan and Dennis Ritchie, the authors of the first book on C.
(Dennis Ritchie created the language, and Brian Kernighan was one of the creators of awk.)
In the mid-1980s, an effort began to produce an international standard for C. This work
culminated in 1989, with the production of the ANSI standard for C. This standard becamean ISO standard in 1990. In 1999, a revised ISO C standard was approved and released.
Where it makes sense, POSIX awk is compatible with 1999 ISO C.

Glossary

529

Glossary
ActionA series of awk statements attached to a rule. If the rule’s pattern matches
an input record, awk executes the rule’s action. Actions are always enclosed inbraces. (See Section 7.3 [Actions], page 153.)

Ada

A programming language originally defined by the U.S. Department of Defense
for embedded programming. It was designed to enforce good Software Engineering practices.

Amazing awk Assembler
Henry Spencer at the University of Toronto wrote a retargetable assembler
completely as sed and awk scripts. It is thousands of lines long, including
machine descriptions for several eight-bit microcomputers. It is a good example
of a program that would have been better written in another language.
Amazingly Workable Formatter (awf)Henry Spencer at the University of Toronto wrote a formatter that accepts alarge subset of the ‘nroff -ms’ and ‘nroff -man’ formatting commands, using
awk and sh.
Anchor

The regexp metacharacters ‘^’ and ‘$’, which force the match to the beginningor end of the string, respectively.

ANSI

The American National Standards Institute. This organization produces many
standards, among them the standards for the C and C++ programming languages. These standards often become international standards as well. See also“ISO.”

Argument An argument can be two different things. It can be an option or a file namepassed to a command while invoking it from the command line, or it can besomething passed to a function inside a program, e.g. inside awk.
In the latter case, an argument can be passed to a function in two ways. Either it
is given to the called function by value, i.e., a copy of the value of the variable
is made available to the called function, but the original variable cannot be
modified by the function itself; or it is given by reference, i.e., a pointer to theinterested variable is passed to the function, which can then directly modify
it. In awk scalars are passed by value, and arrays are passed by reference. See“Pass By Value/Reference.”Array

A grouping of multiple values under the same name. Most languages just provide sequential arrays. awk provides associative arrays.

Assertion

A statement in a program that a condition is true at this point in the program.
Useful for reasoning about how a program is supposed to behave.

Assignment
An awk expression that changes the value of some awk variable or data object.
An object that you can assign to is called an lvalue. The assigned values arecalled rvalues. See Section 6.2.3 [Assignment Expressions], page 130.

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Associative Array
Arrays in which the indices may be numbers or strings, not just sequential
integers in a fixed range.
awk Language
The language in which awk programs are written.
awk Program
An awk program consists of a series of patterns and actions, collectively known
as rules. For each input record given to the program, the program’s rules are
all processed in turn. awk programs may also contain function definitions.
awk Script Another name for an awk program.
Bash

The GNU version of the standard shell (the Bourne-Again SHell). See also
“Bourne Shell.”

BinaryBase-two notation, where the digits are 0–1. Since electronic circuitry works“naturally” in base 2 (just think of Off/On), everything inside a computer iscalculated using base 2. Each digit represents the presence (or absence) of apower of 2 and is called a bit. So, for example, the base-two number 10101 is
the same as decimal 21, ((1 × 16) + (1 × 4) + (1 × 1)).
Since base-two numbers quickly become very long to read and write, they areusually grouped by 3 (i.e., they are read as octal numbers), or by 4 (i.e., they areread as hexadecimal numbers). There is no direct way to insert base 2 numbers
in a C program. If need arises, such numbers are usually inserted as octal orhexadecimal numbers. The number of base-two digits that fit into registers
used for representing integer numbers in computers is a rough indication of
the computing power of the computer itself. Most computers nowadays use 64
bits for representing integer numbers in their registers, but 32-bit, 16-bit and8-bit registers have been widely used in the past. See Section 6.1.1.2 [Octal andHexadecimal Numbers], page 120.

Bit

Short for “Binary Digit.” All values in computer memory ultimately reduce
to binary digits: values that are either zero or one. Groups of bits may beinterpreted differently—as integers, floating-point numbers, character data, addresses of other memory objects, or other data. awk lets you work with floatingpoint numbers and strings. gawk lets you manipulate bit values with the built-infunctions described in Section 9.1.7 [Bit-Manipulation Functions], page 216.
Computers are often defined by how many bits they use to represent integer
values. Typical systems are 32-bit systems, but 64-bit systems are becoming
increasingly popular, and 16-bit systems have essentially disappeared.

Boolean Expression
Named after the English mathematician Boole. See also “Logical Expression.”
Bourne Shell
The standard shell (/bin/sh) on Unix and Unix-like systems, originally writtenby Steven R. Bourne at Bell Laboratories. Many shells (Bash, ksh, pdksh, zsh)
are generally upwardly compatible with the Bourne shell.

Glossary

Braces531

The characters ‘{’ and ‘}’. Braces are used in awk for delimiting actions, compound statements, and function bodies.

Bracket Expression
Inside a regular expression, an expression included in square brackets, meant
to designate a single character as belonging to a specified character class. A
bracket expression can contain a list of one or more characters, like ‘[abc]’, arange of characters, like ‘[A-Z]’, or a name, delimited by ‘:’, that designates aknown set of characters, like ‘[:digit:]’. The form of bracket expression enclosed between ‘:’ is independent of the underlying representation of the character themselves, which could utilize the ASCII, EBCDIC, or Unicode codesets,
depending on the architecture of the computer system, and on localization. See
also “Regular Expression.”
Built-in Function
The awk language provides built-in functions that perform various numerical,
I/O-related, and string computations. Examples are sqrt() (for the squareroot of a number) and substr() (for a substring of a string). gawk provides
functions for timestamp management, bit manipulation, array sorting, typechecking, and runtime string translation. (See Section 9.1 [Built-in Functions],
page 193.)
Built-in Variable
ARGC, ARGV, CONVFMT, ENVIRON, FILENAME, FNR, FS, NF, NR, OFMT, OFS, ORS,RLENGTH, RSTART, RS, and SUBSEP are the variables that have special meaning
to awk. In addition, ARGIND, BINMODE, ERRNO, FIELDWIDTHS, FPAT, IGNORECASE,
LINT, PROCINFO, RT, and TEXTDOMAIN are the variables that have special meaning to gawk. Changing some of them affects awk’s running environment. (See
Section 7.5 [Predefined Variables], page 163.)
C

The system programming language that most GNU software is written in. The
awk programming language has C-like syntax, and this book points out similarities between awk and C when appropriate.
In general, gawk attempts to be as similar to the 1990 version of ISO C as makes
sense.

C Shell

The C Shell (csh or its improved version, tcsh) is a Unix shell that was createdby Bill Joy in the late 1970s. The C shell was differentiated from other shells
by its interactive features and overall style, which looks more like C. The C
Shell is not backward compatible with the Bourne Shell, so special attention isrequired when converting scripts written for other Unix shells to the C shell,
especially with regard to the management of shell variables. See also “Bourne
Shell.”C++

A popular object-oriented programming language derived from C.

Character Class
See “Bracket Expression.”
Character List
See “Bracket Expression.”

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Character Set
The set of numeric codes used by a computer system to represent the characters (letters, numbers, punctuation, etc.) of a particular country or place. The
most common character set in use today is ASCII (American Standard Code forInformation Interchange). Many European countries use an extension of ASCII
known as ISO-8859-1 (ISO Latin-1). The Unicode character set is increasingly
popular and standard, and is particularly widely used on GNU/Linux systems.
CHEM

A preprocessor for pic that reads descriptions of molecules and produces pic
input for drawing them. It was written in awk by Brian Kernighan and Jon
Bentley, and is available from http://netlib.org/typesetting/chem.

Comparison Expression
A relation that is either true or false, such as ‘a < b’. Comparison expressions
are used in if, while, do, and for statements, and in patterns to select which
input records to process. (See Section 6.3.2 [Variable Typing and Comparison
Expressions], page 135.)
Compiler

A program that translates human-readable source code into machine-executableobject code. The object code is then executed directly by the computer. See
also “Interpreter.”Complemented Bracket Expression
The negation of a bracket expression. All that is not described by a given
bracket expression. The symbol ‘^’ precedes the negated bracket expression.
E.g.: ‘[^[:digit:]]’ designates whatever character is not a digit. ‘[^bad]’designates whatever character is not one of the letters ‘b’, ‘a’, or ‘d’. See“Bracket Expression.”
Compound Statement
A series of awk statements, enclosed in curly braces. Compound statements
may be nested. (See Section 7.4 [Control Statements in Actions], page 154.)
Computed Regexps
See “Dynamic Regular Expressions.”
Concatenation
Concatenating two strings means sticking them together, one after another,producing a new string. For example, the string ‘foo’ concatenated with thestring ‘bar’ gives the string ‘foobar’. (See Section 6.2.2 [String Concatenation],
page 129.)
Conditional Expression
An expression using the ‘?:’ ternary operator, such as ‘expr1 ? expr2 : expr3’.
The expression expr1 is evaluated; if the result is true, the value of the wholeexpression is the value of expr2; otherwise the value is expr3. In either case,only one of expr2 and expr3 is evaluated. (See Section 6.3.4 [Conditional Expressions], page 141.)
Control Statement
A control statement is an instruction to perform a given operation or a setof operations inside an awk program, if a given condition is true. Control

Glossary

533

statements are: if, for, while, and do (see Section 7.4 [Control Statements in
Actions], page 154).
CookieA peculiar goodie, token, saying or remembrance produced by or presented toa program. (With thanks to Professor Doug McIlroy.)

Coprocess A subordinate program with which two-way communications is possible.
Curly Braces
See “Braces.”
Dark Corner
An area in the language where specifications often were (or still are) not clear,
leading to unexpected or undesirable behavior. Such areas are marked in thisbook with the picture of a flashlight in the margin and are indexed under theheading “dark corner.”
Data Driven
A description of awk programs, where you specify the data you are interested
in processing, and what to do when that data is seen.
Data Objects
These are numbers and strings of characters. Numbers are converted into stringsand vice versa, as needed. (See Section 6.1.4 [Conversion of Strings and Numbers], page 126.)
Deadlock

The situation in which two communicating processes are each waiting for theother to perform an action.

Debugger

A program used to help developers remove “bugs” from (de-bug) their programs.

Double Precision
An internal representation of numbers that can have fractional parts. Double
precision numbers keep track of more digits than do single precision numbers,
but operations on them are sometimes more expensive. This is the way awk
stores numeric values. It is the C type double.
Dynamic Regular Expression
A dynamic regular expression is a regular expression written as an ordinary
expression. It could be a string constant, such as "foo", but it may also be an
expression whose value can vary. (See Section 3.6 [Using Dynamic Regexps],
page 60.)
Empty String
See “Null String.”
Environment
A collection of strings, of the form ‘name=val’, that each program has available
to it. Users generally place values into the environment in order to provide information to various programs. Typical examples are the environment variables
HOME and PATH.

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Epoch

The date used as the “beginning of time” for timestamps. Time values inmost systems are represented as seconds since the epoch, with library functions
available for converting these values into standard date and time formats.
The epoch on Unix and POSIX systems is 1970-01-01 00:00:00 UTC. See also
“GMT” and “UTC.”

Escape Sequences
A special sequence of characters used for describing nonprinting characters,such as ‘\n’ for newline or ‘\033’ for the ASCII ESC (Escape) character. (See
Section 3.2 [Escape Sequences], page 52.)
Extension An additional feature or change to a programming language or utility not defined by that language’s or utility’s standard. gawk has (too) many extensions
over POSIX awk.
FDL

See “Free Documentation License.”

Field

When awk reads an input record, it splits the record into pieces separated
by whitespace (or by a separator regexp that you can change by setting thepredefined variable FS). Such pieces are called fields. If the pieces are of fixed
length, you can use the built-in variable FIELDWIDTHS to describe their lengths.
If you wish to specify the contents of fields instead of the field separator, you
can use the predefined variable FPAT to do so. (See Section 4.5 [Specifying
How Fields Are Separated], page 73, Section 4.6 [Reading Fixed-Width Data],
page 80, and Section 4.7 [Defining Fields by Content], page 83.)

Flag

A variable whose truth value indicates the existence or nonexistence of somecondition.

Floating-Point Number
Often referred to in mathematical terms as a “rational” or real number, this isjust a number that can have a fractional part. See also “Double Precision” and“Single Precision.”
FormatFormat strings control the appearance of output in the strftime() andsprintf() functions, and in the printf statement as well. Also, data
conversions from numbers to strings are controlled by the format strings
contained in the predefined variables CONVFMT and OFMT. (See Section 5.5.2[Format-Control Letters], page 103.)

Fortran

Shorthand for FORmula TRANslator, one of the first programming languages
available for scientific calculations. It was created by John Backus, and has
been available since 1957. It is still in use today.

Free Documentation LicenseThis document describes the terms under which this book is published and may
be copied. (See [GNU Free Documentation License], page 553.)
Free Software Foundation
A nonprofit organization dedicated to the production and distribution of freely
distributable software. It was founded by Richard M. Stallman, the authorof the original Emacs editor. GNU Emacs is the most widely used version of
Emacs today.

Glossary

535

FSF

See “Free Software Foundation.”

Function

A part of an awk program that can be invoked from every point of the program,to perform a task. awk has several built-in functions. Users can define theirown functions in every part of the program. Function can be recursive, i.e.,they may invoke themselves. See Chapter 9 [Functions], page 193. In gawk it isalso possible to have functions shared among different programs, and included
where required using the @include directive (see Section 2.7 [Including Other
Files into Your Program], page 46). In gawk the name of the function thatshould be invoked can be generated at run time, i.e., dynamically. The gawk
extension API provides constructor functions (see Section 17.4.4 [Constructor
Functions], page 415).

gawk

The GNU implementation of awk.

General Public LicenseThis document describes the terms under which gawk and its source code may
be distributed. (See [GNU General Public License], page 541.)
GMT“Greenwich Mean Time.” This is the old term for UTC. It is the time of day
used internally for Unix and POSIX systems. See also “Epoch” and “UTC.”

GNU“GNU’s not Unix”. An on-going project of the Free Software Foundation tocreate a complete, freely distributable, POSIX-compliant computing environment.

GNU/Linux
A variant of the GNU system using the Linux kernel, instead of the Free Software Foundation’s Hurd kernel. The Linux kernel is a stable, efficient, fullfeatured clone of Unix that has been ported to a variety of architectures. It is
most popular on PC-class systems, but runs well on a variety of other systems
too. The Linux kernel source code is available under the terms of the GNU
General Public License, which is perhaps its most important aspect.
GPL

See “General Public License.”

HexadecimalBase 16 notation, where the digits are 0–9 and A–F, with ‘A’ representing 10, ‘B’representing 11, and so on, up to ‘F’ for 15. Hexadecimal numbers are written
in C using a leading ‘0x’, to indicate their base. Thus, 0x12 is 18 ((1 × 16) + 2).
See Section 6.1.1.2 [Octal and Hexadecimal Numbers], page 120.
I/O

Abbreviation for “Input/Output,” the act of moving data into and/or out of arunning program.

Input Record
A single chunk of data that is read in by awk. Usually, an awk input record
consists of one line of text. (See Section 4.1 [How Input Is Split into Records],
page 65.)
Integer

A whole number, i.e., a number that does not have a fractional part.

Internationalization
The process of writing or modifying a program so that it can use multiple
languages without requiring further source code changes.

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Interpreter
A program that reads human-readable source code directly, and uses the instructions in it to process data and produce results. awk is typically (but notalways) implemented as an interpreter. See also “Compiler.”Interval Expression
A component of a regular expression that lets you specify repeated matches ofsome part of the regexp. Interval expressions were not originally available inawk programs.
ISO

The International Organization for Standardization. This organization produces international standards for many things, including programming languages, such as C and C++. In the computer arena, important standards likethose for C, C++, and POSIX become both American national and ISO international standards simultaneously. This book refers to Standard C as “ISO C”
throughout. See the ISO website for more information about the name of theorganization and its language-independent three-letter acronym.

Java

A modern programming language originally developed by Sun Microsystems(now Oracle) supporting Object-Oriented programming. Although usually implemented by compiling to the instructions for a standard virtual machine (theJVM), the language can be compiled to native code.

Keyword

In the awk language, a keyword is a word that has special meaning. Keywords
are reserved and may not be used as variable names.
gawk’s keywords are: BEGIN, BEGINFILE, END, ENDFILE, break, case, continue,default, delete, do...while, else, exit, for...in, for, function, func,
if, next, nextfile, switch, and while.

Korn Shell
The Korn Shell (ksh) is a Unix shell which was developed by David Korn at Bell
Laboratories in the early 1980s. The Korn Shell is backward-compatible with
the Bourne shell and includes many features of the C shell. See also “Bourne
Shell.”
Lesser General Public LicenseThis document describes the terms under which binary library archives orshared objects, and their source code may be distributed.
LGPL

See “Lesser General Public License.”Linux

See “GNU/Linux.”

Localization
The process of providing the data necessary for an internationalized program
to work in a particular language.
Logical Expression
An expression using the operators for logic, AND, OR, and NOT, written ‘&&’,‘||’, and ‘!’ in awk. Often called Boolean expressions, after the mathematician
who pioneered this kind of mathematical logic.

Glossary

537

LvalueAn expression that can appear on the left side of an assignment operator. In
most languages, lvalues can be variables or array elements. In awk, a field
designator can also be used as an lvalue.

Matching

The act of testing a string against a regular expression. If the regexp describes
the contents of the string, it is said to match it.

Metacharacters
Characters used within a regexp that do not stand for themselves. Instead,they denote regular expression operations, such as repetition, grouping, or alternation.
Nesting

Nesting is where information is organized in layers, or where objects containother similar objects. In gawk the @include directive can be nested. The “natural” nesting of arithmetic and logical operations can be changed using parentheses (see Section 6.5 [Operator Precedence (How Operators Nest)], page 144).

No-op

An operation that does nothing.

Null String
A string with no characters in it. It is represented explicitly in awk programsby placing two double quote characters next to each other (""). It can appear
in input data by having two successive occurrences of the field separator appear
next to each other.
Number

A numeric-valued data object. Modern awk implementations use double precision floating-point to represent numbers. Ancient awk implementations usedsingle precision floating-point.

Octal

Base-eight notation, where the digits are 0–7. Octal numbers are written in C
using a leading ‘0’, to indicate their base. Thus, 013 is 11 ((1 × 8) + 3). See
Section 6.1.1.2 [Octal and Hexadecimal Numbers], page 120.

Output Record
A single chunk of data that is written out by awk. Usually, an awk output
record consists of one or more lines of text. See Section 4.1 [How Input Is Split
into Records], page 65.
Pattern

Patterns tell awk which input records are interesting to which rules.
A pattern is an arbitrary conditional expression against which input is tested.
If the condition is satisfied, the pattern is said to match the input record. A
typical pattern might compare the input record against a regular expression.
(See Section 7.1 [Pattern Elements], page 147.)

PEBKAC An acronym describing what is possibly the most frequent source of computer
usage problems. (Problem Exists Between Keyboard And Chair.)
Plug-inSee “Extensions.”

POSIX

The name for a series of standards that specify a Portable Operating Systeminterface. The “IX” denotes the Unix heritage of these standards. The mainstandard of interest for awk users is IEEE Standard for Information Technology,
Standard 1003.1TM -2017 (Revision of IEEE Std 1003.1-2008). The 2018 POSIX

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standard can be found online at https://pubs.opengroup.org/onlinepubs/
9699919799/.
Precedence
The order in which operations are performed when operators are used without
explicit parentheses.
PrivateVariables and/or functions that are meant for use exclusively by library functions and not for the main awk program. Special care must be taken whennaming such variables and functions. (See Section 10.1 [Naming Library Function Global Variables], page 242.)

Range (of input lines)
A sequence of consecutive lines from the input file(s). A pattern can specify
ranges of input lines for awk to process or it can specify single lines. (See
Section 7.1 [Pattern Elements], page 147.)
RecordSee “Input record” and “Output record.”

Recursion When a function calls itself, either directly or indirectly. If this is clear, stop,and proceed to the next entry. Otherwise, refer to the entry for “recursion.”
Redirection
Redirection means performing input from something other than the standard
input stream, or performing output to something other than the standard output stream.
You can redirect input to the getline statement using the ‘<’, ‘|’, and ‘|&’operators. You can redirect the output of the print and printf statements toa file or a system command, using the ‘>’, ‘>>’, ‘|’, and ‘|&’ operators. (See
Section 4.10 [Explicit Input with getline], page 89, and Section 5.6 [Redirecting Output of print and printf], page 108.)
Reference Counts
An internal mechanism in gawk to minimize the amount of memory needed
to store the value of string variables. If the value assumed by a variable isused in more than one place, only one copy of the value itself is kept, andthe associated reference count is increased when the same value is used by anadditional variable, and decreased when the related variable is no longer in use.
When the reference count goes to zero, the memory space used to store thevalue of the variable is freed.
RegexpSee “Regular Expression.”

Regular Expression
A regular expression (“regexp” for short) is a pattern that denotes a set of
strings, possibly an infinite set. For example, the regular expression ‘R.*xp’
matches any string starting with the letter ‘R’ and ending with the letters ‘xp’.
In awk, regular expressions are used in patterns and in conditional expressions.
Regular expressions may contain escape sequences. (See Chapter 3 [Regular
Expressions], page 51.)Glossary

539

Regular Expression ConstantA regular expression constant is a regular expression written within slashes, suchas /foo/. This regular expression is chosen when you write the awk programand cannot be changed during its execution. (See Section 3.1 [How to UseRegular Expressions], page 51.)
Regular Expression Operators
See “Metacharacters.”
Rounding

Rounding the result of an arithmetic operation can be tricky. More than one
way of rounding exists, and in gawk it is possible to choose which methodshould be used in a program. See Section 16.4.5 [Setting the Rounding Mode],
page 398.

Rule

A segment of an awk program that specifies how to process single input records.
A rule consists of a pattern and an action. awk reads an input record; then, foreach rule, if the input record satisfies the rule’s pattern, awk executes the rule’s
action. Otherwise, the rule does nothing for that input record.

RvalueA value that can appear on the right side of an assignment operator. In awk,
essentially every expression has a value. These values are rvalues.

ScalarA single value, be it a number or a string. Regular variables are scalars; arrays
and functions are not.

Search Path
In gawk, a list of directories to search for awk program source files. In the shell,a list of directories to search for executable programs.
sed

See “Stream Editor.”

Seed

The initial value, or starting point, for a sequence of random numbers.

Shell

The command interpreter for Unix and POSIX-compliant systems. The shellworks both interactively, and as a programming language for batch files, or shell
scripts.

Short-Circuit
The nature of the awk logical operators ‘&&’ and ‘||’. If the value of the entire expression is determinable from evaluating just the lefthand side of these
operators, the righthand side is not evaluated. (See Section 6.3.3 [Boolean
Expressions], page 140.)
Side EffectA side effect occurs when an expression has an effect aside from merely producing a value. Assignment expressions, increment and decrement expressions,and function calls have side effects. (See Section 6.2.3 [Assignment Expressions],
page 130.)
Single Precision
An internal representation of numbers that can have fractional parts. Single
precision numbers keep track of fewer digits than do double precision numbers,
but operations on them are sometimes less expensive in terms of CPU time.
This is the type used by some ancient versions of awk to store numeric values.
It is the C type float.

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GAWK: Effective AWK ProgrammingSpace

The character generated by hitting the space bar on the keyboard.

Special FileA file name interpreted internally by gawk, instead of being handed directly tothe underlying operating system—for example, /dev/stderr. (See Section 5.8[Special File names in gawk], page 112.)
Statement An expression inside an awk program in the action part of a pattern–action
rule, or inside an awk function. A statement can be a variable assignment, anarray operation, a loop, etc.
Stream Editor
A program that reads records from an input stream and processes them one ormore at a time. This is in contrast with batch programs, which may expect toread their input files in entirety before starting to do anything, as well as withinteractive programs which require input from the user.
StringA datum consisting of a sequence of characters, such as ‘I am a string’. Constant strings are written with double quotes in the awk language and may
contain escape sequences. (See Section 3.2 [Escape Sequences], page 52.)

Tab

The character generated by hitting the TAB key on the keyboard. It usually
expands to up to eight spaces upon output.

Text Domain
A unique name that identifies an application. Used for grouping messages thatare translated at runtime into the local language.
Timestamp
A value in the “seconds since the epoch” format used by Unix and POSIX
systems. Used for the gawk functions mktime(), strftime(), and systime().
See also “Epoch,” “GMT,” and “UTC.”
Unix

A computer operating system originally developed in the early 1970’s at AT&T
Bell Laboratories. It initially became popular in universities around the world
and later moved into commercial environments as a software development system and network server system. There are many commercial versions of Unix,
as well as several work-alike systems whose source code is freely available (suchas GNU/Linux, NetBSD, FreeBSD, and OpenBSD).

UTC

The accepted abbreviation for “Universal Coordinated Time.” This is standard
time in Greenwich, England, which is used as a reference time for day and datecalculations. See also “Epoch” and “GMT.”

Variable

A name for a value. In awk, variables may be either scalars or arrays.

Whitespace
A sequence of space, TAB, or newline characters occurring inside an input
record or a string.

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Each version is given a distinguishing version number. If the Program specifies thata certain numbered version of the GNU General Public License “or any later version”
applies to it, you have the option of following the terms and conditions either of that
numbered version or of any later version published by the Free Software Foundation.
If the Program does not specify a version number of the GNU General Public License,
you may choose any version ever published by the Free Software Foundation.
If the Program specifies that a proxy can decide which future versions of the GNU
General Public License can be used, that proxy’s public statement of acceptance of aversion permanently authorizes you to choose that version for the Program.
Later license versions may give you additional or different permissions. However, no
additional obligations are imposed on any author or copyright holder as a result of yourchoosing to follow a later version.
15. Disclaimer of Warranty.
THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT WHEN OTHERWISE STATED IN
WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES PROVIDETHE PROGRAM “AS IS” WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIEDWARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULARPURPOSE. THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCEOF THE PROGRAM IS WITH YOU. SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, REPAIR OR
CORRECTION.
16. Limitation of Liability.
IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO INWRITING WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO
MODIFIES AND/OR CONVEYS THE PROGRAM AS PERMITTED ABOVE, BE
LIABLE TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR
INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO
LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM
TO OPERATE WITH ANY OTHER PROGRAMS), EVEN IF SUCH HOLDER OROTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.
17. Interpretation of Sections 15 and 16.
If the disclaimer of warranty and limitation of liability provided above cannot be given
local legal effect according to their terms, reviewing courts shall apply local law thatmost closely approximates an absolute waiver of all civil liability in connection with
the Program, unless a warranty or assumption of liability accompanies a copy of theProgram in return for a fee.

GNU General Public License 551

END OF TERMS AND CONDITIONS
How to Apply These Terms to Your New Programs
If you develop a new program, and you want it to be of the greatest possible use to the public,
the best way to achieve this is to make it free software which everyone can redistribute andchange under these terms.
To do so, attach the following notices to the program. It is safest to attach them to thestart of each source file to most effectively state the exclusion of warranty; and each fileshould have at least the “copyright” line and a pointer to where the full notice is found.
one line to give the program's name and a brief idea of what it does.
Copyright (C) year name of author
This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or (atyour option) any later version.
This program is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
General Public License for more details.
You should have received a copy of the GNU General Public Licensealong with this program. If not, see https://www.gnu.org/licenses/.

Also add information on how to contact you by electronic and paper mail.
If the program does terminal interaction, make it output a short notice like this when it
starts in an interactive mode:
program Copyright (C) year name of author
This program comes with ABSOLUTELY NO WARRANTY; for details type ‘show w’.
This is free software, and you are welcome to redistribute it
under certain conditions; type ‘show c’ for details.

The hypothetical commands ‘show w’ and ‘show c’ should show the appropriate parts of
the General Public License. Of course, your program’s commands might be different; for a
GUI interface, you would use an “about box”.
You should also get your employer (if you work as a programmer) or school, if any, tosign a “copyright disclaimer” for the program, if necessary. For more information on this,and how to apply and follow the GNU GPL, see https://www.gnu.org/licenses/.
The GNU General Public License does not permit incorporating your program into proprietary programs. If your program is a subroutine library, you may consider it more usefulto permit linking proprietary applications with the library. If this is what you want to do,use the GNU Lesser General Public License instead of this License. But first, please read
https://www.gnu.org/philosophy/why-not-lgpl.html.

GNU Free Documentation License

553

GNU Free Documentation License
Version 1.3, 3 November 2008c
Copyright
2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.
https://fsf.org/
Everyone is permitted to copy and distribute verbatim copiesof this license document, but changing it is not allowed.
0. PREAMBLE
The purpose of this License is to make a manual, textbook, or other functional anduseful document free in the sense of freedom: to assure everyone the effective freedom
to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way
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This License is a kind of “copyleft”, which means that derivative works of the document
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License, which is a copyleft license designed for free software.
We have designed this License in order to use it for manuals for free software, because
free software needs free documentation: a free program should come with manuals
providing the same freedoms that the software does. But this License is not limited tosoftware manuals; it can be used for any textual work, regardless of subject matter orwhether it is published as a printed book. We recommend this License principally forworks whose purpose is instruction or reference.
1. APPLICABILITY AND DEFINITIONSThis License applies to any manual or other work, in any medium, that contains a
notice placed by the copyright holder saying it can be distributed under the termsof this License. Such a notice grants a world-wide, royalty-free license, unlimited in
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A “Modified Version” of the Document means any work containing the Document or
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relationship could be a matter of historical connection with the subject or with related
matters, or of legal, commercial, philosophical, ethical or political position regardingthem.
The “Invariant Sections” are certain Secondary Sections whose titles are designated, as
being those of Invariant Sections, in the notice that says that the Document is released

554

GAWK: Effective AWK Programmingunder this License. If a section does not fit the above definition of Secondary then it is
not allowed to be designated as Invariant. The Document may contain zero Invariant
Sections. If the Document does not identify any Invariant Sections then there are none.
The “Cover Texts” are certain short passages of text that are listed, as Front-CoverTexts or Back-Cover Texts, in the notice that says that the Document is released under
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be at most 25 words.
A “Transparent” copy of the Document means a machine-readable copy, represented
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Transparent file format whose markup, or absence of markup, has been arranged tothwart or discourage subsequent modification by readers is not Transparent. An image
format is not Transparent if used for any substantial amount of text. A copy that is
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Examples of suitable formats for Transparent copies include plain ascii without
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available DTD, and standard-conforming simple HTML, PostScript or PDF designed
for human modification. Examples of transparent image formats include PNG, XCF
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The “Title Page” means, for a printed book, the title page itself, plus such following
pages as are needed to hold, legibly, the material this License requires to appear in thetitle page. For works in formats which do not have any title page as such, “Title Page”
means the text near the most prominent appearance of the work’s title, preceding the
beginning of the body of the text.
The “publisher” means any person or entity that distributes copies of the Document
to the public.
A section “Entitled XYZ” means a named subunit of the Document whose title either
is precisely XYZ or contains XYZ in parentheses following text that translates XYZ in
another language. (Here XYZ stands for a specific section name mentioned below, suchas “Acknowledgements”, “Dedications”, “Endorsements”, or “History”.) To “Preserve
the Title” of such a section when you modify the Document means that it remains asection “Entitled XYZ” according to this definition.
The Document may include Warranty Disclaimers next to the notice which states that
this License applies to the Document. These Warranty Disclaimers are considered to
be included by reference in this License, but only as regards disclaiming warranties:
any other implication that these Warranty Disclaimers may have is void and has noeffect on the meaning of this License.
2. VERBATIM COPYING

GNU Free Documentation License

555

You may copy and distribute the Document in any medium, either commercially or
noncommercially, provided that this License, the copyright notices, and the license
notice saying this License applies to the Document are reproduced in all copies, andthat you add no other conditions whatsoever to those of this License. You may not use
technical measures to obstruct or control the reading or further copying of the copies
you make or distribute. However, you may accept compensation in exchange for copies.
If you distribute a large enough number of copies you must also follow the conditions
in section 3.
You may also lend copies, under the same conditions stated above, and you may publicly
display copies.
3. COPYING IN QUANTITY
If you publish printed copies (or copies in media that commonly have printed covers) of
the Document, numbering more than 100, and the Document’s license notice requires
Cover Texts, you must enclose the copies in covers that carry, clearly and legibly, allthese Cover Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on
the back cover. Both covers must also clearly and legibly identify you as the publisherof these copies. The front cover must present the full title with all words of the title
equally prominent and visible. You may add other material on the covers in addition.
Copying with changes limited to the covers, as long as they preserve the title of theDocument and satisfy these conditions, can be treated as verbatim copying in otherrespects.
If the required texts for either cover are too voluminous to fit legibly, you should put
the first ones listed (as many as fit reasonably) on the actual cover, and continue therest onto adjacent pages.
If you publish or distribute Opaque copies of the Document numbering more than 100,
you must either include a machine-readable Transparent copy along with each Opaquecopy, or state in or with each Opaque copy a computer-network location from which
the general network-using public has access to download using public-standard network
protocols a complete Transparent copy of the Document, free of added material. If
you use the latter option, you must take reasonably prudent steps, when you begin
distribution of Opaque copies in quantity, to ensure that this Transparent copy willremain thus accessible at the stated location until at least one year after the last time
you distribute an Opaque copy (directly or through your agents or retailers) of that
edition to the public.
It is requested, but not required, that you contact the authors of the Document wellbefore redistributing any large number of copies, to give them a chance to provide youwith an updated version of the Document.
4. MODIFICATIONS
You may copy and distribute a Modified Version of the Document under the conditionsof sections 2 and 3 above, provided that you release the Modified Version under precisely
this License, with the Modified Version filling the role of the Document, thus licensing
distribution and modification of the Modified Version to whoever possesses a copy of
it. In addition, you must do these things in the Modified Version:
A. Use in the Title Page (and on the covers, if any) a title distinct from that of theDocument, and from those of previous versions (which should, if there were any,

556

GAWK: Effective AWK Programming

be listed in the History section of the Document). You may use the same title asa previous version if the original publisher of that version gives permission.
B. List on the Title Page, as authors, one or more persons or entities responsible forauthorship of the modifications in the Modified Version, together with at least fiveof the principal authors of the Document (all of its principal authors, if it has fewer
than five), unless they release you from this requirement.
C. State on the Title page the name of the publisher of the Modified Version, as the
publisher.
D. Preserve all the copyright notices of the Document.
E. Add an appropriate copyright notice for your modifications adjacent to the othercopyright notices.
F. Include, immediately after the copyright notices, a license notice giving the public
permission to use the Modified Version under the terms of this License, in the formshown in the Addendum below.
G. Preserve in that license notice the full lists of Invariant Sections and required CoverTexts given in the Document’s license notice.
H. Include an unaltered copy of this License.
I. Preserve the section Entitled “History”, Preserve its Title, and add to it an item
stating at least the title, year, new authors, and publisher of the Modified Version
as given on the Title Page. If there is no section Entitled “History” in the Document, create one stating the title, year, authors, and publisher of the Document
as given on its Title Page, then add an item describing the Modified Version as
stated in the previous sentence.
J. Preserve the network location, if any, given in the Document for public access toa Transparent copy of the Document, and likewise the network locations given in
the Document for previous versions it was based on. These may be placed in the“History” section. You may omit a network location for a work that was publishedat least four years before the Document itself, or if the original publisher of theversion it refers to gives permission.
K. For any section Entitled “Acknowledgements” or “Dedications”, Preserve the Titleof the section, and preserve in the section all the substance and tone of each of thecontributor acknowledgements and/or dedications given therein.
L. Preserve all the Invariant Sections of the Document, unaltered in their text andin their titles. Section numbers or the equivalent are not considered part of thesection titles.
M. Delete any section Entitled “Endorsements”. Such a section may not be includedin the Modified Version.
N. Do not retitle any existing section to be Entitled “Endorsements” or to conflict intitle with any Invariant Section.
O. Preserve any Warranty Disclaimers.
If the Modified Version includes new front-matter sections or appendices that qualify
as Secondary Sections and contain no material copied from the Document, you may atyour option designate some or all of these sections as invariant. To do this, add theirGNU Free Documentation License

557

titles to the list of Invariant Sections in the Modified Version’s license notice. These
titles must be distinct from any other section titles.
You may add a section Entitled “Endorsements”, provided it contains nothing but
endorsements of your Modified Version by various parties—for example, statements ofpeer review or that the text has been approved by an organization as the authoritative
definition of a standard.
You may add a passage of up to five words as a Front-Cover Text, and a passage of up
to 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified
Version. Only one passage of Front-Cover Text and one of Back-Cover Text may be
added by (or through arrangements made by) any one entity. If the Document alreadyincludes a cover text for the same cover, previously added by you or by arrangement
made by the same entity you are acting on behalf of, you may not add another; but
you may replace the old one, on explicit permission from the previous publisher that
added the old one.
The author(s) and publisher(s) of the Document do not by this License give permission
to use their names for publicity for or to assert or imply endorsement of any Modified
Version.
5. COMBINING DOCUMENTS
You may combine the Document with other documents released under this License,
under the terms defined in section 4 above for modified versions, provided that you
include in the combination all of the Invariant Sections of all of the original documents,
unmodified, and list them all as Invariant Sections of your combined work in its license
notice, and that you preserve all their Warranty Disclaimers.
The combined work need only contain one copy of this License, and multiple identicalInvariant Sections may be replaced with a single copy. If there are multiple Invariant
Sections with the same name but different contents, make the title of each such section
unique by adding at the end of it, in parentheses, the name of the original author or
publisher of that section if known, or else a unique number. Make the same adjustment
to the section titles in the list of Invariant Sections in the license notice of the combinedwork.
In the combination, you must combine any sections Entitled “History” in the various original documents, forming one section Entitled “History”; likewise combine any
sections Entitled “Acknowledgements”, and any sections Entitled “Dedications”. You
must delete all sections Entitled “Endorsements.”
6. COLLECTIONS OF DOCUMENTS
You may make a collection consisting of the Document and other documents released
under this License, and replace the individual copies of this License in the various
documents with a single copy that is included in the collection, provided that you
follow the rules of this License for verbatim copying of each of the documents in allother respects.
You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted
document, and follow this License in all other respects regarding verbatim copying ofthat document.

558

GAWK: Effective AWK Programming

7. AGGREGATION WITH INDEPENDENT WORKS
A compilation of the Document or its derivatives with other separate and independent
documents or works, in or on a volume of a storage or distribution medium, is called
an “aggregate” if the copyright resulting from the compilation is not used to limit thelegal rights of the compilation’s users beyond what the individual works permit. When
the Document is included in an aggregate, this License does not apply to the otherworks in the aggregate which are not themselves derivative works of the Document.
If the Cover Text requirement of section 3 is applicable to these copies of the Document,
then if the Document is less than one half of the entire aggregate, the Document’s CoverTexts may be placed on covers that bracket the Document within the aggregate, or theelectronic equivalent of covers if the Document is in electronic form. Otherwise theymust appear on printed covers that bracket the whole aggregate.
8. TRANSLATION
Translation is considered a kind of modification, so you may distribute translationsof the Document under the terms of section 4. Replacing Invariant Sections withtranslations requires special permission from their copyright holders, but you may
include translations of some or all Invariant Sections in addition to the original versionsof these Invariant Sections. You may include a translation of this License, and all thelicense notices in the Document, and any Warranty Disclaimers, provided that you
also include the original English version of this License and the original versions of
those notices and disclaimers. In case of a disagreement between the translation andthe original version of this License or a notice or disclaimer, the original version will
prevail.
If a section in the Document is Entitled “Acknowledgements”, “Dedications”, or “History”, the requirement (section 4) to Preserve its Title (section 1) will typically requirechanging the actual title.
9. TERMINATION
You may not copy, modify, sublicense, or distribute the Document except as expressly
provided under this License. Any attempt otherwise to copy, modify, sublicense, or
distribute it is void, and will automatically terminate your rights under this License.
However, if you cease all violation of this License, then your license from a particular
copyright holder is reinstated (a) provisionally, unless and until the copyright holder
explicitly and finally terminates your license, and (b) permanently, if the copyrightholder fails to notify you of the violation by some reasonable means prior to 60 days
after the cessation.
Moreover, your license from a particular copyright holder is reinstated permanently if
the copyright holder notifies you of the violation by some reasonable means, this is thefirst time you have received notice of violation of this License (for any work) from thatcopyright holder, and you cure the violation prior to 30 days after your receipt of thenotice.
Termination of your rights under this section does not terminate the licenses of parties
who have received copies or rights from you under this License. If your rights havebeen terminated and not permanently reinstated, receipt of a copy of some or all of thesame material does not give you any rights to use it.

GNU Free Documentation License559

10. FUTURE REVISIONS OF THIS LICENSE
The Free Software Foundation may publish new, revised versions of the GNU Free
Documentation License from time to time. Such new versions will be similar in spirit
to the present version, but may differ in detail to address new problems or concerns.
See https://www.gnu.org/copyleft/.
Each version of the License is given a distinguishing version number. If the Document
specifies that a particular numbered version of this License “or any later version”
applies to it, you have the option of following the terms and conditions either of thatspecified version or of any later version that has been published (not as a draft) by
the Free Software Foundation. If the Document does not specify a version number ofthis License, you may choose any version ever published (not as a draft) by the Free
Software Foundation. If the Document specifies that a proxy can decide which futureversions of this License can be used, that proxy’s public statement of acceptance of aversion permanently authorizes you to choose that version for the Document.
11. RELICENSING“Massive Multiauthor Collaboration Site” (or “MMC Site”) means any World Wide
Web server that publishes copyrightable works and also provides prominent facilities
for anybody to edit those works. A public wiki that anybody can edit is an example ofsuch a server. A “Massive Multiauthor Collaboration” (or “MMC”) contained in thesite means any set of copyrightable works thus published on the MMC site.
“CC-BY-SA” means the Creative Commons Attribution-Share Alike 3.0 license published by Creative Commons Corporation, a not-for-profit corporation with a principal
place of business in San Francisco, California, as well as future copyleft versions of thatlicense published by that same organization.
“Incorporate” means to publish or republish a Document, in whole or in part, as partof another Document.
An MMC is “eligible for relicensing” if it is licensed under this License, and if all works
that were first published under this License somewhere other than this MMC, andsubsequently incorporated in whole or in part into the MMC, (1) had no cover texts
or invariant sections, and (2) were thus incorporated prior to November 1, 2008.
The operator of an MMC Site may republish an MMC contained in the site under
CC-BY-SA on the same site at any time before August 1, 2009, provided the MMC is
eligible for relicensing.

ADDENDUM: How to use this License for your documents
To use this License in a document you have written, include a copy of the License in the
document and put the following copyright and license notices just after the title page:
Copyright (C) year your name.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3
or any later version published by the Free Software Foundation;with no Invariant Sections, no Front-Cover Texts, and no Back-CoverTexts. A copy of the license is included in the section entitled ``GNUFree Documentation License''.

If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the“with. . . Texts.” line with this:

560

GAWK: Effective AWK Programmingwith the Invariant Sections being list their titles, with
the Front-Cover Texts being list, and with the Back-Cover Texts
being list.

If you have Invariant Sections without Cover Texts, or some other combination of thethree, merge those two alternatives to suit the situation.
If your document contains nontrivial examples of program code, we recommend releasingthese examples in parallel under your choice of free software license, such as the GNU
General Public License, to permit their use in free software.

Index 561

Index
!

*

! (exclamation point),! operator . . . . . . . . . . . . . . . . . . 141, 144, 149, 287!= operator . . . . . . . . . . . . . . . . . . . . . . . . . . . 138, 144!~ operator . . 51, 60, 63, 121, 138, 139, 145, 148* (asterisk),* operator,
as multiplication operator . . . . . . . . . . . . . . 144
as regexp operator . . . . . . . . . . . . . . . . . . . . . . 56
null strings, matching . . . . . . . . . . . . . . . . . . 205** operator . . . . . . . . . . . . . . . . . . . . . . . . . . . 129, 144**= operator. . . . . . . . . . . . . . . . . . . . . . . . . . 132, 145*= operator . . . . . . . . . . . . . . . . . . . . . . . . . . . 132, 145"" (double quote),
in regexp constants . . . . . . . . . . . . . . . . . . . . . . . . . 60
in shell commands . . . . . . . . . . . . . . . . . . . . . . . . . . 22#
# (number sign),#! (executable scripts) . . . . . . . . . . . . . . . . . . . . . . 19
commenting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20$
$ (dollar sign),$ field operator . . . . . . . . . . . . . . . . . . . . . . . . 69, 144
incrementing fields and arrays . . . . . . . . . . . . . 133
regexp operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55%
% (percent sign),% operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144%= operator . . . . . . . . . . . . . . . . . . . . . . . . . . . 132, 145&& (ampersand),&& operator . . . . . . . . . . . . . . . . . . . . . . . . . . . 141, 145gsub()/gensub()/sub() functions and . . . . . 205’' (single quote) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
in gawk command lines . . . . . . . . . . . . . . . . . . . . . 19
in shell commands . . . . . . . . . . . . . . . . . . . . . . . . . . 21
vs. apostrophe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
with double quotes . . . . . . . . . . . . . . . . . . . . . . . . . 22(() (parentheses),in a profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 350
regexp operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55+
+ (plus sign),+ operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144++ operator . . . . . . . . . . . . . . . . . . . . . . 133, 134, 144+= operator . . . . . . . . . . . . . . . . . . . . . . . . . . . 132, 145
regexp operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56,
, (comma), in range patterns . . . . . . . . . . . . . . . . . 149–- (hyphen),- operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144-- end of options marker . . . . . . . . . . . . . . . . . . . 39-- operator . . . . . . . . . . . . . . . . . . . . . . . . . . . 134, 144-= operator . . . . . . . . . . . . . . . . . . . . . . . . . . . 132, 145
file names beginning with . . . . . . . . . . . . . . . . . . . 34
in bracket expressions . . . . . . . . . . . . . . . . . . . . . . 58--assign option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34--bignum option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37--characters-as-bytes option . . . . . . . . . . . . . . . . 34--copyright option . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35--csv option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36--debug option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35--disable-extensions configuration option . . . 497--disable-lint configuration option. . . . . . . . . . 497--disable-mpfr configuration option. . . . . . . . . . 497--disable-nls configuration option . . . . . . . . . . . 497--dump-variables option . . . . . . . . . . . . . . . . . . . . . . 35
using for library functions . . . . . . . . . . . . . . . . . 242--enable-versioned-extension-dir configuration
option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 497--exec option. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35--field-separator option . . . . . . . . . . . . . . . . . . . . . 33--file option. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33--gen-pot option . . . . . . . . . . . . . . . . . . . . . . . . . 36, 361--help option. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36--include option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36--lint option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33, 37--lint-old option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39--load option. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

562

GAWK: Effective AWK Programming--no-optimize option . . . . . . . . . . . . . . . . . . . . . . . . . 39--non-decimal-data option . . . . . . . . . . . . . . . 37, 335
strtonum() function and . . . . . . . . . . . . . . . . . . 335--optimize option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38--posix option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38--traditional option and . . . . . . . . . . . . . . . . . . 38--pretty-print option . . . . . . . . . . . . . . . . . . . . . . . . 38--profile option . . . . . . . . . . . . . . . . . . . . . . . . . 38, 348--re-interval option . . . . . . . . . . . . . . . . . . . . . . . . . 39--sandbox option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
disabling system() function . . . . . . . . . . . . . . . 210
input redirection with getline. . . . . . . . . . . . . . 89
output redirection with print,
printf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108--source option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35--trace option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36--traditional option . . . . . . . . . . . . . . . . . . . . . . . . . 34--posix option and. . . . . . . . . . . . . . . . . . . . . . . . . 38--use-lc-numeric option . . . . . . . . . . . . . . . . . . . . . . 38--version option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
-b option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
-c option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
-C option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
-d option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
-D option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
-e option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35, 40-E option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
-f option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18, 33
multiple uses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
-F option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
-Ft sets FS to TAB . . . . . . . . . . . . . . . . . . . . . . . . . 39
command-line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
-g option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
-h option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
-i option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
-I option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
-k option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
-l option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
-L option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
-M option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
-n option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
-N option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
-o option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
-O option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
-p option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
-P option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
-r option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
-s option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
-S option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
-v option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34, 125-V option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
-W option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34.
. (period), regexp operator . . . . . . . . . . . . . . . . . . . . . 55
.gmo files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 358
specifying directory of . . . . . . . . . . . . . . . . 358, 360.mo files, converting from .po . . . . . . . . . . . . . . . . . 365.po files. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 358, 361
converting to .mo . . . . . . . . . . . . . . . . . . . . . . . . . . 365.pot files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357// (forward slash),/ operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144/= operator . . . . . . . . . . . . . . . . . . . . . . . . . . . 132, 145
vs. /=.../ regexp constant . . . . . . . . . . . . . 133
patterns and . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148
to enclose regular expressions . . . . . . . . . . . . . . . 51/= operator vs. /=.../ regexp constant. . . . . . . . 133/dev/... special files . . . . . . . . . . . . . . . . . . . . . . . . . 112/dev/fd/N special files (gawk) . . . . . . . . . . . . . . . . . 112/inet/... special files (gawk) . . . . . . . . . . . . . . . . . 346/inet4/... special files (gawk) . . . . . . . . . . . . . . . . 346/inet6/... special files (gawk) . . . . . . . . . . . . . . . . 346:: (colon),:: namespace separator . . . . . . . . . . . . . . . . . . . 383?: operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145;; (semicolon),
AWKPATH variable and . . . . . . . . . . . . . . . . . . . . . . 499
separating rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
separating statements in actions . . . 30, 153, 154<< (left angle bracket),< operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138, 144< operator (I/O). . . . . . . . . . . . . . . . . . . . . . . . . . . . 91<= operator . . . . . . . . . . . . . . . . . . . . . . . . . . . 138, 144== (equals sign),= operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130== operator . . . . . . . . . . . . . . . . . . . . . . . . . . . 138, 144>> (right angle bracket),> operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138, 144> operator (I/O) . . . . . . . . . . . . . . . . . . . . . . . . . . 108>= operator . . . . . . . . . . . . . . . . . . . . . . . . . . . 138, 144>> operator (I/O). . . . . . . . . . . . . . . . . . . . . 109, 144

Index 563?

\v escape sequence . . . . . . . . . . . . . . . . . . . . . . . . . 52
\w operator (gawk) . . . . . . . . . . . . . . . . . . . . . . . . . 61
\W operator (gawk) . . . . . . . . . . . . . . . . . . . . . . . . . 62
\x escape sequence . . . . . . . . . . . . . . . . . . . . . . . . . 52
\y operator (gawk) . . . . . . . . . . . . . . . . . . . . . . . . . 62
as field separator . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
continuing lines and . . . . . . . . . . . . . . . . . . . . . . . . 28
comments and . . . . . . . . . . . . . . . . . . . . . . . . . . 29
in csh . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
gsub()/gensub()/sub() functions and . . . . . 205
in bracket expressions . . . . . . . . . . . . . . . . . . . . . . 58
in escape sequences . . . . . . . . . . . . . . . . . . . . . 52, 53
POSIX and . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
in regexp constants . . . . . . . . . . . . . . . . . . . . . . . . . 60
in shell commands . . . . . . . . . . . . . . . . . . . . . . . . . . 21
regexp operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

? (question mark),?: operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145
regexp operator . . . . . . . . . . . . . . . . . . . . . . . . . 56, 62[[] (square brackets), regexp operator. . . . . . . . . . . 55^
^ (caret),^ operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144^= operator . . . . . . . . . . . . . . . . . . . . . . . . . . . 132, 145
in bracket expressions . . . . . . . . . . . . . . . . . . . . . . 58
in FS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
regexp operator . . . . . . . . . . . . . . . . . . . . . . . . . 55, 62_ (underscore),
C macro . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 358
in names of private variables . . . . . . . . . . . . . . . 242
translatable strings . . . . . . . . . . . . . . . . . . . . . . . . 360_gr_init() user-defined function . . . . . . . . . . . . . 271_ord_init() user-defined function . . . . . . . . . . . . 247_pw_init() user-defined function . . . . . . . . . . . . . 268{{} (braces) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 350
actions and. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
regexp operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
statements, grouping . . . . . . . . . . . . . . . . . . . . . . 154@@ (at-sign),@-notation for indirect function calls . . . . . . . 232@include directive . . . . . . . . . . . . . . . . . . . . . . . . . . 46
@load directive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
@namespace directive . . . . . . . . . . . . . . . . . . . . . . 384
BEGIN, BEGINFILE, END, ENDFILE and . . . 384|| (vertical bar) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
| operator (I/O) . . . . . . . . . . . . . . . . . . 92, 109, 144|& operator (I/O) . . . . . . . . . . . . 94, 109, 144, 344
pipes, closing . . . . . . . . . . . . . . . . . . . . . . . . . . 115|| operator . . . . . . . . . . . . . . . . . . . . . . . . . . . 141, 145\\ (backslash) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21\" escape sequence . . . . . . . . . . . . . . . . . . . . . . . . . 53\' operator (gawk) . . . . . . . . . . . . . . . . . . . . . . . . . 62\/ escape sequence . . . . . . . . . . . . . . . . . . . . . . . . . 53\< operator (gawk) . . . . . . . . . . . . . . . . . . . . . . . . . 62\> operator (gawk) . . . . . . . . . . . . . . . . . . . . . . . . . 62\` operator (gawk) . . . . . . . . . . . . . . . . . . . . . . . . . 62
\a escape sequence . . . . . . . . . . . . . . . . . . . . . . . . . 52
\b escape sequence . . . . . . . . . . . . . . . . . . . . . . . . . 52
\B operator (gawk) . . . . . . . . . . . . . . . . . . . . . . . . . 62
\f escape sequence . . . . . . . . . . . . . . . . . . . . . . . . . 52
\n escape sequence . . . . . . . . . . . . . . . . . . . . . . . . . 52
\nnn escape sequence . . . . . . . . . . . . . . . . . . . . . . . 52
\r escape sequence . . . . . . . . . . . . . . . . . . . . . . . . . 52
\s operator (gawk) . . . . . . . . . . . . . . . . . . . . . . . . . 61
\S operator (gawk) . . . . . . . . . . . . . . . . . . . . . . . . . 61
\t escape sequence . . . . . . . . . . . . . . . . . . . . . . . . . 52
\u escape sequence . . . . . . . . . . . . . . . . . . . . . . . . . 53~
~ (tilde), ~ operator . 51, 60, 63, 121, 138, 139, 145,
148

564

GAWK: Effective AWK ProgrammingA
aaa (amazing awk assembler) program . . . . . . . . . 529
accessing fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
accessing global variables from extensions . . . . . 429
account information . . . . . . . . . . . . . . . . . . . . . . 266, 270
actions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
control statements in . . . . . . . . . . . . . . . . . . . . . . 154
default . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
empty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Ada programming language . . . . . . . . . . . . . . . . . . . 529
adding,features to gawk . . . . . . . . . . . . . . . . . . . . . . . . . . . 516
fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
advanced features,
fixed-width data . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
gawk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335
network programming . . . . . . . . . . . . . . . . . . . . . 346
nondecimal input data. . . . . . . . . . . . . . . . . . . . . 335
processes, communicating with . . . . . . . . . . . . 343
specifying field content . . . . . . . . . . . . . . . . . . . . . 83
Aho, Alfred . . . . . . . . . . . . . . . . . . . . . . . . . . . 6, 488, 510
alarm clock example program . . . . . . . . . . . . . . . . . 308
alarm.awk program . . . . . . . . . . . . . . . . . . . . . . . . . . . 308
algorithms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 526
all source files, show in debugger . . . . . . . . . . . . . . 378
allocating memory for extensions . . . . . . . . . . . . . . 413
amazing awk assembler (aaa) . . . . . . . . . . . . . . . . . . 529
amazingly workable formatter (awf) . . . . . . . . . . . 529
ambiguity, syntactic: /= operator vs. /=.../ regexp
constant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
ampersand (&),&& operator . . . . . . . . . . . . . . . . . . . . . . . . . . . 141, 145gsub()/gensub()/sub() functions and . . . . . 205
anagram.awk program . . . . . . . . . . . . . . . . . . . . . . . . . 329
anagrams, finding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 328
and Boolean-logic operator . . . . . . . . . . . . . . . . . . . . 140
and() function (gawk) . . . . . . . . . . . . . . . . . . . . . . . . . 216
AND bitwise operation. . . . . . . . . . . . . . . . . . . . . . . . 216
ANSI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 529
API,
informational variables . . . . . . . . . . . . . . . . . . . . 444
version . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 443
API, ownership of MPFR and GMP values . . . . 416
arbitrary precision . . . . . . . . . . . . . . . . . . . . . . . 389, 390
integers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 399
archaeologists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 505
arctangent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194
ARGC/ARGV variables . . . . . . . . . . . . . . . . . . . . . . . . . . . 166
command-line arguments . . . . . . . . . . . . . . . . . . . 40
how to use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173
portability and . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
ARGIND variable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166
command-line arguments . . . . . . . . . . . . . . . . . . . 40
arguments,
command-line . . . . . . . . . . . . . . . . . . . . . 40, 166, 173
invoking awk . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
in function calls . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259

ARGV array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
ARGV array, indexing into . . . . . . . . . . . . . . . . . . . . . . . 40
arithmetic operators . . . . . . . . . . . . . . . . . . . . . . . . . . 128
array manipulation in extensions . . . . . . . . . . . . . . 433
array members . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
array scanning order, controlling . . . . . . . . . . . . . . 182
array subscripts,
null string as . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186
numbers as . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185
arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177
arrays of arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . 189
as parameters to functions . . . . . . . . . . . . . . . . . 227
associative . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178
library functions and . . . . . . . . . . . . . . . . . . . 242
deleting entire contents . . . . . . . . . . . . . . . . . . . . 187
elements,
assigning values . . . . . . . . . . . . . . . . . . . . . . . . 180
deleting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186
order of access by in operator . . . . . . . . . . 182
retrieving number of . . . . . . . . . . . . . . . . . . . 196
that don’t exist . . . . . . . . . . . . . . . . . . . . . . . . 179
example of using . . . . . . . . . . . . . . . . . . . . . . . . . . 180
for statement and. . . . . . . . . . . . . . . . . . . . . . . . . 181
IGNORECASE variable and. . . . . . . . . . . . . . . . . . . 179
indexing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178
merging into strings . . . . . . . . . . . . . . . . . . . . . . . 249
multidimensional . . . . . . . . . . . . . . . . . . . . . . . . . . 188
scanning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189
number of elements . . . . . . . . . . . . . . . . . . . . . . . . 199
numeric subscripts . . . . . . . . . . . . . . . . . . . . . . . . 185
referencing elements . . . . . . . . . . . . . . . . . . . . . . . 179
scanning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181
sorting,asort() function (gawk) . . . . . . . . . . . . . . . 341
asorti() function (gawk) . . . . . . . . . . . . . . 341
IGNORECASE variable and . . . . . . . . . . . . . . . 342
sparse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178
subscripts, uninitialized variables as . . . . . . . 186
unassigned elements . . . . . . . . . . . . . . . . . . . . . . . 179
artificial intelligence, gawk and . . . . . . . . . . . . . . . . 492
ASCII . . . . 52, 59, 119, 248, 308, 345, 486, 487, 490,
531, 534
asort() function (gawk) . . . . . . . . . . . . . . . . . . 196, 341
arrays, sorting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 341
side effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 341
asorti() function (gawk) . . . . . . . . . . . . . . . . 196, 341
arrays, sorting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 341
side effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 341
assert() function (C library) . . . . . . . . . . . . . . . . . 244
assert() user-defined function . . . . . . . . . . . . . . . . 245
assertions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244
assign values to variables, in debugger . . . . . . . . . 376
assignment operators . . . . . . . . . . . . . . . . . . . . . . . . . 130
evaluation order . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
lvalues/rvalues . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
assignments as file names . . . . . . . . . . . . . . . . . . . . . 259
associative arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178

Index 565

asterisk (*),
* operator,
as multiplication operator . . . . . . . . . . . . . . 144
as regexp operator . . . . . . . . . . . . . . . . . . . . . . 56
null strings, matching . . . . . . . . . . . . . . . . . . 205** operator . . . . . . . . . . . . . . . . . . . . . . . . . . . 129, 144**= operator. . . . . . . . . . . . . . . . . . . . . . . . . . 132, 145*= operator . . . . . . . . . . . . . . . . . . . . . . . . . . . 132, 145
at-sign (@),@-notation for indirect function calls . . . . . . . 232@include directive . . . . . . . . . . . . . . . . . . . . . . . . . . 46
@load directive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
@namespace directive . . . . . . . . . . . . . . . . . . . . . . 384
BEGIN, BEGINFILE, END, ENDFILE and . . . 384
atan2() function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194
automatic displays, in debugger . . . . . . . . . . . . . . . 377
awf (amazingly workable formatter) program . . 529
awk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5, See also gawk
function of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
gawk and . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5, 7
history of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
implementation issues,
pipes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
implementations . . . . . . . . . . . . . . . . . . . . . . . . . . . 510
limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
invoking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
language, POSIX version . . . . . . . . . 133, 161, 222
namespace . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383
identifier name storage . . . . . . . . . . . . . . . . . 385
use for indirect function calls . . . . . . . . . . . 385
new vs. old . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
OFMT variable . . . . . . . . . . . . . . . . . . . . . . . . . . 127
POSIX and . . . . . . . . . . . . 5, See also POSIX awk
profiling, enabling . . . . . . . . . . . . . . . . . . . . . . . . . . 38
regexp constants and . . . . . . . . . . . . . . . . . . . . . . 139
terms describing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
uses for . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5, 17, 30
versions of . 473, See also Brian Kernighan’s awk
changes between SVR3.1 and SVR4 . . . . 474
changes between SVR4 and POSIX awk . 474
changes between V7 and SVR3.1 . . . . . . . 473
awk programs . . . . . . . . . . . . . . . . . . . . . . . . . . . 17, 19, 27
complex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
debugging, enabling . . . . . . . . . . . . . . . . . . . . . . . . 35
documenting. . . . . . . . . . . . . . . . . . . . . . . . . . . 20, 242
examples of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279
execution of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
internationalizing . . . . . . . . . . . . . . . . . . . . . 220, 359
lengthy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
assertions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244
location of . . . . . . . . . . . . . . . . . . . . . . . . . . 33, 35, 36
one-line examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
profiling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 348
running . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17, 18
from shell scripts . . . . . . . . . . . . . . . . . . . . . . . . 17
without input files . . . . . . . . . . . . . . . . . . . . . . 18
shell variables in . . . . . . . . . . . . . . . . . . . . . . . . . . 152
awk to Go translator . . . . . . . . . . . . . . . . . . . . . . . . . . 512

AWK_HASH environment variable . . . . . . . . . . . . . . . . . 45
awka compiler for awk . . . . . . . . . . . . . . . . . . . . . . . . . 511
AWKBUFSIZE environment variable. . . . . . . . . . . . . . . 45
awkcc, awk to C translator . . . . . . . . . . . . . . . . . . . . 512
AWKgo. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 512
AWKLIBPATH environment variable . . . 37, 43, 47, 49,
441, 456, 477, 495, 496
AWKPATH environment variable . 42, 46, 49, 322, 326,
369, 477, 478, 495, 496, 499, 504
awkprof.out file . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 348
AWKREADFUNC environment variable . . . . . . . . . . . . . 45
awksed.awk program . . . . . . . . . . . . . . . . . . . . . . . . . . 321
awkvars.out file . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

B
b debugger command (alias for break) . . . . . . . . 373
backslash (\) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21\" escape sequence . . . . . . . . . . . . . . . . . . . . . . . . . 53\' operator (gawk) . . . . . . . . . . . . . . . . . . . . . . . . . 62\/ escape sequence . . . . . . . . . . . . . . . . . . . . . . . . . 53\< operator (gawk) . . . . . . . . . . . . . . . . . . . . . . . . . 62\> operator (gawk) . . . . . . . . . . . . . . . . . . . . . . . . . 62\` operator (gawk) . . . . . . . . . . . . . . . . . . . . . . . . . 62
\a escape sequence . . . . . . . . . . . . . . . . . . . . . . . . . 52
\b escape sequence . . . . . . . . . . . . . . . . . . . . . . . . . 52
\B operator (gawk) . . . . . . . . . . . . . . . . . . . . . . . . . 62
\f escape sequence . . . . . . . . . . . . . . . . . . . . . . . . . 52
\n escape sequence . . . . . . . . . . . . . . . . . . . . . . . . . 52
\nnn escape sequence . . . . . . . . . . . . . . . . . . . . . . . 52
\r escape sequence . . . . . . . . . . . . . . . . . . . . . . . . . 52
\s operator (gawk) . . . . . . . . . . . . . . . . . . . . . . . . . 61
\S operator (gawk) . . . . . . . . . . . . . . . . . . . . . . . . . 61
\t escape sequence . . . . . . . . . . . . . . . . . . . . . . . . . 52
\u escape sequence . . . . . . . . . . . . . . . . . . . . . . . . . 53\v escape sequence . . . . . . . . . . . . . . . . . . . . . . . . . 52
\w operator (gawk) . . . . . . . . . . . . . . . . . . . . . . . . . 61
\W operator (gawk) . . . . . . . . . . . . . . . . . . . . . . . . . 62
\x escape sequence . . . . . . . . . . . . . . . . . . . . . . . . . 52
\y operator (gawk) . . . . . . . . . . . . . . . . . . . . . . . . . 62
as field separator . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
continuing lines and . . . . . . . . . . . . . . . . . . . . . . . . 28
comments and . . . . . . . . . . . . . . . . . . . . . . . . . . 29
in csh . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
gsub()/gensub()/sub() functions and . . . . . 205
in bracket expressions . . . . . . . . . . . . . . . . . . . . . . 58
in escape sequences . . . . . . . . . . . . . . . . . . . . . 52, 53
POSIX and . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
in regexp constants . . . . . . . . . . . . . . . . . . . . . . . . . 60
in shell commands . . . . . . . . . . . . . . . . . . . . . . . . . . 21
regexp operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
backtrace debugger command . . . . . . . . . . . . . . . . 377
Beebe, Nelson H.F. . . . . . . . . . . . . . . . . . . . 12, 194, 511
BEGIN pattern . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74, 150@namespace directive and . . . . . . . . . . . . . . . . . . 384
assert() user-defined function and . . . . . . . . 246
Boolean patterns and . . . . . . . . . . . . . . . . . . . . . . 148
exit statement and . . . . . . . . . . . . . . . . . . . . . . . 162

566

GAWK: Effective AWK Programming

getline and. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
headings, adding . . . . . . . . . . . . . . . . . . . . . . . . . . 100
next/nextfile statements and . . . . . . . 151, 161
OFS/ORS variables, assigning values to . . . . . . 101
operators and . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150
print statement and . . . . . . . . . . . . . . . . . . . . . . 151
profiling and . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 348
pwcat program . . . . . . . . . . . . . . . . . . . . . . . . . . . . 268
running awk programs and . . . . . . . . . . . . . . . . . 280
TEXTDOMAIN variable and. . . . . . . . . . . . . . . . . . . 360
beginfile() user-defined function . . . . . . . . . . . . 255
BEGINFILE pattern . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151@namespace directive and . . . . . . . . . . . . . . . . . . 384
Boolean patterns and . . . . . . . . . . . . . . . . . . . . . . 148
Bentley, Jon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 532
Benzinger, Michael. . . . . . . . . . . . . . . . . . . . . . . . . . . . 489
Berry, Karl. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12, 487
binary input/output . . . . . . . . . . . . . . . . . . . . . . . . . . 163
bindtextdomain() function (C library) . . . . . . . . 358
bindtextdomain() function (gawk) . . . . . . . 220, 360
portability and . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363
BINMODE variable . . . . . . . . . . . . . . . . . . . . . . . . . 163, 499
bit-manipulation functions . . . . . . . . . . . . . . . . . . . . 216
bits2str() user-defined function . . . . . . . . . . . . . 217
bitwise,AND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216
complement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216
operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216
OR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216
shift . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216
XOR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216
body,in actions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
in loops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
Boolean expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
as patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148
boolean function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194
Boolean operators, See Boolean expressions
Bornstein, Dan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 510
Bourne shell, quoting rules for. . . . . . . . . . . . . . . . . . 21
braces ({}) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 350
actions and. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
regexp operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
statements, grouping . . . . . . . . . . . . . . . . . . . . . . 154
bracket expressions . . . . . . . . . . . . . . . . . . . . . . . . . 55, 58
character classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
character lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
collating elements . . . . . . . . . . . . . . . . . . . . . . . . . . 59
collating symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
complemented . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
equivalence classes . . . . . . . . . . . . . . . . . . . . . . . . . . 59
non-ASCII . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
range expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
break debugger command . . . . . . . . . . . . . . . . . . . . . 373
break statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
breakpoint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 368
at location, how to delete . . . . . . . . . . . . . . . . . . 373
commands to execute at . . . . . . . . . . . . . . . . . . . 374

condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373
delete by number . . . . . . . . . . . . . . . . . . . . . . . . . . 373
how to disable or enable . . . . . . . . . . . . . . . . . . . 373
setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373
show all in debugger . . . . . . . . . . . . . . . . . . . . . . . 377
Brennan, Michael . . . . . . . . . . . 2, 3, 12, 187, 321, 510
Brian Kernighan’s awk . . 30, 54, 62, 68, 75, 93, 130,
151, 159, 160, 162, 187, 204, 205, 208
extensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 475
source code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 510
Brini, Davide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 330
Brink, Jeroen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Broder, Alan J. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 489
Brown, Martin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 489
BSD-based operating systems . . . . . . . . . . . . . . . . . 540
bt debugger command (alias for backtrace) . . . 377
Buening, Andreas . . . . . . . . . . . . . . . . . . . . 12, 489, 509
buffering,
input/output . . . . . . . . . . . . . . . . . . . . . . . . . 211, 344
interactive vs. noninteractive . . . . . . . . . . . . . . 209
buffers,
flushing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208, 211
operators for . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
bug reports, email address, bug-gawk@gnu.org . 507
bug-gawk@gnu.org bug reporting address . . . . . . 507
built-in functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193
evaluation order . . . . . . . . . . . . . . . . . . . . . . . . . . . 193
BusyBox Awk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 511
BWK awk, See Brian Kernighan’s awk
BWK awk,
interval expressions in . . . . . . . . . . . . . . . . . . . . . . 57
bytes,
counting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304

Cc.e., See common extensions
C library functions,assert() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244
bindtextdomain() . . . . . . . . . . . . . . . . . . . . . . . . 358
endgrent() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274
endpwent() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269
getaddrinfo() . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347
getgrent() . . . . . . . . . . . . . . . . . . . . . . . . . . . 270, 274
getgrgid() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273
getgrnam() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273
getgruser() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273
getopt() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259
getpwent() . . . . . . . . . . . . . . . . . . . . . . . . . . . 266, 269
getpwnam() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269
getpwuid() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269
gettext() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 358
textdomain() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357
call by reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227
call by value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227
call stack,display in debugger . . . . . . . . . . . . . . . . . . . . . . . . 377
explanation of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367

Index 567

caret (^),
^ operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144^= operator . . . . . . . . . . . . . . . . . . . . . . . . . . . 132, 145
in bracket expressions . . . . . . . . . . . . . . . . . . . . . . 58
regexp operator . . . . . . . . . . . . . . . . . . . . . . . . . 55, 62
case keyword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157
case sensitivity,
array indices and . . . . . . . . . . . . . . . . . . . . . . . . . . 179
converting case . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204
example programs . . . . . . . . . . . . . . . . . . . . . . . . . 241
gawk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
regexps and . . . . . . . . . . . . . . . . . . . . . . . . . . . 63, 164
string comparisons and . . . . . . . . . . . . . . . . . . . . 164
CGI, awk scripts for . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
character classes, See bracket expressions
character lists, See bracket expressions
character sets (in regular expressions), See bracket
expressions
character sets (machine character
encodings) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248, 531
characters,
counting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304
transliterating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311
values of as numbers . . . . . . . . . . . . . . . . . . . . . . 247
Chassell, Robert J. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
chdir() extension function . . . . . . . . . . . . . . . . . . . 457
checking for MPFR . . . . . . . . . . . . . . . . . . . . . . . . . . . 400
chem utility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 532
chr() extension function . . . . . . . . . . . . . . . . . . . . . . 463
chr() user-defined function . . . . . . . . . . . . . . . . . . . 247
clear debugger command . . . . . . . . . . . . . . . . . . . . . 373
Cliff random numbers . . . . . . . . . . . . . . . . . . . . . . . . . 247
cliff_rand() user-defined function . . . . . . . . . . . 247
close file or coprocess . . . . . . . . . . . . . . . . . . . . . . . . . 208
close() function . . . . . . . . . . . . . . . . . . . . . . . . . 113, 208
portability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
return value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
two-way pipes and. . . . . . . . . . . . . . . . . . . . . . . . . 344
Close, Diane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11, 488
Collado, Manuel . . . . . . . . . . . . . . . . . . . . . . . . 12, 84, 85
collating elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
collating symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
Colombo, Antonio . . . . . . . . . . . . . . . . . . . . . . . . . 12, 489
colon (:),:: namespace separator . . . . . . . . . . . . . . . . . . . 383?: operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145
columns,aligning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
cutting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279
comma (,), in range patterns . . . . . . . . . . . . . . . . . 149
comma separated values (CSV) data,--csv option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
-k option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
generating CSV data . . . . . . . . . . . . . . . . . . . . . . 253
Comma separated values (CSV) data,
parsing with FPAT . . . . . . . . . . . . . . . . . . . . . . . . . . 83
parsing with FPAT library . . . . . . . . . . . . . . . . . . . 85
records and fields . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

command completion, in debugger . . . . . . . . . . . . 381
command line,
arguments . . . . . . . . . . . . . . . . . . . 40, 166, 168, 173
directories on . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
formats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
FS on, setting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
invoking awk from . . . . . . . . . . . . . . . . . . . . . . . . . . 33
option -f . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
end of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259
string extraction . . . . . . . . . . . . . . . . . . . . . . . 361
variables, assigning on . . . . . . . . . . . . . . . . . . . . . 125
commands debugger command . . . . . . . . . . . . . . . . . 374
commands to execute at breakpoint . . . . . . . . . . . 374
commenting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
backslash continuation and . . . . . . . . . . . . . . . . . 29
common extensions,** operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128**= operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133/dev/stderr special file . . . . . . . . . . . . . . . . . . . 112/dev/stdin special file . . . . . . . . . . . . . . . . . . . . 112/dev/stdout special file . . . . . . . . . . . . . . . . . . . 112\u escape sequence . . . . . . . . . . . . . . . . . . . . . . . . . 53
\x escape sequence . . . . . . . . . . . . . . . . . . . . . . . . . 52
BINMODE variable . . . . . . . . . . . . . . . . . . . . . . . . . . 499
delete to delete entire arrays . . . . . . . . . . . . . . 187
func keyword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222
length() applied to an array . . . . . . . . . . . . . . 199
RS as a regexp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
single character fields . . . . . . . . . . . . . . . . . . . . . . . 75
comp.lang.awk newsgroup . . . . . . . . . . . . . . . . . . . . 507
comparison expressions . . . . . . . . . . . . . . . . . . . . . . . 135
as patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148
string vs. regexp. . . . . . . . . . . . . . . . . . . . . . . . . . . 139
compatibility mode (gawk),
extensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 475
file names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
hexadecimal numbers . . . . . . . . . . . . . . . . . . . . . . 121
octal numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
specifying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
compiled programs . . . . . . . . . . . . . . . . . . . . . . . 525, 532
compiling gawk,
for Cygwin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 501
for MS-Windows . . . . . . . . . . . . . . . . . . . . . . . . . . 499
for OpenVMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 501
compl() function (gawk) . . . . . . . . . . . . . . . . . . . . . . 216
complement, bitwise . . . . . . . . . . . . . . . . . . . . . . . . . . 216
component name. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383
naming rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 384
compound statements, control statements and . 154
concatenating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
condition debugger command . . . . . . . . . . . . . . . . 373
conditional expressions . . . . . . . . . . . . . . . . . . . . . . . . 141
configuration option,--disable-extensions . . . . . . . . . . . . . . . . . . . . 497--disable-lint . . . . . . . . . . . . . . . . . . . . . . . . . . . 497--disable-mpfr . . . . . . . . . . . . . . . . . . . . . . . . . . . 497

568

GAWK: Effective AWK Programming--disable-nls . . . . . . . . . . . . . . . . . . . . . . . . . . . . 497--enable-versioned-extension-dir . . . . . . 497
configuration options, gawk. . . . . . . . . . . . . . . . . . . . 496
constants,
nondecimal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335
numeric . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
regexp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
string . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
types of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
continue debugger command . . . . . . . . . . . . . . . . . 374
continue program, in debugger . . . . . . . . . . . . . . . . 374
continue statement . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
control statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
controlling array scanning order . . . . . . . . . . . . . . . 182
converting,dates to timestamps . . . . . . . . . . . . . . . . . . . . . . . 213
integer array subscripts to strings . . . . . . . . . . 185
numbers to strings . . . . . . . . . . . . . . . . . . . . 126, 217
string to lower case . . . . . . . . . . . . . . . . . . . . . . . . 204
string to numbers . . . . . . . . . . . . . . . . 126, 202, 217
string to upper case . . . . . . . . . . . . . . . . . . . . . . . 205
CONVFMT variable . . . . . . . . . . . . . . . . . . . . . . . . . 126, 163
array subscripts and . . . . . . . . . . . . . . . . . . . . . . . 185
cookie . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 533
coprocesses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109, 344
closing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
getline from . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
cos() function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194
cosine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194
counting words, lines, characters, and bytes . . . 304
cppawk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 512
csh utility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28|& operator, comparison with . . . . . . . . . . . . . . 344
POSIXLY_CORRECT environment variable . . . . . 40
CSV (comma separated values) data,--csv option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
-k option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
generating CSV data . . . . . . . . . . . . . . . . . . . . . . 253
parsing with CSVMODE library. . . . . . . . . . . . . . . . 85
parsing with FPAT . . . . . . . . . . . . . . . . . . . . . . . . . . 83
records and fields . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
CSVMODE library for gawk . . . . . . . . . . . . . . . . . . . . . . . 85
ctime() user-defined function . . . . . . . . . . . . . . . . . 224
Curreli, Marco . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 489
currency symbols, localization . . . . . . . . . . . . . . . . . 359
current namespace, pushing and popping . . . . . . 384
current source file, show in debugger . . . . . . . . . . 378
current system time . . . . . . . . . . . . . . . . . . . . . . . . . . . 212
custom.h file . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 498
customized input parser . . . . . . . . . . . . . . . . . . . . . . . 420
customized output wrapper . . . . . . . . . . . . . . . . . . . 424
customized two-way processor . . . . . . . . . . . . . . . . . 426
cut utility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279
cut.awk program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 280

D
d debugger command (alias for delete) . . . . . . . 373
d.c., See dark cornerdark corner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10, 533"0" is actually true . . . . . . . . . . . . . . . . . . . . . . . . 135/= operator vs. /=.../ regexp constant . . . . 133^, in FS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
ARGV variable, value of . . . . . . . . . . . . . . . . . . . . . . 20
array subscripts . . . . . . . . . . . . . . . . . . . . . . . . . . . 186
backslash continuation on command line . . . . 29
break statement . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
close() function . . . . . . . . . . . . . . . . . . . . . . . . . . 115
command line, backslash continuation on . . . 29
command-line arguments . . . . . . . . . . . . . . . . . . 125
continue statement . . . . . . . . . . . . . . . . . . . . . . . 160
CONVFMT variable . . . . . . . . . . . . . . . . . . . . . . . . . . 126
empty programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
escape sequences . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
for metacharacters . . . . . . . . . . . . . . . . . . . . . . 54
exit statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162
field separators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
FILENAME variable. . . . . . . . . . . . . . . . . . . . . . 94, 167
FNR/NR variables . . . . . . . . . . . . . . . . . . . . . . . . . . . 173
format-control characters . . . . . . . . . . . . . 103, 104
FS as null string . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
input files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
invoking awk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
length() function . . . . . . . . . . . . . . . . . . . . . . . . . 199
locale’s decimal point character . . . . . . . . . . . . 127
multiline records . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
NF variable, decrementing . . . . . . . . . . . . . . . . . . . 72
OFMT variable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
parameter name restrictions . . . . . . . . . . . . . . . 221
range patterns, line continuation and . . . . . . 150
regexp as second argument to index() . . . . . 198
regexp constants . . . . . . . . . . . . . . . . . . . . . . . . . . 122/= operator and . . . . . . . . . . . . . . . . . . . . . . . . 133
as arguments to user-defined functions . 122
split() function . . . . . . . . . . . . . . . . . . . . . . . . . . 202
string continuation . . . . . . . . . . . . . . . . . . . . . . . . 120
strings, storing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
value of ARGV[0] . . . . . . . . . . . . . . . . . . . . . . . . . . 166
data, fixed-width . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
data-driven languages . . . . . . . . . . . . . . . . . . . . . . . . . 526
database,group, reading. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270
users, reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266
date utility,
GNU . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211
POSIX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215
dates,
converting to timestamps . . . . . . . . . . . . . . . . . . 213
information related to, localization. . . . . . . . . 359
Davies, Stephen . . . . . . . . . . . . . . . . . . . . . . . . . . . 12, 488
Day, Robert P.J. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12dcgettext() function (gawk) . . . . . . . . . . . . . 220, 359
portability and . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363
dcngettext() function (gawk) . . . . . . . . . . . . 220, 360

Index 569

portability and . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363
deadlocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 344
debugger,capabilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367
command completion . . . . . . . . . . . . . . . . . . . . . . 381
commands,b (break) . . . . . . . . . . . . . . . . . . . . . . . . . 370, 373
backtrace . . . . . . . . . . . . . . . . . . . . . . . . 370, 377
break . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 370, 373
breakpoint . . . . . . . . . . . . . . . . . . . . . . . . . . . . 370
bt (backtrace) . . . . . . . . . . . . . . . . . . . 370, 377
c (continue). . . . . . . . . . . . . . . . . . . . . . . . . . . 374
clear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373
commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374
condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373
continue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374
d (delete) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373
delete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373
disable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373
display . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375
down . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377
dump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 379
e (enable) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373
enable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373
end . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374
eval . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375
f (frame) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377
finish . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374
frame . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377
h (help) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 380
help . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 380
i (info) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377
ignore . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374
info . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377
l (list) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 380
list . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 380
n (next) . . . . . . . . . . . . . . . . . . . . . . . . . . 371, 374
next . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371, 374
nexti . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375
ni (nexti) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375
o (option) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 378
option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 378
p (print) . . . . . . . . . . . . . . . . . . . . . . . . . 370, 376
print . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 370, 376
printf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 376
q (quit) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 380
quit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 380
r (run) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375
return . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375
run . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 370, 375
s (step) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375
set . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 376
si (stepi) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375
silent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374
step . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375
stepi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375
t (tbreak) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374
tbreak . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374

trace . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 380
u (until) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375
undisplay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 376
until . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375
unwatch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 376
up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377
w (watch) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 376
watch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 376
where (backtrace) . . . . . . . . . . . . . . . . . . . . . 377
concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367
default list amount . . . . . . . . . . . . . . . . . . . . . . . . 378
history expansion. . . . . . . . . . . . . . . . . . . . . . . . . . 381
history file . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 378
history size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 378
how to start . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369
instruction tracing. . . . . . . . . . . . . . . . . . . . . . . . . 378
interaction with namespaces . . . . . . . . . . . . . . . 388
limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 381
options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 378
printing all array elements . . . . . . . . . . . . . . . . . 371
printing single array elements . . . . . . . . . . . . . . 371
prompt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369, 378
read commands from a file . . . . . . . . . . . . . . . . . 378
repeating commands . . . . . . . . . . . . . . . . . . . . . . 372
running the program . . . . . . . . . . . . . . . . . . . . . . 370
save commands to a file . . . . . . . . . . . . . . . . . . . 378
setting a breakpoint . . . . . . . . . . . . . . . . . . . . . . . 370
stack frames, showing . . . . . . . . . . . . . . . . . . . . . 370
debugging,awk programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367
example session . . . . . . . . . . . . . . . . . . . . . . . . . . . 369
debugging gawk, bug reports . . . . . . . . . . . . . . . . . . 505
decimal point character, locale specific . . . . . . . . . 38
decrement operators . . . . . . . . . . . . . . . . . . . . . . . . . . 134
default keyword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157
Deifik, Scott . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12, 488
delete array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187
delete breakpoint,
at location . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373
by number . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373
delete debugger command . . . . . . . . . . . . . . . . . . . 373
delete statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186
delete watchpoint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 376
deleting,
elements in arrays . . . . . . . . . . . . . . . . . . . . . . . . . 186
entire arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187
Demaille, Akim . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
describe call stack frame, in debugger . . . . . . . . . 377
differences in awk and gawk,
ARGC/ARGV variables . . . . . . . . . . . . . . . . . . . . . . . 174
ARGIND variable . . . . . . . . . . . . . . . . . . . . . . . . . . . 166
array elements, deleting . . . . . . . . . . . . . . . . . . . 187
AWKLIBPATH environment variable . . . . . . . . . . . 43
AWKPATH environment variable. . . . . . . . . . . . . . . 42
BEGIN/END patterns . . . . . . . . . . . . . . . . . . . . . . . . 151
BEGINFILE/ENDFILE patterns . . . . . . . . . . . . . . 151
BINMODE variable . . . . . . . . . . . . . . . . . . . . . . 163, 499
close() function . . . . . . . . . . . . . . . . . . . . . 114, 115

570

GAWK: Effective AWK Programming

command-line directories . . . . . . . . . . . . . . . . . . . 97
ERRNO variable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167
error messages. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
FIELDWIDTHS variable . . . . . . . . . . . . . . . . . . . . . . 163
FPAT variable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164
FUNCTAB variable . . . . . . . . . . . . . . . . . . . . . . . . . . 168
function arguments . . . . . . . . . . . . . . . . . . . . . . . . 193
getline command . . . . . . . . . . . . . . . . . . . . . . . . . . 89
IGNORECASE variable . . . . . . . . . . . . . . . . . . . . . . . 164
implementation limitations . . . . . . . . . . . . . 94, 110
indirect function calls . . . . . . . . . . . . . . . . . . . . . 231
input/output operators . . . . . . . . . . . . . . . . 94, 109
length() function . . . . . . . . . . . . . . . . . . . . . . . . . 199
line continuations . . . . . . . . . . . . . . . . . . . . . . . . . 142
LINT variable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164
match() function . . . . . . . . . . . . . . . . . . . . . . . . . . 200
print/printf statements . . . . . . . . . . . . . . . . . . 105
PROCINFO array . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168
read timeouts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
record separators . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
regexp constants . . . . . . . . . . . . . . . . . . . . . . . . . . 122
regular expressions . . . . . . . . . . . . . . . . . . . . . . . . . 63
retrying input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
RS/RT variables . . . . . . . . . . . . . . . . . 67, 68, 89, 171
single-character fields . . . . . . . . . . . . . . . . . . . . . . . 75
split() function . . . . . . . . . . . . . . . . . . . . . . . . . . 201
strings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119, 120
storing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
SYMTAB variable . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
TEXTDOMAIN variable . . . . . . . . . . . . . . . . . . . . . . . 165
trunc-mod operation . . . . . . . . . . . . . . . . . . . . . . 129
directories,
command-line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
searching,
for loadable extensions . . . . . . . . . . . . . . . . . . 43
for source files . . . . . . . . . . . . . . . . . . . . . . 42, 332
disable breakpoint . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373
disable debugger command . . . . . . . . . . . . . . . . . . 373
display debugger command . . . . . . . . . . . . . . . . . . 375
display debugger options . . . . . . . . . . . . . . . . . . . . . . 378
division . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
do-while statement . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
use of regexps in. . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
documentation,building,
HTML . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 498
Info files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 498
PDF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 498
of awk programs . . . . . . . . . . . . . . . . . . . . . . . . . . . 242
online . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
documents, searching . . . . . . . . . . . . . . . . . . . . . . . . . 307
dollar sign ($),
$ field operator . . . . . . . . . . . . . . . . . . . . . . . . 69, 144
incrementing fields and arrays . . . . . . . . . . . . . 133
regexp operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
double quote ("),
in regexp constants . . . . . . . . . . . . . . . . . . . . . . . . . 60
in shell commands . . . . . . . . . . . . . . . . . . . . . . . . . . 22

double-precision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 390
down debugger command . . . . . . . . . . . . . . . . . . . . . . 377
Drepper, Ulrich . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Duman, Patrice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
dump all variables of a program . . . . . . . . . . . . . . . . 35
dump debugger command . . . . . . . . . . . . . . . . . . . . . . 379
dupword.awk program . . . . . . . . . . . . . . . . . . . . . . . . . 308
dynamic profiling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 351
dynamically loaded extensions . . . . . . . . . . . . . . . . 405

E
e debugger command (alias for enable) . . . . . . . 373
EBCDIC . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248, 486, 490
effective group ID of gawk user . . . . . . . . . . . . . . . . 168
effective user ID of gawk user . . . . . . . . . . . . . . . . . . 168
Eggert, Paul . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
egrep utility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58, 284
egrep.awk program . . . . . . . . . . . . . . . . . . . . . . . . . . . 285
elements in arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
assigning values . . . . . . . . . . . . . . . . . . . . . . . . . . . 180
deleting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186
order of access by in operator . . . . . . . . . . . . . 182
scanning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181
email address for bug reports,
bug-gawk@gnu.org . . . . . . . . . . . . . . . . . . . . . . . . . 507
empty array elements . . . . . . . . . . . . . . . . . . . . . . . . . 179
empty pattern . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
empty regexps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
empty strings, See null strings
EMRED. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 346
enable breakpoint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373
enable debugger command . . . . . . . . . . . . . . . . . . . 373
end debugger command . . . . . . . . . . . . . . . . . . . . . . . 374
END pattern . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150@namespace directive and . . . . . . . . . . . . . . . . . . 384
assert() user-defined function and . . . . . . . . 246
Boolean patterns and . . . . . . . . . . . . . . . . . . . . . . 148
exit statement and . . . . . . . . . . . . . . . . . . . . . . . 162
next/nextfile statements and . . . . . . . 151, 161
operators and . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150
print statement and . . . . . . . . . . . . . . . . . . . . . . 151
profiling and . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 348
endfile() user-defined function . . . . . . . . . . . . . . . 255
ENDFILE pattern . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151@namespace directive and . . . . . . . . . . . . . . . . . . 384
Boolean patterns and . . . . . . . . . . . . . . . . . . . . . . 148
endgrent() function (C library). . . . . . . . . . . . . . . 274
endgrent() user-defined function . . . . . . . . . . . . . 274
endpwent() function (C library). . . . . . . . . . . . . . . 269
endpwent() user-defined function . . . . . . . . . . . . . 269
English, Steve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335
ENVIRON array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166
environment variables,
AWK_HASH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
AWKBUFSIZE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
AWKLIBPATH . . 37, 43, 47, 49, 441, 456, 477, 495,
496

Index 571

AWKPATH . . . . 42, 46, 49, 322, 326, 369, 477, 478,
495, 496, 499, 504
AWKREADFUNC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
GAWK_LOCALE_DIR . . . . . . . . . . . . . . . . . . . . . . 45, 359
GAWK_MSEC_SLEEP . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
GAWK_MSG_SRC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
GAWK_NO_DFA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
GAWK_PERSIST_FILE . . . . . . . . . . . . . . . . . . . . 44, 352
GAWK_READ_TIMEOUT . . . . . . . . . . . . . . . . . . . . . 44, 96
GAWK_SOCK_RETRIES . . . . . . . . . . . . . . . . . . . . 44, 117
GAWK_STACKSIZE . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
in ENVIRON array . . . . . . . . . . . . . . . . . . . . . . . . . . 166
INT_CHAIN_MAX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
LANG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 365
LANGUAGE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 359, 365
LC_ALL . . . . . . . . . . . . . . . . . . . . . . . . . . 294, 365, 506
LC_MESSAGES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 365
Path . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 499
PATH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 520
PMA_VERBOSITY . . . . . . . . . . . . . . . . . . . . . . . . 44, 353
POSIXLY_CORRECT . . . . . . . . . . . . . . . 40, 44, 49, 127
REALLY_USE_PERSIST_MALLOC . . . . . . . . . . . . . . 352
STR_CHAIN_MAX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
TIDYMEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
TZ. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167
used by gawk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
environment, definition of . . . . . . . . . . . . . . . . . . . . . 533
epoch, definition of . . . . . . . . . . . . . . . . . . . . . . . . . . . 533
equals sign (=),
= operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130== operator . . . . . . . . . . . . . . . . . . . . . . . . . . . 138, 144
EREs (Extended Regular Expressions) . . . . . . . . . 58
ERRNO variable . . . . . . . . . . . . . . . . . . . . . . . . . . . 167, 347
with BEGINFILE pattern . . . . . . . . . . . . . . . . . . . 152
with close() function . . . . . . . . . . . . . . . . . . . . . 115
with getline command. . . . . . . . . . . . . . . . . . . . . 89
error handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
ERRNO variable and . . . . . . . . . . . . . . . . . . . . . . . . 167
error output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
escape processing,gsub()/gensub()/sub() functions . . . . . . . . . 205
escape sequences. . . . . . . . . . . . 52, See also backslasheval debugger command . . . . . . . . . . . . . . . . . . . . . . 375
evaluate expressions, in debugger . . . . . . . . . . . . . . 375
evaluation order . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
concatenation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193
examining fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
example debugging session . . . . . . . . . . . . . . . . . . . . 369
exclamation point (!),
! operator . . . . . . . . . . . . . . . . . . . . . . . 141, 144, 287!= operator . . . . . . . . . . . . . . . . . . . . . . . . . . . 138, 144!~ operator . . 51, 60, 63, 121, 138, 139, 145, 148
exit debugger command . . . . . . . . . . . . . . . . . . . . . . 380
exit statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162
exit status, of gawk. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
on OpenVMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 504
exit the debugger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 380

exp() function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194
expand utility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Expat XML parser library . . . . . . . . . . . . . . . . . . . . 468
exponent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194
expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
as patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
assignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
Boolean . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
conditional . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
selecting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
expressions, matching, See comparison expressions
Extended Regular Expressions (EREs) . . . . . . . . . 58
extension API . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 407
informational variables . . . . . . . . . . . . . . . . . . . . 444
interaction with namespaces . . . . . . . . . . . . . . . 388
version number . . . . . . . . . . . . . . . . . . . . . . . 170, 443
extensions,
Brian Kernighan’s awk . . . . . . . . . . . . . . . . 475, 486
common,** operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128**= operator . . . . . . . . . . . . . . . . . . . . . . . . . . . 133/dev/stderr special file . . . . . . . . . . . . . . . . 112/dev/stdin special file . . . . . . . . . . . . . . . . . 112/dev/stdout special file . . . . . . . . . . . . . . . . 112\u escape sequence . . . . . . . . . . . . . . . . . . . . . . 53
\x escape sequence . . . . . . . . . . . . . . . . . . . . . . 52
BINMODE variable . . . . . . . . . . . . . . . . . . . . . . . 499
delete to delete entire arrays . . . . . . . . . . 187
fflush() function . . . . . . . . . . . . . . . . . . . . . 208
func keyword . . . . . . . . . . . . . . . . . . . . . . . . . . 222
length() applied to an array. . . . . . . . . . . 199
RS as a regexp . . . . . . . . . . . . . . . . . . . . . . . . . . 67
single character fields . . . . . . . . . . . . . . . . . . . 75
in gawk, not in POSIX awk . . . . . . . . . . . . . . . . 475
loadable,
allocating memory . . . . . . . . . . . . . . . . . . . . . 413
array manipulation in . . . . . . . . . . . . . . . . . . 433
distributed with gawk . . . . . . . . . . . . . . . . . . 457
example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 447
gawkextlib project . . . . . . . . . . . . . . . . . . . . 467
loading, @load directive . . . . . . . . . . . . . . . . . 47
registration . . . . . . . . . . . . . . . . . . . . . . . . . . . . 417
search path . . . . . . . . . . . . . . . . . . . . . . . . . . . . 447
mawk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 486
extract.awk program . . . . . . . . . . . . . . . . . . . . . . . . . 318
extraction, of marked strings(internationalization) . . . . . . . . . . . . . . . . . . . . . . . 361

572

GAWK: Effective AWK ProgrammingFf debugger command (alias for frame) . . . . . . . . 377
false, logical . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
FDL (Free Documentation License) . . . . . . . . . . . 553
features,adding to gawk . . . . . . . . . . . . . . . . . . . . . . . . . . . . 516
deprecated . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
undocumented. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
Fenlason, Jay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6, 488
fflush() function . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208
field numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
field operator $ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
field operators, dollar sign as . . . . . . . . . . . . . . . . . . . 69
field separator . . . . . . . . . . 73, 164, 165, See also OFS
backslash (\) as . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
choice of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
FIELDWIDTHS variable and . . . . . . . . . . . . . . . . . 163
FPAT variable and . . . . . . . . . . . . . . . . . . . . . . . . . 164
FS variable and . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
in multiline records . . . . . . . . . . . . . . . . . . . . . . . . . 87
on command line . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
POSIX and. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
regular expression as . . . . . . . . . . . . . . . . . . . . . . . 74
spaces as . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281
whitespace as. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65, 69, 526
adding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
changing contents of . . . . . . . . . . . . . . . . . . . . . . . . 71
cutting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279
examining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
number of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
printing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
separating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
single-character . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
FIELDWIDTHS variable. . . . . . . . . . . . . . . . . . . . . . 80, 163
file descriptors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
file inclusion, @include directive . . . . . . . . . . . . . . . 46
file names,
assignments as . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259
distinguishing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166
in compatibility mode . . . . . . . . . . . . . . . . . . . . . 113
standard streams in gawk . . . . . . . . . . . . . . . . . . 112
FILENAME variable . . . . . . . . . . . . . . . . . . . . . . . . . 65, 167
getline, setting with . . . . . . . . . . . . . . . . . . . . . . . 94
files,.gmo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 358
specifying directory of . . . . . . . . . . . . . 358, 360.mo, converting from .po . . . . . . . . . . . . . . . . . . 365.po . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 358, 361
converting to .mo . . . . . . . . . . . . . . . . . . . . . . 365.pot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357/dev/... special files . . . . . . . . . . . . . . . . . . . . . . 112/inet/... (gawk) . . . . . . . . . . . . . . . . . . . . . . . . . 346/inet4/... (gawk) . . . . . . . . . . . . . . . . . . . . . . . . 346/inet6/... (gawk) . . . . . . . . . . . . . . . . . . . . . . . . 346
awk programs in . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
awkprof.out . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 348

awkvars.out . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
closing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208
descriptors, See file descriptors
group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270
initialization and cleanup . . . . . . . . . . . . . . . . . . 254
input, See input files
log, timestamps in . . . . . . . . . . . . . . . . . . . . . . . . . 211
managing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254
data file boundaries . . . . . . . . . . . . . . . . . . . . 254
message object . . . . . . . . . . . . . . . . . . . . . . . . . . . . 358
converting from portable object files . . . . 365
specifying directory of . . . . . . . . . . . . . 358, 360
multiple passes over . . . . . . . . . . . . . . . . . . . . . . . . 41
multiple, duplicating output into . . . . . . . . . . 298
output, See output files
password . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266
portable object . . . . . . . . . . . . . . . . . . . . . . . 358, 361
converting to message object files . . . . . . 365
generating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
template file (.pot) . . . . . . . . . . . . . . . . . . . . 357
processing, ARGIND variable and . . . . . . . . . . . . 166
reading. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256
multiline records . . . . . . . . . . . . . . . . . . . . . . . . 86
searching for regular expressions . . . . . . . . . . . 284
skipping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257
source, search path for. . . . . . . . . . . . . . . . . . . . . 332
splitting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293
Texinfo, extracting programs from . . . . . . . . . 317
find substring in string . . . . . . . . . . . . . . . . . . . . . . . . 198
finding extensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 447
finish debugger command . . . . . . . . . . . . . . . . . . . 374
Fish, Fred . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 488
fixed-width data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
flag variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141, 299
floating-point,
numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 389
arbitrary-precision . . . . . . . . . . . . . . . . 389, 390
double-precision . . . . . . . . . . . . . . . . . . . . . . . 390
single-precision . . . . . . . . . . . . . . . . . . . . . . . . 390
flush buffered output . . . . . . . . . . . . . . . . . . . . . . . . . . 208
fnmatch() extension function . . . . . . . . . . . . . . . . . 460
FNR variable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65, 167
changing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173
for statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
looping over arrays . . . . . . . . . . . . . . . . . . . . . . . . 181
fork() extension function . . . . . . . . . . . . . . . . . . . . . 461
format specifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
mixing regular with positional specifiers . . . 362
printf statement . . . . . . . . . . . . . . . . . . . . . . . . . 103
strftime() function (gawk) . . . . . . . . . . . . . . . 213
format time string . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212
formats, numeric output . . . . . . . . . . . . . . . . . . . . . . 102
formatting,output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202
forward slash (/),
/ operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144/= operator . . . . . . . . . . . . . . . . . . . . . . . . . . . 132, 145

Index 573

vs. /=.../ regexp constant . . . . . . . . . . . . . 133
patterns and . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148
to enclose regular expressions . . . . . . . . . . . . . . . 51
FPAT variable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83, 164frame debugger command . . . . . . . . . . . . . . . . . . . . . 377
frawk. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 511
Free Documentation License (FDL) . . . . . . . . . . . 553
Free Software Foundation (FSF) . 10, 491, 534, 535
FreeBSD. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 540
FS variable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73, 164--field-separator option and . . . . . . . . . . . . . 33
changing value of . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
containing ^ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
in multiline records . . . . . . . . . . . . . . . . . . . . . . . . . 87
null string as . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
running awk programs and . . . . . . . . . . . . . . . . . 280
setting from command line . . . . . . . . . . . . . . . . . 77
TAB character as . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
FSF (Free Software Foundation) . 10, 491, 534, 535
fts() extension function . . . . . . . . . . . . . . . . . . . . . . 458
FUNCTAB array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168
function arguments, show in debugger . . . . . . . . . 377
function calls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
indirect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231@-notation for . . . . . . . . . . . . . . . . . . . . . . . . . . 232
function definition example . . . . . . . . . . . . . . . . . . . 223
function definitions, list in debugger . . . . . . . . . . . 377
function pointers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231
functions,arrays as parameters to . . . . . . . . . . . . . . . . . . . . 227
built-in . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142, 193
evaluation order . . . . . . . . . . . . . . . . . . . . . . . 193
defining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221
library . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241
assertions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244
associative arrays and . . . . . . . . . . . . . . . . . . 242
C library . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259
character values as numbers . . . . . . . . . . . . 247
Cliff random numbers . . . . . . . . . . . . . . . . . . 247
command-line options . . . . . . . . . . . . . . . . . . 259
example program for using . . . . . . . . . . . . . 322
group database, reading . . . . . . . . . . . . . . . . 270
managing data files . . . . . . . . . . . . . . . . . . . . 254
managing time . . . . . . . . . . . . . . . . . . . . . . . . . 249
merging arrays into strings . . . . . . . . . . . . . 249
rounding numbers. . . . . . . . . . . . . . . . . . . . . . 246
user database, reading . . . . . . . . . . . . . . . . . 266
names of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221
recursive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222
string-translation . . . . . . . . . . . . . . . . . . . . . . . . . . 220
undefined . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228
user-defined . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221
calling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224
counts, in a profile . . . . . . . . . . . . . . . . . . . . . 350
library of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241
next/nextfile statements and . . . . 161, 162

G
G-d . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
G., Daniel Richard . . . . . . . . . . . . . . . . . . . . . . . . 12, 509
Garfinkle, Scott . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 488
gawk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5, See also awk
ARGIND variable in . . . . . . . . . . . . . . . . . . . . . . . . . . 40
awk and . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5, 7
bitwise operations in . . . . . . . . . . . . . . . . . . . . . . 216
break statement in . . . . . . . . . . . . . . . . . . . . . . . . 159
character classes and . . . . . . . . . . . . . . . . . . . . . . . 59
coding style in . . . . . . . . . . . . . . . . . . . . . . . . . . . . 516
command-line options, regular expressions
and . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
configuring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 497
options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 496
continue statement in. . . . . . . . . . . . . . . . . . . . . 160
distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 492
dynamic profiling . . . . . . . . . . . . . . . . . . . . . . . . . . 351
ERRNO variable in . . . . . . . . 89, 115, 152, 167, 347
escape sequences . . . . . . . . 54, See also backslash
extensions, disabling . . . . . . . . . . . . . . . . . . . . . . . . 38
features,adding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 516
advanced . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335
field separators and. . . . . . . . . . . . . . . . . . . . . . . . 164
FIELDWIDTHS variable in . . . . . . . . . . . . . . . 80, 163
file names in . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
format-control characters . . . . . . . . . . . . . 103, 104
FPAT variable in. . . . . . . . . . . . . . . . . . . . . . . . 83, 164
FUNCTAB array in . . . . . . . . . . . . . . . . . . . . . . . . . . 168
function arguments and . . . . . . . . . . . . . . . . . . . 193
hexadecimal numbers and . . . . . . . . . . . . . . . . . 121
IGNORECASE variable in . . 63, 164, 179, 196, 342
implementation issues . . . . . . . . . . . . . . . . . . . . . 515
debugging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 515
downward compatibility . . . . . . . . . . . . . . . . 515
limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
pipes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
installing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 491
internationalization, See internationalizationinterpreter, adding code to . . . . . . . . . . . . . . . . 455
interval expressions and . . . . . . . . . . . . . . . . . . . . 57
line continuation in . . . . . . . . . . . . . . . . . . . . . . . . 142
LINT variable in . . . . . . . . . . . . . . . . . . . . . . . . . . . 164
list of contributors to . . . . . . . . . . . . . . . . . . . . . . 488
MS-Windows version of. . . . . . . . . . . . . . . . . . . . 499
newlines in . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
octal numbers and. . . . . . . . . . . . . . . . . . . . . . . . . 121
OpenVMS version of . . . . . . . . . . . . . . . . . . . . . . 501
predefined variables and . . . . . . . . . . . . . . . . . . . 163
PROCINFO array in . . . . . . . . . . . . 40, 168, 212, 345
profiling programs . . . . . . . . . . . . . . . . . . . . . . . . . 351
regexp constants and . . . . . . . . . . . . . . . . . . . . . . 122
regular expressions,case sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . 63
operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
precedence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
RT variable in . . . . . . . . . . . . . . . . . . . 67, 68, 89, 171

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GAWK: Effective AWK Programming

source code, obtaining . . . . . . . . . . . . . . . . . . . . . 491
splitting fields and . . . . . . . . . . . . . . . . . . . . . . . . . . 86
string-translation functions . . . . . . . . . . . . . . . . 220
SYMTAB array in . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
TEXTDOMAIN variable in . . . . . . . . . . . . . . . . . . . . 165
timestamps. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211
uses for . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
version of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170
printing information about . . . . . . . . . . . . . . 39
word-boundary operator . . . . . . . . . . . . . . . . . . . . 62
GAWK_LOCALE_DIR environment variable . . . . 45, 359
GAWK_MSEC_SLEEP environment variable . . . . . . . . . 44
GAWK_MSG_SRC environment variable . . . . . . . . . . . . 45
GAWK_NO_DFA environment variable . . . . . . . . . . . . . 45
GAWK_PERSIST_FILE environment variable . . 44, 352
GAWK_READ_TIMEOUT environment variable . . . 44, 96
GAWK_SOCK_RETRIES environment variable . . 44, 117
GAWK_STACKSIZE environment variable . . . . . . . . . . 45
gawkbug utility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 507
gawkextlib project . . . . . . . . . . . . . . . . . . . . . . . . . . . 467
gawklibpath_append shell function . . . . . . . . . . . . 496
gawklibpath_default shell function . . . . . . . . . . 496
gawklibpath_prepend shell function . . . . . . . . . . 496
gawkpath_append shell function . . . . . . . . . . . . . . . 496
gawkpath_default shell function . . . . . . . . . . . . . . 496
gawkpath_prepend shell function . . . . . . . . . . . . . . 496
General Public License, See GPL
generate time values . . . . . . . . . . . . . . . . . . . . . . . . . . 212
gensub() function (gawk) . . . . . . . . . . . . . . . . 122, 197
escape processing . . . . . . . . . . . . . . . . . . . . . . . . . . 205
getaddrinfo() function (C library) . . . . . . . . . . . 347
getgrent() function (C library) . . . . . . . . . . 270, 274
getgrent() user-defined function . . . . . . . . . 270, 274
getgrgid() function (C library). . . . . . . . . . . . . . . 273
getgrgid() user-defined function . . . . . . . . . . . . . 273
getgrnam() function (C library). . . . . . . . . . . . . . . 273
getgrnam() user-defined function . . . . . . . . . . . . . 273
getgruser() function (C library) . . . . . . . . . . . . . 273
getgruser() user-defined function . . . . . . . . . . . . 273
getline command . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
_gr_init() user-defined function . . . . . . . . . . 271_pw_init() function. . . . . . . . . . . . . . . . . . . . . . . 269
BEGINFILE/ENDFILE patterns and . . . . . . . . . . 152
coprocesses, using from . . . . . . . . . . . . . . . . 94, 113
deadlock and . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 344
explicit input with . . . . . . . . . . . . . . . . . . . . . . . . . . 89
FILENAME variable and . . . . . . . . . . . . . . . . . . . . . . 94
from a file . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
into a variable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
return values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
variants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
getlocaltime() user-defined function . . . . . . . . . 249
getopt() function (C library) . . . . . . . . . . . . . . . . . 259
getopt() user-defined function . . . . . . . . . . . 261, 262
getpwent() function (C library) . . . . . . . . . . 266, 269
getpwent() user-defined function . . . . . . . . . 266, 269
getpwnam() function (C library). . . . . . . . . . . . . . . 269
getpwnam() user-defined function . . . . . . . . . . . . . 269

getpwuid() function (C library). . . . . . . . . . . . . . . 269
getpwuid() user-defined function . . . . . . . . . . . . . 269
gettext library . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357
locale categories . . . . . . . . . . . . . . . . . . . . . . . . . . . 358
gettext() function (C library) . . . . . . . . . . . . . . . . 358
gettimeofday() extension function . . . . . . . . . . . 466
Ghostbusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205
git utility . . . . . . . . . . . . . . . . . . . . . . 468, 510, 515, 517
Git, use of for gawk source code . . . . . . . . . . . . . . . 519
global variables, show in debugger . . . . . . . . . . . . . 378
GMP values, API ownership of . . . . . . . . . . . . . . . . 416
GNITS mailing list . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
GNU awk, See gawk
GNU Free Documentation License . . . . . . . . . . . . 553
GNU General Public License, See GPL
GNU Lesser General Public License . . . . . . . . . . . 536
GNU long options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
printing list of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
GNU Project. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10, 535
GNU/Linux . . . . . . . . . . . . . . . . . . . . . . . . . . 10, 365, 540
Go implementation of awk. . . . . . . . . . . . . . . . . . . . . 511
goawk. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 511
Gordon, Assaf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 489
GPL (General Public License) . . . . . . . . . . . . . 10, 535
printing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
grcat program. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270
Grigera, Juan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 488
group database, reading . . . . . . . . . . . . . . . . . . . . . . . 270
group file . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270
group ID of gawk user . . . . . . . . . . . . . . . . . . . . . . . . . 169
groups, information about. . . . . . . . . . . . . . . . . . . . . 270
gsub() function . . . . . . . . . . . . . . . . . . . . . . . . . . 122, 198
arguments of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203
escape processing . . . . . . . . . . . . . . . . . . . . . . . . . . 205
Guerrero, Juan Manuel . . . . . . . . . . . . . . . . . . . . 12, 489

Hh debugger command (alias for help) . . . . . . . . . . 380
Hankerson, Darrel . . . . . . . . . . . . . . . . . . . . . . . . . 12, 488
Haque, John . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 489
Hartholz,
Elaine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Marshall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Hasegawa, Isamu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 489
help debugger command . . . . . . . . . . . . . . . . . . . . . . 380
hexadecimal numbers . . . . . . . . . . . . . . . . . . . . . . . . . 120
hexadecimal values, enabling interpretation of . . 37
history expansion, in debugger . . . . . . . . . . . . . . . . 381
histsort.awk program . . . . . . . . . . . . . . . . . . . . . . . 316
Hughes, Phil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
HUP signal, for dynamic profiling . . . . . . . . . . . . . . 351
hyphen (-),
- operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144-- end of options marker . . . . . . . . . . . . . . . . . . . 39-- operator . . . . . . . . . . . . . . . . . . . . . . . . . . . 134, 144-= operator . . . . . . . . . . . . . . . . . . . . . . . . . . . 132, 145
file names beginning with . . . . . . . . . . . . . . . . . . . 34

Index 575

in bracket expressions . . . . . . . . . . . . . . . . . . . . . . 58

I
i debugger command (alias for info) . . . . . . . . . . 377
id utility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 288
id.awk program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 289
if statement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
actions, changing . . . . . . . . . . . . . . . . . . . . . . . . . . 149
use of regexps in. . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
igawk.sh program . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324
ignore breakpoint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374
ignore debugger command . . . . . . . . . . . . . . . . . . . 374
IGNORECASE variable . . . . . . . . . . . . . . . . . . . . . . . . . . 164
array indices and . . . . . . . . . . . . . . . . . . . . . . . . . . 179
array sorting functions and . . . . . . . . . . . . . . . . 342
in example programs . . . . . . . . . . . . . . . . . . . . . . 241
with ~ and !~ operators . . . . . . . . . . . . . . . . . . . . 63
Illumos, POSIX-compliant awk . . . . . . . . . . . . . . . . 511
implementation issues, gawk . . . . . . . . . . . . . . . . . . . 515
debugging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 515
limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94, 110
implicit namespace. . . . . . . . . . . . . . . . . . . . . . . . . . . . 384
in operator . . . . . . . . . . . . . . . . . . . . . . . . . 138, 145, 157
index existence in multidimensional arrays . 188
order of array access . . . . . . . . . . . . . . . . . . . . . . . 182
testing if array element exists . . . . . . . . . . . . . . 179
use in loops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181@include directive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
including files, @include directive . . . . . . . . . . . . . . 46
increment operators . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
index() function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198
indexing arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178
indirect function calls . . . . . . . . . . . . . . . . . . . . . . . . . 231@-notation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232
infinite precision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 389
info debugger command . . . . . . . . . . . . . . . . . . . . . . 377
initialization, automatic . . . . . . . . . . . . . . . . . . . . . . . . 28
inplace extension . . . . . . . . . . . . . . . . . . . . . . . . . . . . 461
input,
data, nondecimal . . . . . . . . . . . . . . . . . . . . . . . . . . 335
explicit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
multiline records . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
splitting into records . . . . . . . . . . . . . . . . . . . . . . . 65
standard. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18, 111
input files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
closing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
counting elements in. . . . . . . . . . . . . . . . . . . . . . . 304
examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
running awk without . . . . . . . . . . . . . . . . . . . . . . . . 18
variable assignments and . . . . . . . . . . . . . . . . . . . 41
input pipeline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
input record, length of . . . . . . . . . . . . . . . . . . . . . . . . 198
input redirection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
input, files, See input files
input/output,
binary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163

from BEGIN and END . . . . . . . . . . . . . . . . . . . . . . . 151
functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208
two-way . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 344
insomnia, cure for. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 308
installing gawk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 491
Cygwin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 501
MS-Windows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 499
OpenVMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 501
instruction tracing, in debugger . . . . . . . . . . . . . . . 378
instructions, trace of internal . . . . . . . . . . . . . . . . . . . 36
int() function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194
INT signal (MS-Windows) . . . . . . . . . . . . . . . . . . . . . 351
INT_CHAIN_MAX environment variable . . . . . . . . . . . 45
integer array indices . . . . . . . . . . . . . . . . . . . . . . . . . . 185
integers,
arbitrary precision . . . . . . . . . . . . . . . . . . . . . . . . . 399
unsigned . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 389
interacting with other programs . . . . . . . . . . . . . . . 209
internationalization . . . . . . . . . . . . . . . . . . . . . . 220, 357
localization . . . . . . . . . . . . . . . . . . . . . . . . . . . 165, 357
character classes . . . . . . . . . . . . . . . . . . . . . . . . 59
currency symbols . . . . . . . . . . . . . . . . . . . . . . 359
gawk and . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357
locale categories . . . . . . . . . . . . . . . . . . . . . . . 358
marked strings . . . . . . . . . . . . . . . . . . . . . . . . . 359
monetary information . . . . . . . . . . . . . . . . . . 359
portability and. . . . . . . . . . . . . . . . . . . . . . . . . 363
internationalizing a program . . . . . . . . . . . . . . . . . . 357
interpreted programs . . . . . . . . . . . . . . . . . . . . . 525, 535
interval expressions, regexp operator . . . . . . . . . . . 56
inventory-shipped file . . . . . . . . . . . . . . . . . . . . . . . . 24
invoke shell command . . . . . . . . . . . . . . . . . . . . . . . . . 209
isarray() function (gawk) . . . . . . . . . . . . . . . . . . . . 219
isnumeric() user-defined function . . . . . . . . . . . . 253
ISO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 536
ISO 8601 date and time standard . . . . . . . . . . 214
ISO 8859-1 character set. . . . . . . . . . . . . . . . . . . 531
ISO Latin-1 character set . . . . . . . . . . . . . . . . . . 531

J
Jacobs, Andrew . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267
Jaegermann, Michal . . . . . . . . . . . . . . . . . . . . . . . 12, 488
Jannick . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 489
Java implementation of awk . . . . . . . . . . . . . . . . . . . 512
Java programming language . . . . . . . . . . . . . . . . . . . 536
jawk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 512
jedi knights . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
Johansen, Chris . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 330
join() user-defined function . . . . . . . . . . . . . . . . . . 249

576

GAWK: Effective AWK ProgrammingK
Kahrs, Jürgen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12, 488
Kasal, Stepan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Kelly, Terence . . . . . . . . . . . . . . . . . . . . . . . 353, 354, 485
Kenobi, Obi-Wan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
Kernighan, Brian . . . . 6, 12, 475, 488, 510, 527, 532
quotes . . . . . . . . . . . . . . . . 10, 76, 92, 129, 241, 331
kill command, dynamic profiling . . . . . . . . . . . . . 351
knights, jedi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
Kwok, Conrad . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 488

Ll debugger command (alias for list) . . . . . . . . . . 380
labels.awk program . . . . . . . . . . . . . . . . . . . . . . . . . . 313
LANG environment variable . . . . . . . . . . . . . . . . . . . . 365
Langston, Peter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335
LANGUAGE environment variable . . . . . . . . . . . 359, 365
languages, data-driven . . . . . . . . . . . . . . . . . . . . . . . . 526
LC_ALL environment variable . . . . . . . . 294, 365, 506
LC_ALL locale category . . . . . . . . . . . . . . . . . . . . . . . . 359
LC_COLLATE locale category . . . . . . . . . . . . . . . . . . . 358
LC_CTYPE locale category . . . . . . . . . . . . . . . . . . . . . . 358
LC_MESSAGES environment variable . . . . . . . . . . . . 365
LC_MESSAGES locale category . . . . . . . . . . . . . . . . . . 358
bindtextdomain() function (gawk). . . . . . . . . 361
LC_MONETARY locale category . . . . . . . . . . . . . . . . . . 359
LC_NUMERIC locale category . . . . . . . . . . . . . . . . . . . 359
LC_TIME locale category . . . . . . . . . . . . . . . . . . . . . . . 359
left angle bracket (<),
< operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138, 144< operator (I/O). . . . . . . . . . . . . . . . . . . . . . . . . . . . 91<= operator . . . . . . . . . . . . . . . . . . . . . . . . . . . 138, 144left shift, bitwise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216
leftmost longest match . . . . . . . . . . . . . . . . . . . . . . . . . 87
length of input record . . . . . . . . . . . . . . . . . . . . . . . . . 198
length of string . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198
length() function . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198
Lesser General Public License (LGPL) . . . . . . . . 536
LGPL (Lesser General Public License) . . . . . . . . 536
libmawk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 512
libraries of awk functions . . . . . . . . . . . . . . . . . . . . . . 241
assertions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244
associative arrays and . . . . . . . . . . . . . . . . . . . . . 242
character values as numbers . . . . . . . . . . . . . . . 247
command-line options . . . . . . . . . . . . . . . . . . . . . 259
example program for using . . . . . . . . . . . . . . . . 322
group database, reading . . . . . . . . . . . . . . . . . . . 270
managing,
data files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254
time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249
merging arrays into strings . . . . . . . . . . . . . . . . 249
rounding numbers . . . . . . . . . . . . . . . . . . . . . . . . . 246
user database, reading . . . . . . . . . . . . . . . . . . . . . 266
line breaks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
line continuations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
gawk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
in print statement . . . . . . . . . . . . . . . . . . . . . . . . 100

with C shell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
lines,
blank, printing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
counting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304
duplicate, removing . . . . . . . . . . . . . . . . . . . . . . . 316
matching ranges of . . . . . . . . . . . . . . . . . . . . . . . . 149
skipping between markers . . . . . . . . . . . . . . . . . 149
lint checking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164
array subscripts . . . . . . . . . . . . . . . . . . . . . . 186, 187
empty programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
issuing warnings . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
POSIXLY_CORRECT environment variable . . . . . 40
undefined functions . . . . . . . . . . . . . . . . . . . . . . . . 228
LINT variable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164
Linux, See GNU/Linuxlist all global variables, in debugger . . . . . . . . . . . 378
list debugger command . . . . . . . . . . . . . . . . . . . . . . 380
list function definitions, in debugger . . . . . . . . . . . 377@load directive. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
loading extensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
@load directive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
local variables,in a function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225
show in debugger . . . . . . . . . . . . . . . . . . . . . . . . . . 378
locale categories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 358
locale decimal point character . . . . . . . . . . . . . . . . . . 38
locale, definition of. . . . . . . . . . . . . . . . . . . . . . . . . . . . 145
localization, See internationalization, localizationlog files, timestamps in . . . . . . . . . . . . . . . . . . . . . . . . 211
log() function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194
logarithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194
logical false/true . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
logical operators, See Boolean expressions
login information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266
long options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
loops . . . . . . . . . . . . . . . 155, See also while statement
break statement and . . . . . . . . . . . . . . . . . . . . . . 159
continue statement and . . . . . . . . . . . . . . . . . . . 157
count for header, in a profile . . . . . . . . . . . . . . . 350
do-while . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
exiting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
for,array scanning . . . . . . . . . . . . . . . . . . . . . . . . . 181
iterative . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
while . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
ls utility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25, 27
lshift() function (gawk) . . . . . . . . . . . . . . . . . . . . . 216
lvalues/rvalues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131

Index 577

M
mail-list file . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
mailing labels, printing. . . . . . . . . . . . . . . . . . . . . . . . 313
mailing list, GNITS . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Malmberg, John . . . . . . . . . . . . . . . . . . . . . . 12, 489, 509
mark parity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248
marked string extraction (internationalization) . 361
Marx, Groucho . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
match regexp in string . . . . . . . . . . . . . . . . . . . . . . . . 199
match() function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199
RSTART/RLENGTH variables . . . . . . . . . . . . . . . . . 199
side effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199
matching,
expressions, See comparison expressions
leftmost longest . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
null strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205
mawk utility . . . . . . . . . . . . . . . . . . 54, 93, 130, 162, 510
maximum precision supported by MPFR
library . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170
McIlroy, Doug . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 533
McPhee, Patrick T.J. . . . . . . . . . . . . . . . . . . . . . . . . . 489
memory, allocating for extensions . . . . . . . . . . . . . 413
message object files . . . . . . . . . . . . . . . . . . . . . . . . . . . 358
converting from portable object files . . . . . . . 365
specifying directory of . . . . . . . . . . . . . . . . 358, 360
messages from extensions . . . . . . . . . . . . . . . . . . . . . 427
metacharacters,
escape sequences for . . . . . . . . . . . . . . . . . . . . . . . . 54
in regular expressions . . . . . . . . . . . . . . . . . . . . . . . 54
minimum precision required by MPFR library . 170
Minshall, Greg . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261
mkbool() function (gawk) . . . . . . . . . . . . . . . . . . . . . 194
mktime() function (gawk) . . . . . . . . . . . . . . . . . . . . . 212
modifiers, in format specifiers . . . . . . . . . . . . . . . . . 105
module, definition of . . . . . . . . . . . . . . . . . . . . . . . . . . 383
monetary information, localization . . . . . . . . . . . . 359
Moon, Sailor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357
Moore, Duncan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
MPFR library, building with . . . . . . . . . . . . . . . . . . 496
MPFR values, API ownership of . . . . . . . . . . . . . . 416
MPFR, checking for . . . . . . . . . . . . . . . . . . . . . . . . . . 400
msgfmt utility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 365
multiple precision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 389
multiple-line records . . . . . . . . . . . . . . . . . . . . . . . . . . . 86Nn debugger command (alias for next) . . . . . . . . . . 374
name management . . . . . . . . . . . . . . . . . . . . . . . . . . . . 385
names,
arrays/variables . . . . . . . . . . . . . . . . . . . . . . . . . . . 242
functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221, 242
namespace,
awk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383
default . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383
definition of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383
example code. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 386
implicit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 384

pushing and popping . . . . . . . . . . . . . . . . . . . . . . 384
standard awk, global . . . . . . . . . . . . . . . . . . . . . . . 383@namespace directive . . . . . . . . . . . . . . . . . . . . . . . . . . 384
namespaces,
backwards compatibility . . . . . . . . . . . . . . . . . . . 388
changing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 384
interaction with,debugger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 388
extension API . . . . . . . . . . . . . . . . . . . . . . . . . 388
pretty printer . . . . . . . . . . . . . . . . . . . . . . . . . . 387
profiler . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 387
naming rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 384
qualified names . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383
naming issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242
functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221
naming rules, namespace and component names . 384
Neacsu, Mircea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 512
NetBSD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 540
networks,
programming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 346
support for . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
newlines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28, 38, 141
as record separators . . . . . . . . . . . . . . . . . . . . . . . . 65
in dynamic regexps . . . . . . . . . . . . . . . . . . . . . . . . . 61
in regexp constants . . . . . . . . . . . . . . . . . . . . . . . . . 61
printing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
separating statements in actions . . . . . . 153, 154
next debugger command . . . . . . . . . . . . . . . . . . . . . . 374
next file statement . . . . . . . . . . . . . . . . . . . . . . . . . . 480
next statement . . . . . . . . . . . . . . . . . . . . . . . . . . 141, 160
BEGIN/END patterns and . . . . . . . . . . . . . . . . . . . 151
BEGINFILE/ENDFILE patterns and . . . . . . . . . . 152
user-defined functions and . . . . . . . . . . . . . . . . . 161
nextfile statement . . . . . . . . . . . . . . . . . . . . . . . . . . . 161
BEGIN/END patterns and . . . . . . . . . . . . . . . . . . . 151
BEGINFILE/ENDFILE patterns and . . . . . . . . . . 152
user-defined functions and . . . . . . . . . . . . . . . . . 162
nexti debugger command . . . . . . . . . . . . . . . . . . . . . 375
NF variable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69, 167
decrementing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
ni debugger command (alias for nexti) . . . . . . . 375
noassign.awk program . . . . . . . . . . . . . . . . . . . . . . . 259
non-existent array elements . . . . . . . . . . . . . . . . . . . 179
nonfatal output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
not Boolean-logic operator . . . . . . . . . . . . . . . . . . . . 140
NR variable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65, 168
changing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173
null strings . . . . . . . . . . . . . . . . . . . . . . . 67, 75, 134, 527
as array subscripts . . . . . . . . . . . . . . . . . . . . . . . . 186
converting numbers to strings. . . . . . . . . . . . . . 126
deleting array elements and . . . . . . . . . . . . . . . . 187
in gawk arguments, quoting and . . . . . . . . . 22, 42
matching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205
number of array elements . . . . . . . . . . . . . . . . . . . . . 199
number sign (#),#! (executable scripts) . . . . . . . . . . . . . . . . . . . . . . 19
commenting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

578

GAWK: Effective AWK Programming

numbers,
as array subscripts . . . . . . . . . . . . . . . . . . . . . . . . 185
as string of bits . . . . . . . . . . . . . . . . . . . . . . . . . . . 217
as values of characters . . . . . . . . . . . . . . . . . . . . . 247
Cliff random . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247
converting . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126, 217
to strings . . . . . . . . . . . . . . . . . . . . . . . . . 163, 165
hexadecimal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
octal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
rounding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246
numeric,
constants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194
output format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136

Oo debugger command (alias for option) . . . . . . . 378
obsolete features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
octal numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
octal values, enabling interpretation of . . . . . . . . . 37
OFMT variable . . . . . . . . . . . . . . . . . . . . . . . . 102, 127, 165
POSIX awk and . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
OFS variable . . . . . . . . . . . . . . . . . . . . . . . . . . 72, 101, 165
op-codes, trace of internal . . . . . . . . . . . . . . . . . . . . . . 36
OpenBSD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 540
OpenSolaris . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 511
operating systems . . 491, See also GNU/Linux, Seealso PC operating systems, See also Unix
BSD-based . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
PC, gawk on. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 499
installing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 499
porting gawk to. . . . . . . . . . . . . . . . . . . . . . . . . . . . 518
operations, bitwise . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216
operators,arithmetic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
assignment . . . . . . . . . . . . . . . . . . . . . . . . . . . 130, 131
evaluation order . . . . . . . . . . . . . . . . . . . . . . . 132
comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
decrement/increment . . . . . . . . . . . . . . . . . . . . . . 133
GNU-specific . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
input/output . . . . . . . . . 91, 92, 94, 108, 109, 144precedence of . . . . . . . . . . . . . . . . . . . . . . . . . 134, 144
short-circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
string . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
string-matching . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
for buffers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
word-boundary (gawk) . . . . . . . . . . . . . . . . . . . . . . 62
operators, Boolean, See Boolean expressions
operators, logical, See Boolean expressions
operators, relational, See operators, comparison
option debugger command . . . . . . . . . . . . . . . . . . . 378
options,
command-line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
end of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
invoking awk . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259

deprecated . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
long . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
printing list of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
or Boolean-logic operator . . . . . . . . . . . . . . . . . . . . . 140
or() function (gawk) . . . . . . . . . . . . . . . . . . . . . . . . . . 216
OR bitwise operation . . . . . . . . . . . . . . . . . . . . . . . . . 216
ord() extension function . . . . . . . . . . . . . . . . . . . . . . 463
ord() user-defined function . . . . . . . . . . . . . . . . . . . 247
order of evaluation, concatenation . . . . . . . . . . . . . 130
ORS variable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101, 165
output,buffering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208, 211
duplicating into files . . . . . . . . . . . . . . . . . . . . . . . 298
files, closing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
format specifier, OFMT . . . . . . . . . . . . . . . . . . . . . . 102
formatted . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
pipes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
records . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
standard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
output field separator, See OFS variable
output record separator, See ORS variable
output redirection . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
output wrapper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424
output, nonfatal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
output, printing, See printingP
p debugger command (alias for print) . . . . . . . . 376
package, definition of . . . . . . . . . . . . . . . . . . . . . . . . . 383
Papadopoulos, Panos . . . . . . . . . . . . . . . . . . . . . . . . . 489
parent process ID of gawk process . . . . . . . . . . . . . 169
parentheses (),in a profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 350
regexp operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
password file . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266
Path environment variable . . . . . . . . . . . . . . . . . . . . 499
PATH environment variable . . . . . . . . . . . . . . . . . . . . 520
patsplit() function (gawk) . . . . . . . . . . . . . . . . . . . 201
patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
Boolean expressions as . . . . . . . . . . . . . . . . . . . . 148
comparison expressions as . . . . . . . . . . . . . . . . . 148
counts, in a profile . . . . . . . . . . . . . . . . . . . . . . . . 350
default . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
empty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
expressions as . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
ranges in . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
regexp constants as . . . . . . . . . . . . . . . 51, 147, 148
types of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
pawk (profiling version of Brian Kernighan’sawk) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 511
pawk, awk-like facilities for Python . . . . . . . . . . . . 512
PC operating systems, gawk on . . . . . . . . . . . . . . . . 499
installing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 499
percent sign (%),
% operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144%= operator . . . . . . . . . . . . . . . . . . . . . . . . . . . 132, 145
performance, checking issues . . . . . . . . . . . . . . . . . . 508

Index 579

period (.), regexp operator . . . . . . . . . . . . . . . . . . . . . 55
Perl . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 521
persistent memory . . . . . . . . . . . . . . . . . . . . . . . . . . . . 352
compiled into gawk . . . . . . . . . . . . . . . . . . . . . . . . 169
Peters, Arno . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 489
Peterson, Hal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 488
pipe,
closing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
pipe output, speeding up . . . . . . . . . . . . . . . . . . . . . . 116
platform running on . . . . . . . . . . . . . . . . . . . . . . . . . . 169
Plauger, P.J. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241
plug-in . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405
plus sign (+),
+ operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144++ operator . . . . . . . . . . . . . . . . . . . . . . 133, 134, 144+= operator . . . . . . . . . . . . . . . . . . . . . . . . . . . 132, 145
regexp operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
PMA memory allocator . . . . . . . . . . . . . . . . . . 169, 352
PMA_VERBOSITY environment variable . . . . . . 44, 353
pointers to functions . . . . . . . . . . . . . . . . . . . . . . . . . . 231
portability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53#! (executable scripts) . . . . . . . . . . . . . . . . . . . . . . 20** operator and . . . . . . . . . . . . . . . . . . . . . . . . . . . 129**= operator and . . . . . . . . . . . . . . . . . . . . . . . . . . 133
ARGV variable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
backslash continuation and . . . . . . . . . . . . . . . . . 28
backslash in escape sequences . . . . . . . . . . . . . . . 54
close() function and . . . . . . . . . . . . . . . . . . . . . . 114
data files as single record . . . . . . . . . . . . . . . . . . . 69
deleting array elements . . . . . . . . . . . . . . . . . . . . 187
example programs . . . . . . . . . . . . . . . . . . . . . . . . . 241
functions, defining . . . . . . . . . . . . . . . . . . . . . . . . . 223
gawk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 518
gettext library and . . . . . . . . . . . . . . . . . . . . . . . 357
internationalization and . . . . . . . . . . . . . . . . . . . 363
length() function . . . . . . . . . . . . . . . . . . . . . . . . . 198
new awk vs. old awk . . . . . . . . . . . . . . . . . . . . . . . 127
next statement in user-defined functions . . . 228
NF variable, decrementing . . . . . . . . . . . . . . . . . . . 73
operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
not in POSIX awk . . . . . . . . . . . . . . . . . . . . . 145
POSIXLY_CORRECT environment variable . . . . . 40
substr() function . . . . . . . . . . . . . . . . . . . . . . . . . 204
portable object,files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 358, 361
converting to message object files . . . . . . 365
generating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
template files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357
porting gawk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 518
positional specifiers, printf statement . . . . 105, 362
mixing with regular formats . . . . . . . . . . . . . . . 362
POSIX,
awk and . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5gawk extensions not included in . . . . . . . . . . . . 475
programs, implementing in awk . . . . . . . . . . . . 279
POSIX awk. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7, 133** operator and . . . . . . . . . . . . . . . . . . . . . . . . . . . 145**= operator and . . . . . . . . . . . . . . . . . . . . . . . . . . 133< operator and . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
| I/O operator and . . . . . . . . . . . . . . . . . . . . . . . . . 93
arithmetic operators and . . . . . . . . . . . . . . . . . . 128
backslashes in string constants . . . . . . . . . . . . . . 54
BEGIN/END patterns . . . . . . . . . . . . . . . . . . . . . . . . 151
bracket expressions and . . . . . . . . . . . . . . . . . . . . . 58
character classes . . . . . . . . . . . . . . . . . . . . . 58, 59
break statement and . . . . . . . . . . . . . . . . . . . . . . 159
changes in awk versions . . . . . . . . . . . . . . . . . . . . 474
continue statement and . . . . . . . . . . . . . . . . . . . 160
CONVFMT variable and . . . . . . . . . . . . . . . . . . . . . . 163
date utility and . . . . . . . . . . . . . . . . . . . . . . . . . . . 215
field separators and . . . . . . . . . . . . . . . . . . . . . . . . . 79
function keyword in . . . . . . . . . . . . . . . . . . . . . . 222
functions and,gsub()/sub() . . . . . . . . . . . . . . . . . . . . . . . . . . 206
length() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198
GNU long options and . . . . . . . . . . . . . . . . . . . . . . 33
interval expressions in . . . . . . . . . . . . . . . . . . . . . . 57
next/nextfile statements and . . . . . . . . . . . . 161
numeric strings and . . . . . . . . . . . . . . . . . . . . . . . 136
OFMT variable and . . . . . . . . . . . . . . . . . . . . . 102, 127
period (.), using. . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
printf format strings and . . . . . . . . . . . . . . . . . 107
regular expressions and . . . . . . . . . . . . . . . . . . . . . 56
timestamps and . . . . . . . . . . . . . . . . . . . . . . . . . . . 211
POSIX mode . . . . 38, 40, 55, 98, 113, 115, 120, 127,
139, 140, 202, 340, 402, 482
POSIXLY_CORRECT environment variable . 40, 44, 49,
127
PREC variable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165
precedence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134, 144
regexp operators. . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
predefined variables . . . . . . . . . . . . . . . . . . . . . . . . . . . 163-v option, setting with. . . . . . . . . . . . . . . . . . . . . . 34
conveying information . . . . . . . . . . . . . . . . . . . . . 165
user-modifiable . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163
pretty printer, interaction with namespaces . . . . 387
pretty printing . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38, 351
profiling, difference with . . . . . . . . . . . . . . . . . . . 351
print debugger command . . . . . . . . . . . . . . . . . . . . . 376
print statement . 99, See also redirection of output
BEGIN/END patterns and . . . . . . . . . . . . . . . . . . . 151
commas, omitting . . . . . . . . . . . . . . . . . . . . . . . . . 100
I/O operators in. . . . . . . . . . . . . . . . . . . . . . . . . . . 144
line continuations and . . . . . . . . . . . . . . . . . . . . . 100
OFMT variable and . . . . . . . . . . . . . . . . . . . . . . . . . 165
sprintf() function and . . . . . . . . . . . . . . . . . . . 246
print variables, in debugger . . . . . . . . . . . . . . . . . . . 376
printf debugger command . . . . . . . . . . . . . . . . . . . 376
printf statement . . 99, 102, See also redirection of
outputcolumns, aligning . . . . . . . . . . . . . . . . . . . . . . . . . . 100
format-control characters . . . . . . . . . . . . . . . . . . 103
I/O operators in. . . . . . . . . . . . . . . . . . . . . . . . . . . 144
modifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

580

GAWK: Effective AWK Programming

positional specifiers . . . . . . . . . . . . . . . . . . . 105, 362
mixing with regular formats . . . . . . . . . . . . 362
sprintf() function and . . . . . . . . . . . . . . . . . . . 246
syntax of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
printing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
list of options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
mailing labels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313
messages from extensions . . . . . . . . . . . . . . . . . . 427
unduplicated lines of text . . . . . . . . . . . . . . . . . . 300
user information . . . . . . . . . . . . . . . . . . . . . . . . . . . 288
private variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242
process group ID of gawk process . . . . . . . . . . . . . . 169
process ID of gawk process . . . . . . . . . . . . . . . . . . . . 169
processes, two-way communications with . . . . . . 343
processing data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 525
PROCINFO array . . . . . . . . . . . . . . . . . . . . . . 168, 212, 266
communications via ptys and . . . . . . . . . . . . . . 345
group membership and . . . . . . . . . . . . . . . . . . . . 270
nonfatal output . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
not flushing pipe buffers . . . . . . . . . . . . . . . . . . . 116
platform running on . . . . . . . . . . . . . . . . . . . . . . . 169
testing the field splitting. . . . . . . . . . . . . . . . . . . 269
user and group ID numbers and . . . . . . . . . . . 288
values of sorted_in . . . . . . . . . . . . . . . . . . . . . . . 183
profiler, interaction with namespaces . . . . . . . . . . 387
profiling awk programs . . . . . . . . . . . . . . . . . . . . . . . . 348
dynamically . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 351
profiling, compiling gawk for . . . . . . . . . . . . . . . . . . 508
profiling, pretty printing, difference with . . . . . . 351
program identifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169
program, definition of . . . . . . . . . . . . . . . . . . . . . . . . . . 17
programming,basic steps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 525
concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 525
programming conventions,--non-decimal-data option . . . . . . . . . . . . . . . 335
ARGC/ARGV variables . . . . . . . . . . . . . . . . . . . . . . . 166
exit statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162
function parameters . . . . . . . . . . . . . . . . . . . . . . . 229
functions,calling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193
writing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222
gawk extensions . . . . . . . . . . . . . . . . . . . . . . . . . . . 450
private variable names . . . . . . . . . . . . . . . . . . . . . 242
programming language, recipe for . . . . . . . . . . . . . . . 6
programming languages,
Ada . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 529
data-driven vs. procedural . . . . . . . . . . . . . . . . . . 17
Go . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 511, 512
Java . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 536
Proulx, Bob . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 509
pwcat program. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266

Q
q debugger command (alias for quit) . . . . . . . . . . 380
QSE awk. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 512
qualified name,definition of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383
use of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383
Quanstrom, Erik . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 308
question mark (?),?: operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145
regexp operator . . . . . . . . . . . . . . . . . . . . . . . . . 56, 62
QuikTrim Awk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 512
quit debugger command . . . . . . . . . . . . . . . . . . . . . . 380
QUIT signal (MS-Windows) . . . . . . . . . . . . . . . . . . . . 351
quoting,for small awk programs . . . . . . . . . . . . . . . . . . . . . 20
in gawk command lines . . . . . . . . . . . . . . . . . . 19, 42
tricks for . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Rr debugger command (alias for run) . . . . . . . . . . . 375
Rakitzis, Byron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316
Ramey, Chet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12, 410
Ramming, J. Christopher . . . . . . . . . . . . . . . . . . . . . 512
rand() function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194
random numbers,
Cliff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247
rand()/srand() functions . . . . . . . . . . . . . . . . . 194
seed of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195
range expressions (regexps) . . . . . . . . . . . . . . . . . . . . 58
range patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
line continuation and . . . . . . . . . . . . . . . . . . . . . . 150
Rankin, Pat . . . . . . . . . . . . . . . . . . . . . . . . . . 12, 132, 488
RapidJson JSON parser library . . . . . . . . . . . . . . . 468
reada() extension function . . . . . . . . . . . . . . . . . . . 465
readable data files, checking . . . . . . . . . . . . . . . . . . . 257
readable.awk program . . . . . . . . . . . . . . . . . . . . . . . 257
readall() extension function . . . . . . . . . . . . . . . . . 465
readdir extension . . . . . . . . . . . . . . . . . . . . . . . . . . . . 463
readfile() extension function . . . . . . . . . . . . . . . . 466
readfile() user-defined function . . . . . . . . . . . . . 251
reading input files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
REALLY_USE_PERSIST_MALLOC environment
variable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 352
recipe for a programming language . . . . . . . . . . . . . . 6
record separators . . . . . . . . . . . . . . . . . . . . . . . . . . 65, 165
changing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
newlines as . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
regular expressions as. . . . . . . . . . . . . . . . . . . . . . . 67
with multiline records . . . . . . . . . . . . . . . . . . . . . . 86
records . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65, 526
multiline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
printing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
splitting input into . . . . . . . . . . . . . . . . . . . . . . . . . 65
terminating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
treating files as . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
recursive functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222
redirect gawk output, in debugger . . . . . . . . . . . . . 378

Index 581

redirection,of input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
of output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
on OpenVMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 504
reference counting, sorting arrays . . . . . . . . . . . . . 342
regexp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
regexp constants . . . . . . . . . . . . . . . . . . . . . 52, 121, 139/=.../,/= operator and . . . . . . . . . . . . . . . . . . . . . . . . 133
as patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148
in gawk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
slashes vs. quotes . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
vs. string constants . . . . . . . . . . . . . . . . . . . . . . . . . 61
regexps, empty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
register loadable extension . . . . . . . . . . . . . . . . . . . . 417
regular expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
anchors in . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
as field separators . . . . . . . . . . . . . . . . . . . . . . . . . . 74
as patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51, 147
as record separators . . . . . . . . . . . . . . . . . . . . . . . . 67
case sensitivity . . . . . . . . . . . . . . . . . . . . . . . . 63, 164
computed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
dynamic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60with embedded newlines . . . . . . . . . . . . . . . . 61
gawk, command-line options . . . . . . . . . . . . . . . . 62
interval expressions and . . . . . . . . . . . . . . . . . . . . 39
leftmost longest match. . . . . . . . . . . . . . . . . . . . . . 60
operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51, 54
for buffers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
for words . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
gawk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
precedence of . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
searching for . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284
regular expressions, constants, See regexp constants
relational operators, See comparison operators
replace in string . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203
retrying input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
return debugger command . . . . . . . . . . . . . . . . . . . 375
return statement, user-defined functions . . . . . . 229
return value, close() function . . . . . . . . . . . . . . . . 115
rev() user-defined function . . . . . . . . . . . . . . . . . . . 223
revoutput extension . . . . . . . . . . . . . . . . . . . . . . . . . . 464
revtwoway extension . . . . . . . . . . . . . . . . . . . . . . . . . . 464
rewind() user-defined function . . . . . . . . . . . . . . . . 256
right angle bracket (>),
> operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138, 144> operator (I/O) . . . . . . . . . . . . . . . . . . . . . . . . . . 108>= operator . . . . . . . . . . . . . . . . . . . . . . . . . . . 138, 144>> operator (I/O). . . . . . . . . . . . . . . . . . . . . 109, 144
right shift, bitwise . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216
Ritchie, Dennis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 527
RLENGTH variable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
match() function and . . . . . . . . . . . . . . . . . . . . . . 199
Robbins,
Arnold . . . . . 78, 93, 267, 308, 410, 490, 509, 521
Bill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
Harry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Jean . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Malka . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357
Miriam . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13, 93, 267
Rommel, Kai Uwe . . . . . . . . . . . . . . . . . . . . . . . . . . . . 488
round to nearest integer. . . . . . . . . . . . . . . . . . . . . . . 194
round() user-defined function . . . . . . . . . . . . . . . . . 246
rounding numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246
ROUNDMODE variable . . . . . . . . . . . . . . . . . . . . . . . 165, 398
RS variable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65, 165
multiline records and . . . . . . . . . . . . . . . . . . . . . . . 87
rshift() function (gawk) . . . . . . . . . . . . . . . . . . . . . 216
RSTART variable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
match() function and . . . . . . . . . . . . . . . . . . . . . . 199
RT variable . . . . . . . . . . . . . . . . . . . . . . . . . 67, 68, 89, 171
Rubin, Paul . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6, 488
rule, definition of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
run debugger command . . . . . . . . . . . . . . . . . . . . . . . 375
rvalues/lvalues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131

Ss debugger command (alias for step) . . . . . . . . . . 375
Sailor Moon, See Moon, Sailor
sample debugging session . . . . . . . . . . . . . . . . . . . . . 369
sandbox mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Saturday Night Live . . . . . . . . . . . . . . . . . . . . . . . . . . 230
save debugger options . . . . . . . . . . . . . . . . . . . . . . . . . 378
scalar or array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219
scalar values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 526
scanning arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181
scanning multidimensional arrays . . . . . . . . . . . . . 189
Schorr, Andrew. . . . . . . . . . . . . . . . . . . . . . . 12, 172, 489
Schreiber,
Bert . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Rita . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
search and replace in strings . . . . . . . . . . . . . . . . . . 197
search for substring . . . . . . . . . . . . . . . . . . . . . . . . . . . 198
search paths. . . . . . . . . . . . . . . . . . . . . . . . . 332, 499, 504
for loadable extensions. . . . . . . . . . . . . . . . . . . . . . 43
for source files . . . . . . . . . . . . . . . . 42, 332, 499, 504
searching,files for regular expressions . . . . . . . . . . . . . . . . 284
for words . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307
sed utility . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79, 321, 529
seeding random number generator . . . . . . . . . . . . . 195
semicolon (;),
AWKPATH variable and . . . . . . . . . . . . . . . . . . . . . . 499
separating rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
separating statements in actions . . . 30, 153, 154
separators,
field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164, 165
FIELDWIDTHS variable and . . . . . . . . . . . . . . 163
FPAT variable and . . . . . . . . . . . . . . . . . . . . . . 164
FS variable and . . . . . . . . . . . . . . . . . . . . . . . . . 74
for records . . . . . . . . . . . . . . . . . . . . . . . . . 65, 67, 165
regular expressions as . . . . . . . . . . . . . . . . . . . 67
for statements in actions. . . . . . . . . . . . . . . . . . . 153
subscript . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165
set breakpoint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373

582

GAWK: Effective AWK Programming

set debugger command . . . . . . . . . . . . . . . . . . . . . . . 376
set directory of message catalogs . . . . . . . . . . . . . . 220
set watchpoint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 376
shadowing of variable values . . . . . . . . . . . . . . . . . . 222
shell function,
gawklibpath_append . . . . . . . . . . . . . . . . . . . . . . 496
gawklibpath_default . . . . . . . . . . . . . . . . . . . . . 496
gawklibpath_prepend . . . . . . . . . . . . . . . . . . . . . 496
gawkpath_append . . . . . . . . . . . . . . . . . . . . . . . . . . 496
gawkpath_default . . . . . . . . . . . . . . . . . . . . . . . . 496
gawkpath_prepend . . . . . . . . . . . . . . . . . . . . . . . . 496
shell quoting, rules for . . . . . . . . . . . . . . . . . . . . . . 21, 42
shells,piping commands into . . . . . . . . . . . . . . . . . . . . . 111
quoting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
rules for . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
scripts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
shift, bitwise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216
short-circuit operators . . . . . . . . . . . . . . . . . . . . . . . . 141
show in debugger,
all source files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 378
breakpoints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377
function arguments . . . . . . . . . . . . . . . . . . . . . . . . 377
local variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 378
name of current source file . . . . . . . . . . . . . . . . . 378
watchpoints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 378
si debugger command (alias for stepi) . . . . . . . 375
side effects . . . . . . . . . . . . . . . . . . . . . . . . . . 130, 133, 134
array indexing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
asort() function . . . . . . . . . . . . . . . . . . . . . . . . . . 341
asorti() function . . . . . . . . . . . . . . . . . . . . . . . . . 341
assignment expressions . . . . . . . . . . . . . . . . . . . . 131
Boolean operators . . . . . . . . . . . . . . . . . . . . . . . . . 140
conditional expressions . . . . . . . . . . . . . . . . . . . . 142
decrement/increment operators . . . . . . . . . . . . 133
FILENAME variable . . . . . . . . . . . . . . . . . . . . . . . . . . 94
function calls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
gsub() function . . . . . . . . . . . . . . . . . . . . . . . . . . . 203
match() function . . . . . . . . . . . . . . . . . . . . . . . . . . 199
statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
sub() function . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203
sidebar,
A Constant’s Base Does Not Affect Its
Value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
A Note About Fuzzers . . . . . . . . . . . . . . . . . . . . . 506
Backslash Before Regular Characters . . . . . . . 53
Beware The Smoke and Mirrors! . . . . . . . . . . . 218
Carriage-Return–Line-Feed Line Endings In
CSV Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
Caveats When Using Regular Expressions for
RS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
Changing FS Does Not Affect the Fields . . . . 78
Changing NR and FNR . . . . . . . . . . . . . . . . . . . . . . 173
Controlling Output Buffering withsystem() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210
Escape Sequences for Metacharacters . . . . . . . 54

FS and IGNORECASE . . . . . . . . . . . . . . . . . . . . . . . . . 80
Interactive Versus Noninteractive Buffering . 209
Matching the Null String . . . . . . . . . . . . . . . . . . 205
Operator Evaluation Order . . . . . . . . . . . . . . . . 134
Piping into sh . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
Pre-POSIX awk Used OFMT for String
Conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
Quoting Shell Variables On The awk Command
Line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
Recipe for a Programming Language . . . . . . . . . 6
Rounding Modes and Conversion . . . . . . . . . . 399
RS = "\0" Is Not Portable . . . . . . . . . . . . . . . . . . 68
So Why Does gawk Have BEGINFILE andENDFILE? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256
Syntactic Ambiguities Between ‘/=’ andRegular Expressions . . . . . . . . . . . . . . . . . . . . 133
Understanding ‘#!’ . . . . . . . . . . . . . . . . . . . . . . . . . 19
Understanding $0 . . . . . . . . . . . . . . . . . . . . . . . . . . 73
Using \n in Bracket Expressions of Dynamic
Regexps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
What About The Empty Regexp? . . . . . . . . . . 56
SIGHUP signal, for dynamic profiling . . . . . . . . . . . 351
SIGINT signal (MS-Windows) . . . . . . . . . . . . . . . . . 351
signals,
HUP/SIGHUP, for profiling . . . . . . . . . . . . . . . . . . 351
INT/SIGINT (MS-Windows) . . . . . . . . . . . . . . . . 351
QUIT/SIGQUIT (MS-Windows) . . . . . . . . . . . . . 351
USR1/SIGUSR1, for profiling . . . . . . . . . . . . . . . . 351
signature program . . . . . . . . . . . . . . . . . . . . . . . . . . . . 330
SIGQUIT signal (MS-Windows) . . . . . . . . . . . . . . . . 351
SIGUSR1 signal, for dynamic profiling . . . . . . . . . . 351
silent debugger command . . . . . . . . . . . . . . . . . . . 374
sin() function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195
sine. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195
single quote (') . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
in gawk command lines . . . . . . . . . . . . . . . . . . . . . 19
in shell commands . . . . . . . . . . . . . . . . . . . . . . . . . . 21
vs. apostrophe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
with double quotes . . . . . . . . . . . . . . . . . . . . . . . . . 22
single records, treating files as . . . . . . . . . . . . . . . . . . 69
single-character fields . . . . . . . . . . . . . . . . . . . . . . . . . . 75
single-precision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 390
single-step execution, in the debugger . . . . . . . . . 374
Skywalker, Luke . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
sleep utility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310
sleep() extension function . . . . . . . . . . . . . . . . . . . 467
Smith, Gavin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Solaris, POSIX-compliant awk . . . . . . . . . . . . . . . . . 511
sort array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196
sort array indices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196
sort function, arrays, sorting . . . . . . . . . . . . . . . . . . 341
sort utility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315
coprocesses and . . . . . . . . . . . . . . . . . . . . . . . . . . . 344
sorting characters in different languages . . . . . . . 358
source code,awka . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 511
awkcc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 512
AWKgo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 512

Index 583

Brian Kernighan’s awk . . . . . . . . . . . . . . . . . . . . . 510
BusyBox Awk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 511
cppawk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 512
embeddable awk interpreter . . . . . . . . . . . . . . . . 512
frawk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 511
gawk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 491
goawk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 511
Illumos awk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 511
jawk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 512
libmawk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 512
mawk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 510
mixing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
pawk (profiling version of Brian Kernighan’sawk) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 511
pawk (Python version) . . . . . . . . . . . . . . . . . . . . . 512
QSE awk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 512
QuikTrim Awk . . . . . . . . . . . . . . . . . . . . . . . . . . . . 512
Solaris awk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 511
source file, show in debugger . . . . . . . . . . . . . . . . . . 378
source files, search path for . . . . . . . . . . . . . . . . . . . 332
sparse arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178
Spencer, Henry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 529
Spengler, Egon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205
split string into array . . . . . . . . . . . . . . . . . . . . . . . . . 201
split utility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293
split() function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201
array elements, deleting . . . . . . . . . . . . . . . . . . . 187
split.awk program . . . . . . . . . . . . . . . . . . . . . . . . . . . 294
sprintf() function . . . . . . . . . . . . . . . . . . . . . . 102, 202
print/printf statements and . . . . . . . . . . . . . 246
sqrt() function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195
square brackets ([]), regexp operator. . . . . . . . . . . 55
square root . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195
srand() function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195
stack frame (debugger). . . . . . . . . . . . . . . . . . . . . . . . 367
Stallman, Richard . . . . . . . . . . . . . . . . 10, 12, 488, 534
standard error . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
standard input . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18, 111
standard output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
starting the debugger . . . . . . . . . . . . . . . . . . . . . . . . . 369
stat() extension function . . . . . . . . . . . . . . . . . . . . . 457
statements,
compound, control statements and . . . . . . . . . 154
control, in actions . . . . . . . . . . . . . . . . . . . . . . . . . 154
multiple . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
step debugger command . . . . . . . . . . . . . . . . . . . . . . 375
stepi debugger command . . . . . . . . . . . . . . . . . . . . . 375
stop automatic display, in debugger . . . . . . . . . . . 376
STR_CHAIN_MAX environment variable . . . . . . . . . . . 45
stream editors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79, 321
strftime() function (gawk) . . . . . . . . . . . . . . . . . . . 212
string,constants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
vs. regexp constants . . . . . . . . . . . . . . . . . . . . . 61
extraction (internationalization) . . . . . . . . . . . 361
length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198
operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
regular expression match of . . . . . . . . . . . . . . . . 199

string-manipulation functions . . . . . . . . . . . . . . . . . 196
string-matching operators . . . . . . . . . . . . . . . . . . . . . . 51
string-translation functions. . . . . . . . . . . . . . . . . . . . 220
strings,
continuation across lines . . . . . . . . . . . . . . . . . . . 120
converting . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126, 217
numbers to. . . . . . . . . . . . . . . . . . . . . . . . 163, 165
converting letter case . . . . . . . . . . . . . . . . . . . . . . 204
empty, See null strings
extracting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 361
for localization . . . . . . . . . . . . . . . . . . . . . . . . . . . . 359
length limitations . . . . . . . . . . . . . . . . . . . . . . . . . 119
merging arrays into . . . . . . . . . . . . . . . . . . . . . . . . 249
null . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
numeric . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
splitting, example . . . . . . . . . . . . . . . . . . . . . . . . . 201
strptime() extension function . . . . . . . . . . . . . . . . 467
strtonum() function (gawk) . . . . . . . . . . . . . . . . . . . 202--non-decimal-data option and . . . . . . . . . . . 335
sub() function . . . . . . . . . . . . . . . . . . . . . . . . . . . 122, 203
arguments of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203
escape processing . . . . . . . . . . . . . . . . . . . . . . . . . . 205
subscript separators . . . . . . . . . . . . . . . . . . . . . . . . . . . 165
subscripts in arrays,
multidimensional . . . . . . . . . . . . . . . . . . . . . . . . . . 188
scanning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189
numbers as . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185
uninitialized variables as . . . . . . . . . . . . . . . . . . . 186
SUBSEP variable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165
multidimensional arrays and . . . . . . . . . . . . . . . 188
substitute in string . . . . . . . . . . . . . . . . . . . . . . . . . . . 197
substr() function . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204
substring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204
Sumner, Andrew . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 511
supplementary groups of gawk process . . . . . . . . . 170
switch statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157
SYMTAB array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
syntactic ambiguity: /= operator vs. /=.../ regexp
constant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
system() function . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209
systime() function (gawk) . . . . . . . . . . . . . . . . . . . . 212

Tt debugger command (alias for tbreak) . . . . . . . 374
tbreak debugger command . . . . . . . . . . . . . . . . . . . 374
Tcl . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242
TCP/IP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 346
support for . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
tee utility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 298
tee.awk program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299
temporary breakpoint . . . . . . . . . . . . . . . . . . . . . . . . . 374
terminating records . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
testbits.awk program . . . . . . . . . . . . . . . . . . . . . . . 217
testext extension . . . . . . . . . . . . . . . . . . . . . . . . . . . . 467
Texinfo . . . . . . . . . . . . . . . . . 9, 241, 307, 317, 493, 517
chapter beginnings in files . . . . . . . . . . . . . . . . . . 55
extracting programs from source files . . . . . . 317

584

GAWK: Effective AWK Programming

text, printing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
unduplicated lines of . . . . . . . . . . . . . . . . . . . . . . 300
textdomain() function (C library) . . . . . . . . . . . . 357
TEXTDOMAIN variable . . . . . . . . . . . . . . . . . . . . . . 165, 359
BEGIN pattern and . . . . . . . . . . . . . . . . . . . . . . . . . 360
portability and . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363
TIDYMEM environment variable . . . . . . . . . . . . . . . . . . 46
tilde (~), ~ operator . 51, 60, 63, 121, 138, 139, 145,
148
time,
alarm clock example program . . . . . . . . . . . . . . 308
localization and . . . . . . . . . . . . . . . . . . . . . . . . . . . 359
managing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249
retrieving . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211
time functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211
timeout, reading input . . . . . . . . . . . . . . . . . . . . . . . . . 95
timestamps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211, 212
converting dates to . . . . . . . . . . . . . . . . . . . . . . . . 213
formatted . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249
tocsv() user-defined function . . . . . . . . . . . . . . . . . 254
tocsv_rec() user-defined function . . . . . . . . . . . . 254
tolower() function . . . . . . . . . . . . . . . . . . . . . . . . . . . 204
toupper() function . . . . . . . . . . . . . . . . . . . . . . . . . . . 205
tr utility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311
trace debugger command . . . . . . . . . . . . . . . . . . . . . 380
trace, internal instructions . . . . . . . . . . . . . . . . . . . . . 36
traceback, display in debugger . . . . . . . . . . . . . . . . 377
translate string . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220
translate.awk program . . . . . . . . . . . . . . . . . . . . . . 311
treating files, as single records . . . . . . . . . . . . . . . . . . 69
troubleshooting,--non-decimal-data option . . . . . . . . . . . . . . . . 37== operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
awk uses FS not IFS . . . . . . . . . . . . . . . . . . . . . . . . 73
backslash before nonspecial character . . . . . . . 54
division . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
fatal errors,
field widths, specifying . . . . . . . . . . . . . . . . . . 80
printf format strings . . . . . . . . . . . . . . . . . . 107
fflush() function . . . . . . . . . . . . . . . . . . . . . . . . . 208
function call syntax . . . . . . . . . . . . . . . . . . . . . . . 142
gawk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 515
bug reports . . . . . . . . . . . . . . . . . . . . . . . . . . . . 505
fatal errors, function arguments . . . . . . . . 193
getline command. . . . . . . . . . . . . . . . . . . . . . . . . 258
gsub()/sub() functions . . . . . . . . . . . . . . . . . . . 204
match() function . . . . . . . . . . . . . . . . . . . . . . . . . . 200
print statement, omitting commas . . . . . . . . 100
printing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
quotes with file names . . . . . . . . . . . . . . . . . . . . . 112
readable data files . . . . . . . . . . . . . . . . . . . . . . . . . 257
regexp constants vs. string constants . . . . . . . . 61
string concatenation . . . . . . . . . . . . . . . . . . . . . . . 129
substr() function . . . . . . . . . . . . . . . . . . . . . . . . . 204
system() function . . . . . . . . . . . . . . . . . . . . . . . . . 210
typographical errors, global variables. . . . . . . . 35
true, logical . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
Trueman, David . . . . . . . . . . . . . . . . . . . . . . . . 6, 12, 488

trunc-mod operation . . . . . . . . . . . . . . . . . . . . . . . . . . 129
truth values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
type,
conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
of variable, typeof() function (gawk) . . . . . . 219
typeof() function (gawk) . . . . . . . . . . . . . . . . . . . . . 219
TZ environment variable . . . . . . . . . . . . . . . . . . . . . . . 167

U
u debugger command (alias for until) . . . . . . . . 375
unassigned array elements. . . . . . . . . . . . . . . . . . . . . 179
undefined functions . . . . . . . . . . . . . . . . . . . . . . . . . . . 228
underscore (_),
C macro . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 358
in names of private variables . . . . . . . . . . . . . . . 242
translatable strings . . . . . . . . . . . . . . . . . . . . . . . . 360
undisplay debugger command . . . . . . . . . . . . . . . . 376
undocumented features . . . . . . . . . . . . . . . . . . . . . . . . . 48
Unicode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248, 487, 531
uninitialized variables, as array subscripts . . . . . 186
uniq utility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300
uniq.awk program . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300
Unix. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 540
awk scripts and . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Unix awk,
backslashes in escape sequences . . . . . . . . . . . . . 54
close() function and . . . . . . . . . . . . . . . . . . . . . . 115
password files, field separators and . . . . . . . . . . 78
unsigned integers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 389
until debugger command . . . . . . . . . . . . . . . . . . . . . 375
unwatch debugger command . . . . . . . . . . . . . . . . . . 376
up debugger command . . . . . . . . . . . . . . . . . . . . . . . . 377
uppercase names, namespace for . . . . . . . . . . . . . . 383
user database, reading . . . . . . . . . . . . . . . . . . . . . . . . 266
user-defined,
function,_gr_init() . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271_ord_init() . . . . . . . . . . . . . . . . . . . . . . . . . . . 247_pw_init() . . . . . . . . . . . . . . . . . . . . . . . . . . . . 268
assert() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245
beginfile() . . . . . . . . . . . . . . . . . . . . . . . . . . . 255
bits2str() . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217
chr() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247
cliff_rand(). . . . . . . . . . . . . . . . . . . . . . . . . . 247
ctime() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224
endfile() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255
endgrent() . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274
endpwent() . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269
getgrent() . . . . . . . . . . . . . . . . . . . . . . . 270, 274
getgrgid() . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273
getgrnam() . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273
getgruser() . . . . . . . . . . . . . . . . . . . . . . . . . . . 273
getlocaltime() . . . . . . . . . . . . . . . . . . . . . . . 249
getopt() . . . . . . . . . . . . . . . . . . . . . . . . . 261, 262
getpwent() . . . . . . . . . . . . . . . . . . . . . . . 266, 269
getpwnam() . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269
getpwuid() . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269

Index 585

isnumeric() . . . . . . . . . . . . . . . . . . . . . . . . . . . 253
join() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249
ord() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247
readfile() . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251
rev() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223
rewind() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256
round() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246
walk_array(). . . . . . . . . . . . . . . . . . . . . . . . . . 274
functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221
counts, in a profile . . . . . . . . . . . . . . . . . . . . . 350
variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
user-modifiable variables . . . . . . . . . . . . . . . . . . . . . . 163
users, information about,printing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 288
retrieving . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266
USR1 signal, for dynamic profiling . . . . . . . . . . . . . 351

Vvalues,numeric . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 526
regexp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
string . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 526
variable assignments and input files . . . . . . . . . . . . 41
variable type, typeof() function (gawk) . . . . . . . 219
variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30, 526
assigning on command line . . . . . . . . . . . . . . . . 125
built-in. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
flag . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
getline command into, using . . . . 90, 92, 93, 94
global,for library functions . . . . . . . . . . . . . . . . . . . . 242
printing list of . . . . . . . . . . . . . . . . . . . . . . . . . . 35
initializing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
local to a function . . . . . . . . . . . . . . . . . . . . . . . . . 225
predefined . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163-v option, setting with . . . . . . . . . . . . . . . . . . 34
conveying information. . . . . . . . . . . . . . . . . . 165
private . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242
setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
shadowing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222
types of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
comparison expressions and . . . . . . . . . . . . 135
uninitialized, as array subscripts . . . . . . . . . . . 186
user-defined . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
version of,gawk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170
gawk extension API . . . . . . . . . . . . . . . . . . . . . . . 170
GNU MP library . . . . . . . . . . . . . . . . . . . . . . . . . . 170
GNU MPFR library . . . . . . . . . . . . . . . . . . . . . . . 170
vertical bar (|) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
| operator (I/O) . . . . . . . . . . . . . . . . . . . . . . . 92, 144|& operator (I/O) . . . . . . . . . . . . . . . . . 94, 144, 344|| operator . . . . . . . . . . . . . . . . . . . . . . . . . . . 141, 145
Vinschen, Corinna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12W
w debugger command (alias for watch) . . . . . . . . 376
w utility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
wait() extension function . . . . . . . . . . . . . . . . . . . . . 461
waitpid() extension function . . . . . . . . . . . . . . . . . 461
walk_array() user-defined function . . . . . . . . . . . 274
Wall, Larry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177, 521
Wallin, Anders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 489
warnings, issuing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
watch debugger command . . . . . . . . . . . . . . . . . . . . . 376
watchpoint (debugger) . . . . . . . . . . . . . . . . . . . . . . . . 368
watchpoints, show in debugger . . . . . . . . . . . . . . . . 378
wc utility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304
wc.awk program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305
Weinberger, Peter . . . . . . . . . . . . . . . . . . . . . 6, 488, 510
where debugger command (alias for backtrace) . 377
while statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
use of regexps in. . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
whitespace,as field separators . . . . . . . . . . . . . . . . . . . . . . . . . . 74
definition of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
functions, calling . . . . . . . . . . . . . . . . . . . . . . . . . . 193
newlines as . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
Williams, Kent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 488
Woehlke, Matthew . . . . . . . . . . . . . . . . . . . . . . . . . . . . 489
Woods, John . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 488
word boundaries, matching . . . . . . . . . . . . . . . . . . . . . 62
word, regexp definition of . . . . . . . . . . . . . . . . . . . . . . 61
word-boundary operator (gawk) . . . . . . . . . . . . . . . . 62
wordfreq.awk program . . . . . . . . . . . . . . . . . . . . . . . 315
words,
counting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304
duplicate, searching for . . . . . . . . . . . . . . . . . . . . 307
usage counts, generating . . . . . . . . . . . . . . . . . . . 314
writea() extension function . . . . . . . . . . . . . . . . . . 465
writeall() extension function . . . . . . . . . . . . . . . . 465

X
xgettext utility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 361
xor() function (gawk) . . . . . . . . . . . . . . . . . . . . . . . . . 216
XOR bitwise operation . . . . . . . . . . . . . . . . . . . . . . . . 216

Y
Yawitz, Efraim . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 489

Z
Zaretskii, Eli . . . . . . . . . . . . . . . . . . . . . . . . . 12, 488, 509
zerofile.awk program . . . . . . . . . . . . . . . . . . . . . . . 258
Zoulas, Christos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 488