Perl 5 Tutorial

First Edition

Chan Bernard Ki Hong

Perl is copyright by Larry Wall.
Linux is a trademark of Linus Torvalds.
Unix is a trademark of AT&T Bell Laboratories.

Perl 5 Tutorial
First Edition
Author: Chan Bernard Ki Hong (webmaster@cbkihong.com)
Web site: http://www.cbkihong.com
Date of Printing: December 24, 2003
Total Number of Pages: 241
Prepared from LATEX source files by the author.

© 2001–2003 by Chan Bernard Ki Hong.

While the author of this document has taken the best effort in enhancing the technical accuracy as well
as readability of this publication, please note that this document is released “as is” without guarantee for
accuracy or suitability of any kind. The full text of the terms and conditions of usage and distribution
can be found at the end of this document.
In order to further enhance the quality of this publication, the author would like to hear from you, thefellow readers. Comments or suggestions on this publication are very much appreciated. Please feel free
to forward me your comments through the email feedback form or the feedback forum on the author’s
Web site.

Contents
1

Introduction to Programming
1.1 What is Perl? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2 A Trivial Introduction to Computer Programming . . . . . . . . . . . . . . . . . . . .
1.3 Scripts vs. Programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.4 An Overview of the Software Development Process . . . . . . . . . . . . . . . . . . . .

1
1
1
3
4

2

Getting Started
2.1 What can Perl do? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 Comparison with Other Programming Languages . . . . . . . . . . . . . . . . . . . .
2.2.1 C/C++ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.2 PHP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.3 Java/JSP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.4 ASP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3 What do I need to learn Perl? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4 Make Good Use of Online Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5 The Traditional “Hello World” Program . . . . . . . . . . . . . . . . . . . . . . . . . .
2.6 How A Perl Program Is Executed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.7 Literals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.7.1 Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.7.2 Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.8 Introduction to Data Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7
7
8
8
8
9
9
9
11
12
15
16
16
16
17

3

Manipulation of Data Structures
3.1 Scalar Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.1 Assignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.2 Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.3 Variable Substitution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.4 substr() — Extraction of Substrings . . . . . . . . . . . . . . . . . . . . . .
3.1.5 length() — Length of String . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2 Lists and Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.1 Creating an Array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.2 Adding Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.3 Getting the number of Elements in an Array . . . . . . . . . . . . . . . . . . .
3.2.4 Accessing Elements in an Array . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.5 Removing Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.6 splice(): the Versatile Function . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.7 Miscellaneous List-Related Functions . . . . . . . . . . . . . . . . . . . . . . .
3.2.8 Check for Existence of Elements in an Array (Avoid!) . . . . . . . . . . . . . .
3.3 Hashes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.1 Assignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

23
23
23
24
25
26
26
26
27
28
29
30
31
32
33
35
38
38

i

ii

CONTENTS
3.3.2 Accessing elements in the Hash . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.3 Removing Elements from a Hash . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.4 Searching for an Element in a Hash . . . . . . . . . . . . . . . . . . . . . . . .
Contexts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Miscellaneous Issues with Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

40
40
41
42
44

4

Operators
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2 Description of some Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.1 Arithmetic Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.2 String Manipulation Operators . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.3 Comparison Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.4 Equality Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.5 Logical Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.6 Bitwise Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.7 Assignment Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.8 Other Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3 Operator Precedence and Associativity . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4 Constructing Your Own sort() Routine . . . . . . . . . . . . . . . . . . . . . . . . .

47
47
48
48
50
51
53
54
56
57
58
59
64

5

Conditionals, Loops & Subroutines
5.1 Breaking Up Your Code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.1 Sourcing External Files with require() . . . . . . . . . . . . . . . . . . . . .
5.2 Scope and Code Blocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.1 Introduction to Associations . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.2 Code Blocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3 Subroutines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3.1 Creating and Using A Subroutine . . . . . . . . . . . . . . . . . . . . . . . . .
5.3.2 Prototypes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3.3 Recursion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3.4 Creating Context-sensitive Subroutines . . . . . . . . . . . . . . . . . . . . . .
5.4 Packages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4.1 Declaring a Package . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4.2 Package Variable Referencing . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4.3 Package Variables and Symbol Tables . . . . . . . . . . . . . . . . . . . . . . .
5.4.4 Package Constructors with BEGIN {} . . . . . . . . . . . . . . . . . . . . . . .
5.5 Lexical Binding and Dynamic Binding . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.6 Conditionals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.7 Loops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.7.1 for loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.7.2 while loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.7.3 foreach loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.7.4 Loop Control Statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.8 Leftovers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

67
67
67
69
69
69
70
71
73
75
78
80
80
81
81
82
82
86
88
88
89
89
91
91

6

References95
6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
6.2 Creating a Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
6.3 Using References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
6.4 Pass By Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
6.5 How Everything Fits Together . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

3.4
3.5

CONTENTS
6.6

iii

Typeglobs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

7

Object-Oriented Programming
105
7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
7.2 Object-Oriented Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
7.2.1 Programming Paradigms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
7.2.2 Basic Ideas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
7.2.3 Fundamental Elements of Object-Oriented Programming . . . . . . . . . . . . 107
7.3 OOP Primer: Statistics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
7.3.1 Creating and Using A Perl Class . . . . . . . . . . . . . . . . . . . . . . . . . . 110
7.3.2 How A Class Is Instantiated . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
7.4 Inheritance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

8

Files and Filehandles
119
8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
8.2 Filehandles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
8.2.1 open a File . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
8.2.2 Output Redirection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
8.3 File Input and Output Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
8.3.1 readline() — Read A Line from Filehandle . . . . . . . . . . . . . . . . . . . 122
8.3.2 binmode() — Binary Mode Declaration . . . . . . . . . . . . . . . . . . . . . 122
8.3.3 read() — Read A Specified Number of Characters from Filehandle . . . . . . . 123
8.3.4 print()/printf() — Output To A FileHandle . . . . . . . . . . . . . . . . . 124
8.3.5 seek() — Set File Pointer Position . . . . . . . . . . . . . . . . . . . . . . . . 126
8.3.6 tell() — Return File Pointer Position . . . . . . . . . . . . . . . . . . . . . . 127
8.3.7 close() — Close An opened File . . . . . . . . . . . . . . . . . . . . . . . . . 127
8.4 Directory Traversal Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
8.4.1 opendir() — Open A Directory . . . . . . . . . . . . . . . . . . . . . . . . . 128
8.4.2 readdir() — Read Directory Content . . . . . . . . . . . . . . . . . . . . . . 128
8.4.3 closedir() — Close A Directory . . . . . . . . . . . . . . . . . . . . . . . . . 128
8.4.4 Example: File Search . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
8.5 File Test Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
8.6 File Locking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132

9Regular Expressions
137
9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
9.2 Building a Pattern . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
9.2.1 Getting your Foot Wet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
9.2.2 Introduction to m// and the Binding Operator . . . . . . . . . . . . . . . . . . 139
9.2.3 Metacharacters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
9.2.4 Quantifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
9.2.5 Character Classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
9.2.6 Backtracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
9.3 Regular Expression Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
9.3.1 m// — Pattern Matching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
9.3.2 s/// — Search and Replace . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144
9.3.3 tr/// — Global Character Transliteration . . . . . . . . . . . . . . . . . . . . 144
9.4 Constructing Complex Regular Expressions . . . . . . . . . . . . . . . . . . . . . . . . 145

iv

CONTENTS

10 Runtime Evaluation & Error Trapping
149
10.1 Warnings and Exceptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
10.2 Error-Related Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
10.3 eval . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150
10.4 Backticks and system() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
10.5 Why Runtime Evaluation Should Be Restricted . . . . . . . . . . . . . . . . . . . . . . 151
10.6 Next Generation Exception Handling . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
10.6.1 Basic Ideas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
10.6.2 Throwing Different Kinds of Errors . . . . . . . . . . . . . . . . . . . . . . . . 155
10.6.3 Other Handlers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
10.7 Other Methods To Catch Programming Errors . . . . . . . . . . . . . . . . . . . . . . 159
10.7.1 The -w Switch — Enable Warnings . . . . . . . . . . . . . . . . . . . . . . . . 159
10.7.2 Banning Unsafe Constructs With strict . . . . . . . . . . . . . . . . . . . . . 160
10.7.3 The -T Switch — Enable Taint Checking . . . . . . . . . . . . . . . . . . . . . 163
11 CGI Programming
169
11.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169
11.2 Static Content and Dynamic Content . . . . . . . . . . . . . . . . . . . . . . . . . . . 169
11.2.1 The Hypertext Markup Language . . . . . . . . . . . . . . . . . . . . . . . . . 169
11.2.2 The World Wide Web . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170
11.3 What is CGI? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172
11.4 Your First CGI Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174
11.5 GET vs. POST . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178
11.6 File Upload . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180
11.7 Important HTTP Header Fields and Environment Variables . . . . . . . . . . . . . . . 182
11.7.1 CGI-Related Environment Variables . . . . . . . . . . . . . . . . . . . . . . . . 182
11.7.2 HTTP Header Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183
11.8 Server Side Includes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184
11.9 Security Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186
11.9.1 Why Should I Care? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187
11.9.2 Some Forms of Attack Explained . . . . . . . . . . . . . . . . . . . . . . . . . 187
11.9.3 Safe CGI Scripting Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . 190
A How A Hash Works
193
A.1 Program Listing of Example Implementation . . . . . . . . . . . . . . . . . . . . . . . 193
A.2 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198
A.3 Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199
A.4 Notes on Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199
B Administration
201
B.1 CPAN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201
B.1.1 Accessing the Module Database on the Web . . . . . . . . . . . . . . . . . . . . 201
B.1.2 Package Managers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201
B.1.3 Installing Modules using CPAN.pm . . . . . . . . . . . . . . . . . . . . . . . . 202
B.1.4 Installing Modules — The Traditional Way . . . . . . . . . . . . . . . . . . . . 203
C Setting Up A Web Server
205
C.1 Apache . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205
C.1.1 Microsoft Windows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205
C.1.2 Unix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210

CONTENTSv

D A Unix Primer
213
D.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213
D.1.1 Why Should I Care About Unix? . . . . . . . . . . . . . . . . . . . . . . . . . . 213
D.1.2 What Is Unix? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213
D.1.3 The Overall Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214
D.2 Filesystems and Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215
D.2.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215
D.2.2 Symbolic Links and Hard Links . . . . . . . . . . . . . . . . . . . . . . . . . . 216
D.2.3 Permission and Ownership . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220
D.2.4 Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222
D.2.5 The Special Permission Bits . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223
E In The Next Edition

225

Index

226

vi

CONTENTSPreface
If you are looking for a free Perl tutorial that is packed with everything you need to know to get started
on Perl programming, look no further. Presenting before you is probably the most comprehensive Perltutorial on the Web, the product of two years of diligence seeking reference from related books and Web
sites.
Perl is a programming language that is offered at no cost. So wouldn’t it be nice if you can also learn it
at no cost? Packed with some background knowledge of programming in C++ and Visual Basic, when I
started learning Perl several years ago, I couldn’t even find one good online tutorial that covered at least
the basics of the Perl language and was free. Most Perl tutorials I could find merely covered the very
basic topics such as scalar/list assignment, operators and some flow control structures etc. On the other
hand, although I have accumulated certain levels of experience in a number of programming languages,
the official Perl manual pages are quite technical with whole pages of jargons that I was not very familiarwith. As a result, the book “Learning Perl” written by Larry Wall, the inventor of the Perl language, naturally became the only Perl textbook available. The O’Reilly Perl Series present the most authoritative
and well-written resources on the subject written by the core developers of Perl. While you are strongly
recommended to grab one copy of each if you have the money, they are not so cheap, though, and that’s
the motive behind my writing of this tutorial — so that more people with no programming backgroundcan start learning this stupendous and powerful language in a more cost-effective way.
Although this tutorial covers a rather wide range of topics, similar to what you can find in some other
Perl guidebooks, you are strongly encouraged to read those books too, since their paedagogies of teaching may suit you more.
Here are several features of this tutorial:?

As this is not a printed book, I will constantly add new materials to this tutorial as needed, thus
enriching the content of this tutorial. Moreover, in order to help me improve the quality of thistutorial, it is crucial for you to forward me your comments and suggestions so that I can makefurther improvements to it.

?

Earlier drafts of this tutorial were published in HTML format on my Web site. In response to requests made from several visitors, this tutorial has been made available in PDF format for download. I hope this will help those who are charged on time basis for connecting to the Internet. Thistutorial is typeset in LATEX, a renowned document typesetting system that has been widely usedin the academic community on Unix-compatible systems (although it is available on nearly any
operating systems you can think of). The HTML version has been discontinued, until a solution
can be found which allows both versions to be generated from the same source base.

?

You will find a list of Web links and references to book chapters after each chapter where applicable which contains additional materials that ambitious learners will find helpful to further yourunderstanding of the subject.
vii

viii

Preface?

Throughout the text there would be many examples. In this tutorial, you will find two typesof examples — examples and illustrations. Illustrations are intended to demonstrate a particular concept just mentioned, and are shorter in general. You will find them embedded inline
throughout the tutorial. On the other hand, examples are more functional and resemble practicalscripts, and are usually simplified versions of such. They usually demonstrate how different partsof a script can work together to realize the desired functionalities or consolidate some importantconcepts learned in a particular chapter.

?

If applicable, there will be some exercises in the form of concept consolidation questions as well as
programming exercises at the end of each chapter to give readers chances to test how much they
understand the materials learned from this tutorial and apply their knowledge through practice.

This is the First Edition of the Perl 5 Tutorial. It primarily focuses on fundamental Perl programming
knowledge that any Perl programmer should be familiar with. I start with some basic ideas behind computer programming in general, and then move on to basic Perl programming with elementary topics
such as operators and simple data structures. The chapter on scoping and subroutines is the gateway
to subsequent, but more advanced topics such as references and object-oriented programming. The
remaining chapters are rather assorted in topic, covering the use of filehandles, file I/O and regular expressions in detail. There is also a dedicated chapter on error handling which discusses facilities that you
can use to locate logical errors and enhance program security. The final chapter on CGI programmingbuilds on knowledge covered in all earlier chapters. Readers will learn how to write a Perl programthat can be used for dynamic scripting on the World Wide Web. However short, the main text alreadyembraces the most important fundamental subjects in the Perl programming language. In the appendices, instructions are given on acquiring and installing Perl modules, setting up a basic but functional
CGI-enabled Web server for script testing, and there is a voluminous coverage of Unix fundamentals.
As much of Perl is based on Unix concepts, I believe a brief understanding of this operating system isbeneficial to Perl programmers. An appendix is also prepared to give my readers an idea of the internal
structure of general hashes. While authoring of this tutorial cannot proceed indefinitely, topics that wereplanned but cannot be included in this edition subject to time constraints are deferred to the Second
Edition. A list of these topics appear at the end of this document for your reference.
In the second release candidate of this tutorial I made an audacious attempt of adding into it two topics
that are rarely discussed in most Perl literature. The first is the Error CPAN module for exception handling. The second attempt, which is an even more audacious one, is an introduction of the finite-state
automaton (FSA) for construction of complex regular expressions for pattern matching. While FSA isa fundamental topic in Computer Science (CS) studies, this is seldom mentioned outside the CS circle.
Although there is a high degree of correspondence between regular expressions and FSA, this may not
be at all obvious to a reader without relevant background, despite I have avoided rigorous treatment of
the topic and tried to explain it in a manner that would be more easily communicable to fellow readers.
I would like to emphasize this topic is not essential in Perl programming, and I only intend to use it as atool to formulate better patterns. Feel free to skip it if that is not comfortable to you and I require yourfeedback of whether these topics can help you as I originally intended.
It is important for me to reiterate that this document is not intended to be a substitute for the official Perlmanual pages (aka man pages) and other official Perl literature. In fact, it is the set of manual pages thatcovers the Perl language in sufficiently fine detail, and it will be the most important set of document
after you have accumulated certain level of knowledge and programming experience. The Perl man
pages are written in the most concise and correct technical parlance, and as a result they are not very
suitable for new programmers to understand. The primary objective of this tutorial is to bridge the gap
so as to supplement readers with sufficient knowledge to understand the man pages. Therefore, this tutorial presents a different perspective compared with some other Perl guidebooks available at your local
bookstores from the mainstream computer book publishers. With a Computer Science background, Iix
intend to go more in-depth into the principles which are central to the study of programming languages
in general. Apart from describing the syntax and language features of Perl, I also tried to draw together
the classical programming language design theories and explained how they are applied in Perl. With
this knowledge, it is hoped that readers can better understand the jargons presented in manual pages
and the principles behind. Perl is attributed by some as a very cryptic language and is difficult to learn.
However, those who are knowledgeable about programming language design principles would agree
Perl implements a very rich set of language features, and therefore is an ideal language for students toexperiment with different programming language design principles taught in class in action. I do hopethat after you have finished reading this tutorial you will be able to explore the Perl horizons on yourown with confidence and experience the exciting possibilities associated with the language more easily.
“To help you learn how to learn” has always been the chief methodology followed in this tutorial.
Time flies. Today when I am revising this preface, which was actually written before I made my initialpreview release in late 2001 according to the timestamp, I am aghast to find that it has already beennearly two years since I started writing it. Indeed, a lot of things have changed in two years. Several Perlmanpages written in tutorial-style have been included into the core distribution, which are written in amore gentle way targeted at beginners. There are also more Perl resources online today than there were
two years ago. However, I believe through preparing this tutorial I have also learnt a lot in the process.
Despite I started this project two years ago, a major part of the text was actually written in a windowof 3 months. As a result, many parts of the tutorial were unfortunately completed in a hasty manner.
However, through constant refinement and rewriting of certain parts of the tutorial, I believe the Second
Edition will be more well-organized and coherent, while more advanced topics can be accommodated
as more ample time is available.
At last, thank you very much for choosing this tutorial. Welcome to the exciting world of Perl programming!

Bernard Chanin Hong Kong, China
15th September, 2003

x

Preface

Typographical Conventions
Although care has been taken towards establishing a consistent typographical convention throughout
this tutorial, considering this is the first time I try to publish in LATEX, slight deviations may be found incertain parts of this document. Here I put down the convention to which I tried to adhere:
Elements in programming languages are typeset in monospace font.
Important terms are typeset in bold.
Profound sayings or quotes are typeset in italic.
In source code listings, very long lines are broken into several lines. ¶ is placed wherever a line breakoccurs.

Release History

30th August, 2003

First Edition, Release Candidate 1

15th September, 2003

First Edition, Release Candidate 2

01st October, 2003

First Edition, Release Candidate 3

31st December, 2003

First Edition

About The Author

Bernard Chan was born and raised in Hong Kong, China. He received his Bachelor’s Degree in Computer Engineering from the University of Hong Kong. His major interests in the field include information system security, networking and Web technologies. He carries a long-term career objective of
becoming an avid technical writer in the area of programming. His most notable publication is the Perl
5 Tutorial.

Chapter 1

Introduction to Programming
1.1 What is Perl?
Extracted from the perl manpage,“Perl is an interpreted high-level programming language developed by Larry Wall.”If you have not learnt any programming languages before, as this is not a prerequisite of this tutorial,
this definition may appear exotic for you. The two keywords that you may not understand are “interpreted” and “high-level”. Because this tutorial is mainly for those who do not have any programming
experience, it is better for me to give you a general picture as to how a computer program is developed.
This helps you understand this definition.

1.2 A Trivial Introduction to Computer Programming
You should know that, regardless of the programming language you are using, you have to write
something that we usually refer to as source code, which include a set of instructions for the computer
to perform some operations dictated by the programmer. There are two ways as to how the sourcecode can be executed by the Central Processing Unit (CPU) inside your computer. The first way is togo through two processes, compilation and linking, to transform the source code into machine code,
which is a file consisting of a series of numbers only. This file is in a format that can be recognized by the
CPU readily, and does not require any external programs for execution. Syntax errors are detected when
the program is being compiled. We describe this executable file as a compiled program. Most software programs (e.g. most EXEs for MS-DOS/Windows) installed in your computer fall within this type.
NOTES
There are some subtleties, though. For example, the compiler that comes with Visual Basic
6 Learning Edition translates source code into p-code (pseudo code) which has to be further
converted to machine code at runtime. Such an EXE is described as interpreted instead.
Therefore, not all EXEs are compiled.
On the other hand, although Java is customarily considered an interpreted language, Javasource files are first compiled into bytecode by the programmer, so syntactical errors can bechecked at compile time.

Another way is to leave the program uncompiled (or translate the source code to an intermediate level1

2

Chapter 1 Introduction to Programming

between machine code and source code, e.g. Java). However, the program cannot be executed on its
own. Instead, an external program has to be used to execute the source code. This external program is
known as an interpreter, because it acts as an intermediary to interpret the source code in a way the
CPU can understand. Compilation is carried out by the interpreter before execution to check for syntax
errors and convert the program into certain internal form for execution. Therefore, the main difference
between compiled programs and interpreted languages is largely only the time of compilation phase.
Compilation of compiled programs is performed early, while for interpreted programs it is usually
performed just before the execution phase.
Every approach has its respective merits. Usually, a compiled program only has to be compiled once,
and thus syntax checking is only performed once. What the operating system only needs to do isto read the compiled program and the instructions encoded can be arranged for execution by the
CPU directly. However, interpreted programs usually have to perform syntax check every time theprogram is executed, and a further compilation step is needed. Therefore, startup time of compiled
programs are usually shorter and execution of the program is usually faster. For two functionallyequivalent programs, a compiled program generally gives higher performance than the interpreted
program. Therefore, performance-critical applications are generally compiled. However, there are anumber of factors, e.g. optimization, that influence the actual performance. Also, the end user of
a compiled program does not need to have any interpreters installed in order to run the program.
This convenience factor is important to some users. On the other hand, interpreters have to beinstalled in order to execute a program that is interpreted. One example is the Java Virtual Machine(JVM) that is an interpreter plugged into your browser to support Java applets. Java source files aretranslated into Java bytecode, which is then executed by the interpreter. There are some drawbacks
for a compiled program. For example, every time you would like to test your software to see if
it works properly, you have to compile and link the program. This makes it rather annoying for
programmers to fix the errors in the program (debug), although the use of makefiles alleviates mostof this hassle from you. Because compilation translates the source code to machine code which canbe executed by the hardware circuitry in the CPU, this process creates a file in machine code that
depends on the instruction set of the computer (machine-dependent). On the other hand, interpreted
programs are usually platform-independent, that is, the program is not affected by the operating
system on which the program is executed. Therefore, for example, if you have a Java applet on a
Web site, it can most probably be executed correctly regardless of the operating system or browser avisitor is using. It is also easier to debug an interpreted program because repeated compilation is waived.

TERMINOLOGY
Instruction set refers to the set of instructions that the CPU executes. There are a number oftypes of microprocessors nowadays. For example, IBM-compatible PCs are now using theIntel-based instruction set. This is the instruction set that most computer users are using.
Another prominent example is the Motorola 68000 series microprocessors in Macintosh
computers. There are some other microprocessor types which exist in minority. The
instruction sets of these microprocessors are different and, therefore, a Windows program
cannot be executed unadapted on a Macintosh computer. In a more technical parlance,
different microprocessors have different instruction set architectures.

Recall that I mentioned that a compiled program consists entirely of numbers. Because a CPU isactually an electronic circuit, and a digital circuit mainly deals with Booleans (i.e. 0 and 1), so it isobvious that programs used by this circuit have to be sequences of 0s and 1s. This is what machinecode actually is. However, programming entirely with numbers is an extreme deterrent to computer1.3 Scripts vs. Programs

3

programming, because numeric programming is highly prone to mistakes and debugging is very
difficult. Therefore, assembly language was invented to allow programmers to use mnemonic names
to write programs. An assembler is used to translate the assembly language source into machinecode. Assembly language is described as a low-level programming language, because the actions of anassembly language program are mainly hardware operations, for example, moving bits of data fromone memory location to another. Programming using assembly language is actually analogous to thatof machine code in disguise, so it is still not programmer friendly enough.
Some mathematicians and computer scientists began to develop languages which were more machineindependent and intuitive to programmers that today we refer to as high-level programming
languages. The first several high-level languages, like FORTRAN, LISP, COBOL, were designed forspecialized purposes. It was not until BASIC (Beginner’s All-purpose Symbolic Instruction Code) wasinvented in 1966 that made computer programming unprecedentedly easy and popular. It was the firstwidely-used high-level language for general purpose. Many programmers nowadays use C++, anotherhigh-level language, to write software programs. The reason why we call these “high-level languages” isthat they were built on top of low-level languages and hid the complexity of low-level languages fromthe programmers. All such complexities are handled by the interpreters or compilers automatically.
This is an important design concept in Computer Science called abstraction.
That’s enough background information and we can now apply the concepts learned above to Perl. Perl
(Practical Extraction and Reporting Language) was designed by Larry Wall, who continues to develop
newer versions of the Perl language for the Perl community today. Perl does not create standalone
programs and Perl programs have to be executed by a Perl interpreter. Perl interpreters are now
available for virtually any operating system, including but not limited to Microsoft Windows (Win32)
and many flavours of Unix. As I quoted above, “Perl is a language optimized for scanning arbitrary
text files, extracting information from those text files, and printing reports based on that information.”
This precise description best summarizes the strength of Perl, mainly because Perl has a powerful setof regular expressions with which programmers can specify search criteria (patterns) precisely. You
are going to see a whole chapter devoted to regular expression in Chapter 9. Perl is installed on many
Web servers nowadays for dynamic Web CGI scripting. Perl programs written as CGI applications are
executed on servers where the Perl source files are placed. Therefore, there is no need to transfer thePerl source to and from the server (as opposed to client-side scripts like JavaScript or Java applets).
Guestbooks, discussion forums and many powerful applications for the Web can be developed using
Perl.
There is one point which makes Perl very flexible — there is always more than one approach toaccomplish a certain task, and programmers can pick whatever approach that best suits the purpose.

1.3 Scripts vs. Programs
There has always been some arguments over whether to use the term “script” or “program” for Perlsource files. In general, a piece of code that is executed by hardware or a software interpreter, written
in some kind of programming languages, is formally called a “program”. This is a general term thatapplies to programs written in machine instructions, or any programs that are compiled or interpreted.
However, it is also common today to hear that people use the term “script” to refer to programs thatare interpreted, especially those executed on the command line. In my opinion, it is not important todraw a distinction between the two terms as both are considered equally acceptable and understandable
nowadays. In many situations people just use both interchangeably.

4

Chapter 1 Introduction to Programming

I am more inclined towards calling Perl programs and CGI programs running on a Perl backend as
scripts, so I will adhere to this terminology in this tutorial.

1.4 An Overview of the Software Development Process
An intuitive software development process is outlined below. Note that this process is not tailoredfor Perl programming in particular. It is a general development process that can be applied to any
programming projects with any programming languages. For additional notes specific to Perl, pleaserefer to the next chapter.
Because this tutorial does not assume readers to have any programming experience, it is appropriate
for me to give you an idea as to the procedure you will most probably follow when you write yourprograms. In general, the process of development of a software project could be broken down into a
number of stages. Here is an outline of the stages involved:?

Requirements Analysis
First you need to identify the requirements of the project. Simply speaking, you will need todecide what your program should do (known as functional requirements), and note down otherrequirements that are important but not related to the functions of your program (known as
non-functional requirements), for example, a requirement that the user interface should be user
friendly. You have to make a list of the requirements, and from it you will need to decide whether
you have the capability to complete them. You may also want to prioritize them such that themost important functionalities are developed first, and other parts can be added subsequently.

?

Systems Design
From the requirements determined you can then define the scope of the project. Instead of putting
the whole program in one piece, we will now organize the program into several components (orsubsystems — a part of the entire system). As we will discuss later in this tutorial, modularization
facilitates code reuse and make correction of bugs (debug) easier. Two major models exist today— decomposition based on functions and decomposition based on objects. After you have fixed
the model, you decide on which functions or object methods are to be associated with whichsource file or object, and determine how these components interact with each other to performthe functionalities. Note that you don’t need to decide on how these source files or objects are
implemented in real source code at this stage — it is just an overall view of the interaction between
the components. We emphasize functional decomposition in the first part of the tutorial, while
object-oriented programming will be covered in a later part of this tutorial.

?

Program Design
After we have determined how the components interact with each other, we can now decide
how each function or object method is implemented. For each function, based on the actions
to perform you have to develop an algorithm, which is a well-defined programming-languageindependent procedure to carry out the actions specified. You may want to use a flowchart orsome pseudocode to illustrate the flow of the program. Pseudocode is expressed in a way resembling real programming source code, except language-dependent constructs are omitted. As
pseudocode is language independent, you can transform an idea from pseudocode to source codein any programming languages very easily. There isn’t a single standardized pseudocode syntax.
In many cases, pseudocode can even be written in English-like statements because pseudocode iswritten to demonstrate how a program is supposed to work, and provided it communicates theidea clearly it suffices. It is up to you as the author to express pseudocode in whatever way thealgorithm is best illustrated.

1.4 An Overview of the Software Development Process

5?

CodingThis is largely the continuation of the Program Design stage to transform your algorithm into
programming language constructs. If you have worked out the algorithm properly this should bea piece of cake.

?

Unit Testing
Unit testing corresponds to Program Design. As each function or object method has a predefined
behaviour, they can be tested individually to see if such behaviour agree to that defined. Most of
the time when we are talking about debugging, we are describing this stage.

?

Systems Testing
Systems Testing corresponds to System Design. This is to test if the components interact with eachother in exactly the same way as designed in the Systems Design stage.

?

Requirements Validation
This corresponds to requirements analysis. The software developed is compared against the requirements to ensure each functionality has been incorporated into the system and works as expected.

?

MaintenanceBy now the software has been developed but you cannot simply abandon it. Most probably westill need to develop later versions, or apply patches to the current one as new bugs are found in
the program. Software for commercial distribution especially needs investment of a lot of timeand effort at this stage capturing user feedback, but software not distributed commercially should
also pay attention to this stage as this affects how well your software can be further developed.

Of course, for the examples in this tutorial that are so short and simple we don’t need such an elaborate
development procedure. However, you will find when you develop a larger-scale project that having awell-defined procedure is essential to keep your development process in order.
This is just one of the many process models in existence today. Discussion of such process models canbe found in many fundamental text for Software Engineering, and are beyond the scope of this tutorial.
Actually, what I have presented was a variant of the Waterfall process model, and is considered one that,
if employed, is likely to delay project schedules and result in increased costs of software development.
The reason I present it here is that the Waterfall model is the easiest model to understand. Because
presentation of process models is out of the scope of the tutorial, some Web links will be presented at
the end of this chapter from which you will find selected texts describing process models, including
the Rational Unified Process which I recommend as an improved process model for larger-scale
development projects. Adoption of an appropriate process model helps guide the development process with optimized usage of resources, increased productivity and software that are more fault-tolerant.

Summary”

Source code include a set of instructions to be executed by the computer through levels of translation.

”

Low-level programming languages involves architecture-dependent programming with machinecode or assembly language.

”

High-level programming languages are built on top of low-level languages and source programs
can usually be platform-independent.

”

Source programs are either explicitly precompiled into object code, or are implicitly compiled
before execution by the interpreter.

6

Chapter 1 Introduction to Programming”

Perl is an interpreted high-level programming language developed by Larry Wall.

”

Following a well-defined software development process model helps keep the development process systematic and within budgets.

Web Links
1.1

Evolution of Programming Languages
http://lycoskids.infoplease.com/ce6/sci/A0860536.html

1.2

Rational Unified Process Whitepapers
http://www.rational.com/products/rup/whitepapers.jsp

Chapter 2

Getting Started
2.1 What can Perl do?
I understand it is a full wastage of time for you to have read through half of a book to find that it is not
the one you are looking for. Therefore, I am going to let you know what you will learn by following thistutorial as early as possible.
If you are looking for a programming language to write an HTML editor that runs on the Windows
platform, or if you would like to write a Web browser or office suite, then Perl does not seem to be an
appropriate language for you. C/C++, Java or (if you are using Windows) Visual Basic are likely to be
more appropriate choices for you.
Although it appears that Perl is not the optimum language for developing applications with a graphicaluser interface (but you can, with Perl/Tk or native modules like WIN::GUI), it is especially strong indoing text manipulation and extraction of useful information. Therefore, with database interfacing it ispossible to build robust applications that require a lot of text processing as well as database management.
Perl is the most popular scripting language used to write scripts that utilize the Common Gateway
Interface (CGI), and this is how most of us got to know this language in the first place. A cursory lookat the CGI Resource Index Web site provided me with a listing of about 3000 Perl CGI scripts, comparedwith only 220 written in C/C++, as of this writing. There are quite many free Web hosts that allow you
to deploy custom Perl CGI scripts, but in general C/C++ CGI scripts are virtually only allowed unless
you pay. In particular, there are several famous programs written in Perl worth mentioning here:?

YaBB is an open source bulletin board system. While providing users with many advanced featuresthat could only be found on commercial products, it remains as a free product. Many webmasters
use YaBB to set up their BBS. Another popular BBS written in Perl is ikonboard, featuring a
MySQL/PostgreSQL database back-end.

?

Thanks to the powerful pattern matching functions in Perl, search engines can also be written in
Perl with unparalleled ease. Perlfect Search is a very good Web site indexing and searching system
written in Perl.

You will learn more about Perl CGI programming in Chapter 11 of this tutorial.

78

Chapter 2 Getting Started

2.2 Comparison with Other Programming Languages
There are many programming languages in use today, each of which placing its emphasis on certain
application domains and features. In the following section I will try to compare Perl with several
popular programming languages for the readers to decide whether Perl is appropriate for you.

2.2.1

C/C++

Perl is written in the C programming language. C is extensively used to develop many system software.
C++ is an extension of C, adding various new features such as namespaces, templates, object-oriented
programming and exception handling etc. Because C and C++ programs are compiled to native code,
startup times of C/C++ programs are usually very short and they can be executed very efficiently. Perlallows you to delegate part of your program in C through the Perl-C XS interface. This Perl-C binding
is extensively used by cryptographic modules to implement the core cryptographic algorithms, becausesuch modules are computation-intensive.
While C/C++ is good for performance-critical applications, C/C++ suffers a number of drawbacks.
First, C/C++ programs are platform dependent. A C/C++ program written on Unix is different fromone on Windows because the libraries available on different platforms are different. Second, because
C/C++ is a very structured language, its syntax is not as flexible as scripting languages such as Perl,
Tcl/Tk or (on Unix platforms) bash. If you are to write two functionally equivalent programs in C/C++
and Perl, very likely the C/C++ version requires more lines of code compared with Perl. And also,
improperly written C/C++ programs are vulnerable to memory leak problems where heap memory
allocated are not returned when the program exits. On a Web server running 24×7 with a lot of visitors,
a CGI script with memory leak is sufficient to paralyze the machine.

2.2.2

PHP

Perl has been the traditional language of choice for writing server-side CGI scripts. However, in recent
years there has been an extensive migration from Perl to PHP. Many programmers, especially those whoare new to programming, have chosen PHP instead of Perl. What are the advantages of PHP over Perl?
PHP is from its infancy Web-scripting oriented. Similar to ASP or JSP, it allows embedding of inline
PHP code inside HTML documents that makes it very convenient to embed small snippets of PHP
code, e.g. to update a counter when a visitor views a page. Perl needs Server Side Includes (SSI) or an
additional package “eperl” to implement a similar functionality. Also, it inherits its language syntax
from a number of languages so that it has the best features of many different languages. It mainly
inherits from C/C++, and portions from Perl and Java. It uses I/O functions similar to that in C, thatare also inherited into Perl, so it is relatively easy for Perl programmers to migrate to PHP.
While PHP supports the object-oriented paradigm, most of its functionalities are provided throughfunctions. When PHP is compiled the administrator decides the sets of functionalities to enable. Thisin turn determines the sets of functions enabled in the PHP installation. I’m personally sceptical of thisapproach, because in practice only a small subset of these functions is frequently used. On the other
hand, Perl only has a small set of intrinsic functions covering the most frequently used functionalities.
Other functionalities are delegated to modules which are only installed and invoked as needed. As I
will introduce shortly and in Appendix B, the Comprehensive Perl Archive Network (CPAN) contains acomprehensive and well-organized listing of ready-made Perl modules that you can install and use very
easily.

2.3 What do I need to learn Perl?

2.2.3

9

Java/JSPSun Microsystems developed the Java language and intended to target it as a general purpose programming language. It is from the ground up object-oriented and platform independent. Functionalities areaccessed through the Java API, consisting of hierarchies of classes similar to that of Perl. Java Server
Pages (JSP) is a Web scripting environment similar to ASP except with a Java syntax. Similar to C/C++,
the Java syntax is very structured and thus are not as flexible as scripting languages like Perl. Also, Java
itself is not just an interpreter, it is a virtual machine over which programmers are totally abstracted
from the underlying operating system platforms, which allows the Java API to be implemented on topof this platform-independent layer. For those who have programmed in Java before, you will probably
find that the Java Virtual Machine takes rather long time to load, especially on lower-end systems withlimited computational power. This defers the possibility of widespread deployment of Java programs.
While Perl is not strictly a general-purpose programming language like Java, I found it difficult tocompare Perl and Java because of their different natures. However, if confined to the purpose of Web
server scripting, I generally prefer Perl to JSP for its flexibility and lightweight performance. Despite
this, I feel that Java is a language that is feature-rich and if time allows, you are strongly encouraged tofind out more about this stupendous language, which is expecting increasing attention in mobile andembedded devices because of its platform independence.

2.2.4

ASP

Active Server Pages (ASP) is only available on Windows NT-series operating systems where Internet
Information Services (IIS) is installed (although alternative implementations of ASP on other systemarchitectures exist, e.g. Sun Chili!Soft ASP, which is a commercial product that runs on Unix, but
generally considered not very stable).
Running on a Windows Web server, ASP can impose a tighter integration with Microsoft technologies,
so that the use of, say, ActiveX data objects (ADO) for database access can be made a lot easier. However,IIS is especially vulnerable to remote attacks when operated as a Web server. Numerous service packs
have been released to patch the security holes in IIS and Windows NT. However, new holes are still
being discovered from time to time that makes the deployment of Windows NT/IIS as the Web server ofchoice not very favourable. On the other hand, Apache, the renowned Web server for Unix and now forother operating systems as well, has far less security concerns and are less susceptible to remote attacks.
Apache also has the largest installation base among all Web server software, taking up more than 60%of the market share.

2.3 What do I need to learn Perl?
You don’t need to pay a penny to learn and use Perl. Basically, a text editor that handles text-only files
and a working installation of the Perl interpreter are all that you will need.
Under Microsoft Windows, Notepad meets the minimum requirement. However, a whole page of codein black is not visually attractive in terms of readability. Some text editors have the feature of syntax
highlighting, with different parts of a statement displayed in different colours. Good colouring makes
the source files more pleasurable to look at (such colouring is used for display only and will not be saved
to file). However, avoid using word processors like Microsoft Word or Wordpad which add proprietarycontrol codes on file save by default. The Perl interpreter does not recognize these special formats. If
you have to use these word processors, ensure that your files are saved as plain text ASCII format so

10Chapter 2 Getting Started

Figure 2.1: Editing a Perl source file with GVIM, running on GNU/Linuxthat the Perl interpreter can access them. AnyEdit and UltraEdit are nice text editors on the Windows
platform. On Unix variants, emacs and vim are stupendous text editors featuring syntax highlighting
profiles for most programming languages with a lot of powerful features. Fig. 2.1 shows a screenshot
of a Perl source file edited with GVIM, a port of vim that runs on Windows, X-Windows with the
GTK library on Unix/Linux and many other platforms. This is my favourite text editor and is used toconstruct my entire Web site.
If you are using one of the mainstream operating systems, the perl interpreter can be downloaded fromthe download section of perl.com. perl.com is the official Web site for the Perl language and you canfind the download links to all available interpreter versions there. Choose the version which matches
your operating system. When you go to the download page you will see two versions, namely thestable production release and the experimental developer’s release. The stable release is the version I
recommend to new users, because the developer’s version is for more advanced users to beta test the
new version. It may still contain bugs and may give incorrect results. The files you have to download
are under the heading “binary distribution”. Do not download the source code distribution unless you
know exactly how to compile and install them. In case you are using an operating system that is notlisted, a good place to find a binary distribution for your operating system is the CPAN, located at here,
which contains a fairly comprehensive list of platforms on which Perl can run.
For Windows users, most probably you should download the Activestate distribution of Perl. It is very
easy to install, with some extra tools bundled in the package for easy installation of new modules.
For GNU/Linux users, most probably Perl is already installed or available as RPM (Redhat Package
Manager) or DEB (Debian packages) formats. As many Linux distributions already have builtin support
for RPM packages, you may look at your installation discs and you are likely to find some RPM binaries
for Perl if it is not yet installed. For other Unix systems, you may find tarballs containing the Perlbinaries. If no binaries are available for your system, you can still build from sources by downloading
the source distribution from the CPAN. To check if perl is installed on your system, simply open a2.4 Make Good Use of Online Resources11

terminal and type perl -v. If Perl is installed you will have the version information of Perl installed
displayed on screen. If error messages appear, you will need to install it.
Installation of Perl will not be covered in this tutorial, and you should look for the installation help
documents for details.

NOTES
Because Perl is an open source software, which releases the source code to the public for free,
you will see the source code distribution listed. Yet for usual programming purposes there isno need to download the source files unless binary distributions are not available for yoursystem.
An exception is if you are using one of the operating systems in the Unix family (including
Linux). There are already compilation tools in these operating systems and you can manuallycompile Perl from sources and install it afterwards. However, note that compilation canbe a very time-consuming process, depending on the performance of your system. If you
are using Linux, binary distributions in the form of RPM or DEB packages can be installedvery easily. Only if you cannot find a binary distribution for your platform that you areencouraged to install from source package.

2.4 Make Good Use of Online Resources
You may need to seek reference while you are learning the language. As a new user you are notrecommended to start learning Perl by reading the man-pages or the reference manuals. They are
written in strict technical parlance that beginners, especially those who do not have prior programming
experience or basic knowledge in Computer Science, would find reading them real headaches. You arerecommended to follow this tutorial (or other tutorials or books) to acquire some basic knowledge first,
and these reference documents will become very useful for ambitious learners to know more about thelanguage, or when you have doubt on a particular subject you may be able to find the answers inside.
In this course I will try to cover some important terms used in the reference materials to facilitate
your understanding of the text. For the time being, you may want to have several books on Perl forcross-referencing purposes. I have tried to write this tutorial in a way that beginners should find it easy
to follow, yet you may need to consult these books if you have any points that you don’t understand fully.
Although you are not advised to read the official reference documents too early, in some later parts
I may refer you to read a certain manpage. A manpage, on Unix/Linux systems, is a help document
on a particular aspect. To read a particular manpage, (bring up a terminal if you are in X-Windows)
type man followed by the name of the manpage, for example, man perlvar, the perlvar manpage will
be displayed. For other platforms, manpages may usually come in the format of HTML files. Consult
the documentation of your distribution for details. There is an online version of the Perl official
documentation at perldoc.com. It contains the Perl man pages as well as documentation of the modules
shipped with Perl. In fact, there are now several manpages that are targeted at novice programmers. For
instance, the perlintro manpage is a brief introduction to the fundamental aspects of the Perl languagethat you should master fully in order to claim yourself a Perl programmer.
You are also reminded of the vast varieties of Perl resources online. There are many Perl newsgroups on
the USENET and mailing lists devoted to Perl. Your questions may be readily answered by expert Perl

12

Chapter 2 Getting Started

programmers there. Of course, try to look for a solution from all the resources you can find including
the FAQs before you post! Otherwise, your question may simply be ignored. Perl Monks is also a very
useful resource to Perl users.
dmoz.org contains a nice selection of Perl-related sites. You can find a similar list of entries on Yahoo!.
Google is the best search engine for programmers. You can usually get very accurate search resultsthat deliver what you need. For example, by specifying the terms “Perl CGI.pm example”, you will get
screenfuls of links to examples demonstrating the various uses of the CGI.pm module. As you will see
later, this module is the central powerhouse allowing you to handle most operations involving CGI
programming with unparalleled ease. For materials not covered in this tutorial or other books, a searchphrase can be constructed in a similar manner that allows you to find documentation and solutions toyour questions at your fingertips.
Of course, don’t forget to experiment yourself! CPAN, the Comprehensive Perl Archive Network is anice place where you can download a lot of useful modules contributed by other Perl programmers. By
using these modules you can enforce code reuse, rather than always inventing code from scratch again.
There are so many modules on the CPAN available that you would be surprised at how active the Perl
community has been. Some CPAN modules are well-documented, some are not. You may need to try
to fit the bits and pieces together and see if it works. This requires much time and effort, but you canlearn quite a lot from this process.

2.5 The Traditional “Hello World” Program
Traditionally, the first example most book authors use to introduce a programming language is what iscustomarily called a “Hello World” program. The action of this program is extremely simple — simply
displays the text “Hello World” to the screen and does nothing else. For all examples in this tutorialof which source code are given in the text, you are encouraged to type them in yourself instead of
executing the examples downloaded from my Web site, since it is more likely that by doing so you would
understand the materials more quickly. Let’s write a “Hello World” program to see the procedures wetake to develop a Perl program.
If you are on Windows, it is a good practice to check if the path to the Perl interpreter has been added
to the path list in C:\Autoexec.bat. In this way, you can change to the path containing your Perl source
files and can run the interpreter without specifying its path. The setup program of your distribution
would probably have done it for you. If it hasn’t, append the path to the end of the list and end itwith a semicolon. A typical path list looks like this, the last one in this example is the path to the perlinterpreter (note that your path may be different):
SET PATH = C:\WINDOWS; C:\WINDOWS\COMMAND; C:\WINDOWS\SYSTEM; C:\PERL\BIN;
For Unix/Linux, check your PATH variable and see if the directory containing the perl executable ispresent (usually /usr/bin). You can look at the list of paths by typing echo $PATH on the commandline (be careful of exact capitalization!). Look for “/usr/bin” in the colon-separated values. On somesystems, the path to perl would be “/usr/local/bin” or something else, so please check carefully. You may
need to modify the startup login scripts like .login, .bashrc, .profile etc. so that you don’t need to set
PATH or specify the full path to perl every time if perl is installed at some weird locations. A convenient
workaround is to create a symbolic link in a directory included in PATH, e.g. /usr/bin that points to theperl executable.

2.5 The Traditional “Hello World” Program

13

NOTES
The use of Autoexec.bat is now obsolete starting from Windows 2000. Setting of environment
variables should be carried out by right-clicking on the “My Computer” icon, and thenchoose the “Properties” option. Now select the “Advanced” tab and then click on the“Environment Variables” button at the bottom. To make the perl interpreter available toall users on the system, the path should be appended to the PATH variable in the “Systemvariables” section. If you modify the PATH variable in the “User variables” section, only theuser concerned (presumably you) will be affected.

EXAMPLE 2.1

1
2
3
4
5

#!/usr/bin/perl -w# Example 2.1 - Hello World# print the text to the screen
print "Hello, World!\n";

Here we outline the steps to create this simple program on Windows and Linux.
Microsoft Windows

1.

Open Notepad (or any other text editor you choose) and type in the source code shown above.
Note that the line numbers on the left are for identification of lines only and do NOT type theminto the text editor. Please make sure word wrap is disabled.

2.

Save the file as hello.pl. A few text editors, like Notepad, usually append the “.txt” extension to thefilename when saving. You may put a pair of double quotes around the filename to circumvent
this behaviour. Also, if you are using Windows 2000 or above and would like to use Notepad,
please ensure that the file encoding is set to ANSI instead of Unicode.

3.

Bring up an MS-DOS prompt window and change to the directory containing your newly created
file. Say if you have saved to ”C:\perl examples”, then type cd C:\perl examples and press
Enter. Put a pair of double quotes around the path if any directories in the path contains spaces(In fact I don’t recommend placing Perl source files in directories with names containing spaces.
It only complicates matters).

4.

Execute the program by typing perl -w hello.pl and press enter.

Unix or GNU/Linux

1.

Open any text editor (vim, emacs, pico, kedit ....) and type in the source code shown above. Notethat the line numbers on the left are for identification only and do NOT type them into the text
editor. Please make sure word wrap is disabled.

2.

Save the file as hello.pl. Note that the path on line 1 has to match the path to perl on your system.
Also, no spaces should precede the ‘#’ character and no empty lines are allowed before this specialline (traditionally known as the ‘shebang’ line).

14

Chapter 2 Getting Started

3.

If you are in X-Windows environment, bring up a terminal window. Change to the directory
containing the newly created file using the cd command.

4.

In order to run it without specifying the perl interpreter, set the file access privilege to user executable by using the chmod command. The command should be chmod u+x hello.pl and press
Enter.

5.

Execute the program by typing ./hello.pl and then press Enter.
NOTES
Even if you are using Unix/Linux, it is not absolutely needed to chmod your perl source
files. In fact, you only need to make those source files executable if you want them to be
directly invoked without specifying the name of the interpreter (i.e. perl). In this case,the shell will look at the first line of the file to determine which interpreter is used, so the#!/usr/bin/perl line must exist. Otherwise, the file cannot be executed. If you only intend
to execute the file in Unix or Linux using perl -w filename.pl, then filename.pl need not
be given an executable permission. As you will learn later, you may have some Perl source
files that are not invoked directly. Instead, they are being sourced from another source fileand don’t need to be executable themselves. For these files, you don’t need to chmod themand the default permission is adequate.

If there is not any errors, you should see the words “Hello, World!” under the command prompt. For
such a simple program it is not easy to make mistakes. If error messages appear, check carefully if you
have left out anything, because a trivial mistake is sufficient to end up with some error messages if you
are not careful. Also check if you are using the latest stable version of Perl 5. All examples in this tutorial
have been tested with Perl 5.8.0 Win32 (ActiveState distribution) and Perl 5.8.0 on GNU/Linux, but itshould work for other distributions or versions as well unless otherwise noted.
The -w is an example of a switch. You specify a switch to enable a particular interpreter feature. -w is
specified so that warning messages, if any, are displayed on screen. Under no circumstances should thisswitch be omitted because it is important, especially as a beginner, to ensure that the code written iscorrect. It also helps catch some mistakes that are otherwise difficult to capture. This is explained in
more detail in Chapter 10.
A Perl script consists of statements, and each statement is terminated with a semicolon (;). A statement
is rather like a sentence in human languages which carries a certain meaning. For computer languages
a statement is an instruction to be performed.
The core of the program is on line 5. It is this statement that prints the text delimited by quotation
marks to the screen (in a more accurate parlance, the text is sent to the standard output, which is thescreen by default). print() is an example of a builtin function. A function usually accepts a numberof parameters, or arguments. Parameters serve to provide a function with additional pieces of datathat are necessary for its operation. In the example, print() takes the text as parameter in order todisplay it on screen. A set of basic functions is provided to you by Perl for performing different actions.
Some of these functions will be introduced as you progress through this tutorial.
Notice the strange \n at the end? It is one of the escape characters which will be described later in moredetail. \n is used to insert a line break. Therefore, you see a blank line before returning to the commandprompt.

2.6 How A Perl Program Is Executed

15

Lines preceded by a # (sharp) sign are comments and are ignored by the perl interpreter. A comment
does not need to be on its own line, it can be put at the end of a line as well. In that case, the remainingof the line (starting from # and until the end of the line) is regarded as a comment. Comments arehelpful for you or other programmers who read your code to understand what it does. You can put
anything you like as comments. Line 1 is also a comment as it is of interest to the shell only instead of
the perl interpreter itself. The switch -w here is the same as that specified under the command line.
This is read together with the path to perl to enable the display of warnings.

GOOD PROGRAMMING PRACTICES

Comments
Comments are meant for human readers to understand the source code without the needof running the program once in your brain. This increases both the readability and maintainability of your source code. Many programmers are lazy to insert comments throughout
the code. But it is very likely when you look at a piece of code wrote earlier you may notunderstand it anymore as it is very complicated without any comments in it.
Therefore, you should include comments in appropriate places. Usually for a single block ofcode performing one particular function we will place a comment briefly describing what
this code block does. You may also want to place a comment on a particular line if themeaning of the line is not immediately obvious. Of course, don’t deluge your source codewith comments. For example, in the source code of the ‘Hello World’ program I placed acomment for the print statement. It is superfluous, in fact, as the meaning of this statement
is pretty obvious. But I included it here because this is your first Perl function learnt. As you
proceed, more constructive comments and less superfluous comments will be found in theexamples.

2.6 How A Perl Program Is Executed
Perl programs are distributed in source files. From the instant you invoke the perl interpreter to executea script, a number of steps were involved before the program is executed.

Preprocessing An optional preprocessing stage transforms the source file to the final form. Thistutorial does not cover source preprocessing. For details please consult the perlfilter manpage.

Tokenizing The source file is broken down into tokens. This process is called tokenization (orlexical analysis). Whitespaces are removed. Token is the basic unit that makes up a statement. By
tokenizing the input parsing is becoming easier because all further processing are carried out on tokens,
independent of whitespace.

Parsing & Optimization (Compilation) Parsing involves checks to ensure the program being
executed conforms to the language specification and builds a parse tree internally which describes the
program in terms of microoperations internal to Perl (opcode). Some optimizations to the parse tree

16

Chapter 2 Getting Started

are performed afterwards. Finally, the parse tree built is used to execute the program.
While in-depth understanding of any of these processes is not essential to practical Perl programming,
the compilation phase will be mentioned in some later chapters that I believe it is a good idea to briefly
introduce the phases involved beforehand.

2.7 Literals
All computer programs have to handle data. In every program there are certain kinds of data that
do not change with time. For example, consider a very much simplified CGI script that checks if thepassword input by the user matches the system. How would you implement it? It seems the simplest
method would be to have the correct password specified in the script, and after the user has entered thepassword and hit the “Submit” button, compare it against the password input by the user. The standard
password specified in this script does not change during the course of execution of the script. This isan example of a literal. Other terms that are also used are invariants and constants. In the previous
Hello World example, the text “Hello, World!\n” on line 5 is also a literal (This piece of data cannot bechanged during the time you are running the program).
Literals can have a number of forms, just because we can have data of different forms. In Perl we canroughly differentiate numbers and strings.

2.7.1

Numbers

There are several classes of numbers: integers and decimals (known as floating-point numbers incomputer literature).
In Perl, integers can be expressed in decimal (base 10), hexadecimal (base 16) or octal (base 8) notation.
Octal numbers are preceded by a 0 (zero), e.g. 023 is equivalent to 19 10 (the subscript 10 denotes
representation in base-10, i.e. decimal form); hexadecimal numbers are preceded by 0x (zero x), e.g.
0xfe is equivalent to 25410 . For hexadecimal digits A - F, it does not matter whether you specify themin lowercase or uppercase. That is, 0xfe is the same as 0xFE.
Integers cannot be delimited by commas or spaces like 10,203,469 or 20 300. However, Perl provides
a nice workaround as a substitute. An example is 4 976 297 305. This is just a facility to make large
numbers easier to read by programmers, and writing 4976297305 is entirely correct in Perl.
Decimals are those carrying decimal points. If the integral portion is 0, the integral portion is optional,
i.e. -0.6 or -.6 work equally fine. Exponents (base 10) can also be specified by appending the letter “e”
and the exponent to the real number portion. e.g. 2e3 is equivalent to 2 x 10 3 = 2000.

2.7.2

Strings

A string is a sequence of characters enclosed (delimited) by either double quotes (”) or single quotes(’). They differ in variable substitution and in the way escape characters are handled. The text “Hello,World!\n” in the hello world example is a string literal, delimited by double quotes.
We will defer variable substitution until we come to variables. Escape characters exist in many
programming languages. An escape character consists of a backslash \ symbol followed by a letter.

2.8 Introduction to Data Structures

17

Every escape character has a function associated with it. Escape characters are usually put inside
double-quoted strings. Table 2.1 summarizes the most important escape characters used in Perl.
These escape characters are predefined and can be used in double-quoted strings. There is anothercase where backslashes have to be used in a string. This is known as character escaping. What doesthat mean? Consider you would like to use double quotes in a double-quoted string. For example you
would like to print this English sentence instead of the “Hello World” phrase in Example 2.1:
Howdy says, ”Give me $500”.
That is, you try to print this sentence by:print "Howdy says, "Give me $500".";If you execute this statement, you will get into trouble, because " is used to mark the beginning and theend of the string literal itself. Perl locates the end of the string by searching forward until the second
double quote is found. If the literal contains double quotes itself, Perl will not know where the string
literal terminates. In the example above, Perl will think the string ends after “Howdy says, ”. Also, after
you have learned variable substitution in the next chapter you will realize that the symbol $ is used forvariable substitution. You have to tell Perl explicitly you would like to use the symbol as is instead of
performing variable substitution. To get around this problem, just place the \ character before the two
symbols concerned, and this is what we mean to “escape” a character. So, the correct way to print thissentence using double quotes is:print "Howdy says, \"Give me \$500\".";
However, wise Perl programmers will not do this, as the backslashes make the whole expression ugly. If
we choose to use single quotes instead, we don’t even have to escape anything:print ’Howdy says, "Give me $500".’;Single-quoted strings do not support variable substitution, so the $ needs not be escaped. Also, because
the symbol " does not carry any significance in the string, it does not need to be escaped as well.
There are only two characters that need to be escaped in single-quoted strings, namely ’ and \. For
double-quoted strings, a number of characters have to be escaped, and it would become clear as you
work through the chapters in this tutorial.
Empty strings are denoted by “” or ‘’, that is, two quotes with nothing in between.

2.8 Introduction to Data Structures
Every programming language has certain kinds of data structures builtin. A data structure can bethought of as a virtual container residing in the memory in which data is stored. Each data structure
is associated with a data type specifying the type of data permitted in the data structure. Data type isimportant in programming languages because data of different types are likely to be treated differently.
For example, numbers are sorted by numerical value; while strings are sorted in alphabetical order.
Many programming languages, like C++, Java and Visual Basic, have a large number of data types,
e.g. integer, double, string, boolean ... just to name a few. They require declaration of a data type

18

Chapter 2 Getting Started

EscapeCharacter

Function\n

NewlineStarts a newline\r

Carriage Return
Returns to the starting point of the line\tTabAnalogous to striking the Tab key on your keyboard; However, using
tab to make formatter output does not always generate the format
expected.

\b

Backspace
Analogous to the Backspace key; erases the last character

\a

Bell
Creates a beep sound from the system buzzer (or sound card)\xnn

ASCII character using hexadecimal notation
Outputs the character which corresponds to the specified ASCII index (each n is a hexadecimal digit)\0nn

ASCII character using octal notation
Outputs the character which corresponds to the specified ASCII index (each n is an octal digit)\cX

Control Character
For example, \cC is equivalent to pressing Ctrl-C on your keyboard\u

Next letter uppercase
The letter immediately following \u is converted to uppercase. For
example, \uemail is equivalent to Email\l

Next letter lowercase
The letter immediately following \l is converted to lowercase. For
example, \lEmail is equivalent to email\U

All subsequent letters uppercase
All the letters immediately following \U are converted to uppercase
until \E is reached\L

All subsequent letters lowercase
All the letters immediately following \L are converted to lowercase
until \E is reached\Q

Disables pattern matching until \EThis would be covered in the “Regular Expressions” chapter.

\E

Ends \U, \L, \Q
Terminates the effect of \U, \L or \Q.

Table 2.1: The most commonly used escape characters in Perl2.8 Introduction to Data Structures

19

when a data structure is created, and this data type cannot be changed afterwards. There are bothadvantages and disadvantages to this approach. As different data types occupy different amount ofstorage in the memory, the underlying machine actually requires some type information to convert
the high-level programming constructs into assembly instructions in the compilation stage. Also, by
having the data type fixed there are less ambiguities as to how the data is to be handled. The most obvious disadvantage, of course, is that explicit data conversion is necessary in such programming languages.
As of Perl 5, Perl officially differentiates only two types of data: scalar data and list data. Moreover, Perldoes not enforce strict type-checking, instead, it is loosely-typed. This may change in Perl 6, but you
will not see it in the near future.
Scalar data represents a piece of data. All literals are scalar data. Variables are also scalar data. As theunderlying machine requires explicit declaration of data types, Perl needs to convert the data between
different data types as needed in the underlying implementation, while a Perl programmer can be
oblivious to such data conversion. In the next chapter you would see example code in practice.
Another type is list data. List data is an aggregation of scalar data. Arrays and hashes belong to thistype. While you may not have a clear picture of how list data look like at this point, you would have aclear idea after reading the next chapter.
Three basic data structures are provided by Perl, namely scalar variables, arrays and associativearrays (hashes). I am going to give an introduction to the three types of data structures at this point,
and in the next chapter you would see the functions and operations associated with these data structures.
A scalar variable, or simply a variable, is a named entity representing a piece of scalar data of which thecontent can be modified throughout its lifetime. What does this mean? A variable is conceptually likea virtual basket, and only one object is allowed in it at any one time. If at some time you would like toplace something else in the basket, you have to replace the existing object with a new one, and the existing object is discarded. In Perl a variable can store a piece of string or number (or a reference, which wehaven’t come to yet). Unlike other programming languages, Perl gives you the flexibility that at one time
you may store a number and at other times you may store a string in the same variable, however, it is agood practice to always store data of a particular type at any time in the same variable to avoid confusion.
Because the value of a variable can be modified at any point, and there can be many variables thatare concurrently in use at a time, we have to specify which one to address. Therefore, variables are
named, and on the other hand literals are unnamed. This name is known as an identifier. By default,
all variables are global, that is, after the variable is first used in the script, it can be referred to at any
time, anywhere until the script terminates. However, it is a good practice not to use global variables
excessively. Instead, most variables are actually used for temporary storage only and can be restricted to
be valid for a limited time. This concerns the lifetime of a variable, and in turn the idea of scope. This
would be discussed in Chapter 5.
Sometimes we are dealing with a set of related data. Instead of storing them separately in variables, we
may store them as list data, which is a collection of scalar data sharing a single identifier (name). Arrays
and hashes are two types of list data.
An array is a named entity representing a list of scalar data, with each item assigned an index. In
an array, the integral index (or subscript) uniquely identifies each item in the array. The first item
has index 0, the one afterwards has index 1, and so on. Each item in an array is a piece of scalardata, and, therefore, (in Perl only) numbers as well as strings may coexist in the array. An array canbe empty, that is, containing no elements (called a null array). A representation of an example ar-20

Chapter 2 Getting Started

ray containing some data is shown below, the column on the left is the index and the data is on the right:
Index
0
1
2
3

Data
“Apple”
36“Hello, World”“School”

Table 2.2: Contents of a sample array in Perl

A hash is a special data structure. It is similar to an array except that the index is not an integer, so theterm “index” is not customarily used for hashes. Instead, a string is used for indexing, and is known asa key. The key is conceptually like a tag which is attached to the corresponding value. The key and thevalue forms a pair (key-value pair). Like an array, the keys in a hash have to be distinct to distinguisha key-value pair from another. Recall that ordering in arrays is determined by the indices of the items(because we can say the first item is the one which has subscript 0, the second item which has subscript
1, and so on). However, in a hash no such ordering is present. You will see in the next chapter that wemay “sort” the hashes by keys or by values for presentation purposes. However, this does not physicallyreorder the keys or the values in the hash. It merely rearranges the key-value pairs for screen output or
to be passed to another process for further processing.
Hashes (or hash tables in Computer Science parlance) are especially useful in dictionary programs.
Assume that the program works as follows. It requires a user to enter an English word into the text
entry box that is to be searched in the dictionary database. Inside the dictionary is actually a long
list of key-value pairs, where the key is the word entry and the value is an ID that the database uses
internally to retrieve the corresponding record (containing the explanations, pronunciation etc.). The
term entered by the user is queried in the dictionary. If the entry matches any key, the corresponding
ID is obtained and is used to retrieve the record for the word specified; Otherwise, the term is notfound and the program returns an error. Hash table is an efficient data structure for data storage. A
well-implemented hash table requires only several comparisons to retrieve the value if the key is in
the hash. More surprisingly, even if a given key does not exist in a hash, it is NOT necessary to searchthrough all the keys in the hash before returning the key-not-found error. The reason for this concerns
the principle behind hash tables. You may find more information in Appendix A or in any textbooks
on data structures and algorithms.
Several entries of a possible hash table for the above dictionary program is shown below:
Key
“Boy”
“Apple”“Kite”
...

Value
342
165
1053
...

Table 2.3: Contents of a sample hash in Perl

In the next chapter, you will learn how to manipulate the data structures discussed. You will know how
to construct an array, and remove items from it, etc.

2.8 Introduction to Data Structures21

Summary”

Perl is especially strong in text manipulation and extraction of information.

”

To create a Perl program, you need only a text editor and the perl interpreter.

”

The -w switch of the interpreter enables the output of warnings.

”

The print() function can be used to output a string to the standard output (screen by default).

”

Interpreter switches are either specified on the command line or in the script itself as a shebangline for Unix systems.

”

Comments are preceded by the # symbol and can be placed anywhere in your program.

”

A Perl program consists of statements, with each statement terminating with a semicolon.

”

Integers can be expressed in hexadecimal, octal or decimal notation.

”

Double-quoted string literals perform variable substitution, and recognize a number of escapecharacters.

”

Single-quoted string literals do not perform variable substitution.

”

Scalar data represents a single piece of data. It can be a literal or a scalar variable.

”

List data represents a set of scalar data. Arrays and hashes belong to this type.

”An array uses a zero-based subscript to refer to the element being referred to, while hash elements
are identified by the key.

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Chapter 2 Getting Started

Chapter 3

Manipulation of Data Structures
3.1 Scalar Variables
I have qualitatively described how a scalar variable looks like in the previous chapter. Now we are goingto look at how you can use it in your program.
You refer to a variable by appending the identifier of the variable to the symbol $. For example, avariable named LuckyNumber is written as $LuckyNumber.

3.1.1

Assignment

In Perl, you are not required to declare a variable before it is being used. However, before you ever use avariable in your program you should give it a default value. To assign a value to the variable we use theassignment operator (=). For example, we can assign the value 18 to the variable $LuckyNumber asfollows:$LuckyNumber = 18;
This statement is interpreted as follows: the value on the right hand side of the assignment operator is
assigned to the data structure on the left. In this case, 18 is assigned to $LuckyNumber.
However, the right hand side of the assignment operator is not confined to a literal only. The righthand side of an assignment operator is actually treated as an expression. An expression consists ofa sequence of operations which evaluate to a value by means of operators. For example, (6 + 5) *2 is an expression consisting of two operations (* denotes multiplication). Evaluating an expression
means to deduce the result of the expression, by evaluating each operand, and applying the operators
in a certain order (subject to operator precedence and associativity) to transform the expression into
the value. The expression in this example evaluates to the scalar value 22. You will learn more about
operators in the next chapter. Therefore, if we have another variable $Num which has the value of 8,
executing the statement $LuckyNumber = $Num causes $Num to be evaluated on the right hand side,and thus its value, that is 8, is assigned to $LuckyNumber. So this is essentially $LuckyNumber = 8.
Cascaded assignment is also allowed, e.g. $a = $b = 8; First, 8 is assigned to $b, and then the valueof $b is assigned to $a. The net effect is that the two variables both have the value 8.
In some other programming languages, to supply a default value before you first use a variable(initialization) is very important. For C/C++, you need to declare a variable before it is used. This23

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Chapter 3 Manipulation of Data Structures

reserves memory space for this variable. However, it is not required in C/C++ that a value needs to be
assigned during variable declaration. A variable in this state is described as undefined. Getting a valueof an undefined variable poses a very subtle source of error (some garbage values are returned — that
is, arbitrary value without any meaning) that yields surprising results, and is very difficult to debug.
In Perl, if you use a variable that is not initialized (for example, printing its value), the value undef(undefined) is returned. This is a special value that gives different values in different contexts. Contexts
will be introduced in the last section of this chapter and, simply put, it is 0 in numeric context (i.e. if anumber is expected), an empty string in string context (i.e. a string is expected) or FALSE in “Boolean”
context (i.e. when either TRUE or FALSE is expected). However, if you have specified the -w switch, theinterpreter should have warned you on using uninitialized variables. The use of uninitialized variables
is not a good practice, and you have to ensure that all variables are given a value before being used.
As already discussed, you can store scalar data of different types in a variable during the lifetime of thescript, so you can now assign the string literal “eight” to $LuckyNumber. Perl will just happily accept it.
Of course, you should try to avoid it, as described in the previous chapter.

NOTES
You may see the terms lvalue and rvalue in some other books or documentation. An lvaluerefers to any valid entities that can be placed on the left hand side of the assignment operator,
while an rvalue refers to any valid entities that can be placed on the right hand side of theassignment operator. A list, an array or scalar variable can be an lvalue. Literals (scalar insense) are not lvalues. Different programming languages have different lvalues and rvalues.
For example, these are lvalues (of course they can be rvalues as well):$var($var, @lst)
These are not:46("mystuff", "apple")

3.1.2

Nomenclature

Before you proceed, I will first talk about how to choose an identifier for a variable (and arrays or
hashes alike). An identifier should start with a letter (A-Z, a-z) or underscore ( ). Subsequent letters
may be alphanumeric characters (A-Z, a-z, 0-9) or an underscore ( ). No spaces are allowed in the
middle of an identifier. There is one more important point. Perl is at all times case-sensitive. Thatmeans it differentiates lowercase and uppercase characters. The print() function you saw in the Hello
World example cannot be replaced by Print, PRINT or anything else. Similarly, $var and $Var are twodifferent variables. The last point to note is that identifiers cannot be longer than 255 characters (longidentifiers are time-consuming to type and difficult to interpret — please avoid them).
Another point to note is that the name of a variable, array or hash is formed by a symbol ($ for variable)
and the identifier. Therefore, $Var, @Var and %Var can coexist. Although they have the same identifier,they are still unique names because the symbols are different. Also, you may use “reserved words” forthe identifier, e.g. $print because the symbol before the identifier tells Perl that this is a variable. There3.1 Scalar Variables

25

NOTES
If you read on, you will discover that Perl has many builtin predefined variables which do notfollow such a nomenclature scheme. For example, $1 - $9 are reserved for backreferencing
in pattern matching (see Chapter 9 “Regular Expressions”), or other more awkward looking
variables like $,, $", $ ... just to name a few. Because devoting a whole chapter just to
introduce them would be too boring, I will introduce them as needed throughout the text.
Alternatively, the perlvar manpage describes these Perl predefined variables in detail.

is thus no ambiguities.

3.1.3

Variable Substitution

It’s time to talk about variable substitution. I told you that single-quoted strings do not allow variable
substitution, while double-quoted strings can. Variable substitution means that variables embedded
in a double-quoted string will be substituted by their values at the time the statement is evaluated.
Consider this example:
EXAMPLE 3.1

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Celsius → Fahrenheit Converter#!/usr/bin/perl -w# c2f.pl# A Celsius->Fahrenheit Converter# Print the prompt
print "Please enter a Celsius degree > ";# Chop off the trailing newline character
chomp($cel = <STDIN>);$fah = ($cel * 1.8) + 32;# print value using variable interpolation
print "The Fahrenheit equivalent of $cel degrees Celsius is $fah\n";Line 9 looks awkward, but here we perform two operations, to accept user input and remove the trailing
newline character. You don’t have to concern much about this statement yet, as this will be described in
Chapter 8.
Line 11 calculates the Fahrenheit temperature, and when execution of the script reaches line 14, it sees
the two variables $cel and $fah. Then Perl replaces them with the values the two variables carry atthat instant, and the resulting string is output to the screen.
There is one problem arising from variable substitution. What if we have some words immediately
following the variable, without even a space? Perl has provided a nice facility to get around thissituation. You may put a pair of curly brackets around the identifier name to separate it from thesurrounding text, e.g.

26

Chapter 3 Manipulation of Data Structures

print "${Num}th Edition";

3.1.4

substr() — Extraction of Substrings

Frequently you have to extract a sequence of characters from a string. In other words, you extract a
substring from it. Perl provides you with the substr() function to extract a substring, provided thatyou already know in advance where to start extracting it.
The syntax of the substr() function is as follows:
substr STRING, OFFSET
substr STRING, OFFSET, LENGTH
substr STRING, OFFSET, LENGTH, REPLACEMENT
As you can see, substr() can take 2-4 parameters depending on your needs. STRING is the string fromwhich extraction is performed. OFFSET is a zero-based offset which indicates the position from whichto start extraction. The first character of any string has an OFFSET 0, and 1 for the second characteretc. In Perl, OFFSET can be negative, which counts from the end. For example, the last character of thestring can be represented by the OFFSET -1. LENGTH is the number of characters to extract. If it is not
specified, it extracts till the end of the string. The extracted substring is returned upon evaluation. Hereare some examples:$string = "This is test.";print substr($string, 5);
print substr($string, 5, 2);# is test.
# is

If REPLACEMENT is specified, the substring is replaced by the string obtained by evaluating REPLACEMENT,and the substring being replaced is returned. Alternatively, you may put substr() on the left handside of an assignment operator, and REPLACEMENT on the right. Here is an example which replaces asubstring of length 0 with the replacement string “not a ”, that implies inserting it at position 8:
substr($string, 8, 0, "not a ");## First method
substr($string, 8, 0) = "not a ";## Second method
print $string;# This is not a test.

3.1.5

length() — Length of String

You can find out the length of a string by using the length() function. The only parameter for thelength() function is the string itself. It returns the number of characters in the string.
Scalar variable is a very simple data structure. In the next section we are going to deal with lists, arrays
and hashes that are a lot more interesting to play with.

3.2 Lists and Arrays
Arrays are named entities, lists are not. The relationship between a list and an array is very muchsimilar to that between a literal and a scalar variable. A list is merely an ordered set of elements. An
array is just like a list but with a name thus can be referenced through an array variable. Studying the
behaviour of lists allow us to progress naturally into arrays and hashes in subsequent sections.

3.2 Lists and Arrays

3.2.1

27

Creating an Array

Each item in the list is called an element. To create a list, simply delimit (separate) the elements withcommas (,) and surround the list with a pair of parentheses. For example a list containing the names ofsome colours can be written as("red", "orange", "green", "blue")
An array can be created by assigning a list to an array variable. An array variable starts with the symbol@ (compare with the case of $ for scalar variables). Therefore, an array can be set up containing the list
above, e.g.

@colors = ("red", "orange", "green", "blue");
Alternatively, you may use the equivalent method of per-item assignment (to be discussed shortly):$colors[0] = "red";$colors[1] = "orange";$colors[2] = "green";$colors[3] = "blue";
This array contains 4 elements. A null array is simply (). Lists can be nested, that is, a list may containother lists or array variables as an element. However, the embedded lists will be expanded and mergedwith the container list. Any null lists or null arrays are removed. For example, if@unix = ("FreeBSD", "Linux");@os = ("MacOS", ("Windows NT", "Windows ME"), @unix);@os is expanded into
@os = ("MacOS", "Windows NT", "Windows ME", "FreeBSD", "Linux");
In the following example, @result will become a null array. Note that null lists are ignored.

@nullarray = ();@result = ((), @nullarray);
A useful operator that worths mentioning here is the range operator (..). If you would like to generate
an array of consecutive integers this operator may come in handy, as you no longer have to use a loop
to do it. Example:@hundrednums = (101 .. 200);
However, the numbers must be in ascending order. If you would like to have an array of consecutive
integers in descending order, you may construct it in ascending order using the range operator, andthen reverse the position of the items using the reverse() function:@hundrednums = reverse (101 .. 200);

28

3.2.2

Chapter 3 Manipulation of Data Structures

Adding Elements

We know from the previous section that if we place an array variable or a sublist as an element ofa list, the array variable or the sublist would be expanded and merged with the parent list, and in
this operation the identity of the original array variables or sublists are lost. That means, you can nolonger tell from the resulting list if a particular element originates from a sublist or an array variable.
Therefore, it is natural to conclude that two arrays can be merged together by this operation:@CombinedArray = (@Array1, @Array2);
The resulting array contains all the elements in @Array1, followed by that of @Array2. To append a
scalar element to the end of an array, you can write, for example,@MyArray = (@MyArray, $NewElement);
We can also append a list of scalar data to the end of an array by using the push() function. The syntaxof the push() function ispush ARRAY, LIST;
where ARRAY is the array to which the list data are to be appended. LIST is a list specifying theelements to be appended to ARRAY. The mechanism of the push function is not much different fromthe interpolation of lists above, and I am more accustomed to the previous one than using the pushfunction, because it is more intuitive to understand.
push is a function. A function returns some values (not necessary scalar, can be list data as well for
certain functions) after the operation is finished. For this function, the number of elements afterelement addition is returned. Consider the example below:$NumElements = push(@MyArray, @list);
Note that I have added the parentheses around @MyArray and @list. The parentheses here are notactually necessary, but I added them here to make it obvious the parameters of the function. The return
value is assigned to $NumElements. In the next chapter, you will learn operator precedence, which
describes in detail when and where you should add parentheses. For the time being, stick to the way I
have been doing and it would be fine.
On the other hand, this operation inserts the element at the beginning of the array:@MyArray = ($NewElement, @MyArray);unshift is a function that inserts a list at the beginning of an array, and returns the number of elements
after the operation. The syntax is

unshift ARRAY, LIST;3.2 Lists and Arrays

29

where ARRAY is the array to which elements are added, and LIST is the list that is inserted at the
beginning of ARRAY. Consider the following example:
EXAMPLE 3.2

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Demonstrating unshift()#!/usr/bin/perl -w@alpha1 = ("a", "b", "c");@alpha2 = ("d", "e", "f");unshift @alpha2, @alpha1;$, = " "; # Prints a space in between elements
print @alpha2;
Again, disregard the line numbers that are for illustration only. Note that the ordering of the items
of @alpha1 is preserved in @alpha2. What is worth noting is on line 7. We assign a space to an
odd-looking variable, but what’s that? This is an example of Perl predefined variables, and $, is known
as the output field separator. Without line 7, you would most likely get an output like “abcdef ”
because by default this variable is an empty string. Note that the print() function can accept a list ofparameters. The output field separator is the string that is added in between list elements in a print
operation. You may assign any valid string literals to this variable. In this example, I put a space inbetween elements to make them easier to read.

3.2.3

Getting the number of Elements in an Array

There are two ways in which you could obtain the number of elements stored in an array.
The first method is to employ the concept of context. By evaluating an array in scalar context, we canobtain the number of elements in an array. You may not understand what this is for the time being, but
we will return to this example when we come to contexts later in this chapter. In the following example,
the number of elements in @colours is asssigned to $numElements:$numElements = @colours;
The second method is a little bit clumsy to explain. In Perl, you can find out the subscript of the lastitem of an array by replacing the symbol @ with $#. For example, the subscript of the last item in@Array is given by $#Array.
For historical reasons, Perl provides a facility for users to specify the subscript of the first element of anarray. This is specified by assigning an integer to the predefined variable $[. This is 0 by default, andthat’s why I said subscripts start from 0. However, some people may be accustomed to using 1 as thestarting index (especially those who have used some “antique” programming languages). This value
is not necessarily 0 or 1. You may set other values as well. However, you cannot assign values to thisvariable more than once. Although Perl provides this facility, you are advised not to use it because of
potential confusions that may arise, especially if your project consists of a number of files.
With both the start index and the index of the last element, we can get the number of elements by using
the formula:

30

Chapter 3 Manipulation of Data Structures

Number of elements = Last Index - Starting Index + 1= $#Array - $[ + 1
As the start index is 0 by default (that is, if you don’t specify otherwise), you may assume that the
number of elements of @Array is given by$#Array + 1

3.2.4

Accessing Elements in an Array

After we have added items to an array or list, we can access any of its elements by using the subscript
operator ([]). For example, if we would like to retrieve the third element (remember that subscripts
count from 0) from @colours and return the value to $col3, we can write$col3 = $colours[2];Note that the symbol is $ instead of @ on the right hand side of the assignment operator. In Perl,because the value returned is a scalar value, the symbol $ is used. Though looking awkward, you candefinitely use the subscript operator on a list, like this:$col3 = ("red", "orange", "green", "blue")[2];
Obviously, using the subscript operator in this way is not really useful.
What about specifying a negative subscript? Negative subscripts count backwards, from the lastelement of the array. The last element has the subscript -1, and the next-to-last element has thesubscript -2. Therefore, the first element of the array @array has the subscript -@colours (dueto scalar context). Normally, negative subscripts are used as a convenience method to retrieve thelast element of an array. At least, @array[-1] is definitely easier to understand than @array[$#array].
An array slice is a subset of elements from all the elements in an array. The subscript operator is notconfined to one subscript only. You may specify a list of subscripts using the comma operator (,) andthe range operator (..). You use the range operator to specify subscripts (must be integral) in a given
range, and the comma operator to specify each subscript individually. Nothing will stop you from using
both operators together, as in the following example:
EXAMPLE 3.3

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Array Slices#!/usr/bin/perl -w@alpha = (’a’ .. ’z’);@slice = @alpha[4, 10 .. 15];$, = " ";print @slice;
In this example, the array slice @slice contains the 5 th element, and 6 elements starting from the 11 thelement of @alpha. The resulting output is thus “e k l m n o p”. Note that because the array slice
contains a list of values, the symbol of @alpha is @ on line 4.

3.2 Lists and Arrays

31

We can, of course, modify the value of any element in an array. To do this, just assign a scalar value tothe corresponding array element. Similar is the case for slice assignment, in which a list of scalar data is
assigned as a list. For example:$colours[2] = "violet";# The followings are identical@colours[2,4] = ("violet", "blue");$colours[2] = "violet"; $colours[4] = "blue";@colours[4,2] = ("blue", "violet");If you specify a subscript that is larger than $#array, the size of the array shall grow in order toaccommodate the newly added element. In this example, because @nums contains originally 3 elements,
assigning a value to the 5th element leaves a “gap” at the 4th element, not assigned any values. The valueof this element is undef.
EXAMPLE 3.4

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Array Expansion#!/usr/bin/perl -w@nums = (3, 4, 5);$nums[4] = 7; # (3, 4, 5, undef, 7) expected$, = "\n";print @nums;
To show that there is an empty gap at the 4 th element, the output list separator is set to \n. So you would
see a line like “Use of uninitialized value at eg0304.pl line 6” between 5 and 7 in the output if warnings are enabled. That’s because retrieving the value of an uninitialized value produces this Perl warning.

3.2.5

Removing Elements

We can use the pop function to remove the last element of an array. It also returns the value of the item
being removed. Syntax:

pop ARRAY;
In this example, the last item of @MyArray is removed and its value is assigned to $retval:$retval = pop(@MyArray);On the other hand, the shift function removes the first element of the array, so that the size of thearray is reduced by 1 and the element immediately after the item being removed becomes the firstelement of the array. It also returns the value of the item being removed. Syntax:

shift ARRAY;
If ARRAY is empty, undef is returned.

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Chapter 3 Manipulation of Data Structures

3.2.6

splice(): the Versatile Function

There is a generalized function for adding and removing elements from an array. The splice function
is so general that it can do what push, pop, shift and unshift does. The syntax is:
splice ARRAY, OFFSET [, LENGTH [, LIST]];In this tutorial, the parts in slanted font denote optional parameters. These parameters are optional in
the sense that in some situations they are optional; but not in other situations. I use [] to label that aparameter is optional. Note that [LIST] is placed within another optional parameter LENGTH. Thismeans that if you have to specify the parameter LIST, you must also specify LENGTH; but not viceversa. If you specify the parameter LENGTH, you may or may not supply the parameter LIST.
In general, this function removes LENGTH elements starting from the element of subscript OFFSET
of ARRAY, and inserts LIST at OFFSET if any. Simply put, the list @ARRAY[OFFSET .. OFFSET +LENGTH - 1] is replaced by LIST. The syntax shows that this function takes three forms, and I am
going to describe them one by one.

splice ARRAY, OFFSET, LENGTH, LIST
This performs exactly the action above. In the example below, an array containing the 26 lowercase
alphabets was built, and 5 elements starting from the 5 th element (i.e. the letter “e”) is converted touppercase. This is done on line 2. First @alpha[4 .. 8] contains the 5 letters that are to be converted
to uppercase (remember that subscripts start at 0). The map function calls the uc function (uppercase)for every element in this list, thus converting ("e", "f", "g", "h", "i") to ("E", "F", "G","H", "I"). The splice function, therefore, replaces the lowercase list with the uppercase one. uc andmap would be covered later.
EXAMPLE 3.5

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splice#!/usr/bin/perl -w# Example 3.5 - splice@alpha = (’a’ .. ’z’);
splice @alpha, 4, 5, map(uc, @alpha[4 .. 8]);$, = ’ ’;
print @alpha;
What you will see on the screen should be all lowercase alphabets except E, F, G, H and I.

splice ARRAY, OFFSET, LENGTH
If you don’t specify the replacement list, the action is merely removing LENGTH elements from ARRAY
starting from subscript OFFSET.

splice ARRAY, OFFSET
If you don’t specify the LENGTH, Perl assumes that all elements starting from OFFSET are removed.
OFFSET is just a subscript and can be negative as well. A negative value specifies the OFFSET is counted

3.2 Lists and Arrays

33

from the end of the array as mentioned earlier, e.g. -1 means the last item, -2 means the second lastitem etc. Therefore, pop @MyArray can be equivalently accomplished by splice @MyArray, -1.
The following table summarizes how you can use splice in place of other functions discussed earlier(still remember that in Perl you always have a number of ways to do the same task?). You may find theequivalent method for push() strange, but after you have learnt the whole theory behind contexts you
would understand it. We would use this illustration again when we come to contexts.
Functionpush(@Array, $x, $y)pop(@Array)
shift(@Array)unshift(@Array, @x)$Array[$x] = $y

Equivalent Method
splice(@Array, @Array, 0, $x, $y)splice(@Array, -1)
splice(@Array, 0, 1)
splice(@Array, 0, 0, @x)
splice(@Array, $x, 1, $y)

Table 3.1: Relationship of some array functions with splice()

3.2.7

Miscellaneous List-Related Functions

There are a number of useful functions that allow you to manipulate list data in Perl.

join STRING, LISTThe join() function concatenates a list of scalars into a single string. It takes a STRING as its firstargument which is the separator to be put in between the list elements LIST. For example,

join ’+’, ’apple’, (’orange’, ’banana’)evaluates toapple+orange+banana
reverse LIST
In a list context, the reverse() function returns a list whose elements are identical to that of LIST
except the order is reversed. For example:

print join " ", reverse ’a’..’e’;# e d c b a

The map() function takes on one of the following two forms:

map BLOCK LIST
map EXPR, LIST

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Chapter 3 Manipulation of Data Structures

The map() function iterates over every item in the LIST, sets $ to the item concerned and executes
BLOCK or EXPR on each iteration. The return value is a list consisting of the result of evaluation of all
iterations. BLOCK is a code block enclosing a sequence of statements to be executed. EXPR can be any
expression. Consider this example:@names = (’ALICE’, ’tOm’, ’JaSON’, ’peter’);print join(’, ’, map { ucfirst(lc($_)); } @names), "\n";The output is

Alice, Tom, Jason, Peter
This example prints out each of the names in @names so that they all start with capital letters while theother characters are in lowercase. This is accomplished by first converting all characters to lowercaseby the lc() function, and the first letter is capitalized using the ucfirst() function. This process isperformed for each name in @names. The same expression may be rewritten as@names = (’ALICE’, ’tOm’, ’JaSON’, ’peter’);print join(’, ’, map (ucfirst(lc($_)), @names)), "\n";The first form of map() is generally preferred to the second one because it is more flexible.
The sort() function can be used to sort a list. By default, the sort() function sorts lexicographically.
The items are ordered by comparing the items stringwise (using the cmp operator, the specifics of which
will be introduced in the next chapter). This comparison is case-sensitive, because it is based on the
ASCII values of each character. The sorted list is returned by the sort() function, while the originallist remains intact.

sort (’bear’, ’Post’, ’ant’);# (’Post’, ’ant’, ’bear’)Note that while comparing stringwise, capital letters are considered “smaller” than lowercase letters.
To make the sort routine generic and allow sorting in any arbitary order, you may override thedefault sort criteria with your own rules. The method is to insert a code block before the list, similar tothe case for map(). The value resulted from evaluation of the block determines how the items are sorted.
The principle of constructing the contents of the block is too advanced at this stage. The rest of thedetails can be found in the next chapter. In the following I list several most commonly used sort criteriathat you are likely to find useful:sort { $a <=> $b } @list;
sort { $b <=> $a } @list;
sort { $a cmp $b } @list;
sort { $b cmp $a } @list;# ascending numerical order# descending numerical order# ascending lexicographical order (default)# descending lexicographical order# case-insensitive ascending lexicographical ordersort { lc($a) cmp lc($b) } @list;# or use uc()

3.2 Lists and Arrays

3.2.8

35

Check for Existence of Elements in an Array (Avoid!)

Many new programmers, and programmers who have programmed in other languages tend to use
arrays so extensively that are sometimes inappropriate from performance considerations. One classicalproblem is to deduce if a particular piece of scalar data matches any of the elements stored in an array(in alternative terminology, check for a hit or a miss). In some languages like BASIC where a hash is
not a builtin type, there is still little excuse to solve this problem by searching an array, although you
can still implement a hash yourself (For those who are interested in implementing a hash themselves,please read Appendix A).
In general, you can check if a certain element exists in an array by linear search. That is, you searchfrom the first element up to the end of the array. In the following code snippet, a random list containing
100 entries is generated randomly and you can enter a number to be searched. The program thensearches for the number. When the search ends, it returns whether it is found, and how many elements
the program has searched.
EXAMPLE 3.6

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Linear Search of an Array#!/usr/bin/perl -w# Linear search of an array# Note that if you later on want to search for something from a# list of values, you shouldn’t have used an array in the first# place.
# Generating 100 integers$NUM = 100;$MAXINT = 5000;# 1 + the maximum integer generated
srand();# initialize the randomize seedprint "Numbers Generated:\n(";for $i (1 .. $NUM) {push @array, sprintf("%d", rand(1) * $MAXINT);
print $array[$i-1];
print ", " unless ($i == $NUM);}
print ")\n\n";print "Please enter the number to search for >> ";chomp($toSearch = <STDIN>);# Linear search here
$counter = 0; $hit = 0;
foreach $num (@array) {$counter++;
if ($num == $toSearch) {print "\"$toSearch\" found at subscript ", $counter - 1, "\n";$hit = 1;
last;

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Chapter 3 Manipulation of Data Structures}}
if ($hit == 0) { print "\"$toSearch\" not found in array.\n"; }print "Number of comparisons: $counter/", scalar(@array), "\n";The code itself is not very important here. In fact, many new constructs used in this program havenot been discussed yet. The intent is for you to run the program instead of reading the source code(but you may do it). Try to look at the list of values printed, and try to enter a number that is not in
the array, a number that appears early in the list, and a number that appears around the end of thelist. A fraction is printed on the screen. The one on the left is the number of comparisons performed,
while the one on the right is the number of elements in the array (which should be 100 in this example). You may also want to increase $NUM to increase the number of integers generated. Of course, inthis case you will need to increase $MAXINT accordingly to minimize the chance of duplication of values.
Note that it is possible to have duplicate values in the array owing to the random nature of the random
number generator. In this program, the first occurrence (i.e. with the lowest subscript) will be returned.
An alternative scheme is to use binary search, a rather classical topic in elementary computer science
texts on algorithms. By using binary search, the whole set of values needn’t be searched in theworst case. In the worst case only log 2 n comparisons are required. However, there is an important
requirement — the list needs to be already sorted. If not, sorting needs to be performed first. Here is
the above example rewritten that employs the binary search algorithm:
EXAMPLE 3.7

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Binary Search of an Array#!/usr/bin/perl -w# Binary search of an array# Note that if you later on want to search for something from a# list of values, you shouldn’t have used an array in the first# place.
# Generating 100 integers$NUM = 100;$MAXINT = 5000;# 1 + the maximum integer generated
srand();# initialize the randomize seedprint "Numbers Generated:\n(";for $i (1 .. $NUM) {push @array, sprintf("%d", rand(1) * $MAXINT);
print $array[$i-1];
print ", " unless ($i == $NUM);}
print ")\n\n";print "Please enter the number to search for >> ";chomp($toSearch = <STDIN>);# First sort it in ascending numerical order3.2 Lists and Arrays
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@sortedArray = sort {$a <=> $b} @array;# Binary search here
$counter = 0;$start = 0; $end = $#sortedArray; $mid = 0; $hit = 0;
$mid = sprintf("%d", ($start + $end)/2);
while ($start <= $end) {$counter++;
print "Searched: ", $sortedArray[$mid];# which element is being ¶searched
print " in #[$start, $end] [";# the subscript range
print $sortedArray[$start], ", ", $sortedArray[$end], "] \n";if ($sortedArray[$mid] == $toSearch) {# a hit!
print "\n\"$toSearch\" found!\n";$hit = 1;
last;} elsif ($sortedArray[$mid] > $toSearch) {# decrease upper boundary -> mid value$end = $mid - 1;
} else {# update lower boundary -> mid value$start = $mid + 1;}
$mid = sprintf("%d", ($start + $end)/2);}
if ($hit == 0) { print "\n\"$toSearch\" not found in array.\n"; }print "Number of comparisons: ", $counter, "/", scalar(@sortedArray), "\n";This implementation is even more complicated than that of the linear search. What is worth notingis the very low number of comparisons required in any cases (For 100 integers the fraction printed inany case should be at most 7/100 1 ). That means among the 100 integers in the list, at most 7 elements
searched is sufficient to deduce whether any specified number exists in the array. In Chapter 5, as an
exercise, you would be asked to convert this program into the recursive form.
Binary search works as follows. The list is sorted in either ascending or descending order (assume wehave the list sorted in ascending order here). In this algorithm we always maintain two importantvariables — $start and $end. Initially they are set to 0 and 99 (the subscript of last element)
respectively. Then the element in the middle (subscript 44) is examined and compared with thenumber to be searched. If the number examined is larger than the input number, that means the input
number, if exists, would have a subscript less than 44, so we can shrink the range by reducing $end to
43. Otherwise it would have a subscript greater than 44, so we increase $start to 45. Such a bisection
process is repeated with this new set of $start and $end, until the number is found or $end is lessthan $start. Note that on each iteration the range examined is halved, and that’s why the maximum
number of comparisons is log2 (n).
An example of binary sort is shown in Figure 3.1. 18 is to be searched and is found on the third
iteration. The red boxes denote the element in the middle of the range being searched, and those in
1 The maximum number of comparisons can be calculated by ceiling(log (n)) where ceiling(y) is the minimum integer
2
that is greater than or equal to y. In this case ceiling(log2 (100)) = 7.

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Chapter 3 Manipulation of Data Structures

grey are those that are bypassed.

Figure 3.1: An illustration of binary sort

Although it seems binary search performs a lot better compared with linear search, there is an importantcatch here — most arrays we encounter are not sorted, so the cost of sorting cannot be simply ignoredin practice.
As you can possibly see in both cases when $NUM is large, neither approach is performing in an efficientmanner, especially when misses occur because in the linear search case, every element in the array has
to be searched in the worst case (when a miss occurs); while in the binary search case, the array needs
to be sorted first, and the average sorting time of an array grows with the array size 2 , so combining
sorting and searching may be even slower than the linear search algorithm. Towards the end of the next
section we shall redo this code with a hash, and you ought to find the code can be made cleaner and a
hit or a miss can be determined faster in general, especially with large array sizes.

3.3 Hashes
Hash is a special kind of data structure. There are several characteristics associated with it. It practicallytakes very short time to deduce whether any specified data exists. Also, the time it takes does notlargely depend on the number of items stored. This is important because hashes are usually used for
applications that handle a large amount of data.
An array is simply a contiguous block of memory and is nothing more than that. In order to support
the characteristic stated above, hashes require a slightly more complicated internal structure. This isoutlined in Appendix A for your reference. It explains the general principles that further Perl knowledge
is not necessary in order to understand it. However, in this section we are dealing with how we use
hashes in Perl, and will not discuss the peculiarities of them here.
As a quick review, each item in a hash has a key and a value. The key, which is a string, uniquely
identifies an item in the hash. The value is any form of scalar data. Hash variables start with the symbol
%.

3.3.1

Assignment

We may assign a list to a hash variable. In this case, the list will be broken up two-by-two, the first one
as the key and the second as the value:
2 In algorithmic analysis, the best sort algorithms can attain the time complexity of O(nlogn). That is, the sorting time t
is in the form t = Knlog(n) where K is a constant and n is the size of the array. The function grows even faster than n, so
performance degrades would be evident at relatively large values of n.

3.3 Hashes

39%Age = (’Tom’, 26, ’Peter’, 51, ’Jones’, 23);
Because a hash contains multiple key-value pairs, this alternative syntax may seem more intuitive tolook at:%Age = (’Tom’ => 26, ’Peter’ => 51, ’Jones’ => 23);
The symbol => is defined as an almost equivalent symbol to the comma, so in general anywhere acomma is needed you can replace it with this symbol. However, the use of => is particularly intuitive fordata that occur in pairs. In addition, Perl allows even less keystrokes by omitting the quotation marks:%Age = (Tom => 26, Peter => 51, Jones => 23);
This is because Perl always interprets the word before the symbol => as a double-quoted string.
However, if you omit the quotes, the key cannot have embedded whitespace (space, tabs and so on).
To specify a key with embedded whitespace, the quotes must be specified. Also, because Perl uses thesemicolon, not a newline, to mark the termination of a statement, you can make the hash assignment
better to look at by writing it in multiline form:%Age = (Tom => 26,
Peter => 51,
Jones => 23,);
NOTES
One note about why we can omit the quotes here. According to the perldata manpage, aword that has no other interpretation in the grammar will be treated as if it were a quoted
string. For example, if an unquoted word is not a reserved word, filehandles, labels etc. Perlwill automatically treat it as a bareword. However, avoid barewords consisting entirely of
lowercase letters, because all Perl reserved words are in lowercase. As Perl is case-sensitive,
this eliminates the possibility of potential name clashes in future versions of Perl.

Note that on the final line there seems to be a superfluous comma there. Yes, it is. The final comma
before the closing parentheses can be omitted. But it is customary to have it there in the above multiline
form because it is likely that you may append more key-value pairs later on. Then you will have to add
the comma back, and it will result in an error if you have omitted to do so. Bear in mind that the four
formats above are identical, and you may choose the form which looks best to you. An empty hash isan empty list assigned to a hash variable, similar to the case of arrays.
Because in the assignment operation Perl expects a list as the rvalue, apart from lists you may as wellassign an array, or even a subroutine returning a list to the hash variable. Note that the hash variable
provides a list context here.
If we assign a hash variable to an array, or anywhere a list is expected, the key-value pairs stored in thehash will be returned in list form. However, because of the way the key-value pairs are stored in a hash,they may be (actually most likely) returned in an order different from when they were put into the hash.

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3.3.2

Accessing elements in the Hash

Accessing an element in a hash is similar to that from an array, except we replace square brackets withcurly ones and instead of an index, the key is used. Here’s an example:

print $Age{Tom};As you will learn in Chapter 10, a word appearing inside the curly braces not enclosed by quotes isevaluated as a string. Therefore, Tom is equivalent to ‘Tom’ in this case. To associate a scalar value to a
key is as simple as:$Hash{’Key’} = $value;
If Key already exists, it is assigned the supplied value; otherwise, a new key-value pair is added to the
hash.

3.3.3

Removing Elements from a Hash

Perl provides the delete function for removing a specified key-value pair from a hash. Here’s an
example:delete $Age{Tom};This function returns the deleted scalar value associated with the specified key(s). So if you print thereturn value in the above statement, the value(s) deleted would be displayed on screen.
To delete all key-value pairs in a hash, you can of course use a loop to do it, but this is slow and it would
be more efficient to use either method below:%Age = ();orundef %Age;
Here is a special example for delete that is worth mentioning:
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%Age = (Tom => 26,
Peter => 51,
Jones => 23,);@temp = delete @Age{’Tom’, ’Peter’};$, = " ";print "Deleted values:", @temp, "\n";print "Remaining keys:", keys @Age;3.3 Hashes

41

On line 6, we delete multiple key-value pairs from %Age. Note that the function returns a list of values
associated with the deleted keys, so the symbol used should be @.
Line 9 uses the keys() function. It returns a list of keys in the specified hash. There is a corresponding
values() function that returns a list of values contained in the hash. Recall the sort() function
mentioned earlier. Apart from arrays, you may use the sort() function together with the keys andvalues function to specify how to order the hash items. For example,%array = (’3’ => ’apple’,’11’ => ’orange’,’5’ => ’banana’,);@key = sort { $a <=> $b } keys %array;@value = sort { $a cmp $b } values %array;

3.3.4# (’3’, ’5’, ’11’)# (’apple’, ’banana’, ’orange’)

Searching for an Element in a Hash

You may test if a particular key exists in a hash by using the exists function. However, even if the key
exists in the hash, the value associated may be undefined. Use the defined function to test if the value
is defined.
Recall earlier we had a program that generated 100 integers in random and you look for the existence ofa particular number in the list. We now present the version using a hash. The number of comparisonstaken is not shown, as you cannot get this information with a builtin hash. Of course if you build a hashyourself you would be able to see the efficiency of hashes. At least, you will find the implementation is alot easier compared with the two previous approaches.
EXAMPLE 3.8

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Searching for an Element in a hash#!/usr/bin/perl -w# Search for an element in a hash
# Generating 100 integers$NUM = 100;$MAXINT = 5000;# 1 + the maximum integer generated
srand();# initialize the randomize seedprint "Numbers Generated:\n(";for $i (1 .. $NUM) {$valueToInsert = sprintf("%d", rand(1) * $MAXINT);$hash{$valueToInsert} = 0; # in fact, any values can be assigned here
print $valueToInsert;
print ", " unless ($i == $NUM);}
print ")\n\n";print "Please enter the number to search for >> ";

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chomp($toSearch = <STDIN>);# Hash search here
if (exists($hash{$toSearch})) {print "\"$toSearch\" found!\n";} else {print "\"$toSearch\" not found!\n";}Notice how clean it is to determine whether the number exists in the hash in this case (line 24). In thisexample, the numbers are stored as keys in the hash. We use the exists() function to check if the key
exists in the hash. This function returns TRUE if the specified key exists. Note that you can as well use
defined() in place of exists() in this example. This function returns true only if the key exists in
the hash, and the value is not undefined (i.e. undef).
Table 3.2 summarizes the difference between defined() and exists():
Functionexists()defined()Key Exists
Yes
Yes
No
Yes
Yes
No

Value Undefined
No
Yes
N/A
No
Yes
N/A

Return Value
TRUE
TRUE
FALSE
TRUE
FALSE
FALSE

Table 3.2: Differences between exists() and defined()

In the example, because 0 (or anything except undef) is assigned as value to each key put into the hash,therefore, exists() and defined() yields the same results.
Owing to the nature of a hash, duplicate keys are not allowed. However, as I have previously noted it ispossible that by using such a random generation scheme duplicate elements may be generated, and in
this case exactly only 1 instance is stored, so the number of elements in the hash in this example can beless than 100.

3.4 Contexts
The idea of contexts in Perl may appear a bit odd at first glance, but you would soon discover that
contexts are actually quite intuitive to understand, because there are many real life examples resembling
contexts in Perl.
First, look at these two phrases. Note that the same word “press” appears in both sentences, but their
meanings in the phrases are entirely different.
Press the button
Freedom of press
The word “press” plays different roles in the two sentences, one acting as a verb and the other as a noun.
Therefore, to deduce the meaning of the word in the phrase, we have to look at the words surrounding3.4 Contexts

43

it, in other words, the context.
I’m sure you would find many other examples that exemplify how contexts play their roles in our daily
lives, so I am not going to spend too much time mentioning things you may have already known. Recall
that earlier in this chapter I obtained the number of elements of an array by using an assignment like this:$numElements = @colours;
You may find this assignment rather peculiar. It seems that I have been assigning list data to scalar data,
and they actually don’t match! How can a scalar data store an array?
Yes. That’s impossible (unless by using references - that we will see at a later part of this tutorial). Also
recall that I mentioned earlier that I was “evaluating an array in scalar context”. Now I am telling youwhat this phrase means.
Because you are now assigning something to a scalar variable, the data type expected on the right handside of the assignment operator is naturally a scalar value. In this way, we have created a scalar context
around the array @colours. Because a scalar value is requested instead of an array, Perl defines that byevaluating an array in scalar context, the number of elements stored in the array is returned and thevalue of which is assigned to $numElements.
It is important to bear in mind that the rules as to how a data type is evaluated in another context
depend on the definition, and there is no general rules of inference. Later on, when you are to write
subroutines, you will be taught to use the wantarray() function to determine the contexts so that you
can specify what to return from your subroutine in different contexts.
Remember I have mentioned Boolean context earlier in this chapter? Actually there is not a “Boolean”
context, because Perl does not have an intrinsic Boolean data type. Perl achieves this by using scalarcontext instead. Perl defines that the numeric zero (0), the empty string (“”) and the undefined value(undef) are interpreted as FALSE, and all other scalar values are interpreted as TRUE. Therefore,
anywhere a Boolean test is expected you can place any scalar value there instead, and Perl shall evaluate
it according to this rule.
Sometimes you may want to evaluate a list in scalar context, but it is in list context instead. A usefulfunction about contexts is scalar, which provides the necessary scalar context, as in the following
example:print scalar(@array);Without the scalar function, the array is evaluated in list context and so the contents of the array
will be printed. However, if we would like to print the number of elements in the array instead, thescalar function provides the necessary scalar context for this purpose.
I do not aim at telling you everything about how different contexts give rise to different behaviours
when we deal with operators at the moment. Instead, it is important to realize that operators can
exhibit different behaviours depending on the context they are in. You are going to have a boring
day probably reading the next chapter, as you are going to know in more details how each operator isaffected by contexts.

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3.5 Miscellaneous Issues with Lists
Before we start a new chapter, let us pay attention to several issues concerning lists which have not yet
been mentioned above.
You have seen in the previous section that, because of evaluation of an array variable in scalar context,
the number of elements in the array is returned:$a = @array;but things get entirely different when you write it this way:$a = (35, 48, 56);
This is a special case, because in scalar context the list on the right of the assignment operator is actuallya list of values delimited by the comma operator (,). This operator is borrowed from C/C++ and has
nothing to do with lists or arrays. The behaviour of this operator is, evaluate each of the expressions(in this case, numbers) delimited by commas, and return the value resulting from the last expression.
Therefore, in this example 56 is returned and assigned to $a. This applies to void context as well. While@array = (35, 48, 56);
the commas there act as list argument separators. As a rule, always bear in mind that a list is only a list
in list context. In other contexts the rules of comma operators apply.
Thanks to the flexible syntax of Perl, we can assign multiple values concurrently, like this (this is called
list assignment):($a, $b) = (11, 22);
Both sides are lists of the same size, so this is just a mapping: 11 is assigned to $a and 22 to $b. Whatabout if the list on the right has more elements than the one on the left? Then the extra elements are
simply ignored. What about if the list on the left has more elements? Then some variables in the left
hand list shall receive the value undef, as you may expect. To swap the values of two variables, anoperation that requires an additional temporary variable in most other programming languages, you
can perform it in Perl simply by:($x, $y) = ($y, $x);
Consider this example and try to guess what happens:($a, @b) = (11, 22, 33, 44);
11 is assigned to $a. The remaining three elements are assigned to @b. But what if you do this?

(@a, $b) = (11, 22, 33, 44);3.5 Miscellaneous Issues with Lists

45

In this case, @a gobbles up all the values in the list, leaving the value undef to $b. We describe thisbehaviour as “greedy”.
In this chapter, you have learnt how to use the three fundamental data structures in Perl, namely
variables, arrays and hashes. I have also tried to give you some ideas on the concept of contexts, which
is of fundamental importance in Perl. In the next chapter, I will introduce to you the operators available
in Perl.

Summary”

Scalar variables are prepended with the symbol $. For array and hash variables, the symbols are@ and % respectively.

”

Scalar variables can be assigned a value by using the assignment operator =. A variable holds thevalue undef by default.

”

Variable names are case-sensitive. They should start with an alphabet or an underscore character( ). Subsequent characters may be decimal digits as well. Variables whose names not conforming
to this nomenclature are predefined by Perl and serve special purposes.

”

Variable substitution means variables embedded in a double-quoted string will be substituted bytheir respective values at the instant the string is evaluated.

”

The substr() function extracts a sequence of characters from a given string.

”

The length() function returns the number of characters in a given string.

”

A list is an ordered set of scalar values. An array is a list associated with a name.

”An array may be created by assigning a list to an array variable.

”

Nested lists are merged to form a single list when the list is evaluated, with null lists removed.

”

The reverse() function returns a list whose items are identical to the input list except the items
are arranged in reverse order.

”

The push() function may be used to append a list of items to the end of an array.

”

The unshift() function prepends a list of items to the beginning of an array.

”

The subscript operator [] can be used to access a subset of elements in a list or an array.

”

The pop() and shift() functions may be used to remove an item from an array. pop() removes
the last item while shift() removes the first item.

”

splice() is a general purpose function to add or remove array elements.

”join() concatenates a list of scalars into a string, inserting a string in between.

”map() executes a given code block for each list element, and the results evaluated are combined
to form an array.

”

Searching an array is not efficient. A hash should be used instead.

”

A hash may be initialized by assigning it a list, whose elements are treated as a list of key-value
pairs.

”To refer to a hash element, use curly braces with the key in between.

”

The delete() function may be used to remove a key-value pair from a hash.

”

You may use the exists() function to test if a given key exists in the hash. The defined()
returns if a value is not undef.

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Chapter 3 Manipulation of Data Structures”

Behaviours of Perl operators and functions are influenced by context. The scalar() function
forces a scalar context in an otherwise list context.

”

The result of evaluating an array in scalar context is the number of items in the array. Evaluating
a list in scalar or void context causes all list items to be evaluated, and the value of the last item isreturned.

Chapter 4

Operators
4.1 Introduction
Operators are important elements in any programming language. They are so called because they
operate on data. For those who are new to computer programming the operators with which they may
be most familiar are the arithmetic operators, like addition, subtraction, multiplication and division.
Yet there are many more varieties of operators in programming languages. Perl provides more operators
than most programming languages I could think of, but most of them fall within one of the several
categories that we would go into detail in this chapter. Although devoting an entire chapter to discuss
operators is rather boring, it is important for you to understand what operators are as they act as theglue to bind pieces of data into an expression.
Arithmetic operators manipulate on numeric scalar data. Perl can evaluate an arithmetic expression,in a way similar to our daily-life mathematics.
Assignment operators are used to assign scalar or list data to a data structure. Apart from = that you
learned earlier, there are other operators, like +=, that perform additional operations at the same time.
Comparison operators are used to compare two pieces of scalar data, e.g. alphabetically or numericallyand returns a Boolean value. For example, you have two variables and you can use a comparison
operator to deduce which one is numerically larger.
Equality operators compares two pieces of scalar data and returns if their values are identical. They
may be considered special cases of comparison operators.
Bitwise operators provide programmers with the capability of performing bitwise calculations.
Logical operators can be used to do some Boolean logic calculations.
String manipulation operators manipulate on strings.
There are some other operators that do not fall into the categories above. Some of them will be coveredin this chapter, and the rest would be introduced as needed in subsequent chapters in this tutorial.
Because Perl classifies all data into one of the two forms, namely scalar and list data, operators can
be classified, similarly according to the number of operands, into two groups. Either the number ofoperands is fixed or variable. An operator that takes on one, two and three operands are referred to asunary, binary and ternary operators, respectively. List operators, on the other hand, can take a list of47

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Chapter 4 Operators

arguments as operands.

4.2 Description of some Operators
In this section we shall study a number of operators. This is not intended to be a full coverage because it
would be better for you to learn the rest later on in the tutorial as you accumulate more Perl knowledge.
If you would like to have a detailed reference of the whole family of operators in Perl, please consult theperlop manpage.

4.2.1

Arithmetic Operators

Arithmetic operators refer to the following operators:

Operator
+
*
/
%
+
++
-**

Description
Addition operator
Subtraction operator
Multiplication operator
Division operator
Modulus operator
Positive sign
Negative sign
Autoincrement operator
Autodecrement operator
Exponentiation operator

The operators +, -, *, / take two operands and return the sum, difference, product and quotient
respectively. Note that the division operation is floating-point division. Unlike C, Perl does not offer
builtin integral division. To get the integral quotient, you may use the int() function. For example,
int(7 / 2) evaluates to 3.
Perl won’t round a number to the nearest integer for you automatically. If you need this, the “correct”
way to do this is$num = 7 / 2;
print int($num+0.5), "\n";The modulus operator is more problematic. The operands of this operand are both integers. If you feed
any floating-point numbers (i.e. decimals) as operands they will be coerced to integers. Assume we arecarrying out $a % $b.
If $b is positive, the value returned is $a minus the largest integral multiple of $b such that the result isstill positive. For example,
63 % 5 = 3 (63 = 12 × 5 + 3, the largest multiple in this case is 12)-63 % 5 = 2 (−63 = −13 × 5 + 2, the largest multiple in this case is −13)

4.2 Description of some Operators

49

If $b is negative, the value returned is $a minus the largest integral multiple of $b such that the result
is still negative. For example,
63 % -5 = -2 (63 = (−13) × (−5) + (−2), the largest multiple in this case is −13)
-63 % -5 = -3 (−63 = 12 × (−5) + (−3), the largest multiple in this case is 12)
Such behaviours are so tedious that most programmers simply use the modulus operator for the firstcase — where both operands are positive integers, which evaluates to the remainder of $a / $b.
The positive/negative signs, just as in our usual mathematics, are unary operators that are affixed before
a number to indicate whether it is positive or negative. Our usual convention is that the unary positive
sign is not specified, because it is the default as expected.
If you know C/C++, the autoincrement and the autodecrement operators are identical to what you
have learned. For those who don’t know, it’s worth to spend a few minutes to repeat all the details here.
These operators can be placed before or after a variable. There are four possible variations:
Expression++$var$var++--$var$var--

Description
Prefix Increment
Postfix Decrement
Prefix Decrement
Postfix Decrement

The first two are autoincrement, while the remaining two are autodecrement. If autoincrement/autodecrement is performed as a statement on its own, the prefix or postfix configurations do not produce
any difference. For example, both ++$a; and $a++; as standalone statements increase the value of $aby 1. However, they are different if the operators are used as part of a statement. Consider the following
examples:A.
B.

$b = ++$a;$b = $a++;

In statement A, $a is first incremented, and then the new value is returned. In statement B, however, thevalue is returned first, and then $a is incremented. Therefore, the value returned (and is thus assigned
to $b) is the value before increment. The two forms differ in the order of increment/decrement andreturn of value. Autodecrement works in the same way, except the variable is decremented instead.
In other words, statement A and statement B are identical in effect as the following respectively:++$a; $b = $a;$b = $a; ++$a;# equivalent to statement A# equivalent to statement B

The exponentiation operator calculates the n th power of a number. For example, 43 , i.e. 4*4*4 isexpressed by 4**3, and the result is 64. Both operands can be floating point numbers.
All the operands discussed in this section take on scalar values only. In other words, they create a scalarcontext for the operands.

50

4.2.2

Chapter 4 Operators

String Manipulation Operators

String manipulation operators include the following:
Operatorx.

Description
String repetition operator
String concatenation operator

The string concatenation operator is used to concatenate two strings. In other words, it glues two piecesof string together. For example,"hello " . "guy"results in the string “hello guy”.
You can concatenate as many pieces of string as you wish by using a series of concatenation operators
together, like this:$username . ", your disk quota is " . $quota . " Megabytes."
In general, the concatenation operator dictates that both operands must be strings, and numeric
operands would be converted to string form before concatenation (still remember that Perl does typeconversions internally if necessary?). However, Perl can become highly confused about whether you
are using the concatenation operator or the decimal point if both operands are numeric literals. For
example:
A.
B.
C.
D.
E.
F.
G.

print "1"."1"; (return: “11”)print "1".1; (return: “11”)print 1."1"; (return: “11”)print 1.1; (return: 1.1)
print 1 . 1; (return: “11”)print 1. 1; (return: error!)print 1 .1; (return: “11”)In case A, B and C, Perl thinks that the dot represents the concatenation operator because one or more
operands is a string literal. Note the difference between cases D and E. Although whitespace does not
influence how Perl interprets an expression in general, this is not the case as shown in this example.
In case D, because the dot follows the first 1 immediately, Perl thinks that you would like to use thedecimal point, and returns the numeric value 1.1. In case E, however, because there is a space before
the dot, Perl thinks that you would like to use the string concatenation operator, and glues them up foryou. So is case G.
However, in case F, Perl thinks that you would like to supply a floating-point number like in case D,
but you supply it with two numbers with no comma in between (note that 1. is identical to 1), so thisis a syntax error! You get the same error if you replace case F with print 2 3;. However, because it isnonsense (although allowed) to concatenate two literals, just take this as some extra information andlittle attention can be paid to it.
In scalar context, the string repetition operator returns the string specified by the left operand repeatedthe number of times specified by the right operand. For example,

4.2 Description of some Operators

51$str = "ha" x 5;
results in the string “hahahahaha” being assigned to $str.
In list context, if the left operand is a list in parentheses, it repeats the list the specified number of times.
Examples:@array = ("a") x 3;

# or even (a) x 3 will work, but not a x 3

results in the list ("a", "a", "a") being assigned to @array.

@array = 3 x @array;replaces all the elements with the value 3. Note that the second operand always has a scalar numericcontext.

4.2.3

Comparison Operators

In Perl, there are two sets of comparison operators. The first set compares the operands numerically:
Operator
<
>
<=
>=
<=>

Description
less thangreater thanless than or equal togreater than or equal togeneral comparison

The second set compares the operands stringwise:
Operator
lt
gt
le
ge
cmp

Description
less thangreater thanless than or equal togreater than or equal togeneral comparison

At this point it is important to tell you one behaviour of Perl. As you know, Perl differentiates only
scalar and list data. That implies that Perl is quite ignorant about whether the value of a given variable(or a scalar element of a list) is a string or a number. This is why we have two sets of comparison
operators defined in Perl, in this way the programmer should choose the appropriate set of comparison
operator for comparison.
Because of the fact that Perl does not differentiate string and numbers much, it allows you to use astring wherever a number is required, and vice versa. However, this involves a conversion that I have to
explain it here. This behaviour is not specific to comparison operators only, but a general rule that you
have to bear in mind at all times when you are writing your own scripts. Consider the two statements
below:

52

Chapter 4 Operators

A.
B.

print "23.1abc" + 4;
print "23.1abc" . 4;

The addition operator (+) requires a numeric context (i.e. it requires numeric values as operands).
Therefore, Perl extracts the leading numeric portion of the double-quoted string until a non-numericcharacter is encountered, and converts this portion into a number (which yields 23.1 in this example).
4 is then added to this number, thus yielding 27.1 for the first statement. But what if the string started
with non-numeric characters? Simply a 0 is returned. Also, if the string-to-number conversion involves
a string that contains any non-numeric characters, Perl will display a warning message if you havewarnings turned on.
The concatenation operator (.) requires a string context. Therefore, Perl converts the second operand(4) into a string and is then appended to the end of the first operand (“23.1abc”), thus yielding theoutput “23.1abc4”. This is how Perl automatically converts between numbers and strings to and forth
as needed.
Comparing two numbers is easy, but what about two strings? How does the computer compare twostrings, possibly with non-alphanumeric characters in it? Perl compares two strings by comparing the
ASCII Code of each individual character in the string. As internally the computer only recognizes
numbers, a way of representing characters with numbers have to be devised. ASCII representation is
one of the several schemes available, and is well adopted. You can go to this website to consult the ASCII
Table, which contains the characters with their associated ASCII code. ASCII codes are in the range 0 127, and there is an extended set in the range 128 - 255 which is not well supported on many systems.
Don’t ask me why the characters are assigned in this way. The ASCII table was defined as such, andthere’s little significance about its origin anyway. Let’s compare the two pieces of string in this example:“Urgent”, “agent”
Perl compares character by character. First compare the first character of the two strings, ‘U’ and ‘a’.
‘U’ has the ASCII code of 85, while ‘a’ is 97. Because 97 is greater than 85, Perl decides that the lattercharacter is greater, and thus “agent” is greater than “Urgent”. In the case of comparing “tooth” and“toothpicks”, the longer one prevails for obvious reasons.
After you have learned how Perl handles string and numeric comparison in general, it’s time to lookat how you specify the comparison with Perl. That’s easy, because you merely put the appropriate
operator in between the two operands. For example, to test if “agent” is less than “Urgent” stringwise,
simply specify "Urgent" lt "agent", and the result is true. Note that Perl does not have the intrinsic
Boolean type, so if you pass the result directly to the print function, it is customary for Perl to output
1 for true, and “” (an empty string) for false. The best way to have the test result displayed properly is
by using the conditional operator ?:. I will talk about this shortly later on in this chapter, but this is
how you can do it:

print "Urgent" lt "agent" ? "true" : "false";
causes the word “true” to be printed if the test is true, and “false” is printed if otherwise.
Here is another example showing how numeric comparison may be used to decide on which block ofcode to be executed:

if ($score >= 90) {4.2 Description of some Operators
print "Well done. Your score is $score.\n";} else {print "Work hard. Your score is $score.\n";
}

53

# A
# B

In the next chapter you will learn how to use the conditional statement like if as presented in thisexample. Basically, the concept is simple. If the value of $score is greater than or equals 90, statement
A is printed; statement B is printed otherwise.
Be aware that you may get different results if you compare two pieces of scalar data numerically or
stringwise!
The <=> and cmp operators can be regarded as general comparison operators. Because of the shape, they
are sometimes referred to in Perl manpages as spaceship operators. <=> compares numerically while cmp
compares stringwise. The action of these two operators are similar, and I shall take <=> as an illustration.
The characteristics of <=> is as follows. Denote the left operand as $a and the right operand $b. If $a< $b, the result is -1. If $a > $b, the result is 1. If both operands are equal, that is, $a == $b (seebelow), the result is 0. This is a handy operator to quickly establish a trichotomy by determining in asingle operation whether a number is greater than, equal to or less than another. Despite its power,
it is seldom used in practice. It is mostly used with the sort() function to sort a list of scalars in aconvenient manner.

4.2.4

Equality Operators

Equality operators include the following:
Operator
==
!=
eq
ne

Description
equal (numeric comparison)
not equal (numeric comparison)equal (stringwise comparison)
not equal (stringwise comparison)

Similar to the case for comparison operators, we have two sets of equality operators. One set fornumeric comparison, the other set for strings. Equality operators can usually be regarded as part of thecomparison operators, but some books may prefer to classify them into two categories. There’s actually
little point to argue which approach is better, as different book authors take different views. Equality
operators compares two pieces of scalar data and return a Boolean value (again, scalar value instead inPerl) that indicates if the two pieces of data are identical.
The first two operators compare numerically, while the remaining two compare stringwise. For theequal operators (==, eq) they return true if the two operands are identical, false if otherwise. The
inequality operators (!=, ne) have an opposite sense, they return false if the two operands are identical,
true if otherwise.
The equality and comparison operators we have covered so far are concluded by these four examples:A.
B.

’true’ == ’false’ # true !!
’add’ gt ’Add’ # true

54

Chapter 4 Operators

C.
D.

’adder’ gt ’add’ # true’10’ lt ’9’ # true

In example A, == requires a numeric context, thus both strings are converted into 0. Both sides areequal and evaluates to true (although you would receive a warning if -w is enabled). In example B, Perlwill stop after checking the first character since ‘a’ is greater than ‘A’. Beware that in the ASCII table
capital characters have smaller ASCII codes than the small letter counterparts! In example C, because
the first three characters are the same and Perl cannot yet deduce whether ‘adder’ is greater than ‘add’
the longer string shall be considered greater. In example D, since we compare with lt, ‘1’ is less than ‘9’,
therefore, the comparison evaluates to true. This example and example A illustrate why in Perl we need
2 sets of comparison operators. Because Perl is loosely typed and does type conversions automatically,
there should be a method for Perl to know whether you would like to compare them as numbers or
strings.

4.2.5

Logical Operators

Logical operators include the following:
Operator|| or&& and! not
xor

Description
Logical OR
Logical AND
Logical NOT, i.e. negation
Logical XOR — Exclusive OR

The logical operators performs Boolean logic arithmetic. We have seen how to do a test using thecomparison and equality operators. But what if you would like to carry out two or more tests and checkif all of them are true? Boolean algebra can do it rather easily. However, it is out of the scope of thistutorial somehow for me to teach you the specifics of Boolean algebra, and I would focus on how to use
the Perl logical operators only.
You may discover that there are two sets of OR, AND and NOT operators. ||, && and ! refer to the C-Styleversion. If you know C/C++, these operators would look familiar to you, and are continued to besupported in Perl. Perl also has its own set, consisting of or, and, not and xor. Note that Perl gives you
an extra xor logical operator, that is not available in C/C++. The two sets differ only by precedence(which you will learn in the next section). The C-style operators have higher precedence, while the Perl
operators have the lowest precedence among all the Perl operators.
In the above example, we would like to see if the results of both tests are true. We can then use either
logical AND operator to do it. The result would be true only if both tests are true, and false if otherwise.
The logical OR operators return false only when both tests are false, and true if otherwise. The logical
NOT operator toggles the truth value. For example, !(13 < 25) is false, because it inverts the truth
value of 13 < 25, which is true.
The exclusive or operator returns true if exactly one of the two operands is false. That is, one is true
while the other is false.
Here is the truth table of the logical operators we have covered so far:
Here are several examples to conclude:4.2 Description of some Operators
test1
true
true
false
false

test2
true
false
true
false

and &&
true
false
false
false
test
true
false

or ||
true
true
true
false

55

xor
false
true
true
false

not !
false
true

Table 4.1: Truth table of various Perl logical operators(4<8) && (16<32) (return: true)(4<8) xor (16<32) (return: false)(4<8) || (16<10) (return: true)
Another important behaviour I haven’t told you yet concerning logical operators is the short-circuiting
property. Take and as an example. Given the expression(4 > 6) and (5 < 7)
you can tell by merely looking at the first expression that the whole expression is false, regardless of thevalue of the second expression. Similar to the case for the or operator, if the first operand evaluates
to true already, it is not necessary for Perl to examine the second expression. Therefore, Perl will justignore that expression and do NOT even attempt to evaluate it. This behaviour is known as shortcircuiting. The logical AND as well as logical OR operators support short-circuiting. xor, for example,
cannot short-circuit because its nature requires the value of both expressions be examined.
Short-circuiting is significant because it eliminates several runtime errors, in particular in the following
example we will not get the “division by zero” error because of short-circuiting:
EXAMPLE 4.1

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16

Safe Division#!/usr/bin/perl -w# safediv.pl# Safe division which detects division-by-zeroprint "== Safe Division ==\n";print "Please enter the dividend > ";chomp($x = <STDIN>);
print "Please enter the divisor > ";chomp($y = <STDIN>);$quotient = ($y || undef) && $x/$y;
if (!defined $quotient) {print "Division by zero!\n";} else {print "$x / $y = $quotient\n";56
17

Chapter 4 Operators}
The core of the program is on line 12, which appears to be a little bit complicated. This example makes
use of short-circuiting property of && and ||. Let’s consider two cases, when $y is non-zero and zero.
Consider the case if $y is non-zero. First $y is evaluated. Because it is not zero, the short-circuiting
property of || causes the second operand undef not to be evaluated. Therefore, the first operand of&& is $y itself. Because it is non-zero, the second operand has to be evaluated, which calculates thequotient, which is assigned to $quotient.
If $y is zero, then the second operand of || has to be evaluated, which is undef. The operand of && isthus undef. Because this value is interpreted as a false value in Boolean context, the second operand of&& is not evaluated and thus undef is assigned to $quotient.
Subsequently, a check of whether $quotient is undef is sufficient to tell whether “division by zero”
occurs. Depending on this result, either an error message or the quotient will be printed.

4.2.6

Bitwise Operators

Bitwise operators refer to the following:

Operator
<<
>>
&
|
ˆ
∼

Description
Binary shift left
Binary shift right
Bitwise AND
Bitwise OR
Bitwise XOR
Bitwise NOT

The first two operators are the binary shift operators. The two operands of these operators must be
integral. As you may know, numbers are represented in binary form internally. The left operand is thenumber to be operated on, while the right operand is the number of bits to be shifted. Let me explain
this with the help of an example.
Say you would like to perform 60 >> 2. First we convert 60 into binary notation, that is 111100 2(subscript 2 means representation in base 2, that is, binary). In this example we intend to shift 2 bits
to the right, that means the two least significant bits (the bits on the far right) are removed, and thus
resulted in 11112 , which is 1510 in decimal notation, so 15 would be returned.
We have used the binary shift right in the example. On the other hand, the binary shift left operator
does the opposite. It shifts the integer specified left a specific number of bytes, filling the least significant
bits with 0. Observant readers may notice that each binary bit shift to the left actually multiplies thenumber by 2. Try it.
The remaining four operators compare the two integral operands bit by bit. As an example, to extract
the 4 least significant bits of the number $num, we can use the expression $num & 15. The returned
value is the integral expression of the last 4 bits of $num. The reason is quite simple. The binary
representation of 15 is 11112 . Say $num takes the value of 37 (1001012 ). The operation is shown below:

4.2 Description of some Operators

57

1 0 0 1 0 1& 0 0 1 1 1 1---------------0 0 0 1 0 1
The bitwise AND operator compares each bit. If both corresponding bits are 1, the resulting bit is 1;
otherwise 0. The above example demonstrates a technique known as bit-masking, and 15 is the maskin this example. To extract specific bits, just set up a mask with the corresponding bits setting to 1, and
0 for other bits that we are not interested in. Note that although 15 is only 4-bit long, you may consider
that it is extended automatically to 6 bits, with the two most significant bits set to 0.
The other bitwise operators have the same semantics as their logical operator counterparts except thetruth value is represented by 1 and 0 for true and false respectively. Therefore, the details of which are
not repeated here.
Bitwise operators are in general rarely used in most scripts. They are usually only used in applications
such as cryptography or binary file access.

4.2.7

Assignment Operators

Assignment operators refer to the following operators:
Operator=+= -= *= /= %= **=.= x=&&= ||=&= |= ˆ= <<= >>=Description
Assignment operator
Arithmetic manipulation with assignment
String manipulation with assignment
Logical manipulation with assignment
Bitwise manipulation with assignment

I have mentioned quite a lot about the ordinary assignment operator = in the previous chapters already.
Now consider this example:$num = $num + 15;
You should understand what this means, do you? The value stored in $num is added to 15, and theresult is again assigned to $num. The net effect is to increment the value of the variable by 15. However,some people may prefer that manipulation and assignment be accomplished by one, instead of two
operators. Therefore, Perl recognizes several shorthand notations as shown above.
In general, if you can write a statement in this format:
op1 = op1 operator op2;
you can have a shorthand version like this:op1 operator= op2;
For instance, the example above is identical to $num += 15; However, note that not all the operators
mentioned above have such a shorthand version. Look at the list above for all the recognized shorthand58

Chapter 4 Operators

operators. As an example, to invert all the bits in an integral scalar variable (that is, on a technicalparlance, to assign the “1s complement” to the scalar variable), we have to write$num = ˜$num;

4.2.8

Other Operators

Here I shall introduce to you some other operators that do not fit in any of the above main categories.
The operators that would be covered here include the conditional operator and the range operator.
The comma operator and => have already been described in detail in the previous chapter. Putting aside
the builtin functions that may be considered operators in Perl, the remaining operators are ->, =∼ and!∼. I have chosen to defer mentioning the remaining operators because they are related to some later
topics and I would cover them in those sections. The first one, the arrow operator is similar in somesense to the pointer-to-member operator in C. This would be introduced in the references chapter. The
other two are used for pattern matching with regular expressions.
The conditional operator ?: is a ternary operator. In other words, it has three operands. The syntax is
as shown below:

test-expr ? expr1 : expr2
The first operand (test-expr) is an expression. If this expression evaluates to true, expr1 will be
returned by the conditional operator; otherwise, expr2 is returned. You are likely to use conditional
operators quite often in the future because it is considered too “bulky” to use the if-else structure all
the time. This is an example using if-else to compare the values of $a and $b, and assign the smaller
value to $c:

if ($a < $b) {$c = $a;} else {$c = $b;}
By using the conditional operator, this 5-line structure (although no one will stop you from writing all
this on one line) can be transformed into the simpler statement:$c = $a < $b ? $a : $b;
The following example is extracted from the perlop manpage which demonstrates how the list context
around the conditional operator propagates to expr1 and expr2.

$a = $ok ? @b : @c;In this example, depending on the value of $ok, the number of elements of either @b or @c is returnedand assigned to $a. First, the assignment to $a creates a scalar context around the conditional operator.
Therefore, the conditional operator is expected to return a scalar value, and so the two possible values
to be returned will be evaluated in scalar context.

4.3 Operator Precedence and Associativity

59

Although the conditional operator is inherited from C, there is one important behaviour that is unique
to Perl. The conditional operator can be assigned to if both expr1 and expr2 are legal lvalues. This isan example:($whichvar ? $var1 : $var2) = $new_value;
In the next section, “Operator Precedence and Associativity”, we would see an example on how you
may obtain unexpected results involving the conditional operator because of operator precedence.
The behaviour of the range operator (..) depends on the context around the operator. The range
operator actually consists of two disparate operators in list context and scalar context. The range
operator in list context is more frequently seen, so I will cover it first.
I have introduced the range operator in list context briefly in the previous chapter, when I described
how you can construct an array. The range operator in list context returns an array consisting of thevalues starting from the left value, with the value of each subsequent element incremented by one until
the right value is reached. For example, @array[0..2] is synonymous with @array[0,1,2] to return
an array slice. Note that both the left and right values would be converted to integer by chopping off
the decimal portions in case they are not already integers.
Not only integers can be used in the range operator, you can experiment with alphabets as well. For
example, (’a’ .. ’z’) and (’A’ .. ’Z’) are two lists representing small letters and capital letters, respectively.

4.3 Operator Precedence and Associativity
Now we have learned several operators, so it’s time for us to put them together. It is not uncommon
that a given statement contains more than one operator. Operator precedence and associativity arise asa result. Recall that in elementary Maths class teachers teach us in a mathematical expression involving
several arithmetic operators, multiplication and division shall be performed before addition andsubtraction. For example, 3 + 6 × 7 is interpreted as 3 + (6 × 7), not (3 + 6) × 7. In this case, we say thatmultiplication and division has higher precedence than addition and subtraction operators. Because
multiplication has a higher precedence than addition, the multiplication operation, involving the two
operands 6 and 7, is performed first, and then the result 42 is added to 3, getting 45 as the result.
In Perl, because there are many operators, the rules of precedence is far more complicated. In orderto describe the relative precedence among the operators, most programming languages would use an
operator precedence table to show the relative precedence of the operators, and Perl is of no exception.
Table 4.2 shows the operator precedence and associativity table, which you can obtain from the perlop
manpage.
The operators are arranged in order of decreasing precedence. That is, the operators at the top (Terms
and List Operators) have the highest precedence, while the operators at the bottom (or, xor) have thelowest precedence. Operators on the same line have the same precedence.
The first column lists the associativity of the operators. Associativity is useful when there are several
operators of the same precedence in a statement. In this situation, the order of evaluation of these
operators depends on the associativity. If the associativity is right, then the rightmost one would be
evaluated first, then the one on the left, and so on. That’s why cascaded assignment is possible. The
idea is the same for left associative. You may also find that some operators are labelled “nonassoc”

60

Chapter 4 Operators

Associativity
left
left
nonassoc
right
right
left
left
left
left
nonassoc
nonassoc
nonassoc
left
left
left
left
nonassoc
right
right
left
nonassoc
right
left
left

Operators
Terms and list operators (leftward)->++ -**! ∼ \ + - (unary)=∼ !∼* / % x+ - .
<< >>named unary operators< > <= >= lt gt le ge== != <=> eq ne cmp
&
| ˆ
&&
||
.. ...
?:= += -= *= etc. (assignment operators), =>List operators (rightward)notandor xorTable 4.2: Operator Precedence and Associativity Table4.3 Operator Precedence and Associativity

61(non-associative). The operators are classified as non-associative if the order of evaluation is notimportant, or not applicable for other reasons. In general, you don’t have to worry about the order of
evaluation of the non-associative operators really much.
To demonstrate the effect of operator precedence and associativity, let’s go over several scripts here.

#!/usr/bin/perl -w$, = "\n";$a = 13, $b = 25;$a += $b *= $c = 35 * 2;print "\$a == $a, \$b == $b, \$c == $c";
This one is practically easy. I would use it to illustrate how precedence and associativity works. The
problematic statement is on line 5, and you shouldn’t have problems with the rest of the script, so I am
focusing on this line only. To deal with this statement, first try to locate the operators concerned. The
operators, by scanning from left to right, are:

+=

*=

=

*

Now locate the operators with the highest precedence, and it is * in this case. Therefore, multiplication
would be performed first. The operands of this operator are 35 and 2, so this multiplication yields 70.
The remaining are all assignment operators having the same precedence, so we look at their associativity. Because the associativity is right, the rightmost one is evaluated first, which is =. The operands
are $c and 70 (the value of the expression 35 × 2), so 70 is assigned to $c. Then, $b *= 70 is evaluated, and $b is eventually assigned 1750 (70 × 25). At last, $a += 1750 causing $a to be assigned 1763.
There is not much confusion here, but let’s consider another more tricky example, of which the result
is not necessarily apparent at first glance.

#!/usr/bin/perl -w$, = ", ";$a = 1, $b = 0;
print $a >= $b ? $b : $a += 6, $a;
I first saw a similar example from a C++ book, and I adapted it to become a Perl script. This is a reallynotorious example, because it includes ?:, which easily leads to unexpected result if you are not careful
enough. I hope you could understand my explanation as this example, though seemingly short, is so
notorious that it is rather difficult for me to explain it well.
Now let’s have a quiz: without actually running it with your perl interpreter, try to deduce the printoutof this example. Let’s give you two choices:

A.
B.

0, 1
6, 1

62

Chapter 4 Operators(don’t look at the answer below before you have an answer in your mind)
If you choose option A, I’m sorry, you’re wrong. But don’t despair, as many people share the same
mistake as you do, and it is common and understandable for people to make mistakes. Althoughnovices are more prone to make mistakes, nothing will stop veterans from making mistakes aswell. Most computer programs we are using have bugs. Some bugs are obvious, but there aremany more hidden ones which are not normally revealed unless your program gives unexpected
output. Trust me, debugging is going to occupy most of your development time, and is the mosttedious job for all programmers. If you choose option B, congratulations, you are correct. Hope thatyou did not get the correct answer by sheer guesswork. Anyway, let’s look at how we arrive at the answer.
If you choose option A, you may have the impression that += has a high precedence, but this is not thecase. In this example, ?: also exists, which has a higher precedence than +=, so ?: is evaluated before+=. Here comes the trouble. The operators, scanning from left to right, are:

print

>=

?:

+=

,

print is a list operator, and has the highest precedence. So Perl looks on the right hand side forits parameters. Then Perl sees >=, which has the second-highest precedence. So $a >= $b is thenevaluated, yielding true. Among the remaining operators, ?: is the highest. The problem is, what are
its operands? As you have learned, this operator has three operands, the so-called “test” part (that is$a >= $b, and have found to be true), the “true” part ($b), and the “false” part. But wait! What is thefalse part? $a or $a += 6? Its the former one, in the contrary to what you may have expected.
As Perl scans from left to right to find the false part, it encounters the += operator, which has a lower
precedence than ?:, and Perl stops and claims that the false part is $a. Then Perl knows what to do, so
evaluates the conditional operator, returning the true part, that is $b (that holds the value 0). Anythingleft? Yes, we still have the dangling += 6 part! It is tricky that the conditional operator returns anlvalue if it can, and so $b += 6 is evaluated, causing $b to hold the value 6 instead. We have finishedevaluating the first argument to the print operator, and the second operand is $a, which has been
untouched. That’s how we arrive at the answer “6, 1”.
Troublesome enough? Two long paragraphs just to explain one statement. If you are still scratching
your head, try to read these two paragraphs repeatedly until you understand. So you see, operator
precedence and associativity can trip you up if you are not careful.
But what if this is not your intention? What if you’d like to have $a += 6 as your false part? Recall how
you do this in arithmetic — adding a pair of parentheses. This applies to Perl as well. This is easily done
as this:print $a >= $b ? $b : ($a += 6) , $a;
In Perl, parentheses are treated as “terms”, which has the highest precedence in the precedence table.
This ensures that all expressions in parentheses are evaluated before other operators do. Parenthesesis the cure for those who enjoy writing complicated statements but wouldn’t like to memorize (orconsult) the operator precedence table. Adding parentheses appropriately not only eliminates a great
deal of effort when you or other programmers read your code at a later time, many unexpected errors
could be eliminated as a result. But sometimes parentheses do not add much clarity to your code.
Here’s an example:

4.3 Operator Precedence and Associativity

63

print(lc(shift(@MyArray)));In this example, all but print are named unary operators. Because named unary operators have only
one parameter, in general there is not much confusion if you just omit the parentheses. In conclusion,use parentheses wherever appropriate. Feel free to insert parentheses as you see fit to minimize
confusion but don’t overdo it.
There are several points to note arising from this example. First, although looking naı̈ve to mention
here, whitespace is NOT a determinant of the order of evaluation. You definitely cannot write the above
statement in this way and expect it to work the same way the parenthesized version does:

print $a >= $b ? $b :$a+=6, $a;This is actually the same as line 5 in the example! So it returns “6, 1”, not “0, 1”. This is because when
the Perl interpreter loads the script, it parses it and removes all intervening whitespace automatically.
Second, you have to be careful about the use of parentheses. Consider these examples:

A.
B.
C.
D.
E.

print 1+2+3;
print (1+2+3);
print 1+(2+3);
print (1+2)+3;
print(1+2)+3;

In the above examples, statements A–C would give the correct answer, 7, while the last two would give 3.
Statements A and B have nothing special. In statement D, however, because the opening brace is placed
immediately after the operator, Perl thinks that this pair of parentheses contain all the parameters to besent to the operator. In this case, 1+2 is its only argument, so 3 is printed. Because the print operator
returns nothing, thus leaving the dangling part “+3” for Perl to evaluate. Since this is a useless addition
operation, a Perl warning would be displayed if you have warnings enabled. Statement E is the same asD (recall that whitespace does not matter?).
For statement C, because you do not have an opening brace immediately following it, Perl does
not think that there are parentheses to contain the parameters as in statement D. The parentheses
in statement C is treated in the normal way, that is, evaluated first, and then the lower-precedence
operators.
Third, notice that in the operator precedence charts there are two entries for list operators. List
operators (leftward) in this tutorial (and in the perlop manpage) refer to the name of the list operator
concerned. Such a high precedence allows Perl to know where such operators occur, so that it knows
where to look for their parameters. On the other hand, list operators (rightward) applies to thecomma-separated expressions to pass to the list operator concerned as parameters. The extremely
low precedence of this part implies that you do not normally need to put parentheses around theparameters. This part acts figuratively like a basket containing all the parameters. The only operators
having lower precedence are the operators not, and, or and xor. For example,

open HANDLE, "$path/.bash_history" or die "Can’t open file\n";64Chapter 4 Operators

Because or has a lower precedence than list operator (rightward), the or part is not treated as part of theoperand for the open() operator. In the example, if the open operation fails, it returns undef, which isinterpreted as false. Because short-circuit evaluation is not possible, the second part is executed, which
outputs the error message and terminates the script. You would learn how to open files later on in thistutorial.

4.4 Constructing Your Own sort() Routine
In the last chapter you have had an overview of using the sort() function to sort a list of values. I also
provided you with a list of common search routines. By now you should have sufficient knowledge tounderstand the rest of the story.
The principle is pretty simple but rather difficult to visualize. You override the default sort criteria bydefining the ordering criteria in the code block of the sort() function. Two special variables $a and$b are defined in the block. In order to sort a list of values, the sort routine needs to establish the totalordering of any two arbitrary items in the list. To do so, it assigns the two items to $a and $b arbitrarily.
The block should perform some comparison operations in the block based on these two values. If
evaluation of the block yields a scalar value that is negative, the value held by $a comes before that of$b. If the result is positive, the value held by $b comes before that of $a. If the result is zero, then thetwo values should be considered equal. The <=> and cmp operators are usually used. The following is
an example which illustrates numeric descending sorting.
Assume the unsorted list is (3, 1, 4). To sort in descending numerical order, an appropriate
expression which satisfies the above characteristics is$b <=> $a
The following table illustrates how you can try to verify if the comparison works. 3 → 1 below means 3
comes earlier than 1 in the resulting list.
$a
3
1
3

$b
1
4
4

$b <=> $a-1
1
1

Ordering
3→1
4→1
4→3

Table 4.3: An Example Illustrating sort()

Therefore, the ordering 4 → 3 → 1 is established.

Summary”

Operators carry out certain operations on data, known as operands.

”

An expression consists of a combination of operators and operands.

”

Unary, binary and ternary operators take on one, two and three operands respectively. List operators can take on a variable number of operands.

”

In Perl, functions are simply treated as unary or list operators.

4.4 Constructing Your Own sort() Routine

65”

Arithmetic operators carry out arithmetic calculations.

”

Assignment operators are used to assign scalar or list value to a data structure.

”

Comparison operators compare two pieces of scalar data and returns a Boolean value.

”

Equality operators are specializations of comparison operators which test if two pieces are considered equal.

”

Bitwise operators can be used to perform binary arithmetic.

”

Logical operators operate on Boolean values.

”

String manipulation operators involve string concatenation operations.

”Operator precedence and associativity determines the order in which operators are evaluated in
an expression.

”

The sort() function sorts a list of scalar values. Ordering is determined by the return value ofthe code block provided in the sort() function.

66

Chapter 4 Operators

Chapter 5

Conditionals, Loops & Subroutines
5.1 Breaking Up Your CodeSo long you have been writing your programs in one piece. You are totally allowed to carry on with thispractice, however, lumping everything in one piece is often considered undesirable for the following
reasons:

Ease of maintain A single source file with tens of thousands of lines is for sure not easy to maintain.
First, navigating around in the source file is messy — page up and page down keys are unlikely to beeffective for files of this size and to locate a certain section of code much traversal is necessary.

Variable Conflicts Variable management is not a trivial affair in practice when the program is a large
one. It is easy for us to keep track of the variables in use for short programs we have been writing for
now, but shortly when the code base grows in size, it becomes increasingly likely that variable nameclash occurs at certain parts of the source code. That is, you use a variable that is defined somewhere else
but you are unaware of it. This may give rise to some unexpected behaviour that are difficult to debug.
Object-oriented programming is a desirable solution. We would discuss the concepts of scope and packages, which are fundamental to object-oriented programming that would be covered in the next chapter.

5.1.1

Sourcing External Files with require()

The require() function can be used to source external files. It acts in this way — when execution
reaches a require() statement, it first checks if the file has already been required. This is to avoid
multiple inclusions and potential file inclusion loops (for example, A includes B, and B includes A etc.).
The predefined variable @INC stores path prefixes the Perl file inclusion system use to find the file to besourced. You may type the following command at the command prompt to output the content of @INC:
perl -e ’print map {"$_\n"} @INC;’
On my Linux system, the list of paths is, in ascending order,
cbkihong@cbkihong:∼/public_html> perl -e ’print map {"$_\n"} @INC;’/usr/lib/perl5/5.8.0/i686-linux-thread-multi/usr/lib/perl5/5.8.0
67

68

Chapter 5 Conditionals, Loops & Subroutines/usr/lib/perl5/site_perl/5.8.0/i686-linux-thread-multi/usr/lib/perl5/site_perl/5.8.0/usr/lib/perl5/site_perl/5.6.1/usr/lib/perl5/site_perl.
There are 7 entries. The last entry is a single dot denoting the current directory. Note that the paths
listed are likely to be different on each system. They are fixed in the Perl installation process.
require() is an overloaded function serving three purposes:?

If the argument is a number, it checks if the perl interpreter version ($]) is greater than the version
specified as the argument. This use generally appears in Perl modules to specify the earliest perlinterpreter version with which the module can be executed;?

If it is a bareword, that is not enclosed in quotation marks, it is assumed to be the package name ofa module with extension “.pm” (see Chapter 7) and any occurrences of ‘::’ are converted to ‘/’ on
Unix-variant OSes or ‘\’ on Windows or MS-DOS. The result is a relative path to a Perl module;?

If the argument is a quoted string, it is treated as the relative path to the file to be included.

For the last two cases, the relative path resulted is appended to each of the paths in @INC in turn to form
a complete path and require() checks if a file exists at this location. The first file that is found is used.
As you can see, the system modules directories are searched first, and at last the current directory is
searched.
If the search yields an existing file, the relative path is added to %INC as the key while the complete pathformed as the value. The %INC hash variable is maintained by Perl which contains a list of files included.
Whenever you invoke the require() function, it checks this hash to determine if the file specified
already exists in %INC. If it does not, an entry is added to it; otherwise, the inclusion is silently bypassed.
For example, say you have a source file which has the following require() statements:

require ’myfile.pl’;require MyModule;
The require() function will append the key-value pairs(’myfile.pl’ => ’/home/cbkihong/somedir/myfile.pl’,’MyModule.pm’ => ’/home/cbkihong/somedir/MyModule.pm’,)
to %INC. Consider the situation if inside the module MyModule it also tries to require ’myfile.pl’,
because an entry with the key ‘myfile.pl’ already exists in the hash, this inclusion is bypassed. Therefore,
multiple file inclusions can be detected and avoided.
If the file required does not return a true value (as a recapitulation — anything other than 0, an empty
string or undef), require() fails with an error. Perl module authors may utilize this characteristic toabort the scripts using the module in case of errors. Therefore, you ought to see in later chapters thatexternal files usually terminate with the statement 1; which returns the true value required.

5.2 Scope and Code Blocks

69

The description above is a rewrite of the require() manpage. It is easy to understand and you areadvised to read it for further information. In Chapter 7 you would learn the use() operator which is awrapper of require() and is used to load Perl modules.

5.2 Scope and Code Blocks
5.2.1

Introduction to Associations

Consider this simple assignment statement:$fruit = "apple";
How would you describe this statement? The string “apple” is assigned to the scalar variable named$fruit. Fine, this is how the statement is interpreted literally, but this description is not sufficientas you progress through the following chapters. A better and more in-depth way of expressing thisstatement is that a data object with the scalar value “apple” is created, and the name “fruit” is now
associated with this data object. Why do we have to bother with this expression? Because up to now
we have been writing programs with each data object associated with one and only one name, whichis the name of the variable. However, in this chapter we would see that due to scoping rules the same
name at different parts of a program can be associated with different data objects. This association is
also known as a binding. In the next chapter we would introduce to you the possibility of establishingadditional associations to a data object, which are references. Therefore, from now on you need to havea clear separation of the data object and its name associations.

5.2.2

Code Blocks

The idea of confining the scope of associations arose from the fact that not all variables need to bevalid throughout the lifetime of the program. This is particularly important in subroutines, which are
self-contained reusable units containing a sequence of statements.
The scope of an association refers to the region in a program in which the association is visible. Up tothe present, we have been writing very simple scripts with scalar variables, arrays and hashes which,once created, could be used anywhere in the script. What we have been using in our examples so
far are all global variables, in the sense that all associations are established during the compilation
phase before execution, and are not destroyed until the program terminates because these variables areresolved from the symbol table, which exists as long as the program is in execution. Note that once you
have created an association there is no way for you to manually destroy it. An association will only bedestroyed when the scope in which it resides terminates. In this sense, a global variable has a global
scope. Excessive use of global variables is considered a poor programming practice, because as yourprogram becomes increasingly complicated, it is not impossible that your program may involve dozens
to even hundreds of variables. Perhaps you may need some of them to be accessible throughout thewhole script, but this is not generally the case. In particular, many variables are simply used to store
data temporarily, for example, as a counter in a loop (as you will see later in this chapter). Introductionof limited scopes is seen as a cure to the problem.
Scopes are defined in terms of environments. An environment is created by means of a code block, or
simply, a block. A code block simply consists of a sequence of statements that makes up an environment,
and is delimited by curly brackets (but the mere presence of { } doesn’t necessarily imply a code block— you will see do{} and eval{} later in this chapter that look like blocks, but they are not strictly one).

70

Chapter 5 Conditionals, Loops & Subroutines

Code blocks can be nested. In other words, one code block can appear in another code block. Like this:# Environment A{# Environment B{# Environment C}# Environment B resumes{# Environment D}# Environment B resumes}# Environment A resumes
As you can see, after { a new environment is started. When } is reached, the current environment
terminates, and the parent environment, that is, the environment that was previously in effect isreinstated. For example, when environment C terminates, environment B will resume. With my andlocal modifiers that we are going to explore in the next section, you can confine the scope of an
association to within the extent of an environment — when the containing code block terminates, theassociation would be destroyed.
NOTES
Please interpret the above statement carefully. Although all associations are destroyed when
the current environment terminates, this does not necessarily mean the objects themselves
are destroyed. This is related to the garbage collection mechanism in Perl, which will becovered in the next chapter when we come to references. In particular, if additional references
to the object exist, the object will not be destroyed.

5.3 Subroutines
Breaking up your source code into multiple files is not the only way you can make your code moremanageable. A complex program can be broken into smaller tasks, each of which carries out awell-defined function. Take an online book catalogue served in a typical library as an example, you canactually split the whole complicated program into smaller parts. For example, these are the functions
that may be implemented in such a system:?

Search the catalogue by title, author, keywords, etc.

?

Allows users to check their circulation record?

Allows users to reserve or renew items?

Perhaps to send a reminder to a borrower automatically if he forgets to return the borrowed items
on time5.3 Subroutines

71

You should be able to think of many more, but this already exemplifies a program will be easier to writeand manage if you break it up into smaller and simpler parts, with each part doing its intended task
only. This shows how useful subroutines are. A subroutine consists of a sequence of statements defining
an environment. Subroutines are just like user-defined functions in contrast to those provided by Perl.
There are two types of subroutines — named subroutines and anonymous subroutines. Anonymous
subroutines are not given a name while named subroutines are. We shall cover anonymous subroutines
in Chapter 6 when we come to the topic of references. From now on, by means of subroutines we referto named subroutines.

5.3.1

Creating and Using A Subroutine

In general, before we can call a subroutine, we need to declare and define it. Declaring the subroutine
makes Perl aware that a subroutine of a particular name exists. Defining means you explicitly describe
what the subroutine does by listing the statements to be executed if the subroutine is being called.
In general, subroutine declaration and definition go together. The syntax used to declare and define asubroutine is as follows:
sub name [(prototype)] block
block is the subroutine definition. It is a code block containing the statements to be executed when thesubroutine is invoked. The rest is the subroutine declaration. It declares a subroutine with the namename. After the subroutine name you may insert an optional prototype part which contains a concise
specification of the types of parameters to pass to the subroutine.
Here is an example using a subroutine to calculate the sum of a list of scalars (presumably numeric):
EXAMPLE 5.1

1
2
3
4
5
6
7
8
9
10
11
12
13
14

#!/usr/bin/perl -w
sub sum (@) {# This subroutine takes a list of numbers as input# and returns the sum
my $sum = 0;
for my $tmp (@_) {$sum += $tmp;}
return $sum;}# calculates 0 + 1 + 2 + ... + 100 and prints the value
print sum 0 .. 100;
# must be 5050. No doubt.
Lines 3-11 contain the declaration as well as the definition of the subroutine sum(). Line 3 tells Perl thatyou are declaring a subroutine named sum, while lines 4-10 are the subroutine definition. Subroutines
have to be declared and defined before being called in the script. If you place the print statement(line 14) before the subroutine declaration, your script simply won’t work as expected. This is because
the perl interpreter reads your script sequentially. If it encounters the token sum before it is declared,
Perl will not know sum is a user-defined subroutine as Perl cannot find it in the system libraries. That
implies you should, in principle, always put subroutine declarations and definitions very early in source

72

Chapter 5 Conditionals, Loops & Subroutines

files. As you can see in the example, the subroutine is put before the source program that calls it.
However, this may not be convenient sometimes. For example, when you have multiple subroutines
which are inter-dependent on one another, it may not be convenient for you to find out a proper orderof declaring the subroutines. Therefore, Perl allows you to make forward declarations, at which point
the subroutines are declared but not defined. Forward declarations are put at the very top to declarethe subroutines, and somewhere later on you give the definitions of the subroutines concerned, which
can then be placed in any order. In the example, in order to make a forward declaration of the sum
subroutine, it can be made like this:sub sum(@);
This statement tells Perl in advance (before parsing the subroutine definition) that sum is the name ofa subroutine accepting a variable number of scalars as arguments, and Perl will know to look for thedefinition of sum in the later part of the program. The forward declaration is the same as the declarationline except we replace the block containing the definition with a semicolon.
Although you may not understand the my modifier and the for loop in the script at the moment, you
may ask a question: why do I have to resort to subroutines if I can implement it directly with a for loop
or a foreach loop? Well, note that the subroutine sum serves a general purpose. It adds up all the input
values and return the result, regardless of the values of the input. Therefore, this subroutine is highly
versatile and flexible — it can be used directly without modification on any occasion you would like toevaluate the sum of a list of scalars. If you write the script as follows, it can only be used to sum up all
the integers between 0 and 100. Whenever you would like to evaluate a sum of something else, you haveto rewrite the code:# calculates 0 + 1 + 2 + ... + 100 and prints the value
my $sum = 0;
for my $tmp (0 .. 100) {$sum += $tmp;}
print $sum;
By writing your code in subroutines, you are enforcing reusability of your code. Code reusability isimportant as we wouldn’t like to write similar pieces of code again and again. Later on you will learn
how to build reusable modules where you can put your subroutines and save them as a file so thatwhenever you need to use the subroutines in another project, you just need to import the module,
and the subroutines can be reused in your new project. This is very convenient. Here is how the sum
subroutine can be readily applied to evaluate the sum in other situations:print sum values %score;$avg = sum(@nums)/@nums;# print the sum of the values of %score# evaluate the average of the values of @nums

Apart from improved reusability, subroutines also help make debugging easier. Once we are certainthat a subroutine is correct, we can safely apply it. It is likely that fewer number of errors would becommitted when we combine the subroutines to form an entire program than writing it without using
any subroutines.
Unlike other languages like C or Java, Perl does not support named parameters. All incoming parameters are combined into one indistinguishable array @ . The first parameter is thus $ [0]. Therefore, if
you have a mixture of arrays and scalars the elements of which would be combined into @ . You may

5.3 Subroutines73

use pass-by-reference to be discussed in the next chapter to avoid it.
Also, note that the elements of @ are not copied by value. The elements of @ are “references” to thedata values. They behave like references but do not look like references in terms of syntax. Basically, theidea is when you modify a value of an element in @ , the corresponding data object will be modified aswell. Consider this example:
sub test {$_[2] = 5;$_[3] = 6;}
my @a = (1, 2, 3);
my $b = 4;
test(@a, $b);
print join(’, ’, @a, $b), "\n";# 1, 2, 5, 6Here, when the third as well as the fourth element of @ are updated, the corresponding elements in @aand $b will be modified. What if we replace $b with the literal 4? Now because the data object cannot
be modified, it will be an error. In principle, changing the value of parameters silently in a subroutine
is a bad programming practice, although sometimes you cannot avoid it. You should document thesecases clearly, for example, as comments in the source files.
Because a parameter can be inadvertently modified in a subroutine, in general, you should not use theelements of @ directly in scripts. You can use the following technique:sub search {my ($myitem, @myarray) = @_;# use $myitem and @myarray thereafter# ...
}my @array = (1, 2, 3);
my $searchFor = 2;
search($searchFor, @array);By using the my modifier, $myitem and @myarray are confined to within the search subroutine only.
Here, the elements are copied by value to $myitem and @myarray. Therefore, the tie between themand the original data objects no longer exists. Even if you inadvertently modify the values or $myitem
or @myarray, the changes won’t be made to the original data objects. Moreover, this emulates named
parameter passing in other programming languages.
A subroutine may return a list of values to the caller. As shown in Example 5.1, this is achieved by the
return function. A subroutine may return a scalar or a list of scalars. Similar to the case for incoming
parameters, multiple arrays or hashes are combined into one single list. Again, you may use references
to circumvent this, though.

5.3.2

Prototypes

Recall that while declaring a subroutine you may put an optional prototype specification. It describes
the number of as well as the type of parameters in a compact form. The prototype gives Perl a clue as to74

Chapter 5 Conditionals, Loops & Subroutines

how the arguments should be handled. Having an accurate grasp of the types of arguments expected isimportant, as illustrated in a mini case study below.
The prototype specification comprises a sequence of symbols indicating the type of each argument.
The symbols should look familiar to you, because they are the same symbols which are used to denotescalar variables, arrays, hashes etc. In front of each symbol you may prepend a backslash \ to indicate
the element is to be passed by reference. This is covered in the next chapter.
Symbol
$
@
%
&
*

Type
Scalar variable
Array
Hash
Anonymous subroutine
Typeglob (Reference to Symbol Table entry)

For example, if the prototype is ($$), that means the subroutine accepts two scalar variables as parameters. ($$@) implies the first two parameters to be evaluated in scalar context while the remaining
parameters would be grabbed by an array variable. Note that you cannot have something like (@$) as
the array variable (or hash variable alike) would take up all the input parameters. Always bear in mind
that multiple parameters, after evaluating in their respective contexts, are combined together to become
one indistinguishable array @ .
A “programmer” who claimed to know Perl was asked by his boss to write a subroutine which inserts alist into an array at a certain position. There is already a splice() function which can do that for him,
so he decided to write a wrapper which calls splice() to do the job. The boss, as a user, would like to
use the subroutine in this format:
insert @array, pos, list
which is identical to the syntax of splice() except without the length parameter. The “programmer”
wrote this:sub insert {# !!WARNING!! This does NOT work!
my (@myarray, $pos, @list) = @_;return splice(@myarray, $pos, 0, @list);}
Without even trying it, he handed it to his boss. The boss tried to use it in this way:@array = (1, 2, 3);insert(@array, 3, 4, 5, 6);
It didn’t work, and he lost his job. Does that sound too stupid for you? Why doesn’t it work? As I
have reiterated a number of times already, because all the parameters are combined into a single list
when they are passed to the subroutine. You can’t really separate them back into the three parameters,
because the first argument is an array which due to its “greediness” would take all the elements passed
into the subroutine, leaving $pos and @list undefined. The proper way to do this is:5.3 Subroutines

75

sub insert (\@$@) {my ($array, $pos, @list) = @_;return splice(@$array, $pos, 0, @list);}
It uses both pass-by-reference with a prototype added to make the types of parameters expected explicit.
The use of @$array causes the original array to be modified, as we’ll cover in the next chapter. As forthe prototype, we indicate the parameters are an array reference, a scalar and then a list. If a subroutine
has a prototype, Perl will try to evaluate the parameters according to the prototype. Consider the case if
the boss uses the subroutine in this way:@array = (1, 2, 3);insert(@array, @array, 4, 5, 6);This is identical to the previous case. Note that in the prototype the second parameter indicates a scalaris expected. Therefore, a scalar context is put around @array which causes the number of elements in@array to be passed as the second parameter, which causes the list specified to be appended to the endof the @array.
You can also specify optional parameters. Compulsory parameters are separated from optional parameters by adding a semicolon in between. For example, say you have a subroutine whose declaration
statement is sub mysub ($$;$$);, and you make the following subroutine call:mysub @array, "3", 9;
Because @array is evaluated in scalar context, @ is the list (scalar(@array), "3", 9). The fourthparameter is empty. By using prototypes you can let Perl check parameter types and evaluate theparameters in the correct contexts.
A sidenote about subroutine invocation. Traditionally, subroutines had to be prefixed with the & symbol
when invoked, and the parentheses are compulsory in this case. For example, &mysub(@array, "3",9); When a subroutine is invoked in this way, the prototype is ignored. Therefore, I recommend not to
use the & form in general. A few situations where you need to use the & form will be covered in the next
chapter.

5.3.3

Recursion

Recursion is more of a technique rather than a feature of a programming language. It refers to thepractice of tackling a problem through dividing it into smaller sub-problems and tackling them independently. Each of these sub-problems may also be subdivided if necessary. In programming languages,
recursion is typically achieved through nested invocation of a subroutine, directly or indirectly. We’ll
examine recursion with the help of an example.
A palindrome is a sequence of characters that is identical regardless you read it in a forward or backward
direction. For example, “dad” and “sees” are examples of palindromes. Here, we tackle the problem ofdetermining whether a given string is a palindrome. For simplicity, we only consider strict palindromesthat are symmetric character by character. Phrases like “Madam, I’m Adam” are generally consideredpalindromes, but we don’t classify them as such.

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Chapter 5 Conditionals, Loops & Subroutines

There are (at least) two ways to tackle this problem, namely the iterative and recursive approach. First,
we present the source program for the iterative approach:
EXAMPLE 5.2

1
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3
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5
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7
8
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12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34

Palindrome#!/usr/bin/perl -w# Determining whether a given string is a palindrome.
# (Iterative approach)
sub isPalindrome($);
print "Enter a string > ";chomp(my $str = <STDIN>);if ($str ne ’’) {print "$str is ", isPalindrome($str)?"":"not ", "a palindrome.\n";} else {print "The string should not be empty!\n";}
sub isPalindrome($) {my $string = $_[0];my $ctr_l = 0;
my $ctr_r = length($string)-1;
while ($ctr_l <= $ctr_r) {# To do case-insensitive comparison, convert both to lowercase if ¶applicable
my $leftchar = lc substr($string, $ctr_l, 1);
my $rightchar = lc substr($string, $ctr_r, 1);
if ($leftchar ne $rightchar) {return 0;} else {$ctr_l++;$ctr_r--;}}return 1;}
Then the recursive approach:

1
2
3
4
5
6
7
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#!/usr/bin/perl -w# Determining whether a given string is a palindrome.
# (Recursive approach)
sub isPalindrome($);
print "Enter a string > ";chomp(my $str = <STDIN>);

5.3 Subroutines
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if ($str ne ’’) {print "$str is ", isPalindrome($str)?"":"not ", "a palindrome.\n";} else {print "The string should not be empty!\n";}
sub isPalindrome($) {my $string = $_[0];# A standalone character or an empty string are by definition symmetric.
# This signifies the deepest recursion stack possible.
if (length($string) <= 1) {return 1;}# Here, what we need to do is to examine the first# and last character, and invoke a new isPalindrome# to deduce whether the string in the middle is a palindrome.
my $leftchar = lc substr($string, 0, 1, "");
my $rightchar = lc substr($string, -1, 1, "");
return $leftchar eq $rightchar && isPalindrome($string);}
The only part of concern is the subroutine definition of isPalindrome(). In the iterative approach,
two pointers are maintained which initially point to the first and the last character, respectively. A loop
is set up which iterates as the pointers move towards each other. When an unmatched character pair isfound the value 0 is returned, which signifies the string is not a palindrome. Finally, when the positionsof the left pointer and right pointer are swapped, that implies the entire string has already been scanned
through and all character pairs matched (or 0 would have been returned), so we can then conclude thestring is a palindrome.
The logic behind the recursive scheme, however, seems to be cleaner and more intuitive. In theiterative approach, a single isPalindrome() invocation tackles the whole of the problem. However,
the recursive approach suggests to break this problem into multiple levels. At each level, we merelycompare the first and last character of the incoming string. The string in the middle is passed to a new
invocation of isPalindrome() to deduce whether it is a palindrome. In other words, we define that apalindrome is one whose first and last character are identical and the substring in the middle is also apalindrome. If both conditions are satisfied we conclude the string is a palindrome; otherwise, it isn’t.
Lines 21-23 in the recursive example handles the case when the incoming string is a single character oran empty string. In recursive schemes one always need to consider the case at which point recursion
should stop. Recursion should not be allowed indefinitely (and in fact, you should avoid recursions ofmany levels, say possibly 500 levels deep because in practice the stack size is limited and you actuallycreate an entry on the call stack (to be described shortly) as you recurse. Exceeding the limit results ina stack overflow error). Note that as recursion proceeds, the first and last character are being taken offof the string before passing to a new isPalindrome() invocation. Therefore, there must be a level atwhich the incoming string is either a single character or empty, depending on the number of charactersin the original string specified by the user. Also note that the ordering of the two conditions on line 29
is significant. Because if we find that the border character pair doesn’t match, we can already claim thestring is not a palindrome without having to test the string in the middle. I used the short-circuiting
property of && to achieve this. If you swap the two conditions, then recursion must always have toproceed to the deepest level and the test is only carried out just before you exit from each level, which is

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just a waste of time.
Depending on the problem nature, recursion may be a better solution compared with an iterative
approach. For example, a program which searches through a directory structure (say, search for certain
files on the hard disk) is nearly always implemented by a recursive scheme because directory structureis hierarchical, or in other words, nested by its very nature. Because the number of nested levels is notknown in advance, an iterative scheme is unlikely to be appropriate.
Recursion generally requires nested subroutine invocation. How is this possible? Subroutine invocation
is internally kept track of by a series of stacks. A stack is a data structure. Think of a stack as a pile ofbooks lying on your desk. Only two operations are allowed on a stack, either you place a book at the
top of the stack, or you take away a book from the top of the stack. You are not allowed to insert into or
remove from it any books in the middle. I will now briefly go into the process of subroutine invocation
because certain concepts will be revisited in later chapters.
The basic idea is that a call stack is maintained by your program. The top of the stack reflects yourcurrent environment. When a subroutine is invoked, a new entry is created on top of the stack
to represent the new environment. Each environment holds a copy of the lexical variables (thosedeclared with my, to be introduced later on) that are specific to its environment. When the current
subroutine terminates, so is the environment and the entry is removed from the top of the call stack.
The environment previously in effect is reinstated. Therefore, every invocation is independently
represented by the corresponding entry on the stack. This is how subroutine invocations are handled
in programming languages. Note that this mechanism does not impose any restrictions to nested
subroutine invocation. Nested subroutine invocation is just performed in exactly the same way as
invocation of another subroutine.

5.3.4

Creating Context-sensitive Subroutines

As we have seen, several builtin functions are context-sensitive. That is, a function call exhibits different
behaviours depending on the context around the function call. Subroutines may exhibit this behaviour
too, with the wantarray() function.
The wantarray() function determines the context around a specific subroutine invocation. Consider
the following example:
EXAMPLE 5.3

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wantarray()#!/usr/bin/perl -w# wantarray.pl# Demonstrates how wantarray() may be used
# Context-sensitive searching:# scalar:# search for first occurrence of substring and# return offset# list:# search for occurrence of substring in incoming list and# return matched strings
sub search {my $substr = shift;5.3 Subroutines
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if (wantarray()) {# remaining parameters are values being searched
my @values = @_;my @retval = ();foreach (@values) {my $index = index($_, $substr);
push @retval, (($index >= 0)?$_:());}
return @retval;} else {# search in incoming scalar string
my $value = shift;
my $index = index($value, $substr);
return ($index >= 0)?$index:undef;}}
my @items = (’systematic’, ’system’);my $scalar = ’delinquency’;
my $search1 = search(’que’, $scalar);print "’que’ ", $search1?’’:’not ’, "found in $scalar\n";my @search2 = search(’tic’, @items);$" = "\n";print "’tic’ found in the following items:\n@search2\n";In this example, search() is a context-sensitive subroutine which exhibits two different behaviours,
depending on scalar or list context. On line 35, $search1 introduces a scalar context to the rvalue.
Therefore, there is a scalar context around search(). Whereas on line 38, there is a list context instead.
The wantarray() function returns a true value in a list context, while undef in a scalar context. Many
constructs in this example are introduced at a later time in this chapter. However, the idea is pretty
simple. In a list context, the subroutine accepts a variable number of parameters which are searched forthe existence of the given substring. Strings with contain the given substring are returned as an array.
On the other hand, in a scalar context, the subroutine accepts two parameters (you may pass additionalparameters, but they are simply ignored) indicating the substring and the string being searched. The
offset associated with the first occurrence of the string is returned using the index() function. I will
not describe the index() function in detail in this tutorial, because its uses can be substituted byregular expressions which I will cover in Chapter 9.

I would like to take this opportunity to introduce to you another predefined variable $" which appears
on line 39. This is called the list separator. As illustrated in the example, substitution also applies toarrays in double-quoted strings. Therefore, you should escape all @ characters in string literals so thatthey are not interpreted as array variable substitution. The list separator is embedded in between list
elements during array variable substitution of double-quoted strings.

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5.4 Packages
When you split your code into multiple files, Perl provides a nice mechanism to ensure that variables indifferent parts of your program do not clash. The mechanism is to divide your program into different
namespaces. The idea is very simple — each namespace has a label which uniquely identifies thenamespace and we prepend to variable names the label so that we can differentiate in case two variablesin two namespaces happen to have the same name. C++ uses the notion of namespace, while in Perlterminology a namespace is called a package instead.
Any variables not explicitly contained in any packages belong to the main package. Therefore, allvariables we have been using in fact belong to the main package. By declaring additional packages wecreate shields so that variables in different packages would not interfere with each other. Packages arefundamental to object-oriented programming because each object is intended to be a self-contained
unit.

5.4.1

Declaring a Package

A package extends from the package declaration up to the end of the enclosing code block, theclosing bracket of eval() (see Chapter 10) or the end-of-file, whichever comes first (see the perlmod
manpage). To declare the start of a package, put
package package name;
Usually package declarations are placed at the beginning of source files to ensure that all variables in thefile are protected. For example,#!/usr/bin/perl -w
package Apple;# Package extends to the end of the fileIf a package declaration is placed inside a code block, the package extends to the end of the code block:#!/usr/bin/perl -w# ‘main’ package{
package Apple;# package ’Apple’ extends to the end of the block$var = 3;}# ‘main’ package
To avoid any misconceptions that may arise as you read on, I would like to remind you that packages
appearing inside code blocks, such as in the example shown above, continue to exist after the block
is closed. Always bear in mind that packages are only intended to prevent inadvertent clashing ofnamespace. It has nothing to do with scoping. In the above example, the variable $Apple::var still5.4 Packages

81

has the value of 3 after the containing block terminates. This is not the case for my or local variables,
though, which we will come to shortly.
Note that a package may be declared within the extent of another package. As illustrated in the above
example, the ‘Apple’ package is declared within the extent of the ‘main’ package. Therefore, you candeclare an ‘Orange’ package within the extent of the ‘Apple’ package, and each package protects thevariables within its respective extent. By convention, the first letter of a package name is capitalized,
except the ‘main’ package.

5.4.2

Package Variable Referencing

If you omit the package name when referencing a variable, e.g. $somevar, it refers to the variable inside
the current package. This also applies to variables in the main package. Therefore, by not explicitly
declaring any packages in previous chapters we have been referring to variables in the current package,that is, the main package.
If you need to refer to a package not contained in the current package, you need to qualify the variablewith the package name prepended, with ‘::’ as the package separator. Therefore, to refer to a scalarvariable $var in the apple namespace you need to write $apple::var. If the package name is empty
but contains the package separator, e.g. $::var, the main package is assumed.
Because you need to explicitly qualify a variable with the package name if you have to refer to it inanother package, you will not modify it inadvertently unless that is your intention. That is exactly how
namespaces work.
There is also an old syntax of using a quote instead of double colon for referring to package variables,for example, $orange’var. This syntax may be deprecated in future versions of Perl. However, because
it will be interpolated in double-quoted strings, you should beware of strings such as “$people’spen”. You should disambiguate by putting a pair of curly braces around the variable name, such as“${people}’s pen”.

5.4.3

Package Variables and Symbol Tables

Each package in Perl maintains its own symbol table. A symbol table keeps track of a list of symbols
defined in the current package and their memory locations at which they can be found. For non-lexical
variables (that is, those not declared with the my modifier) Perl needs to keep track of them because they
are not confined to any environments. You may access the symbol table of a package through a hashwhose name is the name of the package, followed by two colons. For example, the hash representing
symbol table of the main package is %main::. The keys of the hash are the names of symbols defined in
the package. The corresponding values is a scalar representing an internal data structure of Perl known
as a typeglob which in turn holds the references to the actual symbols.
To help you understand it, consider the @INC array that we described earlier in this chapter. This
array is one of the predefined variables in Perl that is automatically listed in the symbol table of themain package. Therefore, there exists a key ‘INC’ in %main::, that is, $main::{’INC’}. The value
is a typeglob which holds a list of references of symbols with the name INC, that is, @INC and %INC.
The typeglob is represented by *main::INC. The typeglob has a number of slots, each of which storesthe references of a different type such as scalar, array, hash, anonymous subroutine, filehandle andtypeglob (which is just a reference to itself). If there isn’t a symbol of one type, the corresponding

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slot is simply null. You can access the reference of @INC and %INC through the symbol table with aso-called *FOO{THING} syntax. In this example, that are *main::INC{ARRAY} and *main::INC{HASH}
respectively.
Typeglobs were mainly used for parameter passing in earlier versions of Perl when references were notyet in the Perl language. In the next chapter you will be taught on how to use pass-by-reference forparameter passing. You may wonder why I need to mention typeglobs at all if it is no longer actively
used. First, in order for you to understand how local or package variables work, you’ll need to knowwhat a symbol table is. And, because the entries in a symbol table are represented by typeglobs, it isdifficult for me not to mention typeglobs at all.

5.4.4

Package Constructors with BEGIN {}In each package you may define one or more subroutines with the name BEGIN, which are automatically
executed when the package is being parsed. During parsing, when perl sees these subroutines, they areautomatically executed in the order these subroutines are defined. That means, these subroutines are
executed before the rest of the program does. Customarily, the sub keyword is omitted.
That being said, these subroutines are not truly subroutines. After they have been executed, thesesubroutines are not registered in the symbol table. Therefore, you cannot manually invoke them duringruntime. One of the major uses of BEGIN blocks is to modify @INC to include additional module searchpaths, especially because use is a compile-time statement (modules are loaded during compile-time as
opposed to runtime — see Chapter 7). Therefore, you have to ensure that @INC gets modified before
any use statements:

BEGIN {
@INC = (@INC, ’./libs’);}use MyLib;# ....

# defined in ./libs

5.5 Lexical Binding and Dynamic Binding
We have already learned how to define environments in a program by establishing code blocks in
Section 5.2.2. Subroutine definition is also placed inside a code block so it also defines an environment
in itself. However, I have not yet explained how you can restrict an association to within a certain
environment. Recall that all variables we have been using are package variables. Once declared, package
variables continue to exist in the symbol table as long as the program is running. Before we go into thedetails of the two types of bindings with respect to associations, let us first examine the general conceptof referencing first.
Let’s execute this program on your system:
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for (keys %main::) {print $_, " => " , $main::{$_}, "\n";}$abc = 3;

5.5 Lexical Binding and Dynamic Binding

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The generated lines that are of interest at this point are shown below, with others omitted:...
stdin => *main::stdin
ARGV => *main::ARGV
INC => *main::INC
ENV => *main::ENV
abc => *main::abc...
The above code dumps the content of the symbol table. On the left of the arrow are the names of thesymbols, while on the right are the corresponding typeglobs. What appears to be interesting is thatan entry for abc exists in the symbol table, regardless of the fact that the statement which assigns 3
to $abc has not yet been executed at the instant the symbol table dump is made. The reason is thata compilation step, despite invisible to users, was performed before the actual execution which scans
the whole program for package variables which are then added to the symbol table. Therefore, before
the program is actually executed the symbol table has already been constructed. As noted previously,
a symbol table keeps track of the symbols that appear in the program. By doing so, the runtime
environment (for example, the perl interpreter) prepares a list of symbols at an early point in time
which facilitates it to arrange for storage space in the memory and, most importantly, to prepare forreferencing operations that occur during execution of the program.
Whenever a symbol appears in a program, a referencing operation is required to be carried out duringexecution to deduce which data object is associated with the given symbol. For example, when thestatement $abc = 3; in the program above was executed, the runtime environment needs to find out
which data object is associated with the scalar variable $abc. The goal of a referencing operation is tolocate this association.
Referencing has been made complicated because of the presence of scopes. Without scopes, referencing
is easy because there can be only one variable of a certain name in each package. For example,
throughout the duration of a program there can be one and only one scalar variable $Apple::varin the Apple package. $Orange::var is already a different scalar variable and do not interfere with
$Apple::var at all. Therefore, throughout the program all references to $Apple::var always referto the same data object. However, this may not apply to those variables which are confined by scoping
rules. In Perl, two major types of scoping rules are supported, namely lexical scoping and dynamic
scoping. Both scoping systems base on definition of environments such as code blocks, but the wayreferencing is performed is different.
Most modern programming languages only support lexical scoping. An association that is lexicallyscoped is visible from the point in the environment in which it is defined, and all environments thatappear inside the extents of that environment, until when another lexically scoped variable with thesame name appears in those environments. In Perl, a variable declared with the my modifier is lexicallyscoped. Consider this example:
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my $a = "Hello ";{$a .= "World\n";print $a;
# Hello World
my $a = "Bye!\n";print $a;
# Bye!
}

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print $a;

# Hello World

When execution proceeds to line 3, Perl needs to find out which data object $a refers to. At this instant,the current environment, that is the code block between line 2 and 5, is known as the local referencing
environment. Perl first finds out that up to this point there is not any lexical variable with the name ain the local referencing environment. As local referencing fails, the referencing operation proceeds tosearch for one in the nonlocal referencing environments, by proceeding all the way up through parent
environments. Here, we find a lexical $a in the parent environment, and the data object associated withthat variable is used. Therefore, the string “World\n” is appended to the scalar value held by that data
object.
However, on line 5 a new lexical $a is declared at this point. Therefore, referencing operation performed
at line 6 resolves to this lexical. Note that the lexical in the parent environment is untouched, butcannot be accessed anymore in the current environment. It is described as being hidden. When thecode block terminates, all local associations are destroyed. Because of the reference-based garbage
collection mechanism, the data object associated with the lexical on line 5 is also destroyed. The parent
environment that is once invisible springs into existence again, and lexicals that were once hidden are
visible again. Therefore, the last print() outputs “Hello World”.
my expects a scalar or a list as its argument. We have seen a scalar used as the argument in the examples.
Using a list as an argument with optional assignment looks like this:my ($a, $b) = (’Hello’, ’World’);
If the variable list is not assigned, then they are given the values of undef.
Lexical scoping models are recommended for several reasons. First, the use of lexical variables is fastercompared with dynamically scoped ones. That is because lexical scoping can be solely determined fromthe nesting of code blocks, which is already fixed during the compilation phase. Therefore, referencingof lexical variables can be performed during compilation instead of at runtime. Dynamic scoping, on
the other hand, also takes into account the dynamic factor of subroutine invocations. In Perl, local
variables are dynamically scoped. As the use of dynamically scoped variables share similar problems as
global variables in other programming languages, and the scope is dependent on the call stack which isdetermined by how the program calls subroutines at runtime, they make debugging more difficult, andare relatively slower.
my variables are never listed in the symbol table. This fact is important as you go on and learn how to
use typeglobs.
Dynamic scoping, on the other hand, is based on the call stack instead of nesting of environments. In
Perl, local variables are dynamically scoped. These variables are package variables and appear in thesymbol table of the respective packages. The idea is, when a local variable is declared, the current
value as can be accessed through the symbol table is saved temporarily in a hidden stack, and a new data
object is created to hold the new value. When the current environment terminates, the current symbol
table entry is removed, and the value that was previously saved is reinstated. Consider this example:
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sub greeting {print $a;}$a = "Hello ";# Bye!

5.5 Lexical Binding and Dynamic Binding
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{
$a .= "World\n";print $a;
# Hello World
local $a = "Bye!\n";&greeting;}print $a;

# Hello WorldThis is similar to the example I used above to introduce my variables in Perl, except now local is used
and the second print is put in a subroutine. On line 9, the original symbol table entry for $a (withthe scalar value “Hello World”) is replaced with a newly created data object whose value is “Bye!”. In
the subroutine greeting(), because $a cannot be resolved in the local environment, the symbol tableentry is used, and therefore “Bye!” is displayed. When the block terminates, the original symbol tableentry saved is reinstated. Therefore, the value printed is “Hello World”.
There are a few points to note here. Here is a slightly modified version of the above program forillustration:
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my $a = "Hello ";sub greeting {print $a;}

# Hello World!!!!

{$a .= "World\n";print $a;
local $::a = "Bye!\n";&greeting;}print $a;
When you run this program, you would find that all three print() result in the string “Hello World”
being displayed. The reason is that during compilation phase all references to $a within the lexicalscope have already been associated with the lexical variable. If you swap the positions of the lexical
variable declaration and the subroutine, you will find that the localized package variable $a isprinted in this case.
Also, as shown on line 10, when you try to localize a variable when a lexical variable of the same nameexists, you have to explicitly use the double-colon form to indicate the package symbol table entry. Thatis because, as explained, $a is tied to the lexical variable and trying to localize a lexical variable is aruntime error.
Apart from a package variable, you can localize a member of composite type. For example, you can
have

local $ENV{’PATH’} = ’/home/cbkihong/bin’;
which causes the original value to be saved and temporarily replaced by the new given value. When theenvironment terminates the original value is restored.

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Because of potential confusions that may arise when you use local variables, one is generally notrecommended to use local variables.

5.6 Conditionals
A programming language is practically not useful if the statements are only allowed to run from thevery first line to the last. Therefore, in this section we are going to talk about loops and conditionals.
You have used the comparison and logical operators in the previous chapter. By using conditionals,
you can specify a block of code to be executed if a particular condition (test) is satisfied. This is what
conditionals exactly do.
The if-elsif-else structure is the most basic conditional structure. The general form is:
if (EXPR1) BLOCK 1[elsif (EXPR2) BLOCK 2] ...
[else BLOCK n]
The parts in square brackets denote the optional parts. The if-elsif-else structure works as follows:
if EXPR1 evaluates to true, statements in BLOCK 1 are executed, and the remaining elsif or else parts
are bypassed. Otherwise, Perl jumps to the next elsif or else part, if any.
Perl goes to the elsif part if the previous condition is not met (i.e. false). If EXPR2 evaluates to true,
BLOCK 2 is executed and the remaining parts are bypassed. There can be as many elsif parts as you
like, and Perl will test each condition successively until any test evaluates to true. The else part isplaced at last, handling the situation when all the previous tests failed. The BLOCK n will be executed inthis situation.
Figure 5.1 presents the flowchart showing the sequence of actions performed inside an if-elsif-else
conditional structure.
The following is a simple program in which the user inputs a number, and the program deduces
whether it is an even number, odd number or zero.
EXAMPLE 5.4

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#!/usr/bin/perl -w$numtype = "";print "Please enter an integer > ";chomp($num = <STDIN>);if ($num % 2 == 1) {$numtype = "an odd number.";} elsif ($num == 0) {$numtype = "zero.";} else {$numtype = "an even number.";}

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Figure 5.1: If-elsif-else Flowchart
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print $num . " is " . $numtype . "\n";This program has all the three parts, forming a complete if-elsif-else structure. Because 0 is
customarily considered neither odd nor even, we have taken this special case into consideration. Firstit tests if the number is odd by checking if the modulus (remainder) is 1. If this is true, line 9 would beexecuted. Otherwise, it jumps to line 10 to test if the number is 0. If this test fails again, we know forsure that it should be an even number.
Perl also has an unless conditional structure. The following example illustrates its use:

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#!/usr/bin/perl -w
print "Please enter your age > ";chomp($in = <STDIN>);
unless ($in < 18) {print "You are an adult.\n";} else {print "You are less than 18 years old.\n";}
If you use unless, the sense of the test is reversed. Line 6 is executed if the expression evaluates tofalse. If the expression evaluates to true, Perl executes the else part. In fact, the unless structure issomehow redundant. However, Perl gives you the flexibility to do your job in alternative ways. That is
an exemplification of the Perl motto “There Is More Than One To Do It”. You can replace line 5 withif (!($in < 18)) {... or even

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if ($in >= 18) {to achieve the same effect.

5.7 Loops
Sometimes we would like to have a mechanism to execute a sequence of statements repeatedly for a
specific number of times or under a particular condition. A loop is the answer. First, I will introduce
the for loop.

5.7.1

for loop

The for loop is inherited from C/C++. The general syntax isfor ([init-expr]; [cond-expr]; [loop-expr]) BLOCKFirst, the initial expression init-expr is executed. In this part usually a variable would be definedthat acts as a counter to keep track of the number of times executed. Then the conditional expression
cond-expr is evaluated. If the expression evaluates to anything other than undef, empty string (“”) or
the numeric 0 (i.e. the three scalar values that are defined as false), the BLOCK is executed. After the
BLOCK has been executed, the loop-expr is evaluated. Then, a new cycle starts, and the cond-expr isevaluated again until the cond-expr evaluates to false, then the loop terminates.
The process described above could best be visualized using a block diagram as shown in Figure 5.2.

Figure 5.2: for-loop Flowchart

We would now write a script that prints a special pattern on the screen. It consists of two isosceles
triangles pointing vertically towards each other.
EXAMPLE 5.5

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Double Triangles#!/usr/bin/perl -w

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print "Please input the width of the base (1-50) > ";chomp($input = <STDIN>);if ($input < 1 or $input > 50) {die "Input must be in the range (1..50)!\n";}for ($trend = 0, $i = $input; $i <= $input; ($trend)?($i+=2):($i-=2)) {if ($i == 1 or $i == 2) {$trend = 1;}
print " " x (($input - $i)/2) . "*" x $i . "\n";}In the example, lines 5-7 handles the case if the user enters a number out of the range (1..50). In
particular, the die() operator on line 6 outputs the error message specified to the standard error(STDERR), which is the screen by default, and then terminate the program.
On lines 9-14 I used the for loop to print the asterisks line by line. The $i variable stores the numberof asterisks to be printed at each cycle, while $trend keeps track of whether you decrease or increase
the number of asterisks by 2 after each loop. Initially 0 is assigned to $trend, so that the conditional
operator decrements $i by 2 after each loop. The upper triangle is completed when $i takes the valueof 1 or 2, depending on whether the user has entered an odd or even number. At this point I assign 1
instead, so that in the next loop $i is incremented by 2. This loop is repeated until when $i is greaterthan the width of the base specified. In this case, the loop stops.
Note that although line 9 looks a bit complicated and strange, this is grammatically correct. The
init-expr part consists of two expressions separated with a comma operator. As you have learned
from the previous chapter, both expressions would be evaluated while returning the value of the last
expression. However, this return value is actually ignored in this case.

5.7.2

while loop

A for loop is not the only type of loop structure available. Another form of the loop structure I would
like to mention is the while loop. This structure is simpler compared with for loop, and the syntax ofwhich is as follows:
while (cond-expr) BLOCK
How does it work? First, cond-expr is evaluated. If it evaluates to true, BLOCK is executed. Afterthat cond-expr is tested again, and the loop just goes on indefinitely until cond-expr evaluates to false.

5.7.3

foreach loop

Now I would introduce to you another loop structure that works closely with the list data structure.
The general syntax of a foreach loop is as follows:foreach [[my] $loop var] ( list ) BLOCK

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Chapter 5 Conditionals, Loops & Subroutines

In every cycle of a foreach loop, an element from the specified array or list (list) is retrieved andassigned to a temporary local variable $loop var, and BLOCK is executed. Looping continues until
all the elements in list have been enumerated. For example, if we would like to check if a particularelement exists in an array, we can use a foreach loop and iteratively checks if the returned element
matches the data we are looking for, as in the example below:
1
2
3
4
5
6
7
8
9
10
11
12
13

#!/usr/bin/perl -w$searchfor = "Schubert";@composers = ("Mozart", "Tchaikovsky", "Beethoven", "Dvorak", "Bach","Handel", "Haydn", "Brahms", "Schubert", "Chopin");$prompt = "$searchfor is not found!\n";foreach $name (@composers) {if ($name eq $searchfor) {$prompt = "$searchfor is found!\n";
last;
}
}
print $prompt;
As mentioned previously, we should have used a hash in the first place, but we use a loop to demonstrate
the use of foreach loop anyway. In this example, each of the names in @composers is compared with
“Schubert” in turn, the name we are looking for. The loop keeps on going unless the name specified isfound in the list, or all the elements have been exhausted without resulting in a match. It is apparent
that “Schubert” is in the array, so we must always obtain a positive result.
In each foreach cycle, an element from the specified list or array is assigned to the scalar variable
specified. If the variable is omitted (note that $loop var is an optional argument), it defaults to $ .
This is a special variable that Perl, in general, assigns temporary data to if no scalar variable is specifiedin certain operations. This special variable is used quite often in later chapters, like regular expressions,to shorten the length of the script. However, in my opinion, although you are allowed to make yourscripts shorter by omitting specifying certain variables in Perl, it may cause your script to look more
cryptic than necessary. However, many Perl programmers use such shorthands, and you should know
how to interpret them, and this is the reason why I cover this here.
The variable $loop var is in the form of a local variable. However, you may want to restrict it to belexical scoped. In this case, put my after foreach, as shown in the syntax above.
Some Perl programmers are lazy to type 7 characters for foreach so you may use the shorthand for
instead. Perl can differentiate whether you use the for loop or foreach loop from the syntax.
Also notice line 10. The last statement is one of the loop control statements. A loop control statement
controls the execution of the loop. In the next subsection we would explore the loop control statements
available in Perl.
You may attach an optional continue block after a loop block, such as while or foreach, which will
be executed before starting another iteration to evaluate the conditional expression. This is similar inpurpose to loop-expr in the for loop, except this is a block instead of an expression. This is handy if
you have to preform a number of operations before starting a new iteration that is otherwise clumsy
or cannot be represented by an expression. Here is the double triangle example earlier rewritten with acontinue block:5.8 Leftovers

EXAMPLE 5.6

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19

91

Double Triangles (with continue block)#!/usr/bin/perl -w
print "Please input the width of the base (1-50) > ";chomp(my $input = <STDIN>);if ($input < 1 or $input > 50) {die "Input must be in the range (1..50)!\n";}{my ($trend, $i) = (0, $input);
while ($i <= $input) {if ($i == 1 or $i == 2) {$trend = 1;}
print " " x (($input - $i)/2) . "*" x $i . "\n";} continue {$i += ($trend? 2 : -2);
}
}

5.7.4

Loop Control Statements

Loop control statements can only be used inside loops to control the flow of execution.
The next statement causes the rest of the code block to be bypassed and starts the next loop iteration.
If a continue block is present, it is also executed before starting another iteration.

?

For for loops, loop-expr is evaluated, and then cond-expr is evaluated;?

For while loops, cond-expr is evaluated;?

For foreach loops, the next element is taken from list.

The last statement causes the rest of the code block to be bypassed and the loop then terminates.
Execution starts at the statement immediately following the BLOCK. Any continue block is also
bypassed.

5.8 Leftovers
Before we end this chapter, we will look at some miscellaneous issues which have not yet been covered.
A do block contains a sequence of statements which are sequentially executed, while returning the value
resulting from the last statement. A do block is usually used with a modifier. Perl has defined a numberof modifiers which can be attached to the end of a statement. As an example:$i = 0;do {92Chapter 5 Conditionals, Loops & Subroutines

print $i++, "\n";} while ($i <= 30);
which prints all the integers from 0 up to 30. If a modifier is attached to a do block, the block is
executed once before evaluating the condition. If the condition is true, then the block is executed again.
Otherwise, execution resumes on the next statement. The possible modifiers are:if EXPR
unless EXPR
while EXPR
until EXPR
foreach EXPR

# same as "if (!EXPR)"# same as "while (!EXPR)"

If a modifier is not attached to a do block, as in the following example, the expression in the modifier isevaluated before executing the rest of the statement:$i = 0;
print $i++, "\n" while ($i <= 30);

# prints 0 to 30

This is actually equivalent to$i = 0;while ($i <= 30) {print $i++, "\n";}Summary”

The require() function is used to source in external files.

”

The @INC array stores path prefixes Perl uses to find an external source file.

”

An association exists between a variable name and the underlying data object.

”

The scope of an association refers to the region in which the association is visible.

”

Scopes are determined by nesting of environments. An environment is established inside a pairof curly braces.

”

A subroutine consists of a sequence of statements defining an environment, which may accept alist of scalars as parameters and return a list of values to the caller.
•
•
•
•
•

”

Subroutines enforce reusability of code snippets and make debugging easier.
Parameters are merged into a single list and passed into a subroutine which may be retrieved
from the array @ .
A subroutine may return a scalar value or a list of values to the caller by using the return
function.
Prototypes allow type checking of subroutine invocations at compile time.
In a subroutine you may determine the context around the subroutine invocation by using
the wantarray() function.

Recursion refers to the technique of tackling a problem by defining the solution in terms of itself.
In programming languages this is achieved by nested subroutine invocation.

5.8 Leftovers”

93

Package, or namespace, is a mechanism to minimize variable name clashes.
•
•
•

•
•

Variables of the same name may exist concurrently in each package.
Variables not within the extents of any packages belong to the main package.
Package variables are referenced by qualifying the variable name with the package name,followed by ::.
* Defaults to the main package if :: is present but the package name is not specified.
* Defaults to the current package in effect if both the package name and :: are absent.
Package variables are resolved from the symbol table.
Each package may have multiple BEGIN blocks which are executed at compile time.

”

An association that is lexically scoped is visible from the point in the environment where it isestablished and all environments that exist within the extents of the current environment, until
the end of the current environment. my variables are lexically scoped.

”

The scope of an association that is dynamically scoped depends on the call stack. A dynamicallyscoped association exists as long as the current environment is not destroyed. local variables are
dynamically scoped.

Web Links
5.1

Coping with Scoping
http://perl.plover.com/FAQs/Namespaces.html

QuestionsA.

In Example 3.7 I presented a binary search program to check if a user-specified integer exists inthe list of 100 randomly-generated integers. Introduce the following changes to the program:

?
?
?
?

convert it from iterative to recursive approach;
only the search routine is sufficient. You are not required to write the parts which accept
user input and generate integers in random;
use lexical variables wherever appropriate;
the search routine should be placed inside a package with name Search, in an external file“binarysearch recursive.pl”;

There should be at least a binarySearch() subroutine which is in the Search package (namespace), which can be invoked as follows:

require ’binarysearch_recursive.pl’;
my $exists = Search::binarySearch(@array, $integer);where @array is any array containing a list of integers, $integer is the integer to search for, and$exists is 1 if the integer is found, undef otherwise.
You may save the number of comparisons taken to reach a conclusion as a package variable$numSteps in the Search package that can be accessed directly or through a subroutine.

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Chapter 5 Conditionals, Loops & Subroutines

Chapter 6

References
6.1 Introduction
We have covered the use of scalars in the first few chapters of this tutorial. However, we haven’t covereda very important type of scalar yet — references. Perl references are prevalently used nowadays, thanks
to the official support of object-oriented programming starting from Perl 5. You need to have a goodcommand of references before heading towards the topic of object-oriented programming.
References look cryptic from the start. It resembles the concept of “pointers” in C/C++, and “hardlinks” in Unix filesystems (you may flip to Appendix D for a crash course on basic Unix). The useof references is in some sense a peculiar concept in programming. However, it turns out to be very
useful in parameter passing and serves as the basis for constructing complex data structures in Perl
(and therefore, it is central to object-oriented programming in Perl). Towards the end you will also
learn to use typeglobs in your programs. Typeglobs were used to pass data structures to and fromsubroutines when references were not yet available in earlier versions of Perl. Today, they have been
largely superseded by references, and the section on typeglobs is merely presented for your information
only.
A reference is a special form of scalar variable that stores the location (address) of a data structure.
Fundamentally, when something needs to be stored in the memory, a memory address is required. It
is only with the knowledge of the address of the object that we can access it. This is synonymous withyour postal address in mail delivery, or your IP address for packet routing on the Internet.

6.2 Creating a ReferenceTo create a reference, prefix the \ operator to the data object. For example,$a = \100;
This creates a reference variable $a that points to a newly created data object which holds the literal
100. If the memory address of the data object is stored is at location 0x8101B8C, the reference $a would
have an rvalue of 0x8101B8C. The following diagram is a pictorial representation of the situation.
Note that the memory address varies from system to system, and because of the relocatable nature ofprocesses, the address may probably vary on every execution. I just made it up here for the purpose ofillustration.

95

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Chapter 6 References

Figure 6.1: Scalar Reference

In the diagram, the association is represented by a solid arrow. The real data object is represented by acloud shape while the reference by a rectangle. You can further create a reference $b which points to$a, by$b = \$a;# scalar reference

Figure 6.2: A Chain of Scalar References

Apart from references of scalars, you can also create references of hashes, arrays, subroutines andtypeglobs. Subroutine reference (or code reference/anonymous subroutines) and typeglob reference
will be revisited afterwards.

$arrayref = \@array;$hashref = \%hash;$coderef = \&subroutine;$globref = \*typeglob;# array reference# hash reference# subroutine reference# typeglob reference

Though appearing awkward to do so, you may create a chain of references in one go without involving
intermediate variables by prefixing multiple backslashes to the data object. For example, the above twostatements are functionally equivalent to:$b = \\100;
Be careful of enumerated lists as a special case! As indicated on the perlref manpage, taking a referenceof an enumerated list evaluates to a list of references of the list elements:@list = \($a, $b, $c);
# Actually (\$a, \$b, \$c)@list2 = \($a, @b);
# Actually (\$a, \$b[0], \$b[1], ...)# Actually \$c. Remember list operator in scalar context?
$scalarref = \($a, $b, $c);
Recall that in an enumerated list all arrays or hashes are expanded to the constituent elements to forma single list. To create a reference to an enumerated list directly without creating an intermediate arrayfirst, which is called an anonymous array, enclose the list in [ ] instead:6.3 Using References97$listref = [$a, $b, $c];
Similarly, to construct an anonymous hash, enclose the key-value pairs in { } instead:$hashref = {’key1’ => ’value1’,’key2’ => ’value2’,’key3’ => ’value3’,};
Because a reference is simply a scalar value, elements of an anonymous array (or values of anonymoushash) can also be a hash reference or array reference. In this way, we will be able to implement somecomplex data structures easily, which we shall explore later in this chapter as well as in the next chapter
when I introduce object-oriented programming. For example,$hashref = {’values’ => [
’a’,
’b’,
’c’,
],
’device’ => ’screen’,’options’ => {’indent’ => TRUE,’color’ => ’0xFFFF00’,},};
which creates an anonymous hash containing three key-value pairs. The values key maps to an
anonymous array, while the options key maps to an anonymous hash. The device key maps to asimple scalar string.
References to subroutines behave like functors in C. They can be created in two ways. The first way isto prepend \& to the name of the subroutine. The & is required when you are referring to the name of a
subroutine. For example, if you have a subroutine named somesub() you can take a reference to it by\&somesub.
You can also create an anonymous subroutine directly, by using sub without specifying the name of thesubroutine:$subref = sub {# This is a subroutine};

6.3 Using References
If you try to print() a reference variable, the type as well as the memory location will be displayed, forexample CODE(0x814f3a0) for an anonymous subroutine.

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Chapter 6 References

Once you have the reference variables, you may dereference them to access the underlying data objects.
To dereference a reference variable, simply put a pair of curly braces around the reference variable andprepend it with the symbol which stands for the underlying type. For example, to dereference thereference variable $a in our earlier examples, we can write ${$a}. Other examples:@array = @{$arrayref};$scalar = ${$arrayref}[0];%hash = %{$hashref};$scalar = ${$hashref}{’KEY’};&{$coderef}(’a’, ’b’);# array# Return the first element of array above# hash# Return the value whose key is ’KEY’ of hash above# subroutine invocation

In general, you can omit the curly braces around the reference variables in dereferencing operations.
Situations that require them will be described at a later time.
You can also dereference multiple levels deep. For example, consider the chain of references $b wesaw at the beginning of this chapter, we can access the data object which holds the literal 100 by
dereferencing it two times, by$a = \100;$b = \$a;print $$$b;# prints 100

However, creating a reference to a literal makes it read-only. For example, trying to modify its value in
dereferencing is a runtime error:$a = \100;$b = \$a;$$$b = 90;# same as $$a = 90

Modification of a read-only value attempted at test.pl line 3.
Because the value pointed to by $a is read-only, we can only change it by creating a new literal of thedesired value, and point $a to it instead. $b would now reflect the new value when dereferenced:

print "Before reference \$a changes: $$$b\n";$a = \200;
print "After reference \$a changes: $$$b\n";
The output is

Before reference $a changes: 100
After reference $a changes: 200
This is different from the case below, which you may change the underlying value because because thereference is taken on a scalar variable, not a literal:6.3 Using References

99

Figure 6.3: Change in Reference Chain$a = 100;$b = \$a;$$b = 90;
Note that subroutine prototypes are ignored when you invoke a subroutine through its reference,
because prototypes are only used during compile time, at which point perl cannot yet resolve thesubroutine to which the reference variable points.
Recall that to dereference a reference variable we put a pair of curly braces around it and prepend abackslash to it. I then told you that the braces are customarily omitted. The fact is, the curly braces may
contain anything, provided it returns a reference matching the expected type. For example,${ $$ref{’KEY’} }dereferences the hash reference $ref, take the value associated with key KEY which is presumably ascalar reference, and dereference it. Note that this is not the same as $$$ref{’KEY’}, which is actually
identical to${${$ref}}{’KEY’}# $$ref->{’KEY’}(see below)

which dereferences the scalar reference $ref to get the underlying hash reference, dereference itand then access the value associated with the key KEY. In other words, when the curly braces areomitted, access of array or hash is performed at last. As the reference chain gets more complicatedthe dereferencing expression can get very confusing. Therefore, Perl also supports a special syntax
resembling the C pointer-to-member (arrow) operator. Table 6.1 summarizes the alternative forms.
Operation$$arrayref[$index]$$hashref{$key}&$coderef(@args)

Alternative Method$arrayref->[$index]$hashref->{$key}$coderef->(@args)

Table 6.1: Alternative Syntax for Dereferencing

The arrow operator can be cascaded, for example,$hashref->{’files’}->[0] = ’index.html’;100

Chapter 6 References

presumably $hashref is a hash reference, and $hashref->{’files’} yields an array reference. However, Perl is smart in that by executing the above statement it will automatically create all the necessary
data structures and references to fulfill this statement if they do not yet exist. This is known as autovivification. In other words, Perl essentially executes the following statement when $hashref doesn’t exist:$hashref = {’files’ => [’index.html’,],};

6.4 Pass By Reference
Recall that Perl combines the input parameters into a single list. Similarly is the case when a subroutine
returns a list of scalars to the caller. Therefore, you cannot pass multiple lists to a subroutine as you
cannot separate them into respective lists. References are useful in this regard because, no matter
how complicated the underlying data object is, they are simply represented by a scalar, and you canpass scalars easily as usual. Moreover, passing by reference — even if you copy it to another variable— allows you to access and possibly modify the underlying data object. Pass by reference is efficientbecause only a scalar value is involved in parameter passing. When used with subroutine prototypes,
subroutine invocation can never be more flexible than ever.
Passing a reference is easy. Simply take the reference, and pass it as a parameter and get it back from@ in the subroutine. With prototypes (and it is not invoked using the & form) the caller does not evenneed to take the reference himself/herself if the corresponding symbol in the prototype is prefixed withthe backslash \ symbol. The following example demonstrates both methods:

sub passArgs (\%$) {my ($hashref1, $hashref2) = @_;# Both variables are now hash references}%myhash = (’key1’ => ’value1’, ’key2’ => ’value2’);
passArgs(%myhash, \%myhash);
However, because prototypes may not be observed, depending on the way the subroutine is invoked,
you may wish to require the caller to take the reference and pass the reference instead of relying on
prototypes, which is in my opinion the safest way to go at present.
Perl does not support named parameter passing, but some programmers may also wish to emulate it asfollows, which is recommended if you have to pass a number of parameters to a subroutine.

sub somesub (%) {my %params = shift;# All parameters now in %params.
my $filename = $params{’FILE’};
my @args = @{$params{’ARGS’}};}6.5 How Everything Fits Together

101somesub(’FILE’ => ’/bin/ls’,’ARGS’ => [
’-l’,
’-R’,
’/home/cbkihong’,],);
The advantage is you don’t need to remember and follow a predetermined ordering of parameters
because there is no ordering in a hash.

6.5 How Everything Fits Together
Here, let us consolidate the concepts you have learned in this and the previous chapter. This is also anappropriate opportunity to form a more complete picture as to how they all fit together.
By now you should already have a clear separation of the names and their underlying data objects. Dataobjects exist independently from variable names as seen in programs. They are linked together by anassociation, or in other words, a binding. Bindings of lexical variables are determined and fixed in thecompilation phase, before a program is even executed. Which data object a lexical variable is bound tois solely determined by the nesting of environments. Bindings of dynamic variables, including thosedeclared with the local modifier are resolved at runtime from the symbol table, and are dependent on
the call stack during program execution.
Creating references to a data object actually implies establishing additional access paths to it. In general,
you can access a data object in two ways. The first way is to access it through a variable which is visible
and is bound to the data object. The second way is to access it through dereferencing a reference
variable which points to the data object. Perl maintains the number of associations to every data object
to determine when they should be freed. Simply put, Perl would destroy a data object when the numberof associations to a data object drops to 0. This indicates it cannot be accessed from within the program
anymore, and can therefore be safely destroyed. Consider this example:
sub createArray () {my @array = (1, 2, 3);
return \@array;}{
my $arrayref = createArray();
print scalar @$arrayref, "\n";}# $arrayref and underlying object destroyed
In the createArray() subroutine, a data structure in the form of an array is created which holds thethree elements, and a lexical array variable @array is created that associates with the array. Before
the return function is executed, a reference to the array is created, which adds an association to thearray. By the time the return function has been executed, the lexical association to the array has been

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Chapter 6 References

destroyed. However, because a reference to the array still exists, the array is not deallocated. Therefore,
when the number of elements of the array as accessible through the lexical array reference is printed
on line 8 the value displayed is still 3. However, at the end of the block the lexical array reference isdestroyed, so the reference count to the array is now 0, and the array object is also destroyed.

6.6 Typeglobs
In the previous chapter we had a brief introduction to typeglobs. A typeglob represents an entry on
the symbol table and stores a list of references to data objects with the same name. In this section, we
describe how you may use typeglobs in your programs. In many cases, you seldom have to mess with
typeglobs anymore as references are more convenient and flexible.
Package variables, including those declared with local are resolved from symbol tables. You can
use typeglobs to create symbol table aliases. For example, *array = *myarray; causes referencing
operations on the symbol array to be resolved through the symbol table entry of myarray instead.

Figure 6.4: Symbol Table AliasingThis causes $array to refer to $myarray, @array to @myarray and &array to &myarray etc. However,
you rarely would like to create an alias for all types in practice. You can actually assign a referenceof the desired type to a typeglob to replace the existing one. In Section 5.3.2 we wrote an insert()
subroutine that allows you to insert a list into any given array at any arbitrary position. You may also
rewrite it in this way to make use of typeglobs:
sub insert (\@$@) {local *myarray = shift;# not my!!
my ($pos, @list) = @_;return splice(@myarray, $pos, 0, @list);}
Note that, unlike references, accessing through a typeglob does not require any dereferencing. With a
typeglob entry in place you simply replace the * symbol with whatever type symbol that is required for6.6 Typeglobs

103

the type. Some people may consider it convenient as a result.
Also note that you cannot use a lexical variable, that is one declared with my, to represent a typeglob
because a typeglob is a symbol table entry which by nature cannot be lexical. However, this does notprevent you from assigning a lexical reference variable to a typeglob, though.
Finally I would introduce to you the *foo{THING} notation, which you have already seen in theprevious chapter in passing using *INC as an example. By using this notation you can get the individualreferences held in a given typeglob foo. Replace THING with the name of the type. The table below lists
the possible types and their names:

THING
SCALAR
ARRAY
HASH
CODE
GLOB
FORMAT
IO

Type
Scalar reference (\$foo)
Array Reference (\@foo)
Hash reference (\%foo)
Subroutine reference (\&foo)
Typeglob reference (\*foo)
Format Variables (Not covered in this tutorial)
Filehandle

The notations in parentheses represent an alternative way of accessing them, using references. Becauseof the absence of alternative ways to get the reference to a filehandle, *foo{IO} is most probably the
only one that is more widely used. This is used when a file handle needs to be passed to and from asubroutine. We will talk about filehandles and the generic input/output mechanism at a later time.
While typeglobs are still useful in modern Perl programming, you are generally advised to use them
only if you have good reasons to do so, especially if that purpose can be fulfilled with other means suchas references. First, typeglobs cannot be lexical variables while reference variables can. Also, typeglobs
cannot be used to create complex data structures that are possible with references.
Now you should have the fundamental knowledge to proceed to object-oriented programming.

Summary”

Reference is a scalar value whose value is the address of a data structure.

”

A reference is created by prefixing the \ symbol to a data object.

”

An anonymous array is an array reference, which can be constructed by a pair of square brackets[].

”

An anonymous hash is a hash reference, which can be constructed by a pair of curly braces {}.

”An anonymous subroutine is a subroutine reference, constructed by a subroutine definition without specifying a name.

”

A data structure of arbitrary complexity can be easily implemented in Perl by using hash and arrayreferences as building blocks.

”

You may access the underlying data object through a reference by dereferencing it.

104

Chapter 6 References”

Pass by reference refers to the practice of passing a reference of a data structure as argument toa subroutine, which is the best way to pass an array or hash to a subroutine while not having its
elements merged into @ .

”Named parameter passing may be emulated by passing all name-value pairs through a hash.

”Creating references to a data object actually implies establishment of additional access paths to it.
Perl maintains a reference count for all data objects and they are not destroyed until the reference
count drops to 0.

”

A typeglob represents an entry of the symbol table and stores a list of references to data objects
with the same name.

”

References stored in a typeglob may be accessed using the *FOO{THING} syntax.

Chapter 7

Object-Oriented Programming
7.1 Introduction
Object-oriented programming (OOP) is a popular term in the programming community. It represents an alternative approach of programming to cope with program development in the large.
As I outlined in the previous chapter, maintenance of large scale programming projects become
increasingly difficult as the size of code base increases. Object-oriented programming is seen by
many in the Software Engineering community as a desirable solution to keep complex programming
projects in order. In the text below we would explore the rationale behind this argument and point out
how object-oriented model can help alleviate some of the deficiencies in the plain old procedural model.
Perl started to support the notion of object-oriented programming in Perl 5. In fact, most of thenecessary concepts that you need to understand object-oriented programming in Perl has already been
covered in the previous chapters, and frankly, not much content are left for this chapter. However,
because of the importance of object-oriented programming, and in order to avoid making the previous
chapters too long if I lump them together, I have to dedicate a chapter to object-oriented programming.
Compared with previous chapters, you will find longer and more complete examples in this chapter, so
as to help you familiarize with OOP in a more practical context. You should flip back to the previous
chapters if you have not read them carefully, because the Perl implementation of OOP is based on
subjects studied in the previous chapters. In other words, knowledge of packages, scope, subroutinesand references is a prerequisite to understanding OOP in Perl.
If you have previously programmed in some other object-oriented programming languages like Javaand C++, you may safely skip the section “Object-oriented Concepts” below. However, because thePerl implementation of OOP is very much different from other languages like PHP, Java and C++, you
should not skim read the remaining sections as you would find the Perl approach to object-oriented
programming alien to you (it happened to me as well when I learned Perl with some knowledge inC++), and there are some traps to which programmers who have written object-oriented programs inother languages are vulnerable. Please keep this in mind when you are reading this chapter.

7.2 Object-Oriented Concepts
7.2.1

Programming Paradigms

The syntax of a programming language is largely influenced by the programming paradigm on which
the language is based. A programming paradigm represents a framework which describes in a general,language-independent way how syntactic language elements are organized and processed. Without
delving deep into a formal definition of programming paradigms, which is far beyond the scope of
105

106

Chapter 7 Object-Oriented Programming

this tutorial, I would rather state in a more casual way that a programming paradigm is supported bya school of thoughts to address a specific subset of programming tasks. Therefore, it is generally not
appropriate to strictly claim that one paradigm is always better than the other. However, I personallybelieve if properly implemented, a properly-written object-oriented program is easier to maintain, andallows for a larger basis of extension by leveraging the power of object-oriented programming, forreasons that I would explain later in this section.
There are various programming paradigms in use today. However, the majority of programmers
adopt either one of two major paradigms, namely procedural programming and object-oriented
programming.
Our discussion so far has been solely based on the procedural programming paradigm. A complicatedprogram is broken down into smaller pieces by delegating pieces of the source code to subroutines.
and external source files. Variables are created and updated directly to maintain program state duringexecution.
Object-oriented programming should not be considered a total revamp of procedural programming.
Instead, it is best considered one that uses procedural programming as the basis with the emphasis on
the way different logical components (i.e. classes) interact with each other. It enforces a more welldefined way of grouping related subroutines and data into a logical entity, and this is the foundation of
object-oriented programming.
For an executive summary of the various programming paradigms, please visit the Wikipedia.org entry
on programming paradigm. Be prepared, they are conceptual computer science topics that cannot beeasily understood.

7.2.2

Basic Ideas

As I mentioned in the previous section, the idea of object-oriented programming is to group related
subroutines and data into logical entities, each of which constituting its own domain. Such logicalentities are known as classes. Each class defines a framework which describes the set of properties andbehaviours of objects created (instantiated) from the class.
Consider an airplane and a car. Because a car and an airplane have different characteristics and exhibit
different behaviours (e.g. fly vs. move) we model them using two different classes. In other words, eachclass represents a certain type of object. In a class, behaviours are implemented as methods, which
are subroutines associated with a class. For example, an airplane class would very likely include an
ascend() method and a descend() method which contain the program needed for escalation andlanding of an airplane. Also, different classes are likely to have different properties. For example, anairplane is likely to maintain an altitude property to track down the current height of the airplane
above the ground. On the other hand, a car would not have this property. A class only defines theproperties. The values of which are maintained independently in each class instance.
After we have established a class, we need to create a class instance which is called an object. Whena class instance (an object) is created, it possesses all the methods of the class and, as noted in theprevious paragraph, each object holds an instance of the properties. This arrangement allows eachobject to carry its own set of property values. For example, consider a Car class which has only one
property colour and one method move(). When we create several objects from the Car class, eachof them “inherits” the method from the class and maintains a value representing the colour of the object.

7.3 OOP Primer: Statistics

7.2.3

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Fundamental Elements of Object-Oriented Programming

An object-oriented programming language needs to qualify three fundamental properties, namely
encapsulation, inheritance and polymorphism. These principles are fundamental to support thevirtues of object-oriented programming.
Adapted from Wikipedia.org, encapsulation refers to the practice of hiding data structures which represent
the internal state of an object from access except through public methods of that object. Basically, that
implies you should not change a property by directly modifying the internals of an object. Instead, you
should modify it through the interface of the object. An interface is what is expected to be seen fromoutside of the object. In Perl, this includes all object methods. For example, while an airplane object
maintains the altitude property, you should not modify its value directly. Instead, you invoke themethods ascend() and descend() to change its altitude because some actions need to be taken before
climbing up or going down, which are accounted for by the two methods. By interacting with methods
through the interface, the methods can check whether the operation is valid before committing any
changes to the object.
Inheritance allows a class (subclass) to inherit methods from another class (superclass). For example, a
Helicopter class may inherit from the Airplane class the ascend() and descend() methods, while
adding a stationary() method which an airplane doesn’t have and is a characteristic of a helicopter.
By inheriting from another class, you don’t need to write the inherited code again (unless you would
like to override them). Inheritance allows creation of code that can be easily reused.
Polymorphism is a more abstract notion that cannot be easily explained without resorting to examples.
The principle is that it allows programmers to use a consistent interface for method invocation on
objects of different classes.
These concepts would be revisited later on. However, before we go further, let us write a simple Perlprogram with object-oriented perspective so that you can appreciate how an object-oriented program
looks like in Perl.

7.3 OOP Primer: Statistics
In this section we write a simple Perl module Stats.pm which calculates the mean, variance andstandard deviation of a set of input numbers. We then write a perl program stats.pl which uses themodule written to output these statistics given the input of a set of numbers. The program listing isgiven first, and the theories are revisited afterwards.
EXAMPLE 7.1

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Statistics Calculator# Stats.pm# The "Stats" Perl module
package Stats;# Create a new class instance (object)# and return a reference of the object
sub new {my $arg0 = $_[0];my $cls = ref($arg0) || $arg0;

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my $this = {};bless $this, $cls;$this->clear();return $this;}sub clear {my $this = $_[0];$this->{’numlist’} = undef;$this->{’x_sum’} = 0;
$this->{’x2_sum’} = 0;}
# Append a value to the list
sub addValue {my $this = $_[0];my $num = $_[1];if (defined $num) {push @{$this->{’numlist’}}, $num;$this->{’x_sum’} += $num;$this->{’x2_sum’} += $num**2;}}# Calculate total
sub getTotal {my $this = $_[0];return $this->{’x_sum’};}# Calculate mean
sub getMean {my $this = $_[0];my @numlist = @{$this->{’numlist’}};if (!@numlist) { return 0; }return $this->getTotal()/@numlist;}# Calculate variancesub getVariance {my $this = $_[0];my @numlist = @{$this->{’numlist’}};my $n = @numlist;
my $sum_x2 = $this->{’x2_sum’};my $sum_x = $this->{’x_sum’};if (!$n) { return 0; }return ($n*$sum_x2 - $sum_x**2)/($n**2);}# Calculate standard deviation
sub getStdDev {7.3 OOP Primer: Statistics
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my $this = $_[0];return $this->getVariance()**0.5;}# Get list of values
sub getValueList {my $this = $_[0];return @{$this->{’numlist’}};}1;#!/usr/bin/perl -w# stats.pl# This program uses Stats.pm to print out some assorted# statistics on the input numbers
use Stats;# Catch Ctrl-C (SIGINT signal)$SIG{’INT’} = ’getResults’;
my $obj = new Stats;
sub getResults {print "\n\nResults ==============================\n";print "Number of values: ", scalar($obj->getValueList()), "\n";print "Total: ", $obj->getTotal(), "\n";print "Mean: ", $obj->getMean(), "\n";print "Standard Deviation: ", $obj->getStdDev(), "\n";print "Variance: ", $obj->getVariance(), "\n";exit (0);}print qq∼
Statistics Calculator
Calculates several sets of statistics given a sequence of input numbers.
Enter one value on each line.
To exit, press Ctrl-C.
∼;
while (1) {print ">> ";chomp(my $num = <STDIN>);$obj->addValue($num);}
When the program is executed, the output looks like this:1

cbkihong@cbkihong:∼/docs/perltut/src/oop$ perl -w stats.pl

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Statistics Calculator
Calculates several sets of statistics given a sequence of input numbers.
Enter one value on each line.
To exit, press Ctrl-C.
>> 13
>> 26
>>
Results ==============================
Number of values: 2
Total: 39
Mean: 19.5
Standard Deviation: 6.5
Variance: 42.25
In this transcript, 13 and 26 were input and then Ctrl-C was pressed to signal the end of input
list. The results are then displayed. This program catches (intercepts) the SIGINT signal, which is
generated when you press Ctrl-C. By default, if you press Ctrl-C when a program is running it promptly
terminates the program. In this program, I demonstrated how to install a signal handler, which is asubroutine that is automatically invoked when the corresponding signal is caught. The signal handler is
executed instead of terminating the program.
Signals are messages sent by the operating system to a process (a program in execution). Because signal
is a notion from Unix, and the message broadcasting mechanism is different on every operating system,
behaviour of signal handling is platform-specific. Signals are readily supported on Unix platforms byusing the signal handling system calls. Windows has limited support of signals. Support for common
signals like SIGINT seems to be working on Windows, though. The example program works on bothof my testing platforms, namely GNU/Linux and Activestate Perl 5.8.0 on Windows. I chose to use
signal because catching of the SIGINT signal and executing the getResults() subroutine is handled
automatically so we don’t need to place any extra code in the while loop to detect the end of input
list. However, because an explanation of signals is out of the scope of this tutorial, and is not the main
theme of this chapter, my discussion of signals will stop here.

7.3.1

Creating and Using A Perl Class

Perl does not have any specialized syntax for classes as in many other object-oriented programming
languages like C++, Java and PHP. That makes OOP in Perl looks more cryptic than it really is. A Perl
class is contained in a file of extension ‘.pm’ in its own package. This is called a module. The filenameof the module is the name of the class followed by ‘.pm’. The package name is also the name of theclass. Recall from the previous chapter that if the package name contains ‘::’, it is changed to thedirectory separator as the pathname when the module is being sourced. The following table displays
some example package names and the corresponding locations where they need to be saved.
Recall that @INC contains a list of path prefixes that Perl uses to locate a Perl source file. A module
contains all method definitions. Don’t forget to put the 1; at the end of the module. Omitting thisresults in a compile-time error.
Before you use a module you should first import it into your program. You can use require thatyou were taught in Chapter 5. You may import a module in one of two ways. The first way is to pass

7.3 OOP Primer: Statistics
Package Name
Stats
Crypt::CBC

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File Path (relative to @INC)
Stats.pm
Crypt/CBC.pm

Table 7.1: Relationship of Package Names and File Placements

the path to the module (relative to @INC) in quotes to the require function. For example, require"Crypt/CBC.pm"; Another way is to just specify the package name without quotes, for example
require Crypt::CBC; However, the use function is preferred in general for Perl modules. The syntax
of use isuse MODULE [LIST];
It is semantically equivalent to

BEGIN {
require MODULE;
import MODULE LIST;}# indirect object syntax

which not only imports the module MODULE at compile time, it also imports the symbols specified by
LIST into the current namespace (package) using the import class method. import is a special classmethod that may be defined by a class author. Many modules do not have any symbols to be exported,
however, some of them do. If the method cannot be found, it is silently bypassed. For example, users of
the CGI::Carp module may import from it the fatalsToBrowser symbol, which is actually the nameof a subroutine that generates a page in HTML describing the details when an error occurs, which isthen returned to the viewer’s browser. This is used by CGI scripts to easily trap runtime errors. The
use statement required is

use CGI::Carp ’fatalsToBrowser’;Documentation of a class should tell you how to use it, and whether any symbols need to be imported.
Therefore, usually you don’t have to worry about this at all, unless you have to write a class that exports
symbols. The use function is generally preferred to require because modules are imported at compile
time. Therefore, missing modules are discovered at the time the program is compiled before execution,
which saves you from an embarrassing situation where execution is on midway when Perl finds out
some of the modules are missing and the program fails to continue.

7.3.2

How A Class Is InstantiatedAs mentioned previously, to instantiate a class is to create an object that belongs to the class. From theperlobj manpage, a Perl object created is simply a reference that happens to know which class it belongs to.
How does everything fit together?
Let us go back to the new() class method of the Stats class in our example. The most importantstatement is on line 12, that is, the bless() statement. The bless() statement makes the reference$this no longer just an ordinary hash reference. It becomes an object reference of the Stats class, andby doing so you can also access the methods from the reference. Therefore, this method is known as a

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constructor, because it is where the object is created. A constructor is not necessarily called ‘new()’. It
can be of any name, despite it is customarily called new. Object-specific data, or properties, are stored
into the object reference. Let us explain each line one by one.
my $arg0 = $_[0];my $cls = ref($arg0) || $arg0;
In Perl, methods are exactly identical in behaviour to subroutines. However, whether it acts like amethod or a plain subroutine is determined solely by the way it is invoked. A method is a subroutinethat expects an object reference as its first argument. Using the new() method as an example, thefollowing ways of invoking it are identical:
my $obj = new Stats;
my $obj = Stats->new();
my $obj = Stats::new(Stats);# Indirect object syntax# C++-like class member invocation (recommended)# Resembling subroutine invocation

With the indirect object syntax, the name of the method is placed first, followed by the object referenceor name of the class. For example, getTotal $obj (’a’, ’b’) means that the method getTotal()is invoked on the object $obj. Any arguments that follow (’a’, ’b’) are arguments to the method.
This form is generally only recommended for constructors, because this form of method invocation is
not syntactically obvious. The second form is generally recommended as it’s less confusing. The classname or object reference is placed first, followed by ->, the method name and arguments. The lastform is typical subroutine invocation. However, in this form you need to pass the class name or objectreference as the first argument.
Now go back to our example. We put the first argument to $arg0. The next line serves to obtain theclass name from it. The ref() function tests if the parameter is a reference. If it is one, it returns astring indicating its type (see the ref() documentation for details). If it is an object reference, it returns
the package name. By the short-circuiting behaviour of logical operators the package name is assigned
to $cls. This caters for the fact that the new() method can be invoked on the Stats class or a classinstance, although customarily, a constructor is only invoked on a class in most other programming
languages.
my $this = {};bless $this, $cls;
Here, we create an empty hash reference and bless() it to the Stats class. This operation is called
bless, very likely because it works like a wizard playing with his magic wand, turning an ordinaryreference into an object. Very magical indeed. To be an object, Perl only mandates a bless()edreference. It doesn’t require to be a hash reference, although it is most likely chosen for its flexibility.
Please read the perltoot manpage for its coverage on alternative forms of Perl objects. It accepts two
parameters. If the second parameter is missing, it assumes the current package.
The last two lines in the constructor resets the object data and return the object reference so that thecaller of the constructor (in stats.pl as $obj) can save the object reference for use in later operations.
The program reads in a number from each line, and invoke the addValue() method on $obj. This
invokes the method with the object reference itself as the first parameter. The value, as the second

7.4 Inheritance113

parameter, is appended to a list of numbers that is stored with the object reference, and update thesum (x sum) and the sum of squares (x2 sum). These two pieces of statistics are used to calculate thevarious statistics, when the SIGINT signal is detected and the getResults() signal handler invoked.
The variance and standard deviation are given bysnn1Standard deviation (σ) =(n ∑ x2i − ( ∑ xi )2 )
n
i=1
i=1
Variance = σ2
The class provides a number of methods for users of the class to retrieve the list of values, total,mean, variance and standard deviation. Note that these methods are invoked on the blessed object
returned by the constructor instead of on the class. Through the class interface, the class provides
all the necessary methods to interact with without requiring the user to know anything about theimplementation of the class. For example, you do not have to care about how data are stored andprocessed internally. Because an object is simply represented by a reference variable, unlike someother programming languages Perl does not have the notion of protected access specifiers to classify
methods by different access permissions such as “public” or “private”. Users may also be possible tomanipulate the underlying hash directly. However, by documenting the interface clearly class users will
be encouraged to access the object through the interface instead of having to manipulate object data
directly from the reference variable. This fulfills the initiative of encapsulation.

7.4 Inheritance
Inheritance is important in any programming languages because it allows you to reuse portions ofanother source program in your program, so that you don’t have to rewrite them. You save development
effort as a result. Perl implements inheritance in a very simple way. When a method is invoked on
an object and the method cannot be found in the current package, the packages whose names are in
the variable @ISA are searched in order for the missing method. In object-oriented terminology, we
describe this situation as a derived class inheriting from a base class, because the derived class contains
methods defined in itself as well as from the base classes. A base class is also called a superclass or a
generic class, while a derived class is also called a subclass.
When you put the name of a package into your package’s @ISA, all the methods defined in that package,
together with any defined in their base classes as well as the base classes of base classes etc. are available
in your package. You may wish to redefine some of the inherited methods because they may not suit
your current class anymore. To redefine a method, which is called overriding a method, simply rewrite
the method in the way you desire and put it in the current package. If the method is invoked on thecurrent package or its subclasses, the overriding method will be invoked instead of the overridden ones
(that is, those in the base classes).
As an example, we will write a Stats2 class which inherits from Stats with the added functionalityof deducing the maximum and minimum among the list of numbers input. Also attached below is themodified stats2.pl which uses Stats2 with the additional functionalities:
EXAMPLE 7.2

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package Stats2;@ISA = (’Stats’);use Stats;# inherits Stats# Note that by using the 2-argument form of# bless() the constructor can also be inherited# Override addValue()
sub addValue {my ($this, $num) = @_;if (!defined $this->{’min’}) {$this->{’min’} = $num;$this->{’max’} = $num;} else {$this->{’min’} = ($num < $this->{’min’})?$num:$this->{’min’};$this->{’max’} = ($num > $this->{’max’})?$num:$this->{’max’};}# invoke the base class version of addValue()$this->SUPER::addValue($num); # OR $this->Stats::addValue($num)}# Find minimum
sub getMinimum {my $this = $_[0];return $this->{’min’};}# Find maximum
sub getMaximum {my $this = $_[0];return $this->{’max’};}
1;#!/usr/bin/perl -w# stats2.pl# This program uses Stats2.pm to print out some assorted# statistics on the input numbers
use Stats2;# Catch Ctrl-C (SIGINT signal)$SIG{’INT’} = ’getResults’;
my $obj = new Stats2;
sub getResults {print "\n\nResults ==============================\n";7.4 Inheritance
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print "Number of values: ", scalar($obj->getValueList()), "\n";print "Minimum: ", $obj->getMinimum(), "\n";print "Maximum: ", $obj->getMaximum(), "\n";print "Total: ", $obj->getTotal(), "\n";print "Mean: ", $obj->getMean(), "\n";print "Standard Deviation: ", $obj->getStdDev(), "\n";print "Variance: ", $obj->getVariance(), "\n";exit (0);}print qq∼
Statistics Calculator
Calculates several sets of statistics given a sequence of input numbers.
Enter one value on each line.
To exit, press Ctrl-C.
∼;
while (1) {print ">> ";chomp(my $num = <STDIN>);$obj->addValue($num);}
Note that by placing the name of a package in @ISA does not free you from the need of adding the use
statement to import the contents of the package. Stats is the base class while Stats2 is the derived
class. Here, the addValue() method is overridden to check whether the current input number isthe minimum or maximum and update the counters internally if it is one to always keep track of theminimum and the maximum. Then, two new methods are defined that let users retrieve the minimum
and maximum values.
Occasionally an overriding method needs to invoke an inherited but overridden method. addValue()here is an example, because we overrode it to add new functionalities rather than to replace it in its
entirety. You can prefix a package name before the name of the method, separated by two colons, toindicate the class from which to start searching for the method. The default is always to start searchingfrom the package to which an object belongs. Therefore, you always have access to the current,
overriding version. You can access the version defined in the Stats package, that is the base class withrespect to Stats2, by$this->Stats::addValue($num);
However, for convenience a pseudo class SUPER has been defined that always refer to the base classes.
Essentially, the current, overriding version is bypassed, as indicated in the example program.
The Stats2 class can actually make use of the getValueList() method to deduce the maximum andthe minimum values on-the-fly instead of having to memorize two additional pieces of information,and the addValue() method does not need to be overridden as a result. This is left as an exercise forthe readers.

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Even if your class’s @ISA is empty, it still inherits from a class called UNIVERSAL, which is the base classof all classes in Perl. Therefore, methods defined in UNIVERSAL are available in any class. It has threemethods, which are described below.
isa(CLASS)This method allows you to check whether an object belongs to CLASS or a subclass of CLASS. It returns
a true value if it is one, undef otherwise. You may use it in two ways.
Object-oriented form:$obj->isa(’UNIVERSAL’);# must be true, by definition

Procedural form:
use UNIVERSAL ’isa’;isa($obj, ’UNIVERSAL’);can(METHOD)
This method allows you to check whether an object has a method called METHOD. It returns the reference
to the subroutine if there is one, undef otherwise. It also searches the upstream base classes. As an
example, this is an indirect and silly way to invoke the isa method on the current package:sub isSubclass {my ($this, $cls) = @_;my $subref = $this->can(’isa’);$this->$subref($cls);}$obj->isSubclass(’UNIVERSAL’);Can you add methods to a package like UNIVERSAL without modifying the module file? The answer is
you can. Here, I am going to demonstrate how to add to the UNIVERSAL class a constructor so that you
don’t have to define any constructors in your modules.
EXAMPLE 7.3

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sub initialize {# defaults to no-op# override to execute class-specific startup code}1;
# TestObject.pm# Test class module
package TestObject;
sub initialize {
print "TestObject::initialize()\n";$this->{’a’} = 6;}
sub displayMessage {my ($this, $msg) = @_;print $msg;}
1;#!/usr/bin/perl -w# testuniversal.pl
BEGIN {
# Add additional methods to UNIVERSAL
require "universal.pl";}
use TestObject;
my $obj = new TestObject;$obj->displayMessage("Hello World!\n");universal.pl contains the methods to add to the UNIVERSAL class. This file is sourced by a BEGIN
block in testuniversal.pl which ensures that the module definitions are imported at compile time.
Note that the TestObject module does not have to define the constructors as a result. Note that you
may optionally specify an initialize() method in your class module, which is automatically invokedby the constructor (universal.pl::9) where you can place some initialization code. If you do notprovide it, the version provided by universal.pl will be used instead, which is just an empty methodthat does not do anything.
Although you may do it that does not necessarily mean you should do it. In general, I suggest converting
universal.pl into a module which acts as the base class for all your other modules instead of adding
new methods to the UNIVERSAL class. This example is given to demonstrate it works. However, if a userof TestObject forgets to import the universal.pl, then it will be an error as a constructor cannot befound.

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Summary”

Object-oriented programming refers to the practice of grouping related subroutines and data into
a self-contained logical entity, called a class.

”

Each class defines a framework which describes the set of behaviours and properties of objects
instantiated from the class.

”

Methods are subroutines associated with the class. Properties are attributes possessed by objects
instantiated from the class.

”

Encapsulation refers to the practice of hiding internal class implementation from users. Class
users are expected to only interact with it through public methods.

”

Inheritance allows a subclass to inherit methods from a superclass, so that you may save development effort through reuse.

”

A class module in Perl is simply a package, whose file extension is ‘.pm’.

”

The use() function is usually used to load a Perl module at compile time.

”

Every Perl class has a constructor which is responsible for creating objects belonging to the class.

”

The bless() function associates a reference variable with a class in order for the reference tobecome an object.

”

A method inherited from a superclass may be overridden by specifying the definition for the
subclass.

”

In class methods the SUPER pseudo class may be used to invoke an overridden method.

”

All classes inherit a class named UNIVERSAL.

Web Links
7.1

perlboot manpage — Object-Oriented Tutorial for Beginners
http://www.perldoc.com/perl5.8.0/pod/perlboot.html

7.2

perltoot manpage — Tom’s Object-Oriented Tutorial for Perl
http://www.perldoc.com/perl5.8.0/pod/perltoot.html

7.3

perltooc manpage — Tom’s Object-Oriented Tutorial for Class Data in Perl
http://www.perldoc.com/perl5.8.0/pod/perltooc.html

7.4

perlbot manpage — Object-Oriented Bag of Tricks
http://www.perldoc.com/perl5.8.0/pod/perlbot.html

Chapter 8

Files and Filehandles
8.1 Introduction
The ability to read from and write to files is nearly always essential to computer programs. Output
is frequently generated in the course of execution of a program. However, if they are not stored insecondary storage media such as disks, they will disappear once the power is switched off. Therefore,
file access is an important element of the input/output system. In this chapter, we will explore thegeneral Perl input/output system and the functions we can use to access the filesystem.
Similar to the C standard and to be in line with Unix concepts, Perl uses the concept of filehandles torepresent an opened file. They are also known as file descriptors in programming languages like C.
Although I did not mention explicitly, actually you have been working with filehandles throughout the
tutorial. The statement

print "Hello World!";
Appeared in the very first Perl program covered in this tutorial, it implicitly uses a filehandle to get thestring printed on the screen. This statement is actually written as

print STDOUT "Hello World!";
However, the filehandle STDOUT is assumed by default, so we have been omitting it all the way up to
this chapter. Unix has a generalized view of input and output. Apart from files, hardware devices arealso represented as files on a Unix filesystem. Therefore, input and output of hardware devices can be
represented by input and output of files, through a filehandle. This generalized view helps abstract auser or programmer from the peculiarities of the implementation of each device, which are left to thedevice drivers that exist in the operating system kernel, allowing users to interact with each device in a
uniform manner. With a strong Unix tradition, Perl adopts a similar notion as well. Even if you don’t
use any flavours of Unix at all it does not mean you don’t have to read this chapter, because Perl uses
the same concepts on every platform it runs on, so that a considerable degree of platform independence
can be achieved.

8.2 Filehandles
Perl has a number of predefined filehandles, namely STDIN, STDOUT and STDERR that you can use incommand line applications for input/output on screen. STDIN, or standard input, is the filehandle from
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which keyboard input can be read. STDOUT, or standard output, is where you should send the output
of your program. STDERR, or standard error, is mostly used for outputting warning or error messages
because sometimes people wouldn’t like error messages to be displayed. By dumping error messages
to a separate filehandle the user may decide whether to display them or instead redirect them to, forexample, a log file on disk for diagnosis at a later time. These three filehandles are always ready for you
to use and you don’t need to create them manually.
Filehandle is one of the data types available in Perl. As you have learnt in the chapter on references,
filehandle has a separate slot in a symbol table entry. However, unlike other data types you do not haveto prefix a filehandle with a type symbol. By convention, filehandles are in all capital characters to makethem visually stand out from names of functions and subroutines etc.

8.2.1

open a File

Unless you are working on one of the predefined filehandles, the first step you should take is to populatea filehandle. A populated filehandle represents an active stream which allows input or/and outputof data. For the case of file access, a populated filehandle represents an opened file, which is thenused by the input/output functions to read from or write to the file. Filehandle is also used in Perlsocket programming which is used to represent a socket. However, due to time constraints network
programming is not covered in the first edition of this tutorial.
Unix supports two sets of file access functions. One set is provided by the operating system kernel, andthe other set is provided by the standard C libraries installed on the system, which is implemented on
top of the version provided by the kernel. The file access functions in Perl are actually interfaces tothese functions. Perl allows access to both, through the open() function which invokes the version
provided by the C libraries, and the sysopen() function invokes the operating system version. The
use of open() is generally preferred to sysopen(), because it is simpler to use, but sysopen() is more
powerful. sysopen() is not covered in this edition of the tutorial.
You use the open() function to open a file. Usually, the open() function takes on one of the following
forms:
open FILEHANDLE, EXPR
open FILEHANDLE, MODE, EXPR
FILEHANDLE is either a filehandle or a lexical variable with the undef value, which is used by theopen() function to store the reference to the filehandle created. EXPR is a scalar expression whichcontains the name of file to open(), and MODE describes the access mode to apply to the file, forexample, whether read or write access are allowed on the file. If MODE is missing, it defaults to “<”, theread-only mode. Otherwise, MODE should be prepended to EXPR in the first form, if it is not given as astandalone argument. If open() is successful, it returns a nonzero value. Otherwise, undef is returned.
You should always check the return value of file access functions and handle cases of failure to ensure
your program is fault tolerant, especially if your program is to be used by other people instead of you,such as CGI scripts.
Traditionally, a filehandle is usually used instead of a lexical filehandle reference. For example,

my $retval = open(LOG, "<command.log");8.2 Filehandles

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which creates LOG permanently on the symbol table of your current package. This may be acceptable toyou, but you may wish to localize it to a certain subroutine, for example. As a recapitulation, this is
the way to do it:

local *LOG;
my $retval = open(LOG, "<command.log");
However, because filehandles themselves cannot be lexical, and many Perl programmers are not familiar
with the use of typeglobs, a better way would be to use a lexical filehandle reference, which you caneasily pass around without needing any knowledge in typeglobs. An example is

my $fhLOG;
my $retval = open($fhLOG, "<command.log");
Here is a summary of the 6 access modes provided:
MODE
<
+<
>
+>
>>
+>>

Description
Read-only access. Specified file must exist.
Read-write access. Specified file must exist.
Write-only access. File emptied if exists; created otherwise.
Read-write access. File emptied if exists; created otherwise.
Append-only access (file pointer at end-of-file). File created if not exist.
Read-Append access (file pointer at end-of-file). File created if not exist.
Table 8.1: File Access Modes for open()

These access modes determine the operations that can be applied on a file. If you open() a file withread-only access but you try to write data to the corresponding filehandle, a runtime error will occur.
Every open file has a file pointer, which determines the position of the next character read or write. The
first four modes listed above position a file pointer at the beginning of a file, so that read/writes occur
at the beginning of the file. < grants only reads access to the file. > grants only write access to the file,which is automatically created if it does not exist, and empties it before writing. Both +< and +> grants
read-write access to the file, so you may read from as well as write to the filehandle. The difference
between +< and +> is that for +<, the specified file must exist, while for +> the file is automaticallycreated if it does not exist, and empties the content before writing. The last two modes place the file
pointer at the end of a file. Therefore, data are written at the end of the file. A file opened with either ofthese two modes is created where necessary.
Note that +< does in-place writing. If file write occurs in the middle of a file, it simply overwrites theexact number of characters from the file pointer required by the write, growing the file as necessary andother characters in the file are unaffected. The effect is similar to putting your text editor in “replace”
or “overwrite” mode and typing characters at a cursor to overwrite the old text.

8.2.2

Output Redirection

Output redirection allows you to redirect output sent to a certain filehandle to another filehandle.
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simply to throw them away as if they have not been output at all.
To use I/O redirection, specify “>&” as the MODE, and EXPR is the name of the filehandle to which
output to FILEHANDLE is redirected. An example is shown below, which redirects output that are sent
to STDERR to a file that has been previously opened with the filehandle LOG instead.

open (STDERR, ">&LOG");
Text that are sent to STDERR will be redirected to a file instead, so they are no longer output on screen.

8.3 File Input and Output Functions
In this section, I will introduce to you various functions you can use to read from or write to a
filehandle. Note that in functions expecting a filehandle as its argument you can use a lexical filehandle
reference instead of a typeglob. Simply replace the filehandle with the lexical variable, for example,$fhLOG.

8.3.1

readline() — Read A Line from Filehandle

The readline() function accepts a typeglob as parameter to read a line from the filehandle containedin the typeglob. In scalar context, each invocation of readline() reads up to the newline character.
When no more lines can be read, undef is returned. An example is shown below, which copies a text
file File1.txt to File2.txt.

open FILE1, "<File1.txt" or die "Cannot open File1.txt!";
open FILE2, ">File2.txt" or die "Cannot open File2.txt!";
my $line;
while ($line = readline(FILE1)) {print FILE2 $line;}
close FILE1;
close FILE2;
However, customarily readline() is not frequently used because Perl provides an operator<FILEHANDLE> which is an interface of readline(). We can replace readline(FILE1) above with
<FILE1>.
In list context, both readline() and <FILEHANDLE> read all the way until end-of-file occurs, and split
it into lines. The return value is an array with its elements being the lines extracted. This is seldom
used in practice, because if the incoming file is very large, the generated array will also be very large,
consuming a lot of memory space. Therefore, it is a lot safer to set up a loop to read in line by line asshown above.

8.3.2

binmode() — Binary Mode Declaration

Not knowing if you are aware or not, text files actually are stored differently on Unix and MSDOS/Windows systems. The culprit is that the line termination characters used on these platforms are8.3 File Input and Output Functions

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different. That’s why there is an ASCII/Binary transfer mode option in your FTP application. Binary
files do not rely on line termination characters to denote the end-of-line. Therefore, a binary file isrepresented in an identical manner on these platforms. However, because text files uses line termination
characters, and they vary from platform to platform, two files with identical textual content end up
being represented differently on different systems.
To ensure Perl programs have high levels of portability, in general Perl programmers simply have totreat \n as the line termination character. This is what we have readily assumed from the very beginningof this tutorial, and the underlying system carries out all necessary conversions for us automatically.
However, this system does not work for MS-DOS/Windows systems because they distinguish between
text files and binary files. Therefore, on these systems Perl needs to introduce a PerlIO layer on top of
the native file access functions which converts between the underlying line termination characters and\n. This does not pose any problems for text files, as this conversion is actually intentional for text files.
However, binary files should never be altered in any way. binmode() with just a single parameter of
FILEHANDLE essentially instructs all read/write through the filehandle to bypass the conversion layer.
Because binmode() is ignored on other systems, you should generally use it on all binary files for
portability. It should be invoked right after a file is opened, i.e.

open BMP, "<logo.bmp";
binmode BMP;
If binmode() has two parameters, the second parameter indicates the PerlIO layers to apply which
act as conversion filters. The “crlf ” layer is the layer we mentioned above for MS-DOS compatible
systems which carries out line termination conversions. Usage of this form is generally not needed as
the defaults are generally adequate for most programming purposes, and so are not described here.

8.3.3

read() — Read A Specified Number of Characters from Filehandle

The syntax of read() isread FILEHANDLE, SCALAR, LENGTH[, OFFSET]
which reads from FILEHANDLE LENGTH characters, usually equivalent to bytes and assign it to thescalar SCALAR. If OFFSET is given, it specifies the zero-based offset of SCALAR from which to start writing.
This is usually used for binary files, but not necessarily. The file copying program shown above should
generally not be used because binary files are not line oriented, and it does not use binmode() whichcauses file copying errors on MS-DOS compatible systems. Presumably the correct way is as follows:

sub copy ($$) {my ($src, $dest) = @_;open FILE1, "<$src" or die "Cannot open $src!";
open FILE2, ">$dest" or die "Cannot open $dest!";binmode FILE1;
binmode FILE2;
my ($buffer, $numChars); my $bufferSize = 1024;
my $size = 0;
while ($numChars = read(FILE1, $buffer, $bufferSize)) {$size += $numChars;

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print FILE2 $buffer;}close FILE1;
close FILE2;
return $size;

}

8.3.4

print()/printf() — Output To A FileHandle

We have used print() quite a lot so it is not worthwhile repeating all the details here again. However,
if a filehandle is specified, it outputs to the filehandle. Otherwise, the filehandle defaults to STDOUT, as I
mentioned earlier in this chapter.
printf() is similar to print(). However, it is an exceptionally powerful function that lets you
perform varieties of type conversions on the fly. printf() is an artifact from the C standard I/Olibrary. In C, generating a string is not a simple affair, because there is no variable interpolation as in
Perl and there is not an easy and flexible way of string concatenation. Also, because C is strongly typed
and you cannot concatenate a C-style string with other data types, for example, a number, you have toend up performing a lot of type conversions before the desired string can be successfully generated andeventually written to a file descriptor. Therefore, for convenience purpose C provides a set of functions
collectively known as the printf() family of functions which allows generation of many common
forms of string to be completed in one function call.
While this function is very versatile it is also acclaimed as the most complicated function in C. Thisfunction works by constructing a concise but generally cryptic format string, which consists of thestring with some placeholder fields inserted. These placeholder symbols describe the type conversion
operation required for each of the fields to be inserted into the string, which are passed as additional
arguments to the printf() function. printf() is one of the several few C builtin functions which
accept a variable number of arguments. The complexity lies completely in constructing the proper
format string. While Perl has flexible string interpolation and automatic type conversions, printf() is
not as important as in C. Moreover, because printf() involves additional operations, it is less efficientcompared with print(). Therefore, you should avoid it if print() suffices for the purpose. However,
because it is indeed handy for certain kinds of conversion operations, I am going to describe it below.
In Perl’s implementation, another function sprintf() exists. It returns the generated string insteadof directing it to a filehandle. Otherwise, it is identical to printf(). In fact, printf() is internallyimplemented using sprintf(). This is intended to be an introductory description of most frequently
used options only. For more information, please read the perlfunc/sprintf manpage.
The following sprintf() example, however simple, gives you a taste of what it is like:# "Good Morning. The number is 6."
sprintf("%s. The number is %d.", "Good Morning", 6);
The format string is just a string with placeholders inserted. Placeholders start with the % character.
Placeholders are replaced by the corresponding arguments given after the format string with thespecified type conversion operations performed. Here, because we have 2 placeholders, we have two
additional parameters given after the format string. %s indicates the placeholder value is a string, while%d indicates a signed integer. Perl includes an extension to printf() functions that you may specify theplaceholder values in an arbitrary order. That is, the arguments may be given in a different order thanthe order the placeholders appear in the format string. However, because this only complicates matters,
I shall not discuss this and mandate all arguments to be given in the same order that the placeholders

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appear in the format string. s and d here indicate the conversion type. Some other conversion types aredefined as well. The most frequently used ones are:Conversion Type
s
d
u
x
X
f

Description
StringSigned integer
Unsigned integer (decimal)
Unsigned integer (hexadecimal, lowercase)
Unsigned integer (hexadecimal, uppercase)
Floating point number

Here are some examples:

sprintf(’%X’, 12); # C
sprintf(’Absolute value: %u’, -1);

# 4294967295 (see below)

When using the unsigned integer conversion type you have to ensure the value passed as parameter
is not negative. This concerns the way an unsigned integer is represented in the underlying C library.
All integers are represented with a fixed number of bits. On my system, this is 32 bits. Therefore, forunsigned integers 32 bits can represent all integers from 0 up to 2 32 − 1, that is 4294967295. Signed
integers are also represented with 32 bits. The method of representation of negative integers is known as“2s complement”. The range of signed integers becomes -2147483648–2147483647. Negative integers
are represented by inverting the bits of the corresponding positive magnitude and adding 1. Therefore,
1 and -1 are represented as follows:1-100000000 00000000 00000000 00000001
11111111 11111111 11111111 11111111

When you pass -1 as argument while the conversion type is u, the bit sequence of -1 is interpreted as
if it is an unsigned integer. Note that this bit sequence is identical to 4294967295 in unsigned integer
representation. Therefore, the string returned is 4294967295.
Between % and the conversion type character additional options may be applied. We will dicuss some ofthem below. You may mix and match them, although a few of them are mutually exclusive as otherwise
noted.
You may stipulate a prepended space before a numeric conversion if the numeric value is not negativeby inserting a space. For example,

sprintf(’<% d>’, 3);sprintf(’<% d>’, -3);

# < 3># <-3>

By replacing the space with a + symbol, the + sign is prepended instead of a space. Note that the spaceand + are mutually exclusive. If both exists, + is taken. For example,

sprintf(’<%+d>’, 12);sprintf(’<%+f>’, 13.6);
sprintf(’<% +f>’, 13.6);# <+12># <+13.600000># <+13.600000>

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Once an argument has been converted to the string form, by default the corresponding placeholder
is substituted by the string without introducing any padding, as we have seen in the above examples.
However, we may introduce extra padding by specifying the minimum field width. It specifies theminimum length of the placeholder value string. If this number is larger than the length of the string,
padding is added. By default the string is right-aligned so padding appears on the left. However, if the- symbol is prepended to the minimum field width, the string is left-aligned so padding appears on theright. The padding is in terms of spaces. If the minimum field width is smaller than the string length
required, then the field width is ignored. For example,

sprintf(’<%+12f>’, 13.6);
sprintf(’<%+-12f>’, 13.6);
sprintf(’<%+-5f>’, 13.6);# < +13.600000># <+13.600000 ># <+13.600000>

If the string is left-aligned (i.e. format string without the - symbol) you may place a 0 anywhere before
the field width to instruct zero padding instead of spaces. For example,

sprintf(’<%08d>’, 12345);
sprintf(’<%0+12f>’, 13.6);
sprintf(’<%+012f>’, 13.6);# <00012345># <+0013.600000># <+0013.600000>

As you have seen, floating point numbers have a certain precision. The precision is influenced by the
number of decimal places. Subject to fixed storage space of floating point numbers, the precision is
not unlimited as in our usual mathematics. You may specify the precision by specifying the number ofdecimal places, together with a prepending ‘.’. For example,

sprintf(’<%0+12.4f>’, 13.6);
sprintf(’<%12.4f>’, 13.6);# <+000013.6000># <13.6000>

If applied on a string, this specifies the maximum number of characters to fit, somehow like the Perlsubstr() function. For example,

sprintf(’<%.4s>’, ’abcde’);sprintf(’<%10.4s>’, ’abcde’);sprintf(’<%10.4s>’, ’abc’);
sprintf(’<%-10.4s>’, ’abcde’);8.3.5# <abcd>
# <
abcd>
# <
abc>
# <abcd>

seek() — Set File Pointer Position

seek FILEHANDLE, POSITION, WHENCE
You may use the seek() function to set the position of file pointer in a file specified by FILEHANDLE.
POSITION is a signed integer indicating the new position, relative to the position indicated by WHENCE.
WHENCE is an integer which is either 0, 1 or 2 representing the beginning-of-file, the current file pointerposition and the end-of-file respectively. However, because hard-coding these integer values is not
semantically obvious, we usually use the names of the corresponding constants available in the C
standard I/O library instead. These constants are defined in the Perl Fcntl module. You can import
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127

use Fcntl ’:seek’;
The constants are SEEK SET, SEEK CUR and SEEK END respectively. After you have imported them, you
can use them directly in your programs. seek() is frequently used with binary file access. Binary files
usually have their data stored in a certain format that allows efficient access of data. To achieve this,some fields are encoded and saved at fixed positions in a binary file, which you may access directly withthe seek() function. The new position is calculated as the sum of POSITION and the base positionindicated by WHENCE. Here are some examples:seek(FILE, 0, SEEK_SET);
seek(FILE, 5, SEEK_CUR);
seek(FILE, -1, SEEK_END);

8.3.6# Jump to beginning of file# Jump 5 bytes forward# Jump to last byte of filetell() — Return File Pointer Position

The tell() function returns the position of the file pointer, in bytes. This function returns meaningfulvalues only if used on an open()ed file. The position is a zero-based offset from the beginning of thefile specified by the filehandle which is passed as the parameter. It is frequently used with seek() tomove the file pointer about in a file for reading or writing. This is an example which deduces the size ofa file by using seek() and tell():use Fcntl ’:seek’;sub getFileSize ($) {my $filename = $_[0];local *FILE;
open FILE, "<$filename" or return undef;
seek(FILE, 0, SEEK_END);
my $size = tell();# last filehandle read is FILE
close FILE;
return $size;}
my $filename = $ARGV[0];print "$filename\t\t", getFileSize($filename), " Bytes \n";However, on a system that supports stat() this can be satisfactorily replaced by(stat($filename))[7]8.3.7

close() — Close An opened FileAt the end, when you have finished working with a filehandle, you should close() it. Simply pass thefilehandle as the parameter to close().

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8.4 Directory Traversal Functions
In this section, I will introduce to you various functions you can use to traverse the directory structureof your system. Note that you ought to be using File::Find, which is easier to use and more flexible.
In fact, it uses the functions that we cover below. However, some simple directory traversal operations
may be more efficient if you implement them directly.
Description of the functions is presented first. You will find a full example afterwards which uses thesefunctions to build a class which performs file search.

8.4.1

opendir() — Open A Directory

opendir DIRHANDLE, PATH
This function prepares the directory PATH for subsequent directory traversal functions. If the directory
exists, a true value is returned, and populates DIRHANDLE which is a filehandle.

8.4.2

readdir() — Read Directory Content

readdir DIRHANDLE
The readdir() function reads the content of the directory referred to by DIRHANDLE, which is
populated by the opendir() function. In scalar context, each readdir() invocation returns an entry
in the directory. When there is no more entry, undef is returned. This is similar to the behaviour ofread() saw earlier. You may also get all the entries in one go by calling the function in list context, andan array containing all the entries inside will be returned.

8.4.3

closedir() — Close A Directory

When you have finished traversal of the directory concerned, you should closedir() it.

8.4.4

Example: File Search

In this example, we will write a class that allows users to search for a file recursively in a directory tree
whose name matching a pattern specified by the user. The pattern is in the form of a regular expression
for convenience purpose. It is by no means versatile as File::Find, but it will give you an idea of how
to use the directory traversal functions.
EXAMPLE 8.1

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File Search# FileSearch.pm# An Example File Search module
package FileSearch;# Create a new class instance (object)
sub new {my $arg0 = shift;
my $cls = ref($arg0) || $arg0;

8.4 Directory Traversal Functions
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my $this = {};bless $this, $cls;$this->initialize(@_);
return $this;}sub initialize {my $this = shift;
my %options = @_;foreach (keys %options) {$this->{$_} = $options{$_};}}
sub find {my ($this, $path) = @_;my @matches = ();my $pattern = $this->{’PATTERN’};
local *DIRENT;
opendir DIRENT, $path or return ();# Get a list of entries in this directory,# and sort it lexicographicallymy @entries = sort { lc($a) cmp lc($b) } readdir(DIRENT);
foreach my $fn (@entries) {my $entry = "$path/$fn";# Recursive search if the entry is a directory,# We should ignore . and .. in our search
if (-d $entry and $fn !∼ m/ˆ\.\.?$/) {push @matches, $this->find($entry);}$fn =∼ m/$pattern/ and push @matches, $entry;}
closedir DIRENT;
return @matches;}
1;
The program below uses this module, getting the root of the search as well as the search pattern fromuser input, and uses the module to get a listing of files found. Finally, the list of files together with thecorresponding file sizes are displayed:

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#!/usr/bin/perl -w
use FileSearch;

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Chapter 8 Files and Filehandles

my ($pattern, $dirroot);
print "Please specify the filename pattern by means of a regular expression:\n¶>> ";chomp($pattern = <STDIN>);
print "Please specify the root of directory tree to be searched:\n>> ";chomp($dirroot = <STDIN>);
my $searchobj = new FileSearch(’PATTERN’ => $pattern);
my @result = $searchobj->find($dirroot);
foreach (@result) {print $_, " (", (stat($_))[7], " bytes)\n";}
The transcript of a sample session is shown below. It searches for files whose extension is .pm.
Therefore, a list of Perl modules in the directory tree is shown.
cbkihong@cbkihong:∼/docs/perltut/src/files$ perl search.plPlease specify the filename pattern by means of a regular expression:>> ˆ.*\.pm$
Please specify the root of directory tree to be searched:>> /home/cbkihong/docs/perltut/home/cbkihong/docs/perltut/scrap/Number.pm (344 bytes)/home/cbkihong/docs/perltut/src/ch06/Controller.pm (916 bytes)/home/cbkihong/docs/perltut/src/ch06/Stats.pm (1264 bytes)/home/cbkihong/docs/perltut/src/ch06/Stats2.pm (779 bytes)/home/cbkihong/docs/perltut/src/ch06/Stats3.pm (425 bytes)/home/cbkihong/docs/perltut/src/ch06/TestObject.pm (198 bytes)/home/cbkihong/docs/perltut/src/ch06/Timer.pm (1011 bytes)/home/cbkihong/docs/perltut/src/ch06/TrafficLight.pm (642 bytes)/home/cbkihong/docs/perltut/src/files/FileSearch.pm (796 bytes)
The chomp() function removes the trailing newline character from the variable containing an input
string if one is present. Note that the argument must be an lvalue because it performs in-place editingof the string instead of returning the modified string.
The FileSearch::find() method is a recursive subroutine which takes a single parameter, that isthe base directory in which to search for files matching the specified pattern, which is passed to the
FileSearch object upon instantiation. The search itself is a depth-first search. That means at eachlevel, for each directory entry the content of the directory is searched before the next directory is
searched. Note that we have to avoid . and .. in each directory. In Appendix D in my discussion of
hard links I explain what these two entries are. They refer to the current and the parent directory,
respectively. Because they go up instead of go down the directory tree, we should not delve into them.
This program looks fine. However, this search implementation can cause the search to go into an
infinite loop on Unix systems in a certain case. That happens if two path segments on a path point
to the same inode. First, let us agree on the terminology first. A path like /a/b/c/d consists of four
path segments a, b, c and d. Now, let us assume b and d point to the same directory inode (for an
exhaustive explanation of inodes, please read Appendix D). Now c is a directory inside b. However, c8.5 File Test Operators

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is also a directory inside d because b and d are actually just two aliases representing the same inode.
Therefore, /a/b/c/d/c is still a valid path, and we can find the directory entry d inside, so we have thepath /a/b/c/d/c/d. It is obvious that this circularity will continue endlessly, and obviously you will go
into an infinite loop as a result. This is exactly the reason why we have to avoid delving into . and ..
in the program. Therefore, this problem normally should not occur. It only arises when a hard link iscreated manually that makes two directories point to the same inode. The core difficulty is that you
cannot easily identify this circularity in the program because d appears exactly like any other directory
entries. You are hereby asked to think of a way to patch this potential problem. (hint: use stat())

8.5 File Test Operators
Perl provides you with a set of file test operators that you can use to test a file against certain properties
and return a truth value. Table 8.2 lists the most commonly used operators:
Operator
-r
-w
-x
-o
-R
-W
-X
-O
-e
-z
-s
-f
-d
-l
-u
-g
-k

Description
File is readable by effective user or group.
File is writable by effective user or group.
File is executble by effective user or group.
File is owned by effective user.
File is readable by real user or group.
File is writable by real user or group.
File is executble by real user or group.
File is owned by real user.
File exists.
File is empty (zero size).
File is not empty, size in bytes as return value.
File is a regular file.
File is a directory.
File is a symbolic link.
File has setuid bit set.
File has setgid bit set.
File has sticky bit set.

Table 8.2: File Test Operators

Note that many of these options are specific to Unix operating systems. -e, -f, -d, -z and -s can beused on most operating systems. The only parameter is either a filename or, in case the file has alreadybeen opened, its filehandle. For example, to ensure a certain regular file (abc.txt) exists, you can issue
the following statement:(-e "abc.txt" && -f "abc.txt")
or die "abc.txt not found or is not regular file!\n";Notice there are two sets of operators to test whether a file is readable/writable or executable. In general,
you should use the lowercase version instead of the uppercase version because the lowercase version
uses the effective user or group identity, which always reflect the identity of the user or group underwhich the program is executed. You can get further information on effective and real user and group in

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Appendix D with respect to setuid or setgid files.

8.6 File Locking
File locking, or locking in general, is just one of the various solutions proposed to deal with problems
associated with resource sharing. Sharing of resources frequently arises in our everyday life. Drivingon a highway, boarding an elevator etc. are all manifestations of utilization of shared resources. In
a computer program, you — the developer — have access to disks, peripheral devices etc. within aprogram that appears to be self-contained, which in turn leads you into thinking your program has
exclusive access to these resources. No, you don’t. Resource sharing is equally ubiquitous as, if not morethan, in your daily life. In your computer system there may be a few hundred programs in execution(processes). Many of them are hidden so you may not know they are running. However, with just
one Central Processing Unit (CPU) only one of them can be executed at any instant. Therefore, theCPU actually performs a context switch at regular, but very short intervals when a currently-running
process is suspended from execution by the CPU with its execution state saved, and another process
is resumed. This context switch is performed so frequently that to a user there is a perception thatall processes are executed concurrently, but they’re not. This is already one example of resource
sharing, and the solution is to introduce fine-grained time slicing. Similarly, all programs thatare executing have access to the disks. Therefore, it is not surprising that when a program is accessing a disk it is possible you can find another one that is also trying to access the disk at the same time, too.
File locks are introduced to set temporary restrictions on certain files to limit how they can be shared
among different processes. Depending on the nature of an operation, we come up with two types oflocks. The first type is a shared lock, and another one is an exclusive lock. With respect to files, readaccess can be shared by multiple processes because read access does not result in any changes to thestate of the shared resource. Therefore, a consistent view of the shared resource can be maintained.
Write access, however, should by nature be carried out with exclusive access until the write operation iscomplete. Another write operation to the file which occurs before the current one is complete should
be queued (certain cases may require it be cancelled instead, depending on the nature of the operation).
A read access cannot be concurrent with a write access either because the write access will destroy theconsistent view expected by the read access while it is still reading (for example, the write operation may
be to delete the file altogether, while the read operation is still in its midway). Therefore, the solution
is to introduce shared locks for read access, while exclusive locks for write access. Multiple concurrent
shared locks are allowed. However, exclusive locks are mutually exclusive.
On most Unix systems as well as platforms in the Microsoft Windows NT family (including Windows
2000, XP and Windows Server 2003 as of this writing) you may use a handy flock() function. Please
see the “Function Implementations” section of the perlport manpage to see any platform-specific issues
for your platform. Here is an example demonstrating file locking:
1
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4
5
6
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8
9

#!/usr/bin/perl -w# writer.pl# Writes into myfile.dat
use Fcntl ’:flock’;open(FILE, ">>myfile.dat") or die "Cannot open myfile.dat!\n";8.6 File Locking
10
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print "[$$] Requesting exclusive write lock for myfile.dat.\n";flock(FILE, LOCK_EX);
print "[$$] Requested exclusive write lock for myfile.dat.\n";

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#!/usr/bin/perl -w

133

print "[$$] Writing (appending) to myfile.dat.\n";# Any write operations should be put insideforeach ($ARGV[0]..$ARGV[1]) {print "[$$] Writing $_\n";print FILE "$_ (process id $$)\n";sleep 1;# sleep one second}
flock(FILE, LOCK_UN); # release lockprint "[$$] Released exclusive write lock for myfile.dat.\n";close FILE;# reader.pl# Reads from myfile.dat
use Fcntl ’:flock’;open(FILE, "<myfile.dat") or die "Cannot open myfile.dat!\n";print "[$$] Requesting shared read lock for myfile.dat.\n";flock(FILE, LOCK_SH);
print "[$$] Requested shared read lock for myfile.dat.\n";print "[$$] Reading from myfile.dat.\n";while (<FILE>) {sleep 1;
print $_;}
flock(FILE, LOCK_UN); # release lockprint "[$$] Released shared read lock for myfile.dat.\n";close FILE;

This example consists of two programs, namely the writer and the reader. The writer program writes
some sample data into a data file, while the reader program reads from the data file previously written
data. The writer takes two command-line parameters which determine the values to be written to
the data file. It first acquires an exclusive lock. When the lock is acquired, it starts writing the values
to the file. When the write operation is complete, the lock is released. The reader works in a similarmanner. It first acquires a shared lock, and when one is acquired it starts reading from the file. At theend, the shared lock is released. To try this example, open two or more commnad-line windows on
your desktop environment and try different combinations, such as a writer in a window and a reader inanother (writer-reader), reader-reader and writer-writer. Take the writer-reader as an example, when
you execute the writer program in one window by134

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perl writer.pl 1 15
That means 15 lines of records will be written into the data file. You will see that there is a 1-second
pause between each write, because of the sleep() function. It is inserted after each write or readso that you can have ample time to switch to another window to execute another writer or reader
instance. Now switch to another window and start the reader before the writer has finished writing. No
command-line parameters are needed for the reader. You will find that the reader stalls while trying
to acquire a shared read lock because an exclusive lock is already in place. When the writer completes,
the reader will be able to get the shared lock and proceed with reading. If you try the reader-reader
combination, you ought to find that concurrent readers are allowed, which agrees to what I told you
earlier.
flock() accepts two parameters. The first one is the filehandle. The second argument indicates thelocking operation required. Just like the case for seek(), it is just an integer whose mnemonic constants
can be found by importing the Fcntl module, by importing the symbol ‘:flock’. The shared lock isrepresented by LOCK SH and the exclusive lock by LOCK EX. To release a lock you can specify LOCK UN.
You may optionally bitwise-or LOCK NB with either LOCK EX or LOCK SH to indicate non-blocking. As
demonstrated in the example, an flock() call will be blocked until the lock is acquired. With thisoptional flag, you may choose to skip if it fails and do something else, or you may retry it later on. The
predefined variable $$ refers to the current process ID. It is displayed as well as saved to file to let you
easily identify the various processes.

Summary”

A filehandle is a high-level abstraction in order to provide a uniform interface to I/O involving
different media such as file access and socket communication.

”

There are three predefined file handles: STDIN, STDOUT and STDERR.
•••

User input through the keyboard is read from STDIN.
Output of a program are usually directed to STDOUT.
Error messages are usually directed to STDERR.

”

Before performing any operations on a file, you have to open() it. You should close() it when
you are done with it.

”

Output redirection allows you to redirect data sent to a certain filehandle to another filehandle.

”

The readline() function reads a sequence of characters from a given filehandle until a newline
is encountered. The <> operator in Perl serves the same purpose.

”binmode() ensures binary files are not corrupted on MS-DOS based operating systems.

”read() reads a given number of characters from a filehandle.

”

Every open file maintains a file pointer, which you may manipulate with the seek() function.
The file pointer position may also be obtained by the tell() function.

”

The print() and printf() functions output a string to the given filehandle. printf() performs
some transformations on the string beforehand.

”sprintf() is identical to printf() except the resulting string to be returned instead of written
to any filehandles.

”

The readdir() function traverses a given directory, whose filehandle as been populated by theopendir() function. Upon completion closedir() is invoked to close it.

8.6 File Locking

135”

File test operator can be used to test if a file possesses a given property.

”

File locking serves to set temporary restrictions on how certain files may be shared among several
processes. The flock() function is used.

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Chapter 9

Regular Expressions
9.1 Introduction
Now we are marching into probably the most exciting chapter in this tutorial. Regular expressions(regexps, or even RE) are what makes Perl an ideal language for “practical extraction and reporting”,
as the name implies. To give you an idea, let me first give you an overview on how to perform patternmatching in Perl.
First, you construct the regular expression, which is essentially a sequence of characters describing thepattern you would like to match. The term “pattern” may seem a little bit foreign to you, but you may
actually have had some experience of it already. For example, in MS-DOS if you would like to list all
files with the extension .txt (presumably text files), you may issue a command likedir *.txt
On Unix-like operating systems, similarly, you can specify

ls *.txt
The “*.txt” here can be described as a pattern as it is the specification used by the operating system(strictly speaking, the shell, i.e. the program in charge of reading commands from you and displaying
output) to look for the file entries that have to be displayed. This is a very simple pattern, but Perlprovides users with a very powerful set of regular expressions that can be used to specify the patterns,
so you can make your search specification more specific. After constructing the regular expression, you
can then bind the data to be searched (for example, text files or just a line of user input) to the patternusing the binding operators =∼ or !∼. In this process, you have provided the Perl regexp engine withboth the data to search for and the data to be searched. The return value indicates if pattern matchingis successful. If it is successful, you may want to store the data temporarily, or in a file, or export theresults directly to the standard output.
Regular expressions are used in Perl in a number of ways:?

Search for a string that matches a specified pattern, and optionally replacing the pattern foundwith some other strings?

Counting the number of occurrences of a pattern in a string
137

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Chapter 9 Regular Expressions?

Split a formatted string (e.g. a date like 02/06/2001) into respective components (i.e. into day,month and year)?

Validation of fields received from submitted HTML forms by verifying if a piece of data conforms
to a particular format?

... and much more

Regular expressions are not exclusive to Perl only. It is a vital component in Unix and other Unix-like
operating systems like Linux to provide users with powerful text search and replace capabilities. You
may find that many software on Unix, like grep and awk, allows users to use regular expression to
specify search specifications (although their implementations are slightly different from that of Perl).
Although this is still rare in Windows, many new software with regexp capabilities are emerging
because many famous Unix applications are now being ported to Windows and other system platforms.
Moreover, a large set of Perl regexps are now being adopted in JavaScript and JScript implementations
in Netscape and Microsoft Internet Explorer respectively. This is a piece of good news as sophisticatedinput validation can now be performed directly by the end users’ browsers. In this way, invalid data canbe detected without sending anything to the server that causes the transmission delays. In C/C++ there
are also regular expression libraries that programmers can easily use for adding regexp capabilities totheir programs. That explains how useful regular expressions are in programming nowadays.
As you may know there are books dedicating in their entirety to regular expressions in your local
bookstores. Therefore, this chapter is by no means a complete coverage of regular expressions in finedetail. However, after you have finished this chapter you should appreciate how flexible and powerful
regular expressions are in Perl and in Unix-like operating systems, and how they can accomplish tricky
text manipulating tasks on the fly.

9.2 Building a Pattern
9.2.1

Getting your Foot Wet

Now you will learn to build a pattern using the regular expressions offered by Perl. To search for apattern match, simply construct the pattern and put it in between the two slashes of the m// operator.
If you don’t need the bells and whistles, for example, you just need to know if the characters “able”
appear in any given string, the pattern is as simple as:

m/able/Let’s put this to a test. Now, to see if this pattern occurs in the string “Capable”, we bind the twos
together by using the binding operator =∼. Try this script:
if ("Capable" =∼ m/able/) {print "match!\n";} else {# This should NEVER happen
print "no match!\n";}There is not many things special here. Because the pattern “able” is in the string “Capable”, thewords “match!” will be displayed. I intentionally use the literal “Capable” in the example to show that
although the symbol looks like an assignment operator, it is not necessary for a valid lvalue on the left

9.2 Building a Pattern139

hand side of the binding operator (remember lvalue is for assignment operators only). You may put
any piece of scalar data, including scalar variable, in place of the string literal.

9.2.2

Introduction to m// and the Binding Operator

As you have seen in the above example, the m// operator is used for pattern matching. In between theforward slashes // the pattern to match is placed. Additional options, if any, are placed at the end after
the last slash. If an expression is explicitly bound to the operator using the =∼ or !∼ binding operators,
that expression is searched for the pattern specified. If the binding operator is missing, as you will see
in some later examples, =∼ is assumed and $ is taken as the expression to be searched.
In scalar context, the binding operator =∼ returns a true value if the expression matches the pattern, an
empty string (and hence a false value) if otherwise. !∼ simply inverts the logic so that if the expression
matches the pattern a false value is returned, a true value otherwise. Therefore, the following two
expressions are equivalent:!($expression =˜ m/pattern/)$expression !˜ m/pattern/
Similar to double-quoted strings, the pattern may be interpolated. Therefore, you can generate patterns
at runtime and apply them by, for example,$expression =˜ m/$var/
Again, similar to the case of quoted strings, you may use other symbols in place of //. However, if
you use //, you may omit the prefix m. You may wish to use other symbols in place of // if yourpattern is heavily slashed, for example, to match a Unix path name /var/logs/httpd/error log inan expression you have to escape the forward slashes (to be covered later) like this:$expression =˜ m/\/var\/logs\/httpd\/error_log/
In the manual pages, this is described as the leaning toothpick syndrome (LTS) where a lot of forward
and backward slashes are present, making the pattern itself difficult to recognize. If you change thesymbol to, for example, |, then the entire pattern suddenly becomes clear:$expression =˜ m|/var/logs/httpd/error_log|
This is just one of the methods to remove the leaning toothpick syndrome. We will talk about the m//operator in more detail later in this chapter, together with the options you may use. I am just giving
you a taste of it now anyway.

9.2.3

Metacharacters

The list of metacharacters supported in Perl are listed in Table 9.1.
Metacharacters serve specific purposes in a pattern. If any of these metacharacters are to be embeddedin the pattern literally, you should quote them by prefixing it by \, similar to the idea of escaping in

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Chapter 9 Regular Expressions
Metacharacter
\
ˆ
.
$
|
()
[]

Default Behaviour
Quote next character
Match beginning-of-string
Match any character except newline
Match end-of-string
Alternation
Grouping and save subpattern
Character class

Table 9.1: Metacharacters in Perl

double-quoted string. In fact, the pattern in between the forward slashes are treated as a double-quoted
string. For example, to match a pair of empty parentheses and execute a code block if they can befound, the code should look likeif ($string =˜ m/\(\)/) {# ...
}In the previous section we mentioned the leaning toothpick syndrome. Apart from changing thedelimiters of the m// operator, you can suppress the effect of metacharacters by using the \Q ... \Eescape sequence. This does not suppress interpolation, however. This is demonstrated in the following
example:$expression =˜ m/\Q/var/logs/httpd/error_log\E/| specifies alternate patterns where matching of either one of them results in a match. These patterns
are tried from left to right. The first one that matches is the one taken. Usually, | are used together withparentheses () to indicate the groupings preferred. These are some example patterns:
m/for|if|while/# A match if either ’for’, ’if’ or ’while’ foundm/a(a|b|c)a/# A match if either ’aaa’, ’aba’ or ’aca’ found
Apart from indication of grouping, the use of parentheses also carries another behaviour. If there isa pattern match, the expression matched by a grouped pattern is saved. This is called backtracking.
Backtracking is covered in more detail later in this chapter.
The . metacharacter matches any character. By default, it does not match any embedded newline
characters in a multi-line string. However, if the s option of m// is given, embedded newline characterswill be matched. This is convenient if you have to match a pattern across multiple lines.
"a\nb\nc" =˜ m/a.b/"a\nb\nc" =˜ m/a.b/s# Not matched, because . does not match \n# Matched with ’s’ option

The ˆ metacharacter matches the beginning of the string, and $ matches the end of the string. However,if the m option of m// is given, they match the beginning and the end of each line respectively. This isused to match individual lines inside a multi-lined string.
"a\nb\nc" =˜ m/ˆa$/"a\nb\nc" =˜ m/ˆa$/m# Not matched# Matched9.2 Building a Pattern

9.2.4

141

Quantifiers

Quantifiers are used to specify how many times a certain pattern can be matched consecutively. A
quantifier can be specified by putting the range expression inside a pair of curly brackets. The formatof which is{m[,[n]]}Here are the available variations:

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

Match exactly m times
Match m or more times
Match at least m times but not more than n times

This example shows how you can verify if a string is an even number. Note that this example can befurther simplified with the help of character classes, which we will describe next.

$string = $ARGV[0];my $retval = ($string =˜ m/ˆ(\+|-){0,1}(0|1|2|3|4|5|6|7|8|9){0,}(0|2|4|6|8)$/);
printf("$string is%san even integer.\n", $retval?’ ’:’ not ’);With different input values, different messages will be printed indicating whether the number is aneven integer. You may split the pattern into three sections. The first part, (\+|-){0,1} matches thepreceding sign symbol if there is one. Note that the minimum number of times is 0. Therefore, thispart still matches if the sign symbol is absent. Right after the optional sign symbol are the digits. We
establish that an even number has the least significant digit being 0, 2, 4, 6 or 8. Therefore, on the far
right we specify it as the last digit. In between the sign symbols and the least digit there can be zero or
more digits. This is how we arrive at this pattern.
Perl defines three special symbols to represent three most commonly used quantifiers. * represents{0,}; + represents {1,} and ? represents {0,1}. Because + is a quantifier as a result, it has to beescaped in the example pattern above.

9.2.5

Character Classes

A character class includes a list of characters where matching of any of these characters result in amatch of the character class. It is similar in some sense to alternation, but the way they are interpreted
is different. A character class is constructed by placing the characters inside a pair of square brackets.
Here I demonstrate how to rewrite the pattern in the above example using character classes.

my $retval = ($string =˜ m/ˆ[+-]?[0123456789]*[02468]$/);
It’s a lot shortened. Isn’t it? All characters that appear inside the square brackets belong to one characterclass. We have also used the special quantifier symbols described above to further shorten the pattern.
But that’s not the end. You can further shorten the character class comprising all digits by specifying in
the form of a range:

my $retval = ($string =˜ m/ˆ[+-]?[0-9]*[02468]$/);

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You may define multiple ranges in a character class, for example, [a-zA-Z] matches all lowercase anduppercase forms of English alphabets.
Inside a character class, if you prefix the list of characters with ˆ, that means any characters that are notlisted results in a match. For example, [ˆ0-9] matches any character provided it is not numeric.
Perl also defines some special character classes that contain lists of common character combinations inpattern matching.
Character Class
\w
\W
\s
\S
\d
\D

Content
Alphanumeric characters and ([a-zA-Z0-9 ])Neither alphanumeric characters nor ([ˆa-zA-Z0-9 ])Whitespace characters ([ \t\n\r\f])
Non whitespace characters ([ˆ \t\n\r\f])Numeric digits ([0-9])
Non numeric digits ([ˆ0-9])
Table 9.2: Special Character Classes in Perl

Finally, our example pattern to match even integers can be simplified as

my $retval = ($string =˜ m/ˆ[+-]?\d*[02468]$/);
which is now the most compact form you can attain.

9.2.6

Backtracking

Parenthesised patterns have a useful property. When pattern matching is successful, the matching
substrings corresponding to the parenthesised parts are saved, which allow you to save them for further
operations. For example,$string = ’Telephone:1234-5678’;
if ($string =˜ m/ˆTelephone:\s*(\d{4}-\d{4})$/) {print "The telephone number extracted is ’$1’.\n";}In this example, the telephone number extracted is saved as $1. There can be multiple bracketed
patterns in a given pattern. The matched substrings are numbered in ascending order of position of the
opening parentheses. If we change the pattern as follows:$string = ’Telephone:(852) 1234-5678’;
if ($string =˜ m/ˆTelephone:\s*(\((\d+)\)\s*(\d{4}-\d{4}))$/) {print "The telephone number extracted is ’$1’.\n";print "The country code extracted is ’$2’.\n";print "The local phone number extracted is ’$3’.\n";}9.3 Regular Expression Operators

143

The telephone number extracted is ’(852) 1234-5678’.
The country code extracted is ’852’.
The local phone number extracted is ’1234-5678’.
The pattern looks more complicated then it really is. If you examine it carefully, there are threebracketed patterns in it. The first one embracing the telephone number in full, including the countrycode. The second and third are placed inside this bracket to extract the country code and local
phone number separately. The positions of the opening braces determine the ordering. Therefore,
we can observe that in case of nested parentheses, the outer one has a smaller number than the inner one.
There is a very important property with respect to quantifiers that you have to be aware of. Quantifiers
default to being “greedy”. That is, quantifiers try to match as many times as possible. For example when
you match the string “greediness 2003” against the pattern /(.*)(+. )/, $1 has the string “greediness200”, leaving “3” to $2. Therefore, you can see that quantifiers appearing on the left tries to match as
many times as possible while allowing the pattern to match. To work around this situation, you may
append a ? character to the quantifier to suppress this greedy behaviour. Therefore, for the sample
string the best pattern is /(.*?)(\d+)/, which separates all numeric digits at the end from the rest of
the string.

9.3 Regular Expression Operators
9.3.1

m// — Pattern Matching

As we have been using so far, the m// operator performs pattern matching. It supports a number of
options. We have covered m and s, and now it’s time for a revision. The s option treats the string
being searched as if it consists of a single line only. By doing so, . will match an intermediate newline
character. The m option allows matching of individual lines in a multi-line string. Here, I will introduce
several other commonly used options.
The i option matches in a case-insensitive manner. Therefore,’ABCD’ =˜ m/abc/i
results in a match. By default, pattern matching is case sensitive. Another useful option is g,which attempts to carry out a global pattern matching on the string. In scalar context, a search
pointer is maintained. The search pointer is first initialized to the beginning of the string. In eachmatching operation, matching starts from the search pointer. If a match is found, the search pointeradvances to past the end of the matched substring. If matching fails, the search pointer is reset tothe initial position. You can use the pos() function to retrieve the position of the current search pointer.
You can use this option to find out the position of occurrences of certain patterns in the string. The
following example illustrates this point:$string = ’Telephone: 1234-5678’;
while ($string =˜ m/(\d{4})/g) {print "’$1’ found at position " . (pos($string) - length($1)) . ".\n";}

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Chapter 9 Regular Expressions’1234’ found at position 11.
’5678’ found at position 16.
In list context, the m//g operator (with g) returns a list consisting of all parenthesised substrings fromthe matching. Therefore,

my $string = ’Telephone: (852) 1234-5678’;
my @list = ($string =˜ m/ˆTelephone:\s*(\((\d+)\)\s*(\d{4}-\d{4}))$/g);# @list = (’(852) 1234-5678’, ’852’, ’1234-5678’)
results in the list

@list = (’(852) 1234-5678’, ’852’, ’1234-5678’)

9.3.2

s/// — Search and Replace

This operator is a powerful search-and-replace engine that you can use to flexibly search for certain
patterns and replace it with a replacement string. The first argument is the search pattern, just as thecase of m//. The second argument is the replacement string. As you will soon see, backtracking isimmensely useful in this regard.
The options that I mentioned above that applies to m// also apply to s///. But there is a new one. The
option e causes the replacement string to be treated as an expression instead of a double-quoted string.
That is, you can use a combination of operators to generate the desired replacement string at runtime.
Without the option g, only the first occurrence of the pattern is replaced. With the option g, alloccurrences of the pattern are replaced in one go.
Here are some examples:$string =˜ s/\t/’ ’ x 4/eg;$string =˜ s/ˆ(.*)\n$/$1/s;

9.3.3# change all tabs to 4 spaces# like chomp(), to remove trailing newline

tr/// — Global Character Transliteration

tr/// is a convenient and efficient operator that changes a set of characters into another. The firstargument is the character list to search for. The second argument is the character replacement list. It
builds a character translation map at compile time. At runtime, it changes any characters that can befound in the string into the corresponding character in the replacement list. For example,

tr/a-z/A-Z/
is just an alternative way to convert characters to uppercase form without using uc().

9.4 Constructing Complex Regular Expressions

145

9.4 Constructing Complex Regular Expressions
While the general principles of regular expressions are not complicated, constructing the correct pattern
for your specific purpose is not itself a simple affair, especially if it is a complex one. An incorrect
pattern is one which does not match a string intended to match, or match a string that should not bematched. Despite experience develops over time, following a formal, systematic procedure helps you
construct regular expressions with a higher degree of accuracy.
The mechanism is known in the academic community as finite-state automaton (FSA). The definition
is pretty abstract but the idea is rather easy to understand with the help of examples. Basically, weconstruct an FSA diagram. The diagram has a number of states, with each of the states linked together
by arrows which represent transitions. This diagram is also known as a state diagram. At any point intime an FSA system can only be in one of the states. Every input triggers a transition, which may or
may not change the state of the FSA.
As an example, we will construct a pattern which matches all relative path expressions. For example,
images/fsa.gif and docs/perltut are considered valid paths. However, we add in an additional
requirement to invalidate paths containing “.” and “..” as a path segment, because we will reuse thepattern constructed in the next chapter. We also assume each path segment consists of word characters(\w) only.
The constructed FSA diagram is shown below:

Figure 9.1: FSA to Match Valid Relative Paths

How do we interpret this diagram? We regard a path like images/fsa.gif as a sequence of characters.
We break this path into its constituent characters, and feed each of them in sequence to the FSA. In thisdiagram, each of the states is represented by an oval with the name of the state written at the centre.
The initial state is marked as “Initial”, which is the state before any characters are fed to the system.
An arrow represents transition and beside each arrow the character which triggers the transaction isindicated. We also define that the states marked in blue are valid terminate states, namely “Initial”
and “Valid”. At the end of the input stream, if the system is at a valid terminate state, then the string
matches our criteria. Now “i” is fed to the system. Because “i” is a member of \w, a transition to the“Valid” state occurred. Therefore, the state is now “Valid”. Then, the character “m” triggers a loopback
to the “Valid” state. Similar are the characters “a”, “g”, “e” and “s” that follow. The next character, “/”triggers a transition to the “Initial” state. You may follow the remaining characters in a likewise mannerand eventually, after the last character “f ” you will be at the state “Valid”. This is the terminate state,and because it is a valid terminate state, this path matches our criteria.

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Note that any input character not matching any transitions from the current state shown in the diagram
automatically triggers a transition to a hidden illegal state, which is not shown in the diagram. This isbecause we only emphasize valid transitions, not invalid ones.
This diagram was constructed based on the idea that we may break a path into its constituent pathsegments. We start at the initial state. While constructing this diagram, we have to identify that weneed to have special treatment on path segments that start with a dot. We can accept path segmentsthat start with \w. Therefore, we can immediately transit to the “Valid” state from the initial state if it isa word character. If it is a dot, then we follow another path to the “Critical” state. If the next characteris also a dot, then we transit to the “Critical2” state. Word characters appearing afterwards bring thesystem back to the “Valid” state. From the diagram, it can be seen that we allow path segments such as“...” and “..somedir”, but not “..”. Depending on the operating system, you may wish to change thisassumption if this is not acceptable to you (e.g. on Windows three consecutive dots represent two levels
up the directory tree). Note that at the end of each path segment, a “/” brings the system back to the“Initial” state. This allows paths consisting of an arbitrary number of path segments. In order to be
more flexible, the path may or may not be terminated with “/”. Therefore, the “Initial” state has to bemade a valid terminate state instead of just the “Valid” state.
With a correct FSA established, rewriting it into a pattern becomes very easy. We only have to consider
paths from the initial state that lead to the valid states. Note that in this example we may follow threepaths to reach the “Valid” state. The first path is with the input of a word character, that is \w. The
second path passes through the “Critical” state only, consisting of a dot followed by \w, and hence
the path is .\w. The third path passes both critical states, that is ..[.\w]. Therefore, expressed inregular expression format these three paths can be represented by \w|\.\w|\.\.[.\w]. At the “Valid”
state, there is a transition back to itself. That means a character of the character class [.\w] does not
trigger any state changes. This implies you can inject as many characters of this character class withoutchanging the state of the FSA. In regular expressions, this is synonymous to [.\w]*. Finally, please notethat the “Initial” state is also a valid terminate state. Therefore, “/” which transits to the “Initial” state is
optional. With all these combined, the regular expression representing a valid path segment is/(\w|\.\w|\.\.[.\w])[.\w]*\/?/
Because this represents just one path segment, finally we arrive at the following pattern to match full
relative paths:/ˆ((\w|\.\w|\.\.[.\w])[.\w]*\/?)+$/
Construction of an FSA forces you to think about the specifics of the parts that make up the pattern
carefully. For more complicated patterns, this method generally yields more accurate patterns thanpure imagination in your brain because it is easier for you to verify whether the pattern is correct with
the FSA. Usually you may not get the FSA correct at the very first time, but through corrections andrefinements you can come up with a very accurate pattern. When you are highly experienced withregular expressions, it is likely that you do not have to follow this elaborate procedure anymore.

Summary”

Pattern matching refers to the operation of comparing a string against a given pattern to determine if the string matches the pattern.

9.4 Constructing Complex Regular Expressions

147”

In Perl, a pattern is specified in the form of a regular expression. Regular expression is supported
in many programming languages and programs.

”

To perform pattern matching in Perl, a string is bound to the pattern with the binding operator=∼ or !∼.

”

Metacharacters serve special purposes in a pattern.
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\ quotes the next metacharacter so that it is interpreted literally.
ˆ refers to the beginning of a string.
$ refers to the end of a string.
. refers to any character except newline character.
| lets you specify alternative subpatterns. Matching is successful if any of the subpatterns
match.
() are used to group subpatterns. Matched subpatterns are also saved in special variables
starting from $1 for backtracking.
[] establishes a character class. It includes a list of characters where matching of any ofthese characters result in a match of the character class. If prefixed by ˆ, the character classmatches any character not appearing inside the square brackets.
* \w represents a character class containing alphanumeric characters and* \W represents [ˆ\w]* \s represents a whitespace character* \S represents any non-whitespace character, i.e. [ˆ\s]* \d represents a numeric digit* \D represents any non-numeric character{} is a quantifier which specifies how many times a certain pattern can be matched consecutively.
*
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*

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* implies {0,}, i.e. zero or more times? implies {0,1}, i.e. none or single time
+ implies {1,}, i.e. one or more times
A ? character following the above quantifiers disables greedy mode.

The m// operator performs pattern matching.
••••

The m option allows m// to match individual lines in a multi-line string. ˆ and $ match the
beginning-of-line and end-of-line respectively.
The s option causes . to match any newline characters.
the i option performs case-insensitive pattern matching.
The g option enables global pattern matching on the string.

”

The s/// operator performs search and replace, which supports the options m, s, i and g withthe same semantics as m//, the option e is supported which causes the replacement string to betreated as an expression.

”

The tr/// operator performs global character transliteration on a string.

QuestionsA.

Modify the FSA diagram to disallow path segments matching the pattern /ˆ\.+$/ and construct
the corresponding match pattern.

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B.

In Chapter 8 we developed a class FileSearch to search for a file recursively in a directory tree
whose name matching a pattern specified by the user. Now, use this class to write an application
which finds out all classes accessible by Perl and their respective locations on the filesystem. For
example, selected lines of output on my system look like this:Bundle::Apache2 (/usr/local/lib/perl/5.8.0)
Bundle::CPAN (/usr/share/perl/5.8.0)
Bundle::DBD::mysql (/usr/local/lib/perl/5.8.0)
Bundle::DBI (/usr/local/lib/perl/5.8.0)
Bundle::LWP (/usr/share/perl5)
ByteLoader (/usr/lib/perl/5.8.0)
CGI (/usr/share/perl/5.8.0)
CGI::Apache (/usr/share/perl/5.8.0)
CGI::Carp (/usr/share/perl/5.8.0)
CGI::Cookie (/usr/share/perl/5.8.0)
CGI::Pretty (/usr/share/perl/5.8.0)
CGI::Push (/usr/share/perl/5.8.0)
CGI::Switch (/usr/share/perl/5.8.0)
CPAN (/usr/share/perl/5.8.0)
Note that several versions of the same class may appear on the filesystem. The path has to reflect
the location of the version that will be loaded by Perl.

Chapter 10

Runtime Evaluation & Error Trapping
10.1 Warnings and Exceptions
Man is fallible. So are the programs written. In the course of compilation and execution of a program
various kinds of diagnostic messages are generated. You have for sure encountered a lot of these in yourlearning process. This chapter deals mainly with errors and error handling. Before proceeding to therest of the text, it is discreet for us to establish a common terminology first.
We may classify an error by the time at which it occurs. They can be either compilation errors or
runtime errors. When a Perl source file is being compiled, syntax errors are being discovered as part ofthe parsing process. Every language follows a certain set of syntax, which governs how different parts of
the language can be put together. Syntax errors are thus compilation errors. When a compilation error
occurs, the compilation process will be terminated. Errors are also generated when a source file is being
executed. We collectively refer to them as runtime errors. For example, division by zero is a runtime
error.
We may also classify an error by its severity. Some errors are severe enough that the program should
not be allowed to continue. In Perl manpages, this is customarily called fatal exceptions. For example,
division by zero is a fatal exception. There are also some kinds of diagnostic messages that are generated,
but will not terminate the program. They are called warnings. For example, trying to print() thevalue of an undefined variable if the -w switch is in effect results in a warning. Generation of warnings
may not be fatal for your program, but they usually signify there may be some subtle logical errors inyour program. You should try to find out why they occur and try to eliminate them, or at least you haveto ensure that these warnings are expected.
Note that all errors and warnings are customarily redirected to the standard error, which is the screen
by default unless you redirect it to somewhere else, as indicated in Chapter 8. For CGI scripts, thesemessages are usually recorded in Web server log files that you can examine to help locate sources oferrors.

10.2 Error-Related Functions
Perl provides you with two builtin functions to generate fatal exceptions and warnings, namely die()and warn() respectively. Both of them accept one parameter which is usually the message associatedwith the exception or warning. If multiple arguments are given, they are simply concatenated to forma single error message. If the error message is not terminated with the newline character, the file nameand line number are appended. This is to make debugging easier.
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Usually you use die() to signal occurrences of fatal errors in the sense that the program in question
should not be allowed to continue. For example, I mentioned in Chapter 8 that you should check the
return value of open() to see if file open is successful before proceeding to other file operations. Perlprogrammers customarily do this by a statement like

open FILE, "<file.txt" or die "Cannot open file: $!\n";
It uses the short-circuiting behaviour of the or operator. Only if open() fails and returns an undef
value that die() is called. $! is a predefined variable that returns the error message returned by thesystem on the error. Because failure of open() does not constitute a fatal error, for some programs
it may be deemed necessary or appropriate to manually terminate it (however, most sophisticatedprograms prefer to handle the error and skip the relevant parts of the program instead of abruptly
halting it). The die() function serves this purpose.
The warn() function generates a warning instead of a fatal exception. It merely outputs the message tothe standard error and does not terminate the program.

10.3 evalConsider one day you are asked to write a command-line application that allows users to input amathematical expression and the program will then calculate the result and display it to the user.
This seems like an easy task, but it isn’t. You may still manage it if you impose restrictions to allow
only simple expressions like 3 + 4 and 4 * 6. But what if the expression is immensely complex?
The program has to parse the expression. The expression is also checked for syntax errors andevaluated according to the operator precedence. Parsing a complicated expression is not an easy task
without specialized knowledge in compiler construction. It turns out that you may construct a Perlstatement at runtime and evaluate it on-the-fly. Therefore, what you need to do is to have the user
specify the expression according to Perl syntax, and you simply invoke the function eval() to evaluate it. This is very convenient because Perl checks the syntax and evaluate it for you, so you don’t have to.
The program can thus be as simple as this:
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# WARNING: GRAVE SECURITY HOLE!!!
print "Please enter the expression below:\n";chomp(my $expr = <STDIN>);
my $result = eval($expr);
print "The return value is $result.\n";eval() accepts a string as its argument which is parsed as a Perl program. In the above example, if theuser enters

3*(4+5)then this is evaluated, so the evaluated result, 27, is returned and assigned to $result. Note thatyou may put multiple statements in the argument to eval(), delimited by ;, to evaluate multiple
statements in one go. The final ; may be omitted, as illustrated in the example.

10.4 Backticks and system()151

In this form, both parsing and evaluation occur at runtime, because the code to be evaluated are notavailable until at runtime. eval has another form. Instead of passing to it a string containing the code,
a code block is passed embracing the code (note the final semicolon after the code block!). This form
is different from the above form in that because the code itself is static, it can be parsed at compile
time. Therefore, you cannot dynamically construct a Perl program and evaluate it with this form. For
example,eval {$result = $a / $b;};
Then you may wonder what is the use of the second form if the statement(s) inside an eval{} block areparsed statically just as if the eval{} is not in effect. In fact, both forms of eval may be used for error
trapping. In the first form, syntax errors as well as runtime errors occurred when evaluating the code
will be trapped. The error message is sent to the predefined variable $@, and eval() returns with thevalue undef. In the second form, compile-time errors cannot be trapped so they are still displayed andcompilation is halted. Runtime errors will be trapped and redirected to $@ similar to the first form.
Although the first form of eval() is very convenient because it can evaluate a string as if it is a tiny
Perl program, it is also highly dangerous if it is cleverly abused. You will see why shortly after we coverbackticks, an equally powerful and dangerous tool.

10.4 Backticks and system()
You may execute operating system commands in Perl as if you are on the command line of youroperating system. The system() function accepts a list of strings as parameters. The first argument is
the name of program, followed by the arguments. For example,
system(’notepad.exe’, ’myfile.pl’);On a Windows system, this executes the Windows Notepad and opens the file myfile.pl. The system()function doesn’t return until the program terminates.
Backticks ‘ ‘ are more flexible as it simulates the command line. What you need to do is to construct
the command and enclose it in backticks instead of single or double quotes. In US keyboards, thebacktick is located underneath the Escape key, sharing the key with the tilde (∼). An example is$output = ‘ls -l‘;# get long directory listing

Text that are sent to the standard output are collected and assigned to $output.

10.5 Why Runtime Evaluation Should Be Restricted
While eval() is an immensely powerful tool for programmers, it is also one of the most dangerous
weapon frequently taken advantage by abusers to crack the system. Sometimes it may even be possible
to launch catastrophic attacks. When combined with backticks, this is even more dangerous. Consider
this example, which we used to evaluate mathematical expressions on-the-fly:

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Chapter 10 Runtime Evaluation & Error Trapping# WARNING: GRAVE SECURITY HOLE!!!
print "Please enter the expression below:\n";chomp(my $expr = <STDIN>);
my $result = eval($expr);
print "The return value is $result.\n";What an attacker has to enter is ‘rm -fR /‘, which is the Unix command for deleting everything in
the root directory. If this script is executed by root, the hard disk will be wiped spick-and-span in a few
minutes; Otherwise, at least the home directory of the user running the program will be deleted. On
Windows, passing a rmdir command is equally devastating. Therefore, as you can see, eval() poses a
significant security threat if improperly used.

10.6 Next Generation Exception Handling10.6.1

Basic Ideas

While eval{} is the traditional method of exception handling in Perl, it suffers some grave drawbacks.
The major drawback is that with a deep call stack and the exception is caught at a deep level, you usually
have to put the error message as the return value and return() it multiple times as you exit from eachlevel in order to notify the error to a caller at the outermost level. Having to always manually check the
return value of eval{} is not convenient at all.
If you have used certain languages like Java and C++ you may know that exception handling in thoselanguages are performed differently. The mechanism is called a try/catch. While the Perl language
does not officially support this exception handling mechanism, some wise people managed to write a
Perl module that satisfactorily implemented a similar mechanism from mere Perl code. The module ison the CPAN, with the name Error. The latest version as of this writing is 0.15. It is not in the Perldistribution so you have to install it as described in Appendix B. It is a pure Perl module, so you may
easily distribute it with your Perl program if you use it in your program. It is also expected that thisexception handling mechanism, possibly with slight adjustments, will be officially included in the Perl
6 language. This will not come in the near future, though. For your convenience, I have bundled theError.pm module together with my code examples.
As of this writing, the documentation of the latest version of Error.pm can be found at
http://search.cpan.org/author/UARUN/Error-0.15/Error.pm.
To use this module, simply use() the module and import the symbol :try, i.e.

use Error ’:try’;I hereby start my discussion with the try {} block. Similar to eval {}, the code inside the curly
braces are executed. In fact, internally, try{} uses eval{} to handle the exceptions. Therefore, it iscompatible with eval{}. Let us see how we may use try {} in place of eval {} for the following
simple case:

my $retval = eval {open FILE, "<file.dat" or die "file.dat cannot be opened: $!";
while (<FILE>) {# Do something

10.6 Next Generation Exception Handling153}
close FILE;};
if (!defined $retval) {print "ERROR: $@\n";}
We may transform it into the try/catch way by:use Error ’:try’;
try {open FILE, "<file.dat" or die "file.dat cannot be opened: $!";
while (<FILE>) {# Do something}close FILE;} catch Error::Simple with {my $err = shift;print "ERROR: $err";};
Here, the try block is evaluated, and if the file file.dat cannot be found, the die() function raises
a fatal exception. try is actually a subroutine defined in the Error package. When the exception israised, an error object of class Error::Simple will be automatically created to represent the error. Thisis known as throwing the error.
After an exception is thrown, blocks after the try block are checked sequentially to search for a handler
to catch the exception. After the try block you may place some handlers to handle any exceptions
raised. A catch block is a handler which handles only a certain class of error. In the example above,
the only catch block handles only error objects whose class is or inherits Error::Simple. Because theerror object generated by die() in the try block belongs to this class, this handler is executed. The
error message is printed as a result. Note that you have to place a semicolon after the block of the lasthandler. This semicolon marks the end of the entire try/catch structure. Execution resumes afterwards.
What if a matching handler cannot be found in the current environment? The exception is passed
immediately to the parent environment, if any. The current environment terminates. If the parent
environment has a suitable handler, that handler is executed. Otherwise, the exception is passed toits parent environment etc. In other words, the exception travels up the call stack until it finds anappropriate handler, executes it and resumes execution right after the try block. Note that once a
handler is complete, other handlers that follow will be skipped. Here is an example CGI script whichconverts the file “file2html.pl” (which is the script itself) to HTML for viewing in a browser. It uses
try/catch exception handling. You may execute this program from the command line. If you would liketo view it in a browser, please consult the next chapter.
EXAMPLE 10.1

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Text File to HTML Converter#!/usr/bin/perl -w
use Error ’:try’;

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sub openFile;sub toHTML;
print "Content-Type: text/html\n\n";print toHTML(’file2html.pl’);sub openFile {my $mode = shift;
my $filename = shift;
local *FILE;
open FILE, "$mode$filename" or die "File \"$filename\" cannot be opened: $¶!";return *FILE;}# convert to HTMLsub toHTML {my $filename = shift;
my $html = "<html>\n<head><title>$filename</title></head>\n<body>\n";try {local *FILE;*FILE = openFile(’<’, $filename);$html .= "<pre>\n";
my $line = ’’;while ($line = <FILE>) {$line =∼ s/&/&amp;/g;$line =∼ s/</&lt;/g;$line =∼ s/>/&gt;/g;$line =∼ s/\t/’ ’ x 4/ge;$html .= $line;}
close FILE;$html .= "</pre>\n";} catch Error::Simple with {my $err = shift;$html .= "<h1>Error</h1><br>\n<div style=’color: #FF0000’><b>$err</b></¶div>\n";};$html .= "</body>\n</html>\n";}The script reads the content of itself by default. If you change the filename on line 10 to a non-existing
file, then an error page will be generated instead.
In the default case, the open() should be successful, and hence the exception is not thrown. In thiscase, all handlers are skipped. If the file cannot be opened, the exception is thrown on line 17. Because

10.6 Next Generation Exception Handling155

the openFile() subroutine does not have any handlers defined, the subroutine terminates and theexception is passed to the parent environment. Note that openFile() is invoked inside a try block inthe subroutine toHTML() (line 28). On receipt of an exception from openFile(), the rest of the try
block is bypassed and the handlers are searched. Here, we have a matching catch handler. Therefore, it
is executed. Note that handlers internally are also implemented by subroutines, and the error object isalways passed as the first argument of a handler. In the handler, the error message is embedded into theHTML output. Once the handler completes successfully, execution resumes on line 45.
This example demonstrates the elegance of the try/catch approach. You do not have to handle anexception until you would like to. Traditionally, to notify the caller of a subroutine that an error has
occurred inside the subroutine, the error string or error number has to be passed as the return value.
Multiplexing normal return values and error numbers merely disturbs the program logic, and this is
not convenient. It is even more problematic if you have to propagate the error number of string up thecall stack. The try/catch mechanism generates and handles errors through another path so you do nothave to mess with return values in order to perform exception handling.
If an error object is thrown and no matching handler can be found all the way up the call stack, theprogram terminates with the exception message displayed, similar to the behaviour of executing die()that is not inside any eval. In most well-written programs, this should generally not occur because you
will usually have a generic handler that is executed as the last resort. More on this later on.

10.6.2Throwing Different Kinds of Errors

The power of try/catch does not stop here. The ability to generate different kinds of exception objects
is what that makes this mechanism immensely flexible. You won’t need multiple handlers if you simply
wish to handle Error::Simple exception objects. Only one handler as illustrated in the above example
is already sufficient for this purpose.
All exception objects has to belong to a class that is a subclass of Error. In fact, Error::Simple isjust a class that is predefined to represent simple errors. You can derive your own subclass to add extra
functionality or record extra pieces of debugging information. You do not have to create an Error
subclass for each single type of error. The general rule is, if you are likely to handle some kinds of errors
differently than the others (by using different handlers), then you should create an exception class for
each.
Here is an example. Because usually users of CGI applications are not programmers, you may wish toprovide them with additional information other than the regular error messages. For example, in yourerror message you may include a suggested method to rectify the problem, tailored to the situation. I
will now create an exception class that records this piece of extra information.
EXAMPLE 10.2

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package Exception;# The following is equivalent to# use Error; @ISA = (’Error’);

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use base "Error";# Operator overloading --- so that simple error message# is displayed on string interpolation of error object
use overload (’""’ => ’stringify’);sub new {my $this = shift;
my $msg = shift;
my $soln = shift;
local $Error::Debug = 1;

# enable stacktrace# Ensure correct discovery of position where error object thrown# Needed because overriding of constructor
local $Error::Depth = $Error::Depth + 1;
$this->SUPER::new(-text => $msg, -soln => $soln);}# Show solution, if any
sub solution {my $this = shift;
return $this->{’-soln’};}1;Line 9 is an alternative way to say that this class is a subclass of Error. You can use the plain old @ISA
method. I just teach you another way to do it. Line 13 actually utilizes operator overloading, a methodof introducing new meanings to existing operators if the operands are objects. Here, this statement
actually means if you place the object variable inside a double-quoted string, the object variable will
be replaced by the return value of the method stringify(), which is also defined in Error returninga simple error message. This statement is added because we would not like to break this behaviour.
Operator overloading is not covered in this edition of the tutorial. Line 20 enables the production of
stacktrace. A stacktrace is a textual representation of the current call stack. From the stacktrace you
will find at which point in the program an exception is raised, and the hierarchy of callers to the point
in question. Programmers will find this information useful when debugging a program, but it may not
be useful in production environments at all. Because generation of backtrace is not very efficient, this isdisabled by default. in this class, we enabled it. Line 24 is placed to ensure that the point at which the
exception is thrown is resolved correctly. I am not explaining it here because it relates to some internalmechanisms that are not easy to explain with a few words. As a rule of thumb, just include it and you
are fine.
The Error class is a hash reference. Fields that are carried with the object are stored in the hash itself.
On line 26 we assign to it two fields. The -text field carries the basic error message, and -soln is thefield added by us to hold the suggested solution. We also define a solution() method to get this pieceof information from outside of the class. At last, we invoke the constructor of the superclass.
You may also wish to read the source code of Error.pm. It is not very difficult to read, and it is pretty
short. You may find out more information about the workings of the module that are absent from its

10.6 Next Generation Exception Handling157

documentation. In particular, you may read the source code of Error::Simple, which is in the same
file as Error.pm. It gives you some hints as to how to write a subclass of Error.
Now I include a modified version of the text-to-HTML converter program that uses the Exception
class we developed above. Apart from the error message, it displays the stacktrace as well as the suggested action in response to the error. A screenshot showing the generated error page is in Figure 10.1.

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use Exception ’:try’;# Forward declarations
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print "Content-Type: text/html\n\n";print toHTML(’file2html.pl’);sub openFile {my $mode = shift;
my $filename = shift;
local *FILE;
open FILE, "$mode$filename" orthrow Exception("File \"$filename\" cannot be opened: $!", FILEIO_ERR);
return *FILE;}# convert to HTMLsub toHTML {my $filename = shift;
my $html = "<html>\n<head><title>$filename</title></head>\n<body>\n";try {local *FILE;*FILE = openFile(’<’, $filename);$html .= "<pre>\n";
my $line = ’’;while ($line = <FILE>) {$line =∼ s/&/&amp;/g;$line =∼ s/</&lt;/g;$line =∼ s/>/&gt;/g;$line =∼ s/\t/’ ’ x 4/ge;$html .= $line;}
close FILE;$html .= "</pre>\n";

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} catch Exception with {my $err = shift;$html .= "<h1>Error</h1><br>\n<div style=’color: #FF0000’><b>$err</b></¶div>\n";$html .= "<h2>Suggested Action</h2><br>\n<div>" . $err->solution() . "</¶
div>\n";$html .= "<h2>Stacktrace</h2><br>\n<pre>\n" . $err->stacktrace() . "</¶
pre>\n";} catch Error::Simple with {my $err = shift;$html .= "<h1>Error</h1><br>\n<div style=’color: #FF0000’><b>$err</b></¶div>\n";
};

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Chapter 10 Runtime Evaluation & Error Trapping$html .= "</body>\n</html>\n";}Figure 10.1: Web Page Showing Generated Exception

Here, you see an example of using multiple handlers, although only the handler of Exception is usedin this example. Note that the handler of Exception can display more debugging information than
the generic one. In the screenshot we can see a message indicating the course of action that should betaken, followed by the stacktrace.
Line 21 has been changed from die() to throw(). The throw() function is used to generate a new
instance of the exception object and throw it. In this example, the package name of the exception object
is Exception and the two parameters, the basic error message and the suggested action message, areprovided, in accordance with the constructor of Exception we described earlier.
Line 10 declares a named constant. While Perl does not intrinsically has a mechanism for defining

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named constants, the constant module provides you with a nice workaround. Here, the constant
FILEIO ERR refers to the string on the far right. By using constants instead of scalar variables to store
immutable data, you can avoid inadvertently modifying their contents because attempts to modify theirvalues result in compile-time error.

10.6.3

Other Handlers

Now that you have learned the basic knowledge needed to make good use of exception handling. To
end up this topic I will introduce to you the other kinds of handlers you can place to handle exceptions.
The otherwise handler handles all kinds of exception objects. It is generally used as a generic exception
handler that acts as a last resort to catch the exception if an appropriate handler is missing. Therefore,
you should ensure that this is placed after all catch and except handlers, if any. There can be at mostone otherwise handler per try structure.
An except handler is rarely used in practice, because its functionality can largely be achieved using
catch handlers. Please read the documentation if you are interested in knowing more about it.
A finally handler is different from the other handlers in that, if one is present, the block is evaluated if the try block is executed successfully (with no exceptions thrown) or if a handler has completed.
You may throw an exception object in a handler. For example, you may have a handler in the current
environment to handle the exception. However, if the caller also has to be informed of this exception,
you may simply throw() it again in the handler after you have carried out all necessary processing. For
example,...
} catch Error::Simple with {my $err = shift;# Do some processing here w.r.t. the exception$err->throw();# let caller catch it also};

10.7 Other Methods To Catch Programming Errors
Not all errors are fatal exceptions. More often logical or semantic errors arise because of mistaken (butsyntactically acceptable) use of data values. For example, you may think a certain environment variable
contains valid data, but for unknown reasons it turns out that it is actually undef. You are likely to get
unexpected results if you keep on using this value without knowing it is undef. Certain kinds of errors
like this may be trapped by using the following methods.

10.7.1The -w Switch — Enable Warnings

You have been using this switch throughout this tutorial. By enabling this switch, warnings on certain
dubious operations are generated. Here, I list some sample situations where warnings will be generated
if warnings are enabled:

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Using the value of a variable without first giving it a value (or the value undef);?

Applying mathematical operators on an operand which is not a number (e.g. "30ab" + 4);?

Variables that are mentioned only once. Usually, this signals a typographical error. The rationaleis that in practice a variable is mentioned at least twice, that is when it is given a value (define)
and when it is used. Any variables not adhering to this define-and-use pattern are rather likely
misused;?

The use of undefined filehandles. This usually indicates you have not check the return value ofopen() to see if the open is successful;

This is only a selection of the most frequent sources of warnings. Note that this switch determines
whether warnings are generated by Perl internally. Therefore, warnings manually generated by warn()
are not suppressed in the absence of this switch.

10.7.2Banning Unsafe Constructs With strict

You can use strict to disable some unsafe constructs in Perl. It consists of three kinds of checks,
whose imported symbols are represented by refs, vars and subs, respectively. By default, if you insert
use strict; in your program, all of them are performed by default. You may selectively enable someof them but not the others by specifying the corresponding symbols after use strict. For example,

use strict ’refs’, ’subs’;# Omitting ’vars’

If the refs check is in effect, it disables the use of so-called symbolic references. While it bears thename “references”, it does not have much to do with references, or “hard references” as are called bysome people described in Chapter 6. Symbolic references are generally considered undesirable andshould be avoided. The idea is quite simple. Consider this example:$b = 10;$a = \$b;
print ${ $a }, "\n";10 is printed. Recall that anything returning a reference may be placed inside the curly braces. Thisis the basis for dereferencing. However, a symbolic reference is just a plain string which contains thename of another non-lexical variable instead of a reference. Here is an example which uses symbolicreference that is largely functionally equivalent to the above example:$b = 10;$a = "b";print ${ $a }, "\n";The difference is on line 2, that is, the value held by the lexical variable $a. The first example is a proper
reference, but the second one is not. If refs check is in effect, the second example will be flagged as
invalid and a runtime error generated, because you are trying to dereference a string instead of a real

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reference. Note that symbolic references always resolve to non-lexical variables. For instance, in theabove example $b is accessed from the symbol table. If you declare $b as lexical with my on line 1, Perlwill still resolve it from the symbol table, which is undef unless you have it defined somewhere before.
Because the use of non-lexical variables is discouraged, and you may potentially modify the value ofthe Perl predefined variables which hold important runtime data, symbolic reference should be avoided.
The subs check, despite its name, is used to trap the use of so-called barewords. As you may appreciatefrom your journey through this tutorial, Perl gives you a lot of shorthands that you may use to makeyour code more “compact”, which implies a rather informal way of expression that lets you save somekeystrokes provided you know exactly what you are doing.
A bareword is a standalone word in Perl that does not have any special meaning. For example,

my $word = something;
When parsing the assignment statement during compile time, Perl first checks if a subroutine of thename something has already been declared, because a subroutine may be invoked without having &and the parentheses. If it is not, then it is assumed to be a string with the quotes omitted. However, if

sub something {# something ....
}
my $word = something;Then, something is treated as a subroutine invocation as a subroutine of this name exists when theassignment statement is parsed. If you put the assignment statement above the subroutine declaration,
something is treated as a string because at the time of parsing the assignment statement the subroutine
is not declared yet. That is also why it is always a good idea to forward declare all subroutines early inyour program. Apart from giving you the flexibility of defining the subroutines in any order you prefer,
you also help prevent Perl from interpreting subroutine invocations as barewords.
As you can see, a bareword is always subject to two possible interpretations which you cannot explicitly
tell just by looking at the syntax. That explains why most programming languages today, especially very
structured ones like C and Java, never honour flexibility (or ambiguity) like this. You may rule out
the use of barewords that do not refer to subroutines by carrying out the subs check. There a a few
exceptions, however. Barewords on the left hand side of the => operator is always treated as a string
as I mentioned very early in the tutorial when you learned about hash initialization. Also, a bareword
appearing inside curly braces {} is also treated as a string, as in the example below:$hash{file}# $hash{’file’} recommended

The subs check marks these two exceptional cases as valid use of barewords. However, considering
the use of barewords in most cases does not contribute to a major cut to file size, and you are actually
making the code more confusing, I do not recommend the use of barewords altogether. That’s whybarewords are rarely used in this tutorial.
The vars check is likely to affect you most among the three if you enable it. If you access a scalarvariable, an array or a hash that is not lexical (declared with my) and without qualifying it with the
package name when the vars check is in effect, a compile time error is generated. For a long time, we162

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have been using the shortcut that a package variable without being qualified with the package namerefers to the current package that is in force. Therefore, that implies the package main unless you have
set up additional packages. However, when the vars check is in effect, you are forced to refer to lexicaland package variables in different ways. The following summarizes the syntax, which has already been
covered earlier in this tutorial:

package MyPackage;
my $lexical = 4;
local $MyPackage::pkgVar = 10;# instead of just $pkgVar

Special provisions have been made to allow you to refer to package variables $a and $b without the
package name, because they are used by the sort() function to refer to the two arbitrary picked items
for comparison. This also means if you would like to use this strict mode, you should refrain fromusing these two variables because they are not checked for strictness. Also note that typeglobs are
excluded from this check because they cannot be lexical anyway. The rationale behind this strictmode is to cleanly separate the syntax of variable accesses with respect to package variables and lexical
variables. Without strict mode, you are subject to a problem similar to barewords mentioned above,
illustrated below:$x = 4;sub proc1 {$x;}# package variable

my $x = 5;
sub proc2 {$x;}# lexical# package variable# lexical

print proc1, "\n", proc2, "\n";# 4, 5

A similar example has already been given in Chapter 5 to demonstrate how lexical and package variables
are resolved. Without the comments, it is not immediately obvious to distinguish the package variablefrom the lexical. Worse still, you can have a package variable and a lexical of exactly the same name inthe same scope. By replacing the above example with a strict mode friendly version the whole picture
is easier to identify:$::x = 4;
sub proc1 {$::x;}# package variable

my $x = 5;
sub proc2 {$x;}# lexical# package variable# lexical

print proc1, "\n", proc2, "\n";# 4, 5

10.7 Other Methods To Catch Programming Errors

10.7.3163

The -T Switch — Enable Taint Checking

Strictly speaking, this is a mechanism to enhance program security rather than one which helps catch
programming errors. As we have seen above, data passed into your programs from outside can posesignificant threats if they are fed to backticks or functions like eval(). Although you may honour usersof your programs to exercise self-discipline to specify valid data only, you should not carry this attitude.
As demonstrated by the Internet spam mail problem, you can hardly rely on users to use your programs
properly. A programmer who are conscious of security issues will instead regard all incoming data as
if they are malicious, carry out all necessary validations and accept them only if they are valid. This
defensive approach is the basic principle behind taint checking in Perl. Perl marks external data passed
into the program as tainted. Before you feed such kind of data to potentially unsafe functions, you arerequired to manually untaint it. This forces you to think more carefully about whether every piece of
input data is safe.
In other words, you have to untaint a piece of data if both of the following conditions are satisfied:?

it originates from outside of the program (and is thus tainted);?

it, or its derivative, is going to be involved in a potentially unsafe operation. A potentially unsafe
operation is one which may potentially change the state of the system.

Taint checking is more of a concern for CGI scripts and scripts which are executed setuid or setgid (seeAppendix D for more information about setuid and setgid on Unix systems). In fact, for Perl scripts
executed that has the setuid or setgid file permission bit set, taint mode is performed automatically andyou do not have to enable it.
Examples of tainted data include command-line arguments, data read from user input, files or received
from network functions and environment variables. Results of system calls such as readdir() andform data received in CGI applications are also considered tainted.
Examples of potentially unsafe operations are system(), backticks, eval(), opening a file withwritable privilege. Filesystem functions such as unlink() and rename() and the low-level system callsthat are not covered in this tutorial are also unsafe. You may find that all of them have one aspect incommon — they may be potentially used to alter the state of the system. Note that print() itself is notconsidered a potentially unsafe operation. If you can open() the file with writable permission, thenyou are allowed to write into it even if data written into it are tainted.
For example, in a program involving the open() function in write or append mode, if the filename
derives from external data (such as input by user) then you are required to untaint it.
The only way you may untaint a piece of tainted data is to extract from it by means of backtracking
with regular expressions. For example, the following example allows you to get the directory listing as
generated by the ls command of arbitrary directories inside the current directory, but not outside of it.
1
2
3
4
5
6
7

#!/usr/bin/perl -Tw
use strict;
my $path = $ARGV[0];if (defined $path and $path =∼ m/ˆ(((\w|\.\w|\.\.[.\w])[.\w]*\/?)+)$/) {# $1 contains the path

164
8
9
10
11
12

Chapter 10 Runtime Evaluation & Error Trapping$ENV{’PATH’} = ’’;print ‘/bin/ls -l $1‘;
} else {print "Invalid or missing relative path!\n";}A list of environment variables available and their corresponding values can be retrieved from the %ENV
hash. In this example we also demonstrated how to get the command line arguments. Command line
arguments are passed into a Perl program in the @ARGV array. Each command line argument becomes
an element of this array. For example, you may try out this program by either of these commands
below, depending on your situation:# Directory listing of "images/" subdirectory./ls.pl images/
perl -Tw ls.pl images/# Unix with executable chmod# Otherwise

We used the pattern built at the end of the previous chapter to check if the path is a valid relative
path. Note that the PATH environment variable is cleared. In taint mode, you are required to set theenvironment variable PATH in your program to a known value. This is needed if you are to invoke
operations that execute another program, such as backticks and system(). This restriction is mainly
set for applications started from the command line because PATH, just like any other environment
variables, can be set by the user who executes the program. Therefore, its content cannot be trusted.
This environment variable, as introduced in the beginning of this tutorial, is used to specify a sequenceof directories to be searched for executables, so that users do not have to always qualify the executablewith the absolute path for convenience reasons. If the attacker prepends to PATH the path of a directoryunder his or her control, the operating system will first search that directory for executables, followedby the system-defined ones. For example, if the ls program above is not qualified with an absolutepath, and a malicious version of ls is placed in that directory, it will be executed instead. Note that
this not only affects the program started from within the Perl program, it affects external applications
started by that program as well. With a PATH that is known to be valid, this attack will not be successful.
It is not necessary for PATH to be empty. For example, the following is still valid:
1
2
3
4
5
6
7
8
9
10
11
12

#!/usr/bin/perl -Tw
use strict;
my $path = $ARGV[0];if (defined $path and $path =∼ m/ˆ(((\w|\.\w|\.\.[.\w])[.\w]*\/?)+)$/) {# $1 contains the path
$ENV{’PATH’} = ’/bin’;print ‘ls -l $1‘;
} else {print "Invalid or missing relative path!\n";}
You don’t have to worry about modifying the environment variable. In fact this change is local only
to the Perl program in question and other programs that get started from there. Other parts of yoursystem is not affected.

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165

Note that if taint mode is in effect, @INC no longer has the “.” entry by default. Therefore, modules
in the current directory will not be automatically accessible. This arrangement is mainly introduced
to protect Perl scripts that are executed from the command line. In particular, if your script is written
to be executed by everyone on the system you are vulnerable to a certain attack if your script has tosource in external files while having a relative path like “.” in @INC. Say you have the following program
myprog in /usr/local/bin which can be executed by all users on your server system:#!/usr/bin/perl -w
BEGIN {
@INC = (@INC, ’../lib’);}
use MyModule;# ...

# in /usr/local/lib

A relative path is one which depends on the current directory. “..” is still a relative path because it
denotes the parent directory of the current directory. Using a relative path to locate external modules
suffers two problems. The first being the user has to manually change the directory to the directory of
the script in order to execute it, for example,
cd /usr/local/bin
before running the script. Most importantly, this leaves a backdoor for a malicious user to replace thedefinition of MyModule with whatever he or she desires. What he or she needs to do is to run thisprogram from somewhere in his or her home directory:
cd /home/cracker/bin/usr/local/bin/myprog
Because the current directory is /home/cracker/bin, perl will try to load MyModule from/home/cracker/lib. What the attacker has to do is to place his or her own version of MyModule there,
very likely containing malicious code which will be duly executed by Perl. Note that a malicious user
does not have to modify the modules placed in /usr/local/lib, which are not writable for him inorder to launch a successful attack.
Even if you prefer not to use taint mode, you are advised to replace all relative paths in @INC withabsolute paths to avoid the attack outlined in the above example. There are two possible ways to patch
against this security hole. The first way is to replace all relative paths with absolute paths. For example,
BEGIN {
$basedir = ’/usr/local’;@INC = (@INC, "$basedir/lib");}
If taint mode is on, and you need to place some modules in the same directory as the script itself, thendo not forget to append its absolute path as well. The second method is to use the chdir function tochange the current directory with respect to the script. Then it is still safe to use relative paths in the
program. For example,

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BEGIN {
chdir ’/usr/local/bin’ or die "Cannot chdir(): $!\n";@INC = (@INC, ’../lib’);}
It is possible that chdir() may fail. Therefore, to be discreet you may wish to check the return value ofchdir() (a false value if failed) and stop the program if it fails.
For CGI scripts, again, because the script is executed by the Web server and the current directory isusually set to the directory containing the script itself, it is generally safe to append relative paths to@INC in CGI scripts. It is still a good practice to use absolute paths, however.
While taint mode is a means to achieve a higher level of security, it is not the end. There are certain
kinds of attacks that taint mode cannot protect against, such as reading of privacy-sensitive files(containing password entries, for example), because reading a file does not change the state of thesystem, and hence is not considered a potentially unsafe operation. For example,#!/usr/bin/perl -Tw
my $filename = $ARGV[0];open FILE, "</usr/local/mywebapp/$filename" or die "Cannot open file!\n";While you may think the script can only access files inside /usr/local/mywebapp, what an attacker
needs to do is to pass a string containing “..”. For example, if $filename is ../../../etc/passwd
then the attacker will still be able to access /etc/passwd. On some Unix systems, especially older
installations, this file contains the encrypted passwords of users on the system. However, security-aware
personnel know that even if passwords are stored encrypted it is still possible to recover them. The
algorithm used to encrypt these passwords is well known to be easy to break.
You may prefer to hide or obfuscate your source code and believe by doing so you can achieve a higher
level of security. You don’t. Remember, security through obscurity is not an appropriate attitude towards
protecting your system. Once the secret is revealed for whatever reasons, security is gone. The mostsecure system is one which remains impenetrable even if all of its internals are presented before theattackers. This goal is frankly not easy to achieve, and requires the programmers to stay constantly
vigilant over potential ways of abuse. Your level of sophistication in security is very often proportional
to your experience in this field. You should also keep an eye on security mailing lists to keep yourself
aware of the latest kinds of vulnerabilities and attacks discovered.

Summary”

Fatal exceptions are severe errors which halt execution unless trapped with eval{}.

”Warnings are less severe but usually indicate logical errors in your programs.

”

You may raise fatal exceptions and warnings manually with the die() and warn() functions
respectively.

”eval() can be used for runtime evaluation of a given string as a Perl program.

”system() or backticks can be used to execute external programs or shell commands.

”

The Error module on the CPAN can be used for flexible and convenient exception handling.

10.7 Other Methods To Catch Programming Errors
•
•
•
•
•
•
•
•

All exception objects inherit Error.
The try block contains statements to be checked for exceptions.
The catch block is a handler which handles exceptions of a certain class.
The otherwise handler is used as a generic handler which handles any kinds of exceptions.
The finally block is executed if the corresponding try block completes successfully or if a
handler has been executed.
If there is no matching handler in the current environment, the exception traverses up thecall stack until an appropriate handler is present.
A semicolon is needed after the block of the last handler.
An exception object may be manually thrown using the throw() method.

”-w switch enables warnings on dubious operations.

”

The strict pragma disables some unsafe constructs in your program.
•••refs check disables the use of symbolic references.
subs check disables the use of barewords.
vars check requires all non-lexical variables to be qualified with package name.

”@ARGV contains the command-line arguments.

”

Taint mode serves to enhance program security.
•
•
•
•
•
•

167

Data originating from outside the program is marked as tainted.
A potentially unsafe operation may change the state of the system.
Tainted data has to be untainted before involving in a potentially unsafe operation.
The only way to untaint is by means of backtracking in pattern matching.
The “.” directory is absent in @INC in taint mode by default.
The PATH environment variable needs to be set to an untainted value before executing external programs from within your Perl program.

Web Links
10.1

perl.com: Object-Oriented Exception Handling in Perl
http://www.perl.com/pub/a/2002/11/14/exception.html

10.2

perl.com: Beginners Intro to Perl — Part 6
http://www.perl.com/pub/a/2001/01/begperl6.html

10.3

CGI/Perl Taint Mode FAQ
http://gunther.web66.com/FAQS/taintmode.html

10.4

perlsec manpage — Perl Security
http://www.perldoc.com/perl5.8.0/pod/perlsec.html

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Chapter 10 Runtime Evaluation & Error TrappingChapter 11

CGI Programming
11.1 Introduction
Up to this point we have been writing Perl scripts that are to be executed on the command line. As
I pinpointed early in this tutorial, the ability to write CGI programs with Perl is the prime motive
behind learning Perl for many people. In the following sections we will first look at what CGI is, andunderstand how it allows webmasters to create dynamic content. Towards the end, I will introduce
security issues associated with CGI scripting.

11.2 Static Content and Dynamic Content
11.2.1The Hypertext Markup Language

The earliest Web servers only serve static content. Content is served in the form of files that are
prepared and made available by the server administrator. Static content that are being served by
Web servers are primarily in the form of Web pages, or in a more technical parlance, HTML files. A
document written in the Hypertext Markup Language, or HTML, is actually a plain text documentwith extra markup added that indicates the logical structure of the document. For example, consider
the following HTML document:<html><head><title>A Sample HTML Page</title></head><body><p>This is a paragraph.</p><img src="logo.gif" width="80" height="60" alt=""><a href="http://www.cbkihong.com">This is a hyperlink</a></body></html>This is a very simple HTML document. <p>...</p> denotes the text in between is a paragraph. It isthen followed by an image of dimensions 80 × 60. At last, we insert a hyperlink that, when clicked by auser through a browser window displaying this document, will cause the browser to load the resourceat the URL “http://www.cbkihong.com” into the browser window. This is not an HTML tutorial, and if
you are not familiar with HTML you should learn it first before proceeding with this chapter. But what
I would like to demonstrate here is that a markup language serves to differentiate different elements
169

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Chapter 11 CGI Programmingpresent in a document. In this example, the browser, having received and parsed the HTML document,
finds out that the document consists of a paragraph, an image and a link. Then the browser knows
how to render the markup and therefore create the objects specified in the browser window. Because a
hyperlink has different properties and actions from an image display, the browser needs a way to figureout the kinds of objects present in the document — and that’s what HTML is for.

11.2.2

The World Wide Web

HTML documents are special as they contain hyperlinks. Hyperlinks allow readers to jump from one
document to another document with a Uniform Resource Identifier (URI). In order for a document
to be accessible on the World Wide Web, it has to be assigned an address. The URI (a more widely usedterm is Uniform Resource Locator, URL) is the address in this context. For authors, hyperlinks not
only make referencing internal or external destinations more convenient, they also bind these separate
documents together in the form of linkages. Therefore, with a single URI to an HTML document areader not only can have access to the document identified by the URI, but also resources linked to thatHTML document. Such linkages bind all the linked resources on the Internet into a virtual network,and this is the World Wide Web we are using every day.
The World Wide Web utilizes the client-server model. First we need to establish what a server and aclient is. In everyday language, a server usually refers to a mainframe or other powerful computational
devices, in contrast to personal computers. However, in Computer Science parlance, server and clientsare identified by their roles. A server refers to any entities that provides services to clients. In theclient-server model, a client first initiates a request and address it to the server. Upon receipt of therequest from the client, the server carries out any necessary actions to fulfill the request, and then returnthe results as a response to the client. Therefore, typical interactions between a client and server in theclient-server model can be visualized as in Figure 11.1.

Web
ServerClient
(1)
(2)
(3)
(4)

Figure 11.1: A simplified HTTP client-server interaction

Note that in the client-server model, server and client do not necessarily refer to any physical devices.
In previous chapters, you have learned how to construct modules that represent objects, or in theprocedural approach, represents a set of functions under the same namespace. This situation can fit
into the client-server model too. The modules can be thought of as providing services to users of thesemodules. In this case, the modules act as servers and a program that uses these modules acts as the client.
Have you ever thought about what a Web server is? In fact there is nothing mysterious or complicated.

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171

It is merely a system with a suitable Web server daemon installed. A Web server daemon is a small
program that is executed in the background that handles HTTP requests and responses. A Web browser
is actually one of the forms of a user agent. When a user enters a URI in the browser and hit the“Go” button, the browser, the user agent in this case, sends a request to the Web server concerned
encapsulating the command of getting the resource located at the requested URI. The daemon captures
this request, retrieves the specified resource if any and returns the content of which to the client as a
response. The browser receives the resource. If it is an HTML document, it parses and renders it sothat the document is eventually displayed in the browser window. Rendering refers to the process of
converting HTML into the graphical objects displayed in the browser window. The previous figureshown actually illustrates the interaction between the client requesting the HTML document and theserver processing the request. The connections in the figure are numbered such that you can moreeasily refer to the explanation below to understand what each message flow is for.
The interaction between the client and the server is one of the main concerns of this chapter. The
client and the server may run on vastly different system architectures. For instance, the Web server
may be running on Solaris while the client on Microsoft Windows 2000. However, a common protocol
defines the common language of communication between the two parties so that platform-independentinteraction is made possible. On the Internet, a single protocol is defined for the World Wide Web,
which is the Hypertext Transfer Protocol, with the more widely known abbreviation of HTTP.
Enacted by the World Wide Web Consortium, or W3C, HTTP is an open standard that can be freely
implemented on any platforms.
Let us briefly outline how documents can be made accessible on the World Wide Web. The administrator of the Web server sets aside a directory (or folder) on the server. All the documents that are to bemade accessible on the World Wide Web are placed in this directory (and subdirectories if any). When
the URI is received by the Web server, this address is mapped to a location in this directory representing
the resource to be retrieved for the client.
(1) represents the initial HTTP request to the Web server. Before that, the Web browser has to accomplish several preliminary tasks. This include transformations of the human-oriented domain name andhostname in the URI to the IP address necessary for the Internet routing system to deliver the request
to the intended Web server. After the IP address of the Web server is identified, the HTTP request is
encapsulated in a packet and delivered to the Web server. A packet is the container of the message. Notethat an HTTP request is not sent as is. It is put inside the packet as the payload, and the packet header
contains all the necessary information needed to deliver the packet to the destination. This is analogous
to a letter being placed into an envelope before it is posted. The HTTP request is similar to the letter, andthe envelope with the sender’s/recipient’s addresses is similar to the packet and its header in this analogy.
Having received (1), the Web server will retrieve the document specified. From the file extension, theWeb server recognizes the resource as an HTML document and, therefore, returns the content of theHTML document and marks it as of type “text/html”. This is exactly (2).
The client receives the packet containing the returned response. Note that a Web server may return
content of types other than HTML. For example, it may be a PDF document or simply some audio clips.
Therefore, a means have to be in place that allows the Web browser to identify the type of the returned
content. That explains why the returned content in (2) have to be marked of type “text/html”. Upon
knowing this is an HTML document, the browser would parse it and draws the Web page in accordancewith the HTML received. Recall that an HTML document may contain external references (images,
audio/video clips, external style sheets or Javascript, Java applets etc.) that have to be fetched as well inorder to display the Web page properly. In the HTML document shown earlier, the image “logo.gif ” isthe only external reference that has to be fetched. Therefore, a request for this resource is represented

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Chapter 11 CGI Programmingby (3). The Web server, on receipt of (3), would return the resource and mark it as of type “image/gif ”(4). Note that if the HTML document has multiple external references, additional connections has tobe made by the user agent to request such resources. However, usually the user agent will not fetch
such external resources one by one. Consider a Web page consisting of 30 images. It would be too time
consuming to request each resource sequentially as the network can be slow. Usually, the browser will
send multiple requests at a time to the Web server by opening multiple threads. The Web server is also
likely to be multithreaded to allow it to handle multiple incoming requests concurrently.
A Web server, in this way, serves only static content. With the same URI, anybody would be accessing
exactly the same resource at any instance. Also, visitors will not see any differences across visits unlessthe files have been physically modified. Presentation of static content is generally adequate for many
Web sites. However, in order for the World Wide Web to become an interactive media, the ability toserve dynamic content is desired. Dynamic content is usually achieved by writing scripts, especiallyserver scripts. These scripts can generate the output in real time to clients based on input from theclients and data stored in the server database. Therefore, it is possible that every visitor to a Website serving dynamic content may see different layouts and content customized according to theirpreferences. That is what that makes the World Wide Web a powerful and interesting media comparedwith conventional media.

11.3 What is CGI?
In general Computer Science terms, an interface provides a well-defined way of interaction betweena system and external entities. Recall that in the object-oriented programming paradigm, each classexposes itself to the outside through an interface consisting of methods and properties, and users of theclasses do not need to (and should not have to) know the details of the implementation and, instead,access the objects through their interfaces.
In Figure 11.1 we saw the interaction between the Web server and the client. As the resource is static(an HTML document on the filesystem), the Web server can return the specified resources if they exist.
However, if the resource specified is an executable script, the script will then need to be executed before
returning the generated response to the client.

NOTES
Note that not all Web servers have CGI script execution enabled. In fact, the administrator of
the Web server needs to explicitly enable execution of CGI scripts, set up a CGI script handler
and associate file extensions of CGI scripts permitted (e.g. .pl and .cgi) with it such that the
scripts will be executed instead of being fetched and displayed in the client’s browser window!
Execution of CGI scripts pose certain levels of security risks, and are especially dangerous
if either the scripts contain a lot of security vulnerabilities or the Web server is improperly
configured. Therefore, many Web hosting companies do not allow execution of CGI scripts
in free accounts. Some Web hosting companies set up separate filesystems for hosting CGI
scripts to prevent damages due to CGI script attacks or malfunctioned CGI scripts from
affecting the entire filesystem. Later in this chapter I will introduce some techniques to write
more secure Perl programs to be deployed as CGI scripts.

The Common Gateway Interface (CGI) specifies the mechanism through which the Web server shouldpass data pertaining to the HTTP request to the server script, thus allowing the server script to capture

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data from the client.
Compared with other protocol specifications, the specification for CGI is intriguingly simple andshort that looks more like a tutorial rather than a specification 1 . As you will see, the principle behind
the CGI is very easy to understand. Simply speaking, when the Web server receives a request for an
executable CGI program, the program is executed. If the program is an interpreted one, it needs to
invoke an appropriate interpreter to execute it (which is the perl executable for the purpose of this tutorial). The program writes to the standard output, and the content of which is then returned to the client.
In order for a CGI script to have access to certain information that are only known by the Web
server, such as the remote IP address of the client and the languages the client supports (which isimportant to certain sites which serve content with multiple language versions) that are only available
in HTTP request headers, the CGI stipulates the Web server to make available such information by
setting additional environment variables before the Web server executes the CGI program. Figure 11.2
illustrates this point.

Figure 11.2: CGI script execution

The figure shows the process of a client requesting the CGI script “index.pl”. The command displayedis the actual HTTP request being sent by the browser to the Web server, the syntax of which complies
with the HTTP specification. GET is the action that indicates the method used to send the request. We
usually use GET or POST to send HTML form data to the Web server. Following the action is the pathto the resource requested. The browser removes the domain name or IP address from the URI, as it
is not used by the Web server to get the specified resource. At the end, the HTTP version is specified,and in this diagram, HTTP/1.0. Notice the string after the question mark in the path. These are passedon the URI, and the Web server extracts the text after the question mark (if any) until whitespace isencountered. These are parameters and are made available by the script by setting the QUERY STRING
environment variable as indicated in the diagram. The diagram shows several important environment
variables that are usually set and their corresponding values. Note that some other environment
variables are set as well, but the diagram would be too large to fit in and are thus omitted.
As the script is being executed, it writes to the standard output as the CGI response. Upon completionof execution, the Web server collects the CGI response and returns them to the client, inserting any
1 Some efforts of transforming the CGI specification into a formal specification with well-defined grammar is ongoing.
Visit http://cgi-spec.golux.com for details.

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HTTP response headers necessary at the top to comply with the HTTP specifications. In the diagram,
it is assumed the content returned is an HTML document, and the type of which is indicated on thefirst line of the response. This would become part of the HTTP return header. HTTP headers are readand recognized by the browser but hidden from users. The HTTP specification stipulates a blank line
between the header and the content. Therefore, you should put a blank line after the last line of the
HTTP header, and no blank lines should be present before that.
There is a common misconception by many people, especially for those who are not familiar with the
CGI mechanism, to think Perl is CGI, or vice versa, which is highly erroneous. As I have tried to explain
previously, CGI itself is the mechanism that allows executable server programs to access information
pertaining to the HTTP request through standardized means such as environment variables, insteadof being a programming language. Therefore, it is a fallacy to use the terms “Perl” and “CGI”
interchangeably. In fact, any programming languages may support the CGI mechanism. CGI programs
are not confined to interpreted languages like Perl or Python etc., compiled languages like C/C++ may
also be used to develop CGI programs that are used on a Web server. Therefore, the term “CGI script”
is not adequately specific, although most CGI programs are written in interpreted languages, preferably
Perl. Therefore, it is discreet to refer to an executable CGI-enabled Perl script as a “CGI Perl script”.
However, for the purpose of this chapter, I would use the term “CGI script” to refer to a “CGI Perlscript”, for simplicity.

11.4 Your First CGI Program
Having understood all the necessary concepts you need to know for CGI programming, it’s time to getyour feet wet by building your first CGI script in order for you to understand how a typical CGI script
is constructed.
In this section we are going to build an HTML form that contains a textbox for the visitor to input
his/her name, and a “Submit” button. After the user has pressed the submit button on the form, a CGI
script will be invoked that prints a phrase of greeting based on the time of the server. Therefore, boththe form and the script needs to be written.
EXAMPLE 11.1

HTML Form (greeting.html)<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN""http://www.w3.org/TR/REC-html40/loose.dtd"><html><head><title>Greetings</title></head><body><form action="greeting.pl" method="post"><p style="font-weight: bold">Please enter your name:</p><input type="text" name="name" maxlength="30"><input type="submit" value="Submit"></form></body></html>11.4 Your First CGI Program

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Greeting Script (greeting.pl)
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use CGI;use CGI::Carp "fatalsToBrowser";$obj = new CGI;
$params = $obj->Vars;$visitor = $params->{’name’};@timeFields = localtime time;$hour = $timeFields[2];print "Content-Type: text/html\n\n";
if ($hour < 12) {$greeting = ’Good morning’;} else {$greeting = ’Good afternoon’;}if (!defined $visitor or $visitor !∼ /\w/) {$visitor = ’visitor’;}print qq∼<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN""http://www.w3.org/TR/REC-html40/loose.dtd"><html><head><title>Greetings</title></head><body><p>$greeting, $visitor!</p></body></html>∼;In order to execute the CGI script, you will need a Web server account that allows execution of CGI
scripts. There are a couple of free Web hosts on the Internet that lets you deploy self-written scripts. At
the time when this Tutorial goes to press, Beige Tower seems to be attractive, but it is possible for you
to find better bargains elsewhere. Such free CGI hosts are excellent places for you to become familiar
with CGI Perl programming before you are sophisticated enough to set up your own Web server for
development or get more powerful hosting package with paid hosts. If you would like to set up yourown Web server for testing, please flip to Appendix-C for installation and configuration instructions.
Upload both files to your Web server account. Put the script in the same directory as the form. Please
note that some Web servers set aside a “cgi-bin” directory inside your account where CGI scripts
are only allowed to be executed inside. For some accounts, CGI scripts may be placed and executed
anywhere in your account. This is subject to the server configuration and you should consult the systemadministrator for details. Anyway, in short, put both files in the same directory where CGI script
execution is allowed.

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Next, you will need to give your files the correct access permissions if your account is on Unix-variant
systems. Changing access permissions is commonly known as “chmod”. Most probably you would beusing FTP (File Transfer Protocol) clients to upload your files to your Web server account. You willneed to use an FTP client that supports chmod, like WS-FTP, leechFTP, SmartFTP on MS Windows
systems. If you are using Linux with X-Windows installed, most probably you may want to check out
gftp or kbear. If you have telnet/SSH access to your account or you have direct access to your Web
server filesystem, you may chmod on the command line too, but I’m not going into details here as they
are very basic skills Unix users should have already been familiar with.
The chmod values to give to each file and their corresponding verbose representation is:greeting.html: 644 (rw-r--r--)greeting.pl: 755 (rwxr-xr-x)
Now you are ready to test your script. Enter the URI of greeting.html in the address field of the Webbrowser and press Enter. Please check with your system administrator or relevant instructions from thehosting service on the URI to use. The form should be loaded. Enter your name and click “Submit”,
and a message will be displayed if there aren’t any errors.

Figure 11.3: The “greeting” script in action

Both the HTML form and the CGI script are indeed very simple. Compared with earlier scripts executedon the command line, several elements are new. First, we have used the CGI module to fetch the HTML
form data in the form of name-value pairs to the CGI script. Next, on line 13 we print a line containing the content type information. The “real” server response in HTML format is between line 25 and 34.
On line 4, we use the package CGI::Carp. By importing the “fatalsToBrowser” symbol, this package
automatically generates an error page with the error string and debugging information such as thefilename and number of the line on which the error occurs whenever a runtime error occurs. Simple

11.4 Your First CGI Program177

CGI programs usually do not perform extensive exception handling and simply use this package togenerate the error page. However, as I mentioned in the previous chapter that these error messages are
seldom useful to CGI script users who are not the developers, sophisticated scripts usually have theirown exception handling routines builtin such as what I presented in the previous chapter.
The CGI module is the preferred way for a Perl 5 CGI script to handle CGI-related operations. There
are two major operations a CGI script needs to handle in particular. It needs to check if there are any
incoming data being passed to the script. Such data are usually passed to the script as parameters on
the URI (the GET method) or as form data (the POST method). The script is then executed, andresults from execution have to be returned to the client through the Web server. The HTTP user client(presumably the Web browser) captures the response and renders it in the proper way as previously
mentioned. In this example, incoming parameters are fetched by the CGI module and returned as ahash reference ($params) by the Vars() object method. This is usually the most convenient way to get
all the incoming parameters in a single operation. The module actually parses the CGI environment
variable QUERY STRING, which can be obtained by $ENV{’QUERY STRING’}. Many Perl programmers(and some Perl 5 book authors alike) tend to write their own code of getting HTML form data. An
example is quoted below:
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if ($ENV{’REQUEST_METHOD’} eq ’POST’) {read(STDIN, $buffer, $ENV{’CONTENT_LENGTH’});} else {$buffer = $ENV{’QUERY_STRING’};}@pairs = split(/&/, $buffer);
foreach $pair (@pairs) {($name, $value) = split(/=/, $pair);$value =∼ tr/+/ /;$value =∼ s/%([a-fA-F0-9][a-fA-F0-9])/pack("C", hex($1))/eg;$FORM{$name} = $value;}
The problem is evident. It requires too much typing, and you are actually reinventing the wheel.
Because CGI is now a standard module bundled in every Perl distribution, there is hardly a reason for
not using it. Also, the code snippet above is not tolerant against malformed URIs or alternative URI
formats. For example, some scripts use the new convention of separating key-value pairs by ; insteadof the widely-accepted &. Unless you are very familiar with such alternative formats, many of which
are not well documented in standards or specifications, your code will fail with such URIs. On theother hand, the CGI module was carefully developed and has been under constant scrutiny by the Perl
community to recognize as many alternative formats as possible. As a result, it is more reliable andwell-maintained, not to count its easiness of use. It is instructive to understand how we can get the form
data from the environment variables though because in this way you would understand more about the
CGI specification, yet you are not recommended to get the form data manually in your production code.
Recall that earlier I mentioned the Web server would include the content type in the return response.
Line 12 of greeting.pl is doing exactly that. For static content, the Web server knows the type of content
being returned. However, this does not hold for CGI scripts as it is the CGI scripts that decide on thecontent to return. Therefore, the script should supply this piece of information by the Content-Type
header field. Content-Type is one of the most frequently used declaration in HTTP headers. HTTP
headers have to be printed before the real content, and an empty line should exist between the header

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Chapter 11 CGI Programminglines and the content. Web browsers look for the header-content boundary in this way so that it canhide the headers properly from users. As no more headers are specified after the content type line inthis example, the extra empty line is produced by two consecutive \n.
A CGI script does not have to generate all necessary HTTP headers because the Web server should
generate them automatically. An exception is Content-Type, for the reason that I have just explained.
Apart from defined HTTP headers your scripts may generate other custom headers you prefer.
However, because CGI scripts are usually rendered in browsers, unrecognized headers are generally
ignored, anyway.

11.5 GET vs. POST
In the previous example, we have used a form to transmit user-specified data to the server CGI script.
Note that we have used the GET method to pass the form data, as characterized by the method propertyof the form element. However, another method POST is also available. Here we shall discuss what they
are and how they differ from each other.
To start with, let’s investigate how the form data are transmitted in each case. Both methods involve
construction of a query string of the following format:
name1=value1&name2=value2& ...
Recall the HTML form listed in greeting.html. <form> ... </form> encloses the form. Inside the form,
we can find a text entry control, whose name attribute is “name”. Each control in the form should havethe name atttribute set. When data is transmitted to the server script, as there may be several pieces ofdata sent in the same form, every piece of data has to be labeled with a name so that the server script
can differentiate the values. The name is set as the name attribute in the corresponding form control. In
this example, as there is only one control, there is only one name-value pair in the form. Expressed in
the above format, that is:name=Bernard
What if the name or value contains the characters & or =? In fact, non-alphanumeric characters in thename or the value is encoded, in a manner compatible to RFC2396 for a reason that is to be explained
shortly. This document documents the format of Uniform Resource Identifiers, of which Uniform Resource Locators form a subset. The URI encoding scheme stipulates all characters except alphanumericcharacters and a few unreserved characters (section 2.3) should be encoded, and this is achieved by representing the character by the hexadecimal representation of its ASCII value preceded by the character“%”. Examples:
Bernard%20Chan
100%25
A%26B%20Associates# Bernard Chan
# 100%# A&B Associates

However, as the space character frequently occurs in data to be encoded, there exists an alternative
representation of the space character by the character “+”. Therefore, if you are parsing the form data
manually yourself (which you are discouraged to do so as explained earlier) you should ensure “%20”and “+” are both treated as the space character. This also explains why it is desirable to use CGI.pm,
as to decode properly is not a trivial affair in itself. By using CGI.pm, you don’t even have to care suchdetails — it is handled for you automatically.

11.5 GET vs. POST179

Having examined the encoding mechanism used on form data, it’s time to look at how form data issent using methods GET and POST.
In the GET method, the query string is appended to the end of the script URI, separated by the “?”character. The form data is part of the HTTP command. That explains why the URI encoding scheme
is used for the construction of the query string. An example of the exact HTTP command:
GET /temp/greeting.pl?name=Bernard+Chan HTTP/1.1
followed by a series of request HTTP headers. These headers together with the values are made available
to the CGI script by setting environment variables as well. The environment variable name is thecorresponding header field name with the prefix “HTTP ” and all hyphens replaced by underscores.
For example, the server script can retrieve the value of User-Agent in the HTTP header by querying theenvironment variable HTTP USER AGENT.
In the CGI specification, it is explicitly mentioned that the value of the QUERY STRING environment
variable be set to the query string, that is name=Bernard+Chan in this example and the CGI application
should parse this variable to retrieve the form data. Note that by using the GET method, beccause thequery string is embedded as part of the URI, it would also be displayed in the URI in the HTTP response.
On the other hand, by using the POST method the query string is not embedded in the URI. The exact
mechanism is a little bit more complicated. An example of the HTTP command is:
POST /temp/greeting.pl HTTP/1.1
again, followed by a series of HTTP request headers. However, this time we have an additional line at
the end:Content-Type:application/x-www-form-urlencoded

This indicates an HTML form is going to be sent to the server script. The browser then sends thefollowing line:
Content-Length:17This indicates that the forthcoming form data in the form of a query string is of length 17 bytes. The
server script can get the content length by querying the environment variable CONTENT LENGTH. The
query string is then sent to the server script. In the CGI mechanism, the query string is input to theserver script via the standard input (STDIN), and the content length serves as an indication of thenumber of bytes to read.
You should now be able to understand fully the code snippet for parsing form data in section 10.4.
However, as always, use the CGI module whenever possible.
So you may ask, should you use GET or POST? My suggestion is to use POST exclusively for transmission of forms. That is, if you have an HTML form that requires your visitors to fill in, use POST. Recall
that in the CGI mechanism form data received from GET are saved in the QUERY STRING environment
variable. It is known that certain shells may pose a limit to the maximum size of environment variables.
Therefore, it is possible that very long forms are truncated as a result. POST does not have this problem
because form data, having received by the Web server is directly piped to the standard input fromwhich CGI applications can read. GET is also discouraged because of security issues associated with
it. Please read Section 11.9 for security advice concerning the use of the GET form transmission method.

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However, the GET method carries a unique characteristic that makes using it unavoidable in somesituations. In fact, when you click on a hyperlink in an HTML document, you are actually using the
GET method to access it. Because the query string is directly embedded in the URI, when the URI isaccessed by your visitor, the query string is sent to the server script automatically without the need ofcreating an HTML form. This property is vital to CGI applications. Today, some Web sites no longer
use static HTML documents to serve its content, but to generate the pages dynamically using dynamic
scripting. Usually, on these sites there is a single script which is invoked and the page to view is passed
to the script as a query string. The script finds out which page to display by parsing the query string,and the corresponding page is generated and returned to the client. For example, a guestbook CGI
application may have one script file guestbook.pl, and the various functions are differentiated just by
the query string. To read the guestbook you may go through a URI like guestbook.pl?page=view,
while to sign it the URI is guestbook.pl?page=sign. Therefore, you may reach different parts of the
CGI application simply by varying the query string in the URI. This function cannot be accomplishedwith the POST method.

11.6 File Upload
An HTML form is quite powerful in the sense that it also allows Web-based upload of files to the server.
If you have used Webmail (Web-based email) services you may already have the experience of affixing
attachments when composing an email. This makes use of the file upload capabilities of HTML forms.
CGI provides certain facilities for you to write Perl CGI programs that accepts uploading of files fromHTML forms.

NOTES
Note that many Web servers are configured to disable uploading of files. This is especiallythe case for free Web hosting services. This is partly because to allow file upload exposes thesystem to certain security risks. Free Web hosting services usually have a large number ofusers, and without provisions of remuneration providers of such Web hosting services do nothave adequate incentive and staff to carry out all necessary regular security audits and bear
the potential costs of intrusions should security attacks occur. Please read section 11.9 fordetails on the security issues associated with execution of CGI scripts.

Because the details pertaining to file uploading with the HTTP protocol is rather complicated, I will not
go into details the underlying mechanism but to introduce to you the constructs used in this example.
EXAMPLE 11.2

File Upload

HTML Form (upload.html)<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"http://www.w3.org/TR/html4/loose.dtd""><html><head><title>File Upload</title><meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"></head>11.6 File Upload

181<body><form enctype="multipart/form-data" method="POST" action="upload.pl"><p><b>Please specify a file to upload</b></p><p><input type="file" name="filename" size="60"></p><input type="submit" value="Upload"></form></body></html>File upload script (upload.pl)
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use CGI;use CGI::Carp "fatalsToBrowser";my $cgiobj = new CGI;
my $fn = $cgiobj->param(’filename’);my $fh = $cgiobj->upload(’filename’);
my $rInfo = $cgiobj->uploadInfo($fn);
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# filename# filehandle for reading

print qq∼Content-Type: text/html\n\n<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"http://www.w3.org/TR/html4/loose.dtd""><html><head><title>Upload Results</title><meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"><style type="text/css">.Error, .Normal {font-size: 1.5em; font-weight: bold}.Error {color: #FF0000;}</style></head><body>∼;
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while (my $bytes = read($fh, $buffer, 1024)) {$byteCount += $bytes;
print OUTFILE $buffer;
}

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close OUTFILE;
print qq∼\t\t<p class="Normal">Upload Successful.</p>\n∼;print "\t\t<div>File size: $byteCount bytes</div>\n";print "\t\t<div>File type: $rInfo->{’Content-Type’}</div>\n";}print qq∼</body></html>∼;On line 7 and 8 we get the filename and the filehandle from the CGI object respectively. In earlier
sections I introduced to you the Vars() method to get a reference of the hash containing the form data
(field, value) pairs. However, there is also the method param() that you can use to query the value ofa particular field. On line 7 the filename is passed as the value whose field name is “filename”, as we
specified in the HTML form. The upload() method with the field name passed as parameter returns afilehandle from which you may read the uploaded content. This method returns undef if the filehandlecannot be read. While uploading a file the browser usually sends along some additional information,such as the content type, in the headers. We can use the uploadInfo() method returns a reference toa hash containing this information. You have to pass the filename as the parameter because an HTML
form may have multiple file upload fields. On line 45 we try to retrieve the content type using thismethod. The name of the header is Content-Type.
On line 27 we called the cgi error() method to determine whether any CGI errors had occurred.
CGI errors rarely occur in practice, unless, for example, file upload dies in the middle as a result of theclient stops sending data. The second condition we test is the filehandle being undef. This condition
rarely occurs, either. If either of these conditions is satisfied, we should consider the upload a failed one.
However, please note that the CGI module fails to notice if the user keys in a filename which points to a
non-existent file in the file location entry.
Line 31 extracts the filename portion from the filename passed in. Note that the filename does not takeon any definite formats, and is determined by the client browser. Some browsers send the full absolute
path to the script (e.g. on Windows it looks like C:\somedir\file.zip), while some other browsers
submit only the filename. Note that because the path, if one exists, depends on the client system,
we should treat both \ and / as pathname separators. Line 34 enables binary mode, as explained in
Chapter 8. We read the file received in 1024-byte chunks and write them to the output file.

11.7 Important HTTP Header Fields and Environment Variables
In this section, a few other important environment variables and HTTP header fields that are useful
and are made available through the CGI mechanism are outlined below.

11.7.1CGI-Related Environment Variables

REMOTE ADDR is set to the IP address of the client sending the HTTP request to the server. This is usually
the address of the client machine itself. However, presence of intermediate proxy servers between theclient and the server may result in seeing the address of the proxy server instead of the client machine
itself. This is because the proxy server, on receipt of the client HTTP request, replaces the source11.7 Important HTTP Header Fields and Environment Variables

183

address with its own before sending it to the server. Proxy servers are set for various purposes, such as
caching and imposing security control. REMOTE HOST is the fully qualified domain name (hostname
and domain name) of the machine identified by REMOTE ADDR. As the machine concerned may nothave a domain name, this variable may be NULL. You may also access this piece of information througha CGI object by invoking the remote_host() method. If REMOTE_HOST is not available the content of
REMOTE_ADDR is returned instead.
Other CGI environment variables are generally less useful and are not covered in this tutorial. You canfind out more about these environment variables from the CGI specification.

11.7.2

HTTP Header Fields

In this section I will introduce several commonly-used HTTP header fields that are useful to script
authors. For specifics of any of these fields, please consult the HTTP/1.1 specification (RFC2616).
The Accept-Charset and Accept-Language HTTP header fields are sometimes used by script authors
to decide on the language version of the content to return to the client, in case the content is available inmultiple language versions. The corresponding environment variables are HTTP_ACCEPT_CHARSET and
HTTP_ACCEPT_LANGUAGE. Note that language and character set are two separate concepts, although
they are related. A character set is the set of symbols used to encode a piece of text in the computer
system. Multiple languages may be represented by the same character set. In particular, Unicode isdesigned to be a universal character set that embraces all characters used by every language used today,
although it is not yet in widespread use because many legacy systems are not Unicode-capable. Chinese,for example, may be represented by the “Big5” or “GB2312” character sets depending on whether the
traditional or the simplified form is used. Therefore, the same language may be represented by different
character sets. On the other hand, you cannot judge the language only by examining the character set
because multiple languages may be encoded with the same character set. For example, both French andEnglish use the same character set “ISO-8859-1”. Therefore, you generally need to query both header
fields to determine the exact language version requested by the client. A user agent may supply multiple
values in each field, with decreasing order of preference. The choice of language or/and character set isusually a configurable option in most browsers.
The User-Agent field, represented by the environment variable HTTP_USER_AGENT, carries information about the user agent used that is initiating the request. This field allows script authors to guess theuser agent used and return a version of the content optimized for the user agent concerned. This is an
important piece of information for scripts in which certain features are only available to some browsers
but not the others. However, the method of encoding this field is unspecified, and each user agent isfree to encode this field in whatever way it likes. In fact, some browsers, like Opera, allow the user todecide the user agent string to send to the server. This allows such browsers to masquerade themselves
as other browsers to the server. Therefore, the value collected from this field is not reliable. If you haveaccess to the log files of a Web server you will find a lot of variations in the user agent strings collected.
You can also retrieve the value of this field by invoking the user_agent() method on the CGI object.
Some HTTP header fields may be generated by server scripts. Apart from Content-Type, the following
headers are commonly used:
The Location header can be used to redirect a user agent to another location with respect to a resource
request. For example, a script may generate the header byprint "Location: http://www.cbkihong.com/index.pl\n\n";184

Chapter 11 CGI Programmingthen the user agent will try to access the resource from the location specified. Be careful with redirection— make sure an infinite recursion will not be introduced. I have seen some possibly misconfigured
sites with a page redirecting user agents to itself, which gets the browser into an infinite loop. This adds
unnecessary load to the Web server having to entertain a swarm of consecutive incoming connections.
Some browsers are smart enough to notice this error after a few tens of attempts and bail out with anerror message. Other browsers simply keep on making connections to the server incessantly without
the user ever knowing it. Note that you don’t need to generate the Content-Type header anymore if a
Location header is present because there is no data to be returned.
The last HTTP header I am going to cover in this section deals with caches. A cache, if properly used,
may reduce the time needed by a client to retrieve a resource. A cache is a storage device that sits
between the client and the server and intercepts all traffic passing through. It works as follows. When aresource request to a remote server reaches a cache on its way, the cache first checks its internal storage
to check if the resource being requested is already inside. If so, the cache returns its copy to the client
without passing the request to the server. Otherwise, the request is passed on and the returned response
is put into the cache so that it can be used to serve subsequent requests for the same resource. Resources
are only allowed in a cache for a short period of time. When the age of a cached resource reaches
a preconfigured interval, the item is said to be expired and deleted from the cache. This keeps thecache size manageable and prevents some frequently-modified items from being stale and out-of-date.
Because caches are usually installed in close proximity to client systems, resources returned from caches
can reach the client with significantly reduced delays. This also helps reduce traffic over the Internet
backbone and lowers the probability of network congestion. A cache is usually installed at the ISP or
the corporate network of a large organization to intercept outgoing traffic transparent to client users.
However, there is one more cache that you may or may not be aware of. All mainstream browsers also
maintain an internal cache that is checked before making an outgoing connection to fetch a requested
resource. You can usually disable the cache as a browser option, yet this is customarily enabled bydefault and most people do not bother (or are not even aware of its existence) to disable it. After all, it
is useful most of the time. It is only in certain scenarios that it causes problems.
In the old days when nearly all resources on the Web are static resources like files, caching was very
effective because the resource requested by any client behind the cache must be identical. Therefore,
the cache can readily return the cached resource to every client requesting for it. However, consider adynamically generated Web page. Although the layout of the page as requested by every client looks
similar, the data on the page may differ (such as the account information in the case of an online bank).
Therefore, dynamically generated pages should by definition not be cached. However, they are sometimes being erroneously cached. There are a lot of implications about this situation. If a Web page withsensitive information about user A is used to serve a similar request from user B, you should understandhow serious the problem can be. Or at least, the resource returned is simply inaccurate or out-of-date(such as when the server generates a newer version of the page while the cache retains an older version).
To avoid going into this dreadful situation the script author usually places the header
Pragma: no-cachein the generated list of headers. Then, unless a cache purposefully ignores it, the response will not becached.

11.8 Server Side Includes
One of the major reasons why many people consider PHP more convenient than Perl is that you canembed PHP code inside parsed HTML documents. On servers that support it, there exists a feature11.8 Server Side Includes

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that is known as Server Side Includes (SSI). SSI refers to a set of directives that are placed in an HTML
document, and evaluated when the document is parsed by the Web server. The directives are replacedby the result of evaluation.
The capabilities of Server Side Includes is very limited. Among the several functionalities supported, themost commonly used feature is to embed the results of a CGI program in an HTML document. The SSI
directive used is the include directive, which looks like this:<!--#include virtual="counter.pl" -->where counter.pl is the CGI program to be executed. Note that only a file path is supported, it cannot
be a URI. The following shows an HTML page with this SSI directive and the source of the program
counter.pl:
EXAMPLE 11.3

Counters (Server Side Includes)

HTML Test Page (testpage.shtml)<html><head><title>Test Page</title></head><body><p>This is a test page.</p><hr><div style="font-size: 0.8em; font-style: italic">This page has been ¶accessed <!--#include virtual="counter.pl"--> times.</i></div></body></html>Counter script (counter.pl)
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#!/usr/bin/perl
use Fcntl ’:seek’;print "Content-Type: text/html\n\n";
my $path = "data/counter.dat";# Create if not yet exist
if (! -f $path) {open LOG, ">$path";
print LOG "0";close LOG;}
# If cannot be opened, return a ’?’ for display
if (!open(LOG, "+<$path")) {print "?";
exit 0;

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Chapter 11 CGI Programming}
chomp($num = <LOG>);truncate LOG, 0;seek(LOG, 0, SEEK_SET);
defined($num) or $num = 0;++$num;print LOG "$num";close LOG;
print $num;
Note that counter.pl is still executed as a CGI program. Therefore, the program still have to begiven an executable chmod and it should output the content type header. Whenever you load the page
testpage.shtml the counter stored in the data file will be updated and you will see at the bottom of
the page the number of views of the current page.

Figure 11.4: Counter Embedded in HTML With SSI

11.9 Security Issues
CGI and Perl together provides a great deal of flexibility and ease in developing Web-aware scripting
solutions. However, the issue of security is usually overlooked in the script development process
either because the developers are not aware of security issues associated with server-side scripting, or
in order to meet completion deadlines all necessary security audits are unfortunately bypassed. The
consequence is that a large number of functional scripts but full of security holes are being pushed
into the market every day, and these programs are introducing new security backdoors to systems on
which they are being deployed. It is easy to write a program that is functional, yet many security holes
are so subtle that they are difficult to be discovered and thus avoided. In this section, we attempt tohighlight certain varieties of attacks possible and compile a set of crude but useful guidelines that would
hopefully help you write more secure CGI scripts.

11.9 Security Issues

11.9.1

187

Why Should I Care?

Perl programs are distributed in complete source code, making it very easy for potential crackers to
study your source code, locate vulnerabilities and plan for potential exploits. On the other hand,natively compiled programs can only be examined in assembly language and it is notoriously difficult
to trace the instruction flow, not to count locating vulnerabilities precisely from them which is apractically infeasible task. Attacks on compiled programs are usually either discovered on an ad lib
manner or are attacked in general ways by, say, passing a very long sequence of characters to overflow
the input buffer (many programs written in C or C++ are vulnerable to this attack). Attacks oninterpreted languages are generally more well crafted and precise because the source code is available totarget at specific vulnerabilities.
CGI scripts are installed on Web servers and are thus open to visitors anywhere over the Internet. They
are available for attack 24 hours a day, 365 days in a year. For standalone systems or systems in an
intranet, attacks are only possible from a limited subset of users. On the Internet, attacks may originate
from anywhere in the World, hundreds of miles away.
Many CGI scripts are used in business settings or are directly involved in electronic commerce, such as
shopping cart programs. The ability to maintain a high level of security has always been paramountin business applications. That is because data handled by such applications very likely include highlyconfidential personal data of the clients. Such kinds of sensitive data include the PIN of clients, theirrecord of transactions and personal information like address or social security number etc. Failure to
do so not only invites embarrassment to the financial institution(s) concerned, such institutions may
also be held legally liable for improper handling of personal data. Occasional cases have been heard
in that the credit card information of celebrities were being captured by crackers through some illegalmeans and subsequently such data were posted to the World Wide Web, putting the subjects concernedin profound embarrassment. People are unlikely to have confidence in a company being unable to keepsuch data from being improperly manipulated. This directly leads to loss in revenue.

11.9.2

Some Forms of Attack Explained

HTML Form Tampering
In 2000, Internet Security Systems, Inc.
released a security alert on 11 shopping cart applications that were found to be vulnerable to this attack (please visit
http://www.iss.net/issEn/delivery/xforce/alertdetail.jsp?id=advise42 for the full text). This vulnerability vividly demonstrates the ignorance of the script developers concerned in security issues of CGI
programming.
Shopping cart scripts are set up by merchants on Web servers to track down items users have selected
to purchase, usually while browsing online product catalogues. When a user browses the productcatalogue and locates an item of interest, he or she clicks on a button or icon to save the productinformation, usually the product identifier and other items necessary to identify the user such as
the username, to the server database. When he or she has finished adding all the items intended topurchase, he or she checks out and the shopping cart is displayed. A shopping cart is an abstraction of
the list of items a user intends to purchase, and is private to the user concerned, in a sense similar to atypical shopping experience in modern supermarkets (dropping items into a shopping cart and checkout at the cashier). Shopping cart scripts in general implement this by querying the server databasewith the username and extracts all items to purchase specific to the user. The results are displayed in the

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form of an HTML form for the user to confirm the order and optionally enter additional information
required to process the order, such as delivery address, contact phone number, payment card details
etc. (but such kinds of information are generally saved as part of the user profile at user registration sothat such details no longer need to be specified every time he or she purchases) An HTML form that isvulnerable to this attack may look like the following:
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<html>...
<body>...
<form action="checkout.pl" method="put"><input type="hidden" name="username" value="bernardchan"></input><input type="hidden" name="total" value="560"></input><!-- A list of items to purchase, omitted here --><p>After you have confirmed your order, click on the button below.</¶p><center><input type="submit" value="Submit Order"></input></center></form></body></html>Line 6 and 7 shows two hidden fields in the HTML form. Unlike the visible form controls, such as the
text messages and the submit button, hidden fields are not rendered in a browser window. They are
typically used to hold additional data which are not bound to any visible controls but required by theserver script (the shopping cart script) to process the form input. Notice the name and value attributesof the hidden fields. The name-value pairs of hidden fields are also sent to the server script along withthat of other visible controls when the form is submitted. In this example, the shopping cart script
adds the two hidden fields dynamically in generating the HTML form to hold the username and total
amount so that on submit the shopping cart software would arrange the total amount indicated to be
charged on the customer’s credit card account.
And problem comes. Shopping carts with this vulnerability do not verify the ordering information on
submit and blindly assumes the value of the hidden fields are exactly as generated by the script itself.
Therefore, a user may give himeself or herself a discount by replacing the total amount with a smaller
one. Why is this possible? As the form is generated and sent to the user, the browser renders it in thebrowser window. However, a person with little knowledge will know one can browse the HTML sourceand even save it as a disk file. What he or she needs to do is to use a text editor to modify the value, saveit and load the modified version in the browser. A click on the submit button is all it takes to complete
this attack. Because the script does not perform the check on the total, the attack is successful. This
attack may only be discovered one day, possibly in year-end auditing, that the payment amount and theorder do not match, but that would be too late — the subject may have already closed the account andhid up that you could no longer find him or her anymore.
Why did the script developers commit this error in the first place? Possibly the culprit is “convenience”,
or “laziness”. It takes quite many steps to generate the shopping cart HTML form, and the script
developers could make their lives easier by putting all necessary details to process the order on the form
instead of having the checkout script calculate the total amount again, because the calculated total
needs to be displayed on the HTML form anyway. By means of “hidden” fields they probably thought
no one would bother to read the HTML source and discover this security hole. Most people probably

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would not bother to, but it still leaves a backdoor for abuse. As an e-commerce application I believe
having a backdoor like this is totally unacceptable, however pretty or sophisticated the application canbe.

Conclusion Do not trust anything sent over the network. Always carry out all verifications possibleat the best of your knowledge before committing anything.

Privacy Issues
While this is not directly related to Perl CGI programming in general, you should be aware that theHypertext Transfer Protocol (HTTP) that is used for accessing the World Wide Web conveys all its
messages in plaintext. Therefore, when you submit a form with your password, address or credit card
number etc. filled in these fields are all transmitted in plaintext through the Internet. The Internet is agigantic interconnected network of computers. When you send a message to a remote host, for example,
to browse a certain Web page on a remote Web server, the Internet routing system has to find a path
between you and the remote Web server. The scale of the Internet is so large that usually you have togo through many intermediate hosts on your way to the remote host. Any of these hosts is able to readthe content of the messages, or even to modify it. Under certain circumstances, other malicious hosts
may also be able to eavesdrop the traffic through these hosts. Therefore, there is no confidentiality at all.
If you Web site has to collect privacy-sensitive information from your users by means of forms,
the Web server concerned should be configured to serve these forms using the Secure Socket Layer(SSL) protocol or Transport Layer Security (TLS), a close variant of SSL. The Internet uses a layered
architecture. SSL acts between HTTP on top and TCP/IP, the Internet delivery system at the bottom.
Therefore, messages between a Web browser and the Web server are encrypted in transit and thus
malicious hosts, even intermediate hosts on the path are unable to decrypt the messages except thecommunicating parties. For example, when your browser sends a form with your personal information
which is to be sent over SSL, the HTTP message encapsulating the form data are passed to the SSL layer
which encrypts it and then passed to the TCP/IP subsystem to send it to the remote Web server. The
Web server on the remote end reverses these steps to receive the message from the TCP/IP subsystem,
decrypts it and then recover the original HTTP message. Configuration of SSL/TLS is performed at the
Web server and no modification to your CGI programs is needed.
Earlier in this chapter I mentioned the two form submission methods, namely GET and POST. In the
GET method, the form data encoded as a query string are also carried on the URI. This poses a numberof security issues if such form data contain privacy-sensitive information. First, many browsers now
cache recently visited URIs so that users can easily revisit them without having to manually bookmarkthem or otherwise save them. This causes potential privacy violations on those single-user systems (andthus no password is asked to log on the system) or misconfigured multi-user systems. For instance,
consider a typical patronage to a cybercaf é in your neighbourhood. You logged on a PC inside andconducted an online transaction to buy something from an online store. Suppose the GET method
was used and thus form data including your password in plaintext were carried on the URI, and wasthus cached by the browser. Then you logged off and left the caf é. Then another person logged on thePC, and when he or she started the browser, a URI to the online store with your password embedded
appeared in the browsing history. Does this sound scary to you? System configurations of most public
workstations generally should have this issue fixed already. However, this is not a guarantee. Anothersituation where this URI may be divulged is due to a header field in HTTP that is called “Referer”.
When a user agent, e.g. browser, sends an HTTP request message to the remote Web server to retrievea certain resource, the user agent may include in the header field “Referer” the referring URI, that is,

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the URI of the document from which the URI of this resource was obtained. For example, when you
are viewing a Web page on a certain site like http://www.somesite.com/links.html and you clickon one of the hyperlinks there to http://www.anothersite.com, for example, the URI that refers tolinks.html may be included in the HTTP header which is sent to anothersite.com. Therefore, if
the referrer URI refers to a script URI with privacy-sensitive information embedded in it they will also
be carried along with the next URI access.
You, as a CGI script and Web developer, may do your part to protect your customers by using the
POST form transmission method instead of the GET method whenever privacy-sensitive form data are
involved because form data are not carried in the URI. That does not eliminate the need for encrypted
tunnels such as SSL to keep out of prying eyes over the network, however.

eval() and Related Attacks
As we have seen in the previous chapter, eval() and backticks are very dangerous and frequently taken
advantage by attackers to execute on the Web server whatever commands they specify. As noted, enable
taint mode to check for potential insecure input data that may be passed to potentially dangerous
operations. Construct precise patterns to ensure all input data match the patterns intended. Do not use
these operations unless that is absolutely necessary. Also bear in mind that taint mode is not the end.
There are a lot of traps taint mode cannot guard against. Because all of these points have been duly
introduced in the previous chapter, I am not repeating it here again.

11.9.3

Safe CGI Scripting Guidelines

Today, new CGI scripts emerge on the market every day. However, how many of them are really secure?
A script that has a pretty interface does not imply it must be well tested for security bugs. Because mostusers of CGI scripts are not programmers, they cannot protect themselves but rely on you, the script
developers, to safeguard your programs against abuse. Therefore, you have an irrefutable obligation toensure your programs are as secure as possible.
Below I put together a set of guidelines that you can check against your programs to evaluate if they
are secure. Some of the points have been discussed earlier in the tutorial but being repeated here as asummary. These guidelines are not exhaustive, however. Despite the section heading, these guidelines
pertain to any kinds of programs receiving input from outside of the program. Discreet security
personnel are well-educated about these guidelines. So should you.

?

Refrain from eval(), backticks and system() whenever possible. If you do use it, check its
content. Ensure the content is exactly in the form expected and enable taint mode.

?

For incoming data, it is generally more secure to define a pattern that describes what is allowed
than what is disallowed. For example, to check if an email address is syntactically valid you createa pattern which describes the syntax of a valid email address, for example /ˆ\w+@(\w+\.?)+$/.
With this deny-everything-by-default arrangement, an email address that matches this patternmust be a syntactically valid one. This is preferable to a pattern which enumerates the invalidcharacters and test whether the string contains these invalid characters, because given the large
number of possible characters you may not be able to enumerate them.

?

Do not be more permissive than necessary. That is, do not set a file permission value to morethan what you need to get your program running. More permissions imply higher risk.

11.9 Security Issues?

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Implement rings of security where applicable. A single line of defense is generally weak because
if this line of defense is broken, intrusion will be successful. In security analysis, we try to locate
all possible attack paths in the program. An attack path describes a sequence of steps an intruder
may take to launch a successful attack. To implement rings of security implies we should try
to establish multiple lines of defense on these paths — even if a line of defense can be broken
into, there is still another barrier that deters attackers from proceeding. The more barriers tosurmount, the less likely an intrusion attempt will be successful and, very likely, the attacker will
give up and turn to another more vulnerable system or program instead.

Summary”

A document written in HTML is a plain text document with extra markup added that indicatesthe logical structure of the document.

”

The World Wide Web is a content presentation and distribution platform built on top of HTTP
user agents and Web servers.

”

HTTP is the protocol used for World Wide Web access. A protocol defines a common languageof communication between parties.

”

A Web server serves static content by retrieving resources from the filesystem in the form of files
and return them to the client side.

”

A Web server may also serve dynamic content by executing some programs on the server andreturn the output generated to the client side.

”

A Web browser is just an HTTP user agent which initiates HTTP requests, and captures responses
from Web servers and renders them to produce the graphical display as Web pages.

”

CGI, the Common Gateway Interface, defines the method of interaction between the Web server
and an executable server program.

”

CGI stipulates that Web servers should pass certain information pertaining to the HTTP request
received from the client side to server executables by means of environment variables. Generated
output are collected by the Web server and returned to the client side, supplementing any missing
HTTP headers as necessary.

”

The CGI module may be used for a wide variety of CGI-related operations, including form-based
file uploading.

”

The Vars() object method of CGI returns a reference to a hash containing incoming form data.

”

A Perl CGI program is required to generate the Content-Type HTTP header indicating the kindof data returned to the client side.

”

In the GET form transmission mechanism, form data are appended to the URI as parameters. In
the POST mechanism, on the other hand, form data are sent independent of the URI and CGI
programs may obtain them by reading from the standard input.

”

The GET mechanism is not recommended because of limitations imposed by the shell and potential privacy breaches through carrying privacy-sensitive data over to third-party servers by meansof the Referer HTTP header.

”

In both mechanisms, form data are transformed into a query string with special characters encoded before being sent.

”

Server Side Includes is a mechanism that you may use to embed the generated output of a CGI
program in a server-parsed HTML document. Usually, server-parsed HTML documents take on
the extension .shtml instead of .html.

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Server CGI scripting may easily expose a server to security breaches if CGI scripts are not written
in a careful manner.

”

Do not trust anything sent over the network, including form data received from the client. Carryout all verifications at the best of your knowledge before accepting them.

”

HTTP messages are sent in plain text. Security-aware Web applications should serve their content
over encrypted tunnels such as SSL or TLS to protect their users.

”

Refrain from runtime evaluation and system shell access for CGI programs if possible. Otherwise,
enable taint mode and verify input data to ensure they are safe.

QuestionsA.

We have seen how a shopping cart application with the form tampering vulnerability could beabused to give adversaries discounts on items paid for. In the make-up HTML form example
shown, identify another attack that may be performed and devise a mechanism to defend this
attack. Assume on submit that the value received through the “total” field is charged on the user’s
account whose name is “username” immediately without performing any verifications.

Appendix A

How A Hash Works
A.1

Program Listing of Example Implementation

This appendix is a general, language-independent overview of how a hash table (or simply a hash)works. I will present the complete source code of a Perl class module Hash, which is a hash implemented
in Perl. All basic hash operations have been built into the module. After you have learnt how a hashtable works, you can then implement one likewise in another programming language, in case one
is not available. This implementation is written in an object-oriented style so that it can be reusedand extended very easily. Understanding of the source program shown below requires familiarity ofobject-oriented programming in Perl. Therefore, you should be able to understand the whole of thesource code if you have read through this tutorial and, as a result, I will not attempt to describe theprogramming constructs in detail in the text below.
Please note that the given Hash class is intended for illustration purpose only, and is not optimized in any
way for performance or utilization. Therefore, you are not advised to use it in production environments.
EXAMPLE A.1

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package Hash;# Hash.pm# An example demonstrating how a hash works# This hash is not designed for production use and is not# optimized. The Perl builtin hash should be used instead.
# Number of slots should preferably be prime.
use strict;
sub new {my $arg0 = shift;
my $cls = ref($arg0) || $arg0;
my $this = bless {}, $cls;$this->initialize(@_);
return $this;
}

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Appendix A How A Hash Works# Store property values
sub initialize {my $this = shift;
my %params = @_;foreach (keys %params) {$this->{$_} = $params{$_};}# Hash defaults!exists $this->{’NUMSLOTS’} and $this->{’NUMSLOTS’} = 17;}# Add a (key, value) pair to the hash
# i.e. $hash{$key} = $value;
sub add {my $this = shift;my ($key, $value) = @_;# First call the hash function to find where it should go
my $slot = $this->hashFunction($key);if (my $cur = $this->search($key)) {# The specified key exists, replace current# value with the new one$cur->{’value’} = $value;} else {# Prepend to the chain
my $next = $this->{’slots’}[$slot];$this->{’slots’}[$slot] = {’key’ => $key,’value’ => $value,’next’ => $next,’prev’ => undef,# must be head};$next->{’prev’} = $this->{’slots’}[$slot];}}# Remove a (key, value) pair from the hash
# i.e. delete $hash{$key}
sub remove {my $this = shift;
my $key = shift;# First call the hash function to find where it should go
my $slot = $this->hashFunction($key);if (my $toDelete = $this->search($key)) {my $prev = $toDelete->{’prev’};my $newNext = $toDelete->{’next’};
if (defined $prev) {$prev->{’next’} = $newNext;

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} else {$this->{’slots’}[$slot] = $newNext;}$newNext->{’prev’} = $prev;}}# Search for a specific key in the hash; Internal use only
sub search {my $this = shift;
my $key = shift;# First call the hash function to find where it should go
my $slot = $this->hashFunction($key);
my $count = 1;
my $totalNum = $this->getAllKeys();
my $ptr = $this->{’slots’}[$slot];
while (defined $ptr) {if ($ptr->{’key’} eq $key) {$this->{’DEBUG’} and print STDERR "Number of comparisons: $count/¶$totalNum.\n";return $ptr;} else {# advance$ptr = $ptr->{’next’};++$count;}}$this->{’DEBUG’} and print STDERR "Number of comparisons: $count/$totalNum.\¶n";return undef;

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}
# Get value for a specific key# i.e. $hash{$key}
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Appendix A How A Hash Works# i.e. exists $hash{$key}
sub exists {my $this = shift;
my $key = shift;
return ($this->search($key)?1:undef);}# ======================================================================# These three functions let you get a list of keys, values or both# keys %hash
sub getAllKeys {traverse($_[0], ’key’);}# values %hash
sub getAllValues {traverse($_[0], ’value’);}# @list = %hash
sub getAllKeyValuePairs {traverse($_[0], ’value’, ’key’);}# Traversing the hash returning values of certain fields# Internal Use only
sub traverse {my $this = shift;
my @fields = @_;my @retval = ();foreach my $slot (0..$this->{’NUMSLOTS’}-1) {my $ptr = $this->{’slots’}[$slot];
my @tmpList = ();while (defined $ptr) {push @tmpList, @$ptr{@fields};$ptr = $ptr->{’next’};}
push @retval, reverse(@tmpList);}
return @retval;}# Hash function# Generates mapping (Key -> hash slot index)
sub hashFunction {my $this = shift;
my $key = shift;
my $tmp = 0;

A.1 Program Listing of Example Implementation
172
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for(my $i = 0; $i<length($key); $i++) {$tmp += (ord(substr($key, $i, 1)) * ($i*2+11));}return $tmp % $this->{’NUMSLOTS’};}# Total number of slots in this hashsub getTotalSlots {my $this = shift;
return $this->{’NUMSLOTS’};}# Total number of OCCUPIED slots in this hashsub getOccupiedSlots {my $this = shift;
my $count = 0;
for (0..$this->{’NUMSLOTS’}-1) {$this->{’slots’}[$_] and ++$count;}return $count;}# Return a similar string as# scalar %hash
sub getUtilization {my $this = shift;
return $this->getOccupiedSlots() . ’/’ . $this->getTotalSlots();}
1;
In Chapter 3 I tried to compare the efficiency of searching for an item in an array by various methods,
namely linear search and binary search. I mentioned that although you may search for an item in anarray, you should not try to do so because neither method is efficient when the population (numberof items) becomes large. I purported that searching in a hash is more efficient compared with anarray. However, I could not give you any evidence for it because you cannot look at the internalsof Perl builtin hashes to examine the process of searching. Now this self-written implementationlets you examine the process in more detail. By putting the module in “debug” mode, some extra
information reflecting the internal state will be sent to the standard error. In this implementation, when the search() internal method is invoked, the number of comparisons will be output to
let you see how many comparisons is needed to arrive at the conclusion of whether a hit or a miss occurs.
This example program uses the self-implemented hash. A hash is created and populated with 100 (maybe less, if collision occurs, i.e. generation of the same value multiple times) integers and the user inputs
a number to check if it appears in the generated list of integers:

1
2
3
4
5

#!/usr/bin/perl -w# Search for an element in a hashuse Hash;

198
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34

Appendix A How A Hash Works$h = new Hash(’NUMSLOTS’ => ’19’, ’DEBUG’ => ’1’);# Generating 100 integers$NUM = 100;$MAXINT = 5000;
srand();
print "Numbers Generated:\n(";for $i (1 .. $NUM) {$valueToInsert = sprintf("%d", rand(1) * $MAXINT);$h->add($valueToInsert, 0); # in fact, any values can be assigned here
print $valueToInsert;
print ", " unless ($i == $NUM);}
print ")\n\n";print "Slot utilization: ", $h->getUtilization(), "\n\n";print "Please enter the number to search for >> ";chomp($toSearch = <STDIN>);# Hash search here
if ($h->exists($toSearch)) {print "\"$toSearch\" found!\n";} else {print "\"$toSearch\" not found!\n";}

A.2

Overview

A hash table is actually composed of arrays which possess additional instruments to ensure efficientindexing of data. A hash table has three fundamental operations — insertion, searching and deletion.
They can all be performed very efficiently regardless of the population.
Conceptually, a hash table consists of a number of slots. Each hash table is associated with a hashfunction, which transforms the key into a slot index.
To insert a key-value pair into the hash, a slot index is computed from the key, using the hash function.
In this way, the key will be assigned to one of the slots. The key-value pair will be appended to the slot.
It is possible that multiple keys will be mapped to the same slot index. We describe this situation asa collision. This must occur if the population is larger than the number of slots. However, because of
the characteristics of the hash function, it is possible that collision occurs even if the number of slots islarger than the population. Several collision resolution mechanisms exist that may be used to address
this problem. The method that is described in this section is known as chaining. That is, items that aremapped to the same slot are linked together, like a chain.
To locate an item with a particular key in the hash, we essentially repeat the procedure by computingA.3 Principles

199

the slot index. Then, we perform a linear search on the chain attached to the slot. If an item with thiskey exists in the hash, it must exist in this chain. If we traverse the chain and cannot find a matching
key, then we conclude the key is not in the hash. Therefore, with the help of a slot index we reduce thetraversal from all items to just items whose key maps to a single slot index. That truly explains why
searching is very efficient for hashes.
Other hash operations have to depend on searching. For example, to delete a hash element simply
compute the slot index and search for the key in the chain corresponding to the slot index. If one isfound, remove the item and fix the linkages in the broken chain.

A.3

Principles

Let the number of slots in a hash be m and the population be n. Therefore, in the ideal case when the
population is evenly distributed over all available slots, the average number of items on each chain(load factor) is thus n/m, assuming n ≥ m. If n < m, then in the ideal case all m items are distributed
over n of m slots, so some slots are empty while the others have the maximum chain length of 1.
Efficiency of hash access is determined by how well (uniformly) the items are distributed over theavailable slots. Therefore, the hash function used exerts a direct impact. Construction of good hashfunctions is a topic under constant research. However, how well a hash function performs is not only
determined by the hash function itself, but also by the nature of the keys, which depends on the domain
in which the hash is to be used. It is probable that a hash function that performs well in one scenario
does not perform well in another scenario. Therefore, a perfect hash function can hardly be achieved inpractice if the application domain is not known in advance.
An optimal choice of the number of slots is also important. For hash item access, once the slot index iscomputed, a linear search for the key on the chain attached to the slot has to be performed. Therefore,
the shorter is the chain, the more efficient is the hash access. Again, the optimal number of slots also
depends on the application domain.

A.4

Notes on Implementation

In the sample implementation, the default number of slots is 17, but you may override it by specifying
the NUMSLOTS key in the class constructor. The Perl builtin hash is different from this implementationin that the implementation provided by Perl is a dynamic hash table. That means, the number ofslots is not fixed throughout its lifetime. A dynamic hash table is able to increase the number of slots
as items are constantly added to the hash. This keeps the chains short on average so that access is still
efficient when the population grows large. However, because the principles of dynamic hash tables iscomplicated, you should seek external documentation for more information.
The chains in my implementation are constructed in the form of linked lists. This is usually thepreferred way of implementing a chain in programming languages supporting some sort of references.
However, you may also use an array to represent it in case references are not supported in certain
programming languages.

200

Appendix A How A Hash Works

Web Links
A.1

perl.com: How Hashes Really Work
http://www.perl.com/pub/a/2002/10/01/hashes.html

Appendix B

Administration
In this appendix, you would learn how to:?

Install a Perl Module from the CPAN

B.1 CPAN
Perl has a very active user community. This is evinced by the gigantic list of modules available on theComprehensive Perl Archive Network (CPAN). CPAN is the central warehouse where you can searchfor existing Perl modules other Perl programmers have contributed to the community. You will be
surprised that the CPAN contains modules of virtually any category you can think of. By using existing
modules on the CPAN you can enforce code reuse and cut down both development time and cost by
not reinventing the wheel. Even if you are not writing programs in Perl, for example, when you are asystem administrator at a web hosting company offering Perl-enabled hosting packages, you still needto know how to install Perl modules your users may need.

B.1.1

Accessing the Module Database on the Web

You can browse the module list at http://www.cpan.org/modules/01modules.index.html. However,in my opinion the best way to locate modules is to use the module search engine. There are several
engines available on CPAN, and I generally use http://search.cpan.org because the interface is more
neatly, if you know part of the module name already. If you don’t know the name of a module andwould like to search through the module description, then http://kobesearch.cpan.org would be more
useful to you, and it is actually more powerful. You may download the source packages and read the
documentation online. If you intend to install the modules in the traditional way (but manually), you
may also download the source packages there.

B.1.2

Package Managers

Perl Package Manager
As most Windows systems are not equipped with a suitable compiler suite (for example, Microsoft
Visual C++), and some Perl modules contain portions written in C for performance improvements(see below), Perl modules are usually distributed in the form of packages. In case a module has to becompiled before use, it is compiled before being put into the package. The packages are constructedby volunteers who have the compiler to compile the modules, and the packages generated are thencontributed to the community. Therefore, from a user’s point of view this is very convenient as all he
201

202

Appendix B Administrationor she needs to do to install a module is to fetch the package and install it, and it’s then ready for use.
Activestate Perl (most likely on the Windows platform) distribution comes with a package manager
PPM that I found to be quite convenient to work with. Through this package manager, packages can be
automatically fetched from a remote server and installed. It also includes tools to keep your modules
up to date.
To start PPM, select “Perl Package Manager” from the “ActiveState ActivePerl 5.8” program groupon the Start menu. You would see a prompt ppm> which gives you a command-line interface to typeyour maintenance commands. Don’t be frightened by a command-line interface in case it looks
awkward to you (this is a GUI age, after all). It is very easy to use and you can always access help information by typing help alone. To get help information on module upgrades, type help upgrade etc.
To start with, you would first try to search for modules in the module repository. Try to type in search
Crypt to see a list of cryptographic modules available. By default, PPM would searches for modules byname only. You can use logical operators and, or and not in the query string. To match both the module
name and description, use the command search Crypt or ABSTRACT=’Crypt’. To install a package,
note the package name and use the command install, for example install Crypt-TripleDES.
The latest stable version of the Crypt::TripleDES module is automatically downloaded andinstalled. To keep your modules constantly up to date, type upgrade * -install to upgrade any
packages that are out of date to the latest version. You can browse a list of packages installed by query *.
This section is meant to give you an overview of the most commonly performed operations with PPM.
Please read the documentation bundled with ActiveState ActivePerl for further information. However,
because not all modules are already packaged for PPM, you may not be able to use this method forsome modules.

B.1.3

Installing Modules using CPAN.pm

If your Perl distribution does not come with a package manager like PPM, as is the case for most Unix
variants, there is still an easy way to compile and install modules on the CPAN without resorting to
the traditional, but manual method (to be described next). It is to use the CPAN.pm module that isbundled with your Perl distribution (should be, but please check it for sure). This module gives you aconvenient way to automate the regular extract-configure-compile-install steps. It does not give you
as many features as PPM, but it is more than adequate if you would just like to install a module in aneasy way. However, a new module called CPANPLUS.pm is going to replace CPAN.pm in future Perlreleases, offering a few more features. You may wish to use it instead of CPAN.pm. However, as of thiswriting it is not bundled in perl so you have to install it separately. The instructions below apply to
CPANPLUS as well, but please replace CPAN with CPANPLUS.
You can use CPAN.pm in two ways. If you have tried PPM above, you can also have a shell-likecommand line interface where you can type the maintenance commands. Alternatively, to quickly
install a module you may not wish to go into the CPAN.pm shell and you can simply type it on
your system command line. Both methods are covered below. To start the CPAN.pm shell, type
perl -MCPAN -e shell on the command prompt. Note that very likely you need to be the systemadministrator (root) in order to start the CPAN shell.
cbkihong:∼# perl -MCPAN -e shell
cpan shell -- CPAN exploration and modules installation (v1.76)

B.1 CPAN

203

ReadLine support enabledcpan> hDisplay Informationcommand argument
a,b,d,m WORD or /REGEXP/i
WORD or /REGEXP/
r
NONE
ls
AUTHOR

description
about authors, bundles, distributions, modules
about anything of above
reinstall recommendations
about files in the author’s directory

Download, Test, Make, Install...
getdownload
makemake (implies get)test
MODULES,
make test (implies make)install
DISTS, BUNDLES
make install (implies test)
clean
make clean
lookopen subshell in these dists’ directories
readmedisplay these dists’ README files
Other
h,?
o conf [opt]reload cpan
autobundle

display this menu
set and query options
load CPAN.pm again
Snapshot! perl-codeqreload index
force cmd

eval a perl command
quit the cpan shellload newer indicesunconditionally do cmd

To install a module, for example, Crypt::DES, you can use the command install Crypt::DES. The
module would be downloaded from a local package repository, extracted, compiled, tested and finallyinstalled on your system. Meanwhile you would see a lot of messages printed on the screen. Errors,
if any, will also be printed. Therefore, if the process suddenly stops in the middle and there are someerrors printed, you should try your best to locate the source of the error. Usually, it may be that you
have certain prerequisites not met yet. You should look at the documentation that comes with a module
if problems arise to see if they are addressed in there. If you don’t want to use the CPAN shell, you canspecify the command on the command line, for example, perl -MCPAN -e install Crypt::DES.
Apart from modules, you can install bundles through the CPAN module. A bundle is a set of
related modules that are packaged together. Usually a module requires a set of other modules, andin this case, the packager may prefer to package all of them as a bundle. An example of a bundle isBundle::DBD::mysql, which includes the modules DBI, DBD::mysql, Data::ShowTable and MySQL.
Each bundle has a bundle file which lists the modules covered. The autobundle command of your
CPAN shell lets you create a bundle file which lists the modules that are currently installed on yoursystem. For example, if you are a system administrator for a Web host you can maintain a current listof modules on a single system and have the other systems share the bundle file to ensure that all servers
are installed the same versions of Perl modules. For more information, please see the CPAN manpage.

B.1.4

Installing Modules — The Traditional Way

The traditional way is to download the compressed source package from the CPAN as mentioned
above, and extract it. This method should in general only be used if the CPAN.pm module isn’t orcannot be set up properly. The packages are compressed with gzip, a popular compressing tool on

204

Appendix B Administration

the Unix platform. You can extract the package by the command tar xvzf package.tar.gz . If
your version of tar(1) does not support the ‘z’ switch, you will need to try zcat package.tar.gz| tar xvf -. On Windows you can decompress gzipped tarballs with software like Winzip orPowerArchiver. After the package is extracted, change to the directory containing the extracted sourceson the command line using the cd command. Now the Makefile needs to be created. Run the Makefile
generation program by perl Makefile.PL. The program detects the settings of your Perl installationand creates the site-dependent Makefile (that explains why the Makefile is not included in the sources).
To execute the Makefile, you need a make tool which coordinates the whole compilation session. On
most Unix systems you should have a version of make already available. If you are on Windows, youneed to have Visual C++ installed for compilation. nmake is included in the Visual C++ installation.
Because nmake may not be on your PATH, you may need to use the Visual Studio command prompt
which adds the necessary paths to the PATH so that the necessary tools can be invoked without
specifying path. On Unix, type make; while on Windows nmake. The files would then be compiled.
Then type make install or nmake install to copy the necessary files to the correct location. Notethat modules installed in this way are not recognized by any package managers.
Although Perl source files are generally portable, some modules have to make use of the XS mechanism
to delegate part of the program in C for performance considerations, or when they need to interface
with the system native libraries in order to function. That’s why compilation is sometimes needed.
If compilation is needed you need to have a working installation of the C compiler available. For
ActiveState Perl the compiler required is cl.exe of Visual C++, and gcc on Unix platforms. If
you don’t have a compiler, you can still install modules which do not use the XS mechanism (thatis, there are no .xs files in the bundle). On Windows, nmake is free and you can download fromftp://ftp.microsoft.com/Softlib/MSLFILES/nmake15.exe which is a self-extracted executable.

Web Links
B.1

perlmodinstall manpage — Installing CPAN modules
http://www.perldoc.com/perl5.8.0/pod/perlmodinstall.html

Appendix C

Setting Up A Web Server
You have learned how to develop CGI scripts with Perl. It is not very practical to test CGI scripts with
the Perl interpreter alone, because you cannot easily reproduce an environment resembling a real Web
server. There are several Perl-enabled free web hosting services on the Internet. You may wish to test
your scripts there, or on a paid web hosting account that you own. However, that is still inconvenient
having to upload and test every time you would like to test your scripts. Also, using a third party server
for script testing is dangerous. First, if your script goes astray and locks itself in an infinite loop (this is
far more easier to occur than you think), it would become a never-ending process that sits there wasting
CPU time and system resources. Every invocation of the script concerned increases one faulty process.
It does not require many clicks before the server would eventually be brought down. Although you may
start to wonder your script has gone into an infinite loop, you cannot do anything because the processes
are executed as a system user (on a Unix system, quite likely it’s a pseudo user “nobody”) instead of
you and, even if you have shell access to the Web server, you don’t have the privilege to terminate thesefaulty processes. Helplessly, a few hours you receive a furious letter from the Web host administrator
about locking up your account or so. It’s not a story. This was a pathetic experience of mine in myearly years of Perl script development. Also, don’t forget free web hosts usually don’t give you access toerror logs, so you would get no clue why your scripts do not function as expected. A testing Web servercomes to rescue. In fact, you can set up an entirely free Web server in less than half an hour. In thisappendix, I would give you some instructions that guide you through setting up a basic Perl-enabled
Web server that you can use to test your CGI scripts offline. The instructions were tested on my system,
namely Windows 2000 Professional and Debian Linux 3.0 (woody).

C.1

Apache

You can download the latest version of Apache Web server from Apache. For your convenience, the linkto the latest version as of this writing is provided below:
Apache 2.0.47 (Windows binary)
Apache 2.0.47 (Unix source)

C.1.1

Microsoft Windows

Note that the installation file uses the Microsoft Installer for installation. If your Windows version
is ME or 2000, you are ready to execute the installation program. If you use Windows XP, you needto install Windows XP Service Pack 1 first if you haven’t installed it yet. If you have older versionsof Windows, you may need to install MSI Installer. Double click on the .msi file just downloaded. If
that doesn’t work, you don’t have MSI installer set up yet and need to install it first. Please search the
205

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Appendix C Setting Up A Web Server

Microsoft Web site for the download link.
You should be logged in as a user whose privilege allows installation of software, say administrator.
The installation program should start. You should disable any firewall software on your system before
proceeding with the installation. Also, if you have any other Web servers running which binds to port
80 you should disable them first. On Windows 2000, go to the Services window. Right click over theentry “World Wide Web Publishing Service” and select “Stop”.
After you have agreed to the terms of the license agreement and press “next”, specify the network
domain, server name and administrator’s email address. If a domain name is available on your network,
enter it into “network domain”. Otherwise, put localhost for both “network domain” and “servername” like I do. You can change these settings later on in the Apache configuration file manually, so thesettings that I specify in the screenshot is generally adequate.

Figure C.1: Apache Installation Options

Then select installation type. Choose “Typical” here which is generally sufficient, unless you would
like to compile modules on your own later on (quite rare on a Windows system). Then choose theinstallation folder. The default value is usually acceptable. Installation then begins. When installation iscompleted successfully, Apache should have already been started as a service.
Test your Web server by accessing http://localhost. You should see the default Apache test file, as shown(the language of the page may differ, but you should see a similar page with the Apache logo).
Now you can edit the configuration file. Go to the Start Menu >> Apache HTTP Server 2.0.47>> Configure Apache Server >> Edit the Apache httpd.conf Configuration file. Notepad appears
containing the configuration file.

C.1 Apache

207

Figure C.2: Apache Default Index FileIf you would like to run IIS and Apache concurrently, search for the two lines like
Listen 80
ServerName localhost:80
They are at different locations in the file and I just put them here together for convenience. Change thetwo lines to
Listen 8000
ServerName localhost:8000
which instructs Apache to bind to port 8000 instead of port 80. IIS still gets port 80. If you don’t need(or don’t have) IIS, you don’t need this step.
Now we add in support for CGI. Find the line
#AddHandler cgi-script .cgi
and change it to
AddHandler cgi-script .cgi .pl
That is, remove the # at the front and add “.pl” at the end. This makes Apache recognize this fileextension as CGI scripts.

208

Appendix C Setting Up A Web Server

Now enable CGI scripts to execute in a specific directory you prefer. On Windows NT-series (including
2000 and XP) operating systems you are recommended to put your scripts under your own “My
Documents” folder for easy access. Very likely you already have a “My Webs” folder in it that has alreadybeen created for you by your Windows installation that you can use. Find out a section like
UserDir "My Documents/My Website"#<Directory "C:/Documents and Settings/*/My Documents/My Website">#
AllowOverride FileInfo AuthConfig Limit#
Options MultiViews Indexes SymLinksIfOwnerMatch IncludesNoExec#<Limit GET POST OPTIONS PROPFIND>#
Order allow,deny#
Allow from all#</Limit>#<LimitExcept GET POST OPTIONS PROPFIND>#
Order deny,allow#
Deny from all#</LimitExcept>#</Directory>
Remove the # as shown below and correct the path (for example the drive label on my system is F:
instead of C:, and the folder containing the scripts and HTML files is changed from the default “My
Website” to “My Webs”). Change the “Options” line as shown below, and add the ScriptInterpreterSource directive, which instructs Apache to find the path to Perl from the Windows Registry instead offrom the shebang line:
UserDir "My Documents/My Webs"<Directory "F:/Documents and Settings/*/My Documents/My Webs">
AllowOverride FileInfo AuthConfig Limit
Options MultiViews Indexes SymLinksIfOwnerMatch Includes ExecCGI<Limit GET POST OPTIONS PROPFIND>
Order allow,deny
Allow from all</Limit><LimitExcept GET POST OPTIONS PROPFIND>
Order deny,allow
Deny from all</LimitExcept></Directory>
ScriptInterpreterSource Registry-strict
We would like to use the Registry to resolve the Perl installation path because most Perl scripts(especially third-party scripts) today simply use the Unix-style #!/usr/bin/perl shebang line andin order to use these scripts you will need to correct all the shebang lines to the correct Perl path,
which is not convenient. If the Perl path is resolved from the Registry, then the path on the shebang
is ignored. This is generally recommended unless you are not confident with editing the Registry
yourself. If you don’t feel comfortable editing the Registry, then comment out (put a # in front of) the
ScriptInterpreterSource line above. Then, you will need to edit the shebang line of all your scripts toreflect the path to your Perl interpreter, e.g.

C.1 Apache

209#!F:/Perl/bin/perl.exe
or, if your interpreter is on the PATH, you can use instead#!perlNow save the Apache configuration file and restart the Web server by Start Menu >> Apache HTTP
Server 2.0.47 >> Control Apache Server >> Restart.
At last, you will need to edit the Windows Registry. This is dangerous if you did it improperly as proper
functioning of your Windows system relies on the integrity of the Registry. Always stop and think
before you commit your action because Registry operations are irreversible! To run the Registry Editor,
select Start Menu >> Run and in the box type regedit.exe, and press Enter.
Data entries in the Windows Registry are arranged in a tree, rather like a directory structure of afilesystem. The left pane contains a tree containing a hierarchy of keys. When you click on a key in theleft pane, the right pane contains a list of values associated with that key.
In the left pane, under “My Computer” you would find a number of nodes (keys). In earlier versions ofWindows you would not find a “My Computer” root node, but that does not matter. Now expand the“HKEY CLASSES ROOT” node and find the “.pl” node below. If Activestate Perl is installed, the key
should be there. Then right click on the “.pl” node and choose “New” >> “Key”. A new key is createdunder “.pl”. Type “Shell” for the name. Similarly, create “ExecCGI” under “Shell”, and “Command”
under “ExecCGI”. Click on “Command”, and in the right pane double click on the text “(Default)” andenter ”F:\Perl\bin\perl.exe” ”%1”, don’t forget to update the path to the Perl executable if this is notcorrect, and click OK.
When finished, the Registry Editor should look like Figure C.3, with all the nodes expanded.

Figure C.3: Registry After Modification

You can now close the Registry Editor. Create a simple Perl CGI script:
1
2#!/usr/bin/perl

210
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4
5
6
7
8
9
10
11
12
13
14

Appendix C Setting Up A Web Server# You should update the shebang line above if you don’t have
# the necessary Registry entry or have not added the line
# ScriptInterpreterSource Registry-Script to httpd.conf
print "Content-Type: text/html\n\n";print "<html><head><title>Test script</title></head><body><h1>Hello World</h1></body></html>";Recall Apache have been configured to allow execution of Perl CGI scripts in the “My Webs” folder inyour “My Documents” folder. Save your file there with a “.pl” extension, for example on my system it’s
F:\Documents and Settings\Administrator\My Documents\My Webs\test.pl.
Now, access http://localhost/∼administrator/test.pl. Replace “administrator” with the username. If you
have adhered to this tutorial, it would be “administrator”. If you have changed the Apache port number
to 8000 replace “localhost” with “localhost:8000” in the URL. You should see the “Hello World” text inthe browser window. Congratulations! Your Web server has been set up!
If you have disabled IIS temporarily and changed the Apache port number to anything other than 80,
you can now restart IIS if you would like to.

C.1.2

Unix

By far the only uniform way to install Apache on nearly all Unix platforms is to install a source
distribution. Binaries for different Unix variants are not interoperable, and are installed in a different
manner. For example, if you are on mainstream Linux distributions you may be able to find RPMs
(RPM Package Manager) compiled for your distribution. Debian Linux also has its own package
manager. FreeBSD also has a port system. Therefore, you should possibly check the documentation on
your system to see if binaries are available. However, binaries are usually slightly out of date because youneed to wait until packagers package a software into binaries suitable for your platform. Installation
instructions for binary packages are not described here.
You can extract the package by the command tar xvzf httpd-2.0.47.tar.gz. If your version oftar(1) does not support ‘z’ switch, you will need to try zcat httpd-2.0.47.tar.gz | tar xvf-. After the package is extracted, change to the directory containing the extracted sources on thecommand line using the cd command. First, we set up the configuration options of Apache by theconfigure shell script. Use this command:./configure --enable-mods-shared=all --prefix=/usr/local
which installs Apache using the installation prefix /usr/local. All modules are enabled and compiled
as shared libraries, instead of linking them into the main Apache executable. In this way, we only
dynamically load a module when it is needed. The CGI module is one of the modules that would becompiled. You may specify additional options if you like. Type ./configure --help for a list of
configuration options that can be applied. If you don’t see any errors at the end of the messages, thenyou should be fine. Now compile and install it.

C.1 Apache

211

make
make install
If no error messages appear which causes the compilation to stop, then you are lucky. Now Apacheshould be installed. Here I outline the changes to be made to the Apache configuration file. If you
adhere to this tutorial the configuration file should be at /usr/local/etc/httpd.conf. Because this issimilar to that of in the previous section I’m not going to explain the options that have been covered
there. This is the original configuration:
User nobody
Group #-1
ServerAdmin you@your.address#ServerName new.host.name:80#<Directory /home/*/public_html>#
AllowOverride FileInfo AuthConfig Limit Indexes#
Options MultiViews Indexes SymLinksIfOwnerMatch IncludesNoExec#<Limit GET POST OPTIONS PROPFIND>#
Order allow,deny#
Allow from all#</Limit>#<LimitExcept GET POST OPTIONS PROPFIND>#
Order deny,allow#
Deny from all#</LimitExcept>#</Directory>#AddHandler cgi-script .cgi
My modifications are as follows (please read the description below before making your changes):
User wwwGroup wwwServerAdmin abc@def.com
ServerName 127.0.0.1:80<Directory /home/*/public_html>
AllowOverride FileInfo AuthConfig Limit Indexes
Options MultiViews Indexes SymLinksIfOwnerMatch Includes ExecCGI<Limit GET POST OPTIONS PROPFIND>
Order allow,deny
Allow from all</Limit><LimitExcept GET POST OPTIONS PROPFIND>
Order deny,allow
Deny from all</LimitExcept></Directory>
AddHandler cgi-script .cgi .pl212

Appendix C Setting Up A Web Server

On Unix, although Apache has to be executed as root in order to bind to port 80 (as a side note:
you can also install Apache in a user account by modifying the installation prefix --prefix duringconfiguration to somewhere inside your home directory but you need to set the port number tobe larger than 1024 because port numbers below 1024 are so called privileged ports that by Unix
convention can only be bound to by the root user), in most installations once Apache has started up it
would drop its root privilege through the chroot() system call and run as a pseudo user like “www”
or “nobody” that has limited privileges to do much harm to the system should intrusions occur. Thisis a measure to minimize the security risks involved. An appropriate user and group may have beencreated for you by your operating system installation already. If not, you will need to create them. See
the adduser(8) and addgroup(8) manpages for more information. On my system, a “www” user andgroup has been created and I just use it here.
You should specify the email address of the administrator. In Windows this information is already set upby the installation program, but on Unix you will need to set it here. Similarly is the case for ServerName.
With this configuration, you should be able to execute CGI scripts inside the “public html” directory.
Now start the Apache Web server by/usr/local/bin/apachectl startAppendix D

A Unix Primer
In this appendix, you would learn:?

basic concepts of the Unix operating system?

some basic commands in Unix

D.1

Introduction

D.1.1

Why Should I Care About Unix?

You may wonder why I took the time to dedicate an appendix to the Unix operating system in a Perltutorial. “Perl is platform-independent”, you said, “and my Perl programs run fine on my Windows
XP!” Indeed, Perl programs are largely platform independent (platform-dependent Perl programmingis possible, but in most cases you don’t need to, so I am not going into details), and you can develop
and test Perl programs entirely on your platform. However, a couple of Web server surveys consistently
confirm that more than 60% of the Web servers on the Internet run on various flavours of Unix,
outweighing Microsoft Windows with the remaining share of about 30%. Very likely, your Web hostis also running on Unix. Therefore, it is beneficial to you if you can get yourself acquainted with thisoperating system.
Also, Perl has a strong Unix culture and tradition. It was intended to be a flexible scripting language likeshell scripting, awk and sed etc. on Unix platforms. For example, regular expressions have long been
extensively used in various utilities on the Unix platform. Quite a number of Perl’s builtin functions (asseen on the perlfunc manpage) are interfaces to the corresponding Unix commands or closely resemble
functions in the standard C libraries on Unix (not to mention the fact that Dennis Ritchie, the inventorof the C programming language, was also one of the inventors of the core Unix operating system!).

D.1.2

What Is Unix?

Unlike Microsoft Windows, which is solely owned and developed by the Microsoft Corporation, theterm “Unix” does not refer to any specific operating system releases. It is a collective name embracinga family of operating systems sharing certain common characteristics. 1 It was initially developed by
Dennis Ritchie and Ken Thompson from the Bell Labs in 1960s. Over more than 30 years of evolution,
many variants of the Unix operating systems have emerged. Today, the most prominent names in Unix
1 The Open Group has issued The Open Group Base Specifications Issue 6, IEEE Std 1003.1, 2003 Edition that defines astandard operating system interface, which can be regarded as the basis of Unix operating systems.

213

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include Linux, *BSD (FreeBSD/OpenBSD/NetBSD), Solaris and Mac OS X (Jaguar) as a newcomer.
This appendix is intended to be an introduction to the Unix computing environment. Therefore,emphasis is placed on shell commands that are frequently used by users. In particular, X-Windows,
the graphical user interface on Unix, will not be introduced in this appendix. Also, for the sake ofgenerality features or commands that are specific to any particular Unix variant will not be included.
The commands and concepts described in this chapter can be applied on many Unix operating systems.
Readers are advised to consult other Unix literature for more in-depth treatment of the topics covered.

D.1.3

The Overall Structure

A system running Unix can generally be envisioned as having a layered architecture. What does thismean? First, the bottom layer is the system hardware. This ranges from the central processing unit
(CPU), memory system, storage devices, graphics adapter to peripheral devices such as keyboard andprinters. They are interconnected through buses, that are inter-component wires that carry data andcontrol information in between. The operating system lies on top of the hardware layer to coordinate
the hardware components. This part of the operating system which interacts with the hardware directlyis the core of the operating system, called the kernel. On top of the kernel are the user programs.
These programs do not need to interact with the hardware directly anymore. Instead, they only need tointerface with the kernel in case those services are needed, by invoking the necessary system calls, which
are functions provided by the kernel. Filesystem operations, for example, are examples of standard
system calls on Unix. The kernel would in turn perform the appropriate actions to instruct the devices
to commit the operation. When you are using a user program, you are presumably the top among thelayers, on top of the user program. The user program provides you with the user interface with which
you interact. Commands defined by the user program, and in case of graphical user interfaces, mouseclicks and their corresponding pixel coordinates are translated into instructions for the lower layers.
This layered architecture allows a high degree of abstraction between layers. If the interface (or the
Application Programming Interface) of a layer changes, only the layer immediately on top needs to bemodified. Because of abstraction, the user needs not understand the low-level details of the hardware
circuitry, for example, in order to operate a computer system. This is an important principle in allmodern computer systems.
To sum up, the Unix operating system serves to act as an intermediary between a computer user andthe computer hardware so that activities that occur at different components of a computer system arewell coordinated.
Unlike many other operating systems, Unix is a multi-user operating system from the very beginning,allowing multiple users to work on the system concurrently. Multiple users may log on a machine
remotely from other machines via the traditional telnet protocol, which gives you remote access to thecommand-line interface of the machine. Telnet is not secure because data are carried in plain text. SSH(Secure SHell) is a similar protocol but data are encrypted in transit. There is also a protocol called
Virtual Network Computing (VNC) that lets users have access to the graphical user interface from theremote side.

D.2 Filesystems and Processes

D.2

Filesystems and Processes

D.2.1

Overview

215

In modern days, large-capacity secondary storage is important. Floppy disks, hard disks and CD-ROM
etc. are classified as secondary storage devices, in contrast to primary storage, which is just an alias ofthe main memory. Secondary storage media is considered permanent, because data that are written
onto the media are retained when the power is off. When an executable file has to be executed, theoperating system arranges for the executable file to be copied from secondary storage to primary storage(i.e. your RAM). The program is then broken down into instructions and executed from internalcaches. A program in execution is called a process. The reason that the executable file is not executedfrom the secondary storage directly is that accesses of secondary storage devices are very slow comparedwith the main memory because such accesses involve mechanical movement of disk arms, for example,
and the speed of which is subject to mechanical limitations.
A disk is simply a large array of disk blocks of fixed size in which data can be stored. It does notmandate any rules to organize data that are being stored on the disk. In order to better organize storageof data, a large-capacity storage media is usually divided into multiple partitions. This is very common
in systems with multiple operating systems installed, with each operating system installed in its own
partition. However, partitioning simply marks the beginning and the end of each partition, but does
not answer the need for organizing data that are written in each partition. Therefore, the second step isto create a filesystem on each partition. By creating a filesystem, the data structures that are necessary
to index data to allow efficient access are written to the partition. This operation is more well-known
as “formatting” to users of MS-DOS and Windows operating systems. Modern Windows systems use
either one of the two filesystems, namely File Allocation Table (FAT) or New Technology File System(NTFS). On Unix, various choices of filesystems are available depending on the operating systemvariant. Among the Unix variants, Linux supports the largest number of filesystems. The standard
filesystem on Linux is the Second Extended Filesystem (ext2). Other filesystems in widespread use
include reiserfs and ext3, which are both equipped with journalling capabilities. Other Unix variants
mostly use the Unix File System (UFS).
The filesystem determines the directory structure, for example, how files are represented and layout on
the disk. To better organize the files, we introduce a hierarchical directory structure. Logically, files areclassified and put into different directories. Files of a similar kind are put into the same directory. Also,
directories can be nested, so that a directory can be created inside a directory. This allows fine-grainedorganization of files in a well-structured manner for easy access. Because directories are hierarchical,
they are customarily represented in the form of a tree. In Computer Science arena, a tree refers toa hierarchical data structure which best shows the subordinate relationships of directories. This is avery intuitive concept that readers with some experience with operating systems should be familiar with.
The root of the directory structure in Unix is represented by /. The directory root is the only directory
which does not have any parents. All files on the system must rest under the directory root. As you cansee in the figure above, a number of directories appear under the root. Each of these directories serve
its specific purposes. In Table D.1 I list a few more important ones that are present on nearly all Unix
systems 2 .
In Unix terminology, a file is not necessarily a regular file. Directories, symbolic links and even devices
are also represented in the same way as a regular file, differentiated simply by a file type indicator in theinode (see below). A few other file types are defined, yet they are specialized file types that are seldom
2 Please consult the Filesystem Hierarchy Standard, the recommended scheme for compatibility between different Unix
variants.

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Appendix D A Unix Primer

Figure D.1: Directory Structure In Tree Representation

used directly, so I am not going into details here.

D.2.2

Symbolic Links and Hard Links

The concept of links is foreign to users of many other operating systems, e.g. MS-DOS or Windows.
Unix supports two types of links, namely symbolic links and hard links. Symbolic links and hard
links in Unix are similar in sense to symbolic references and references in Perl, respectively. In many
situations, symbolic links and hard links have similar effects. However, under the hood they are
implemented in very different ways which give rise to different behaviours in certain situations. Because
the concept of hard links is not very clearly explained in many Unix literature, I decided to elaborate alittle bit on this concept in this section.

Directory
/bin
/boot
/dev
/etc
/home
/mnt
/root
/sbin
/tmp
/usr
/var

PurposeEssential user programs, e.g. shells, file tools
Core system files for system bootup
Device files (remember devices are represented as files?)System-wide configuration files for system and user programs
User accounts: files private to individual users
Mount point for foreign filesystems
User account of system administrator (root)
System administration commands used only by root
Temporary files
User programs that are installed system-wide, and related files
Data files written by system programs, e.g. email and various logs
Table D.1: Major Unix Standard Directories and Their PurposesD.2 Filesystems and Processes

217

Symbolic links are easier to understand. It works like “shortcuts” (the .lnk) files on Windows. Basically,a symbolic link is a special type of file that stores an alternative access path. When the symbolic linkis accessed, the stored path is used for access. Symbolic links are frequently used to create shortcuts tovery long paths, such that a shorter path is used instead. For software distribution sites, they are also
used to maintain a static URL to the latest release. For example, on a certain system there are directories
and files as follows:/pub/
download/
myprog_current.zip -> /usr/share/devel/myprog/current.zip
usr/share/
devel/
myprog/current.zip -> myprog_3.0.zip
myprog_1.0.zip
myprog_2.0.zip
myprog_2.2.zip
myprog_3.0.zip
The “->” indicates a symbolic link. On the left is the name of the symbolic link (the alias), andon the right is the alternative path to follow. Say myprog is a software developed. In this example, to access the current version of myprog, the path is /pub/download/myprog current.zip.
When the file is accessed through this symbolic link, the system would then try to access it at /usr/share/devel/myprog/current.zip, which in turn is also a symbolic
link to myprog 3.0.zip, the archive of the latest release.
Therefore, essentially, the path/usr/share/devel/myprog/myprog 3.0.zip is used to access the file. This example serves
both purposes. It shortens the URL, and users can always get the latest release with the same URL,
provided the symbolic links are properly maintained to point to the latest release. You may also wonder
why I create two symbolic links instead of having myprog current.zip pointing to myprog 3.0.zip
directly. This is for convenience in management, so that no matter the user is in /pub/download
or /usr/share/devel/myprog he or she can still easily locate the latest version, and notice that by
doing so the symbolic link in /pub/download needs not be updated when a new release is placed in/usr/share/devel/myprog, only current.zip needs to be.
To create a symbolic link, use the ln command, with the -s switch. For example,
cbkihong@cbkihong:∼/test$ cd pub/download
cbkihong@cbkihong:/pub/download$ ln -s /usr/share/devel/myprog/current.zip ¶myprog_current.zip
Hard links are more difficult to understand. Recall that in the previous section I described what afilesystem is. By creating a filesystem, indexing facilities that allow the operating system to quickly
locate which disk blocks a certain file occupies are created on the disk. This serves a similar purpose as
the map in a traveller’s pocket. When a file is created on the disk, an inode (index node) is created forthe file which contains attributes such as the owner and group (see next section) identifiers, the times ofmodification and access, the file type and locations of disk blocks containing the file content (pointers).
Finally, the operating system needs to add to the inode of the containing directory the pointer to the
newly created inode so that the file is added to the directory entry. This pointer is actually a hard link.

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Hard links work at the level of inodes to allow direct access of the file being pointed to. Therefore, forevery inode there is at least one hard link that points to it, from the containing directory file entry. If
we create an additional hard link to a file, that means a new pointer to the file inode is added. Here is asequence of commands which serves as an example:
cbkihong@cbkihong:∼/test$ touch file1.txt
cbkihong@cbkihong:∼/test$ ln file1.txt file2.txt
cbkihong@cbkihong:∼/test$ touch file3.txt
cbkihong@cbkihong:∼/test$ ls -li
342533 -rw-r--r-2 cbkihong cbkihong
0 2003-07-30 17:56 file1.txt
342533 -rw-r--r-2 cbkihong cbkihong
0 2003-07-30 17:56 file2.txt
342531 -rw-r--r-1 cbkihong cbkihong
0 2003-07-30 17:59 file3.txt
cbkihong@cbkihong:∼/test$ echo "ABCD" > file1.txt
cbkihong@cbkihong:∼/test$ ls -li
342533 -rw-r--r-2 cbkihong cbkihong
5 2003-07-30 18:01 file1.txt
342533 -rw-r--r-2 cbkihong cbkihong
5 2003-07-30 18:01 file2.txt
342531 -rw-r--r-1 cbkihong cbkihong
0 2003-07-30 17:59 file3.txt
The touch command creates an empty file. The ln command without any switches creates a hard link.
Here, file2.txt is created as an additional hard link to the inode of file1.txt. The ls command
displays a listing of files. Here, two switches -l and -i are given. If multiple switches are provided
on the command line, you can combine them into -li (ordering does not matter). The -l option
causes the long listing to be displayed, with the permission values, owner, group, date of the lastmodification and the file size. If the -i switch is given, the inode number that the file entry points tois inserted as the first column. We can see that file1.txt and file2.txt points to the same inode,
while file3.txt points to a different inode. The third column is the number of hard links pointing
to the inode. This number is stored at each inode. For regular files, this number is usually one, as
explained previously. However, because we have manually created a new hard link as file2.txt, thenumber displayed is 2. Also notice that the two entries are exactly identical, except the name. Here, thetext “ABCD” (with line terminating character) is written into file1.txt, and both entries are updated.
You may wonder why the filesystem has to keep track of the number of hard links pointing to an inode.
If you have read the chapter on references in Perl you will find a close resemblance with the Perl garbage
collection mechanism. That is, the inode would not be freed (deleted) until the number of hard links toit drops to 0. Consider the example again. If at this point we delete file1.txt, the number drops to 1,
but because we still have a hard link that points to it from the entry file2.txt, the inode is not freed.
It is not until when file2.txt is deleted, that the inode and other disk blocks that are associated withthis file will eventually be freed.
cbkihong@cbkihong:∼/test$ rm file1.txt
cbkihong@cbkihong:∼/test$ ls -li
342533 -rw-r--r-1 cbkihong cbkihong
342531 -rw-r--r-1 cbkihong cbkihong
cbkihong@cbkihong:∼/test$ rm file2.txt
cbkihong@cbkihong:∼/test$ ls -li
342531 -rw-r--r-1 cbkihong cbkihong

5 2003-07-30 18:01 file2.txt
0 2003-07-30 17:59 file3.txt

0 2003-07-30 17:59 file3.txt

Occasionally you will find some files (and directories) with the “.” name prefix, e.g. “.kde3/” and“.vimrc”. These files (the “dot” files) are usually found in private user accounts in the /home tree, butD.2 Filesystems and Processes219

in most cases you won’t see them at all because the “dot” files are hidden by default. These files areusually created by user applications to store user-specific configuration and data files, because the user
account is usually the only directory that applications written by users can write to that is private to
the user. The /etc tree is owned by root and only system-wide configuration can be made there by thesystem administrator, and the /tmp tree are for temporary files only and need to be periodically cleaned
up to avoid accumulation of useless files, thus data files that are to be kept cannot be placed into these
directories. Due to the large number of applications installed in the system, very likely large numbersof these data files have to be created in the user account. To avoid clutter and prevent the user from
accidentally deleting these files, Unix hides these files by default, unless you pass the -a option to the lscommand.
Are you wondering why I suddenly jump from my hard links discussion to “dot” files? That is because
I am going to introduce to you two special “dot” files that are present in every directory, namely “.”
and “..”. MS-DOS also has the notion of these two special files. They are automatically created in a
directory when the directory is created. They are in fact hard links to the current directory file, andthe parent directory file respectively. Therefore, they appear as directories with the ls -la command.
“..” is used to refer to the parent directory in the directory tree. For example, with the cd command,
you can specify the name of a subdirectory to go into it, but when you need to return to the parent
directory, you can use the commandcd ..
As another example, cat ../../README.txt outputs the file README.txt two levels upward. For
example, if you are in the directory /home/cbkihong/docs/perltut when you type the above
command, then the Unix shell will try to find the file at /home/cbkihong/README.txt and display its
content. The “.” directory points to the current directory. It seems to be not useful at all. However, I
can find at least one use of it. For security reasons, many Unix installations would not put the current
directory, that is “.” into the environment variable PATH, especially for the root user. Therefore, if you
have an executable file, say myprog, in your current directory by just typing myprog the program will
not be started at all, because PATH is the executable search path that is used if the path to the executable
is not specified. Directories not listed in this variable will not be searched for executables. In this case,you need to qualify it with the directory where it can be found, by./myprog
If the target file is not an executable, the “./” prefix is generally optional and well understood if absent.
Therefore, cat docs/README is generally understood to be cat ./docs/README. Because of these
two special “dot” files, you will find that the number of hard links pointing to directories is never one.
As a bare minimum there are two, one due to “.” and the other one from the parent directory file inode.
If there are subdirectories, then “..” in the subdirectories will add to the number of hard links pointing
to the current directory inode. If there are extra hard links created manually to the current directory,
then there are even more. For example,

/
bin/
etc/
home/
cbkihong/

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Appendix D A Unix Primer

The /home directory inode has 3 hard links pointing to it. However, there are 5 for /, that is because
there is no parent directory for /, and for consistency the “..” in / also points to itself. Together with “.”
and “..” from the 3 subdirectories, the hard link count is therefore 5.
Symbolic links are more widely used because symbolic links only stores an alternative path nameand can be used provided the alternative path is accessible from the directory tree. That is, symbolic
links may point to a destination that is on a different filesystem. For example, a symbolic link on myLinux reiserfs partition may point to a file on the Windows FAT partition, which does not even havethe notion of hard links (did I tell you that you cannot create a hard link on a mounted Windows
partition at all?). On the other hand, hard links cannot cross filesystems, and they are only supported
on Unix-compatible filesystems, so their areas of application are rather limited.

D.2.3

Permission and Ownership

Because Unix is from the ground up a multi-user operating system, a permission and ownership system
has to be in place to control who have access to resources and how they can access them. In Unix
filesystems, every file has an owner, the user who created the file on the filesystem. Apart from theowner, each file is associated with a group. The ls command with the -l switch causes the directory
listing to be printed in the long format, with the name of the owner and group printed in the thirdand fourth column, respectively. The first column is the access permissions. In this column, the firstcharacter indicates the file type. Customarily, regular files are represented by a hyphen “-”, directories
by “d” and symbolic links by “l”.
cbkihong@cbkihong:∼/docs/perltut$ ls -l
drwxr-xr-x
2 cbkihong users
477 2003-06-20 14:25 images-rw-r--r-1 cbkihong users
657800 2003-06-20 14:28 perltut.pdf-rw-r--r-1 cbkihong users
4012769 2003-06-20 14:28 perltut.ps-rw-r--r-1 cbkihong users
3887 2003-06-19 19:17 perltut.tex
In this example, the files have the owner “cbkihong” and belong to the group “users”. To control how
different users can access the files, three sets of permission bits are assigned to each file which specify
permissions that are given to the owner, group members and everybody else. The permission values arereflected by the first column of the file listing obtained by the ls command above. The first character
indicate the type of the file. The remaining nine characters represent the permission values. Here is asummary of what each character means with respect to a directory and a regular file:Let’s take the first two entries in the example listing above as an example. For the “images” directory,
we divide the permission values into three sets:

file type
d

owner
rwx

group
r-x

everybody elser-xEverybody on the system can read the directory listing and enter the directory. However, only theowner “cbkihong” can add new files or remove files from the directory. To read the directory listingmeans you get a list of names that represent the files (including subdirectories and symbolic linksetc.) in the directory. For example, if you enable the “x” bit but not the “r” bit, the commandls images/ will fail because this operation involves getting the directory listing. However, thecommand cat images/README.txt will be successful if the file README.txt exists in the “images”
directory because directory listing is not involved in the operation. On the other hand, if the “r”

D.2 Filesystems and Processes

Bit
r
w
x

Bit
r
w
x

Value
4
2
1

Value
4
2
1

221

Regular File
Meaning
Read file content
Modify the file content
Execute the file

Directory
Meaning
Read directory listing
Create/delete files in the directoryEnter the directory

Table D.2: Filesystem Permission values

bit is enabled but not the “x” bit, you can see the list of files in the directory, but you cannot accessthem. Changing into the directory with the command cd will also fail. Because such a combination of permission bits is somewhat nonsense in practical use, for directories usually the “r” and“x” bits go together — either you enable or disable both of them, but not enabling one and not the other.
Unix groups are not frequently used in practice, but they can be good for sharing of files among users
on the system in a simple way. For example, a file server in a company may have a “managers” groupwhose members consist of managers from all departments. The server may have a directory called“reports” containing reports prepared by the managerial for all staff. If the directory is not in the/home tree, it is very likely owned by the system administrator (root). However, the administrator
may set the group to “managers” and set the group “w” bit to allow the managers to put their reports
into the directory, while other staff users only have read access. A possible configuration is shown below:
drwxrwxr-x

2 rootmanagers

477 2003-07-20 14:38 reports

For the “perltut.pdf ” file in the sample file listing above, the permission values are as follows:

file type
-

owner
rw-

group
r--

everybody elser--That means everybody can read the file, but only the owner can modify it. Some people havemisconceptions on Unix permissions that one needs directory write permission to modify a file in the
directory. The fact is only the file write permission is needed. Also, some may think to delete a file oneneeds writable permission to that file. Only the directory write permission is needed in this case. If you
understand my description above fully, you are not going to make these kinds of mistakes.
Note that internally (for example, in the inodes) owners and groups are represented by numbers insteadof names such as “cbkihong” and “users”. That is because storage of integers uses less space comparedwith names. On most systems, the /etc/passwd stores the mapping between user ID and username,while /etc/group stores the mapping between group ID and group name. They are used to resolve themnemonic names for display as output of commands such as ps and ls etc.
In Table D.2 you see a column with the heading “Value”. Internally, to store the permission values
in a more compact form the permissions are encoded as a number. Take the “images” directory as

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Appendix D A Unix Primer

an example. The permission value “drwxr-xr-x” may be converted to numeric representation as follows:
Owner (rwx): 4 + 2 + 1 = 7Group (r-x): 4 + 1 = 5
Everybody else (r-x): 4 + 1 = 5
As already described, the character “d” is only an indication that this is a directory, and it’s not apermission value. Therefore, the numeric representation is 755, by concatenating the permission values
for the owner, group and everybody else. Please note that permission values of symbolic links are notused in practice, and are set to 777 (lrwxrwxrwx) by default.

D.2.4

Processes

Once executable permission is applied to a file, it can then be executed by the system. A program in
execution is called a process. Each process has an owner and is associated with a group, similar to thecase of files on a filesystem. Each process is associated with four identifiers. Apart from the user ID
and group ID, a process also has an effective user ID and an effective group ID. In general, when an
executable file is being executed, the user ID is that of the user who executed the program, and thegroup ID is that assigned to the user when the user account was created. In general, the effective user ID
is the same as user ID, and the effective group ID is the same as group ID. They are different only if thefile being executed has either the setuid or setgid bit set, which are two additional permission bits thatare useful only to executable files and will be described in the next section. They are seldom needed,
and their use are usually not justified unless with good reasons.
The enforcement of an ownership system on processes prevents unauthorized users from modifying
the state of the processes, for example, to terminate them. In general, only root or the users whoseuser ID matches the user ID or effective user ID of a process are allowed to change the state of aprocess. You can change the state of a process by sending it a signal, that is, a message sent fromthe operating system kernel to a process. From a user’s point of view, a process can be in one ofseveral states: running, suspended or terminated. We can use the kill command to send a signal
to a process. The signals that are most frequently used include SIGHUP (1), SIGINT (2), SIGKILL(9), SIGSTOP (19) and SIGCONT (18). The way SIGHUP is handled is process-specific. It is generally
widely supported that when a daemon process receives this signal, it rereads its configuration files.
This is convenient for system administrators to effectuate changes made to the configuration files
without restarting the daemon process. The SIGINT signal is what is sent to a process when the userpresses Ctrl-C. In most cases, the process is terminated. However, some processes are defined tocatch the signal and thus prevent it from being terminated. For example, Ctrl-C is a combination
key defined in emacs, so it has to catch the signal. In this case, you may try to send the SIGTERM(15) signal. Runaway processes can generally be abruptly terminated by the SIGKILL signal. The
SIGSTOP signal causes the process to be suspended. Both SIGKILL and SIGSTOP cannot be caught
by any processes, and therefore provided you have the permission to change the state of the processthese two signals should succeed, at least in theory. However, there are a few occasions when theprocess, or even other parts of the operating system are ignoring these signals. This is a sign ofinauspiciousness and you are advised to restart your system if this happens to you (but probably bythen it is already too late). For a suspended process, you can put it back to running state by sending it
a SIGCONT signal. You can send a signal using kill in several forms, as shown in the following examples:
kill -SIGHUP 826
kill -HUP 826
kill -1 826

D.2 Filesystems and Processes

223

You can use the name of the signal or the signal ID to specify the signal to send to the process. Because
all Unix signal names actually start with the prefix “SIG”, you may omit this prefix to reduce theamount of typing. The last argument to kill is the process ID. Each process has an ID to uniquely
identify a process. You should check the process ID by using the ps command, which is displayed on
the far left. Most systems also have a killall command which frees you from the need of looking up
the process ID. For example,

killall -SIGINT myprog
which tries to terminate all instances of the program myprog. However, kill is a more reliable choice
on some Unix variants, or where killall is not available.
A side note about the executable and read permission of executable files. I have recently seen a question
raised on a Unix forum about making a shell script executable but not readable by other users on
the system. The answer is that is not possible to use the permission 711 (rwx–x–x). There are twomain types of executable programs. Either they are compiled into an executable object code (binary)format that can be executed directly (e.g., compiled programs written in C), or shell scripts (e.g. perlor sh scripts). If the executable is in an object format supported by the operating system kernel, it
can be directly loaded and executed by the kernel. Today two main binary formats are supported
on Unix, namely a.out and ELF (Executable and Linkable Format). a.out is well supported, but isquickly replaced by ELF on many Unix platforms. In the case of executable scripts, on the other hand,
the kernel needs to load the interpreter instead, and the interpreter executes the script instead. To
execute the script, the interpreter needs to have read access of the script. That’s why you can’t use
711 as the script permission if everybody needs to execute it. This works for compiled programs, though.

D.2.5

The Special Permission Bits

Apart from the 9 permission bits that can be set for every file, there are three special option bits that
may be applied, two of which pertain to executable files. These bits are known as setuid, setgid andsticky respectively. They may be set using the chmod command.
If you are granted access to a Unix shell account, possibly one of the first commands you will be asked
to use is passwd. This command lets you change your password. On some Unix platforms, some otherfunctionalities are also available, for example, to change the login shell. Traditionally on Unix, user
information are stored in the file /etc/passwd, containing the username, user ID, home directory,login shell and an encrypted password of users. As encrypted passwords in traditional Unixes are wellknown to be easy to break, today many systems are configured to use shadowed passwords, whose
encrypted (strictly speaking, hashed by a cryptographically strong algorithm such as MD5) passwords
are stored in a separate file /etc/shadow which is only readable by root, in contrast to /etc/passwd
which is readable by everybody. The passwd file has to be readable by everybody because its file content
is read by a number of standard Unix programs, such as finger, which can be run by all users on thesystem to display information about the user (it may also be executed remotely, but it’s usually disabled
to minimize security risks). The passwd file is owned by root. Have you wondered how the passwd
program which is executed by a regular user can modify this file which is only writable by root? The
fact is, the passwd is one of the few programs in a standard Unix installation that has the setuid bit set,
which allows it to take on the privilege of the root user.
When an executable with the setuid bit set is being executed, the effective user is set to the owner of
executable instead of the user who executed the program. Similar is the case for the setgid bit. Therefore,
essentially, the setuid and setgid bits are used to escalate the privileges of the executable. It is also for224

Appendix D A Unix Primer

this reason that setuid and setgid bits should not be set unless with strong reasons, and programs withthese bits on should be examined closely to ensure they do not have any inherent vulnerabilities that
may be taken advantage by attackers. For Perl programs, for example, the program suidperl which isitself a setuid version of the perl interpreter is actually used to execute a Perl program with the setuid
or setgid bit set. As mentioned in Chapter 10, runtime taint check is performed automatically. An
executable may be set setuid or setgid using the chmod command:

chmod u+s executablechmod g+s executable# setuid# setgid

For example, passwd, a setuid program shows up as this:-rwsr-xr-x

1 rootroot

24376 2003-09-14 05:53 /usr/bin/passwd

The sticky bit serves a totally different purpose. Many programs in execution need to create large
number of temporary files. In order to avoid cluttering the home directory of the respective users, they
are customarily created under the /tmp tree. A check of the /tmp directory shows that it is owned byroot, and the group is root. Here is the entry displayed by ls -l /:drwxrwxrwt

8 rootroot

4096 2003-09-22 14:47 tmp

Because the directory is writable by everybody (the w bit is enabled for others), programs executed byregular users can use it as a swap space to create temporary files. However, this also creates a problem.
Note that while any regular user may create anything in this directory, they may as well remove anything
in this directory including files and directories created by another user. A malicious user may keep on
removing other users’ files to inconvenience them. The workaround is a sticky bit applied on the /tmp
directory. If the sticky bit is enabled, the system will not allow you to modify any entries in the directory
unless you are the owner specified at the entry, or the root user. You may enable sticky mode by thecommand shown below:

chmod +t somedir/t stands for a sticky bit. You can see from above that it appears instead of x in the listing of /tmp. Hereis an example of some entries in /tmp on my Debian Linux system:
cbkihong@cbkihong:/$ ls -l /tmp
-rw-r--r-1 cbkihong cbkihong
drwx-----2 cbkihong cbkihong

0 2003-09-22 14:25 AcroEiBBIX
4096 2003-09-22 14:23 kde-cbkihong

Unless you are the owner, that is “cbkihong”, you cannot rename or remove these two entries.

Appendix E

In The Next Edition
The following topics or modifications are planned for inclusion in the next edition of this Perl 5
Tutorial. If you have any other suggestions on other topics of interest or amendments to the content of
this edition, please feel free to use my feedback forum or my email feedback form to let me know. The
Web links can be found on page 2.

?

more full code examples and illustrative figures?

more advanced CGI scripting techniques and more on CGI?

threading and fork()ing new processes?

mod perl?

Database access with DBI and DBD::*?

introduction to XS?

internationalization support in Perl and PerlIO layers?

GUI building with Tk?

more examples on complex data structures and algorithms implementations?

socket programming and existing CPAN classes (e.g. LWP, Net::*)?

“heredoc” quotation syntax (I’m not fond of it but some people do use it)?

Regular expressions (Perl extensions)?

Exporter module in object-oriented programming?

more object-oriented design principles and examples?

coverage of File::* and IO::* classes?

operator overloading?

Command line parameter parsing with GetOpt::Long?

ties and object persistence

225

Indexa.out, 223
abstraction, 3algorithm, 4anonymous array, 96
anonymous hash, 97
arguments, 14
Arithmetic operators, 47
array, 19
array slice, 30
ASCII Code, 52
assignment operator, 23
Assignment operators, 47
association, 69
associativity, 59
attack paths, 191
autovivification, 100
backtracking, 140
barewords, 161base class, 113
binary, 47
binary search, 36
binding, 69
bit-masking, 57
Bitwise operators, 47bundles, 203
buses, 214caches, 184
call stack, 78, 84
catch, 153
chaining, 198classes, 106
client, 170
client-server model, 170
code block, 69
collision, 198
collision resolution, 198
comma operator, 44
comments, 15
Common Gateway Interface, 7, 172
Comparison operators, 47

compilation, 1
compilation errors, 149
compiled, 1
conditional operator, 58
constants, 16
constructor, 112
context, 43
context switch, 132
debug, 2
depth-first search, 130
dereference, 98
derived class, 113
dynamic hash table, 199
dynamic scoping, 83
eavesdrop, 189
effective group ID, 222
effective user ID, 222
element, 27
ELF, 223
encapsulation, 107environments, 69
Equality operators, 47
escape characters, 14
expired, 184
expression, 23
fatal exceptions, 149File Allocation Table, 215
file descriptors, 119
file pointer, 121
filehandles, 119
filesystem, 215
finite-state automaton, 145
floating-point numbers, 16
function, 14functional requirements, 4generic class, 113
global, 19
handler, 153, 172
226

INDEX
hard links, 216
hash, 20
hash function, 198
hidden, 84
high-level programming languages, 3hit, 35
hyperlinks, 170
Hypertext Markup Language, 169
Hypertext Transfer Protocol, 171
identifier, 19
index, 19
inheritance, 107
initialization, 23
inode, 217
instantiated, 106
Instruction set, 2
instruction set, 2
instruction set architectures, 2
integers, 16interface, 107, 172interpreter, 2invariants, 16Java bytecode, 2Java Virtual Machine, 2kernel, 214
key, 20
kill, 222
lexical analysis, 15
lexical scoping, 83
linear search, 35
linked lists, 199
linking, 1
list, 26
list data, 19
List operators, 47
list separator, 79
literal, 16
load factor, 199
local referencing environment, 84
Logical operators, 47
loop control statements, 90
lvalue, 24
machine code, 1
methods, 106
miss, 35
modifiers, 91
module, 110

named constant, 158
namespaces, 80
New Technology File System, 215
non-functional requirements, 4nonlocal referencing environments, 84
null array, 19
objects, 106
operator overloading, 156
Operators, 47
output field separator, 29
overriding, 113
package, 80
packet, 171
parameters, 14parse tree, 15partitions, 215
path segments, 130
pattern, 137
pattern matching, 137
platform-independent, 2polymorphism, 107
precedence, 59
primary storage, 215
privileged ports, 212
process, 215
processes, 132
programming paradigm, 105properties, 106
protocol, 171
proxy servers, 182
pseudocode, 4
range operator, 27, 58
Rational Unified Process, 5
Recursion, 75
references, 95
regular expression, 137
regular expressions, 3
render, 170
resource sharing, 132reusability, 72
rings of security, 191
runtime errors, 149
rvalue, 24
Scalar data, 19
scalar variable, 19
scope, 19, 69
Second Extended Filesystem, 215
secondary storage, 215

227

228

INDEX

Secure Socket Layer, 189
server, 170
Server Side Includes, 185setgid, 223
setuid, 223
shadowed passwords, 223
shebang, 13
signal, 222
signal handler, 110
Signals, 110
slots, 198
Software Engineering, 5source code, 1
stack, 78
stacktrace, 156
standard output, 14
state diagram, 145
statements, 14
static content, 169
sticky, 223
stream, 120
string, 16
String manipulation operators, 47
subclass, 113
subroutine, 71
subscript, 19
subscript operator, 30
subsystems, 4superclass, 113
switch, 14
symbol table, 81
symbolic links, 216
symbolic references, 160
syntax errors, 149
syntax highlighting, 9system calls, 214
ternary, 47
threads, 172
throwing, 153
time slicing, 132
Transport Layer Security, 189
typeglob, 81
unary, 47
undef, 24
undefined, 24
Uniform Resource Identifier, 170
Unix File System, 215
user agent, 171
user programs, 214variable substitution, 25
warnings, 149
Web server daemon, 171
World Wide Web, 170
World Wide Web Consortium, 171

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