# awk

## PROLOG



           This manual page is part of the POSIX Programmer's Manual.  The
           Linux implementation of this interface may differ (consult the
           corresponding Linux manual page for details of Linux behavior), or
           the interface may not be implemented on Linux.
    


## NAME



           awk — pattern scanning and processing language
    


## SYNOPSIS



           awk **[** -F _sepstring_**] [** -v _assignment_**]**... _program_ **[**_argument_...**]**
    
           awk **[** -F _sepstring_**]** -f _progfile_ **[** -f _progfile_**]**... **[** -v _assignment_**]**...
                **[**_argument_...**]**
    


## DESCRIPTION



           The _awk_ utility shall execute programs written in the _awk_
           programming language, which is specialized for textual data
           manipulation. An _awk_ program is a sequence of patterns and
           corresponding actions. When input is read that matches a pattern,
           the action associated with that pattern is carried out.
    
           Input shall be interpreted as a sequence of records. By default, a
           record is a line, less its terminating <newline>, but this can be
           changed by using the **RS** built-in variable. Each record of input
           shall be matched in turn against each pattern in the program. For
           each pattern matched, the associated action shall be executed.
    
           The _awk_ utility shall interpret each input record as a sequence of
           fields where, by default, a field is a string of non-<blank>
           non-<newline> characters. This default <blank> and <newline> field
           delimiter can be changed by using the **FS** built-in variable or the
           **-F** _sepstring_ option. The _awk_ utility shall denote the first field
           in a record $1, the second $2, and so on. The symbol $0 shall
           refer to the entire record; setting any other field causes the re-
           evaluation of $0. Assigning to $0 shall reset the values of all
           other fields and the **NF** built-in variable.
    


## OPTIONS



           The _awk_ utility shall conform to the Base Definitions volume of
           POSIX.1‐2017, _Section 12.2_ , _Utility Syntax Guidelines_.
    
           The following options shall be supported:
    
           **-F** _sepstring_
                     Define the input field separator. This option shall be
                     equivalent to:
    
                         -v FS=_sepstring_
    
                     except that if **-F** _sepstring_ and **-v** FS=_sepstring_ are both
                     used, it is unspecified whether the **FS** assignment
                     resulting from **-F** _sepstring_ is processed in command line
                     order or is processed after the last **-v** FS=_sepstring_.
                     See the description of the **FS** built-in variable, and how
                     it is used, in the EXTENDED DESCRIPTION section.
    
           **-f** _progfile_
                     Specify the pathname of the file _progfile_ containing an
                     _awk_ program. A pathname of **'-'** shall denote the standard
                     input. If multiple instances of this option are
                     specified, the concatenation of the files specified as
                     _progfile_ in the order specified shall be the _awk_
                     program. The _awk_ program can alternatively be specified
                     in the command line as a single argument.
    
           **-v** _assignment_
                     The application shall ensure that the _assignment_
                     argument is in the same form as an _assignment_ operand.
                     The specified variable assignment shall occur prior to
                     executing the _awk_ program, including the actions
                     associated with **BEGIN** patterns (if any). Multiple
                     occurrences of this option can be specified.
    


## OPERANDS



           The following operands shall be supported:
    
           _program_   If no **-f** option is specified, the first operand to _awk_
                     shall be the text of the _awk_ program. The application
                     shall supply the _program_ operand as a single argument to
                     _awk_.  If the text does not end in a <newline>, _awk_ shall
                     interpret the text as if it did.
    
           _argument_  Either of the following two types of _argument_ can be
                     intermixed:
    
                     _file_      A pathname of a file that contains the input
                               to be read, which is matched against the set
                               of patterns in the program. If no _file_
                               operands are specified, or if a _file_ operand
                               is **'-'** , the standard input shall be used.
    
                     _assignment_
                               An operand that begins with an <underscore> or
                               alphabetic character from the portable
                               character set (see the table in the Base
                               Definitions volume of POSIX.1‐2017, _Section_
                               _6.1_ , _Portable Character Set_), followed by a
                               sequence of underscores, digits, and
                               alphabetics from the portable character set,
                               followed by the **'='** character, shall specify a
                               variable assignment rather than a pathname.
                               The characters before the **'='** represent the
                               name of an _awk_ variable; if that name is an
                               _awk_ reserved word (see _Grammar_) the behavior
                               is undefined. The characters following the
                               <equals-sign> shall be interpreted as if they
                               appeared in the _awk_ program preceded and
                               followed by a double-quote (**'"'**) character, as
                               a **STRING** token (see _Grammar_), except that if
                               the last character is an unescaped
                               <backslash>, it shall be interpreted as a
                               literal <backslash> rather than as the first
                               character of the sequence **"\""**.  The variable
                               shall be assigned the value of that **STRING**
                               token and, if appropriate, shall be considered
                               a _numeric string_ (see _Expressions in awk_), the
                               variable shall also be assigned its numeric
                               value. Each such variable assignment shall
                               occur just prior to the processing of the
                               following _file_ , if any. Thus, an assignment
                               before the first _file_ argument shall be
                               executed after the **BEGIN** actions (if any),
                               while an assignment after the last _file_
                               argument shall occur before the **END** actions
                               (if any). If there are no _file_ arguments,
                               assignments shall be executed before
                               processing the standard input.
    


## STDIN



           The standard input shall be used only if no _file_ operands are
           specified, or if a _file_ operand is **'-'** , or if a _progfile_ option-
           argument is **'-'** ; see the INPUT FILES section. If the _awk_ program
           contains no actions and no patterns, but is otherwise a valid _awk_
           program, standard input and any _file_ operands shall not be read
           and _awk_ shall exit with a return status of zero.
    


## INPUT FILES



           Input files to the _awk_ program from any of the following sources
           shall be text files:
    
            *  Any _file_ operands or their equivalents, achieved by modifying
               the _awk_ variables **ARGV** and **ARGC**
    
            *  Standard input in the absence of any _file_ operands
    
            *  Arguments to the **getline** function
    
           Whether the variable **RS** is set to a value other than a <newline>
           or not, for these files, implementations shall support records
           terminated with the specified separator up to {LINE_MAX} bytes and
           may support longer records.
    
           If **-f** _progfile_ is specified, the application shall ensure that the
           files named by each of the _progfile_ option-arguments are text
           files and their concatenation, in the same order as they appear in
           the arguments, is an _awk_ program.
    


## ENVIRONMENT VARIABLES



           The following environment variables shall affect the execution of
           _awk_ :
    
           _LANG_      Provide a default value for the internationalization
                     variables that are unset or null. (See the Base
                     Definitions volume of POSIX.1‐2017, _Section 8.2_ ,
                     _Internationalization Variables_ for the precedence of
                     internationalization variables used to determine the
                     values of locale categories.)
    
           _LC_ALL_    If set to a non-empty string value, override the values
                     of all the other internationalization variables.
    
           _LC_COLLATE_
                     Determine the locale for the behavior of ranges,
                     equivalence classes, and multi-character collating
                     elements within regular expressions and in comparisons
                     of string values.
    
           _LC_CTYPE_  Determine the locale for the interpretation of sequences
                     of bytes of text data as characters (for example,
                     single-byte as opposed to multi-byte characters in
                     arguments and input files), the behavior of character
                     classes within regular expressions, the identification
                     of characters as letters, and the mapping of uppercase
                     and lowercase characters for the **toupper** and **tolower**
                     functions.
    
           _LC_MESSAGES_
                     Determine the locale that should be used to affect the
                     format and contents of diagnostic messages written to
                     standard error.
    
           _LC_NUMERIC_
                     Determine the radix character used when interpreting
                     numeric input, performing conversions between numeric
                     and string values, and formatting numeric output.
                     Regardless of locale, the <period> character (the
                     decimal-point character of the POSIX locale) is the
                     decimal-point character recognized in processing _awk_
                     programs (including assignments in command line
                     arguments).
    
           _NLSPATH_   Determine the location of message catalogs for the
                     processing of _LC_MESSAGES_.
    
           _PATH_      Determine the search path when looking for commands
                     executed by _system_(_expr_), or input and output pipes; see
                     the Base Definitions volume of POSIX.1‐2017, _Chapter 8_ ,
                     _Environment Variables_.
    
           In addition, all environment variables shall be visible via the
           _awk_ variable **ENVIRON**.
    


## ASYNCHRONOUS EVENTS



           Default.
    


## STDOUT



           The nature of the output files depends on the _awk_ program.
    


## STDERR



           The standard error shall be used only for diagnostic messages.
    


## OUTPUT FILES



           The nature of the output files depends on the _awk_ program.
    


## EXTENDED DESCRIPTION



       **Overall Program Structure**
           An _awk_ program is composed of pairs of the form:
    
               _pattern_ { _action_ }
    
           Either the pattern or the action (including the enclosing brace
           characters) can be omitted.
    
           A missing pattern shall match any record of input, and a missing
           action shall be equivalent to:
    
               { print }
    
           Execution of the _awk_ program shall start by first executing the
           actions associated with all **BEGIN** patterns in the order they occur
           in the program. Then each _file_ operand (or standard input if no
           files were specified) shall be processed in turn by reading data
           from the file until a record separator is seen (<newline> by
           default). Before the first reference to a field in the record is
           evaluated, the record shall be split into fields, according to the
           rules in _Regular Expressions_ , using the value of **FS** that was
           current at the time the record was read. Each pattern in the
           program then shall be evaluated in the order of occurrence, and
           the action associated with each pattern that matches the current
           record executed. The action for a matching pattern shall be
           executed before evaluating subsequent patterns. Finally, the
           actions associated with all **END** patterns shall be executed in the
           order they occur in the program.
    
       **Expressions in awk**
           Expressions describe computations used in _patterns_ and _actions_.
           In the following table, valid expression operations are given in
           groups from highest precedence first to lowest precedence last,
           with equal-precedence operators grouped between horizontal lines.
           In expression evaluation, where the grammar is formally ambiguous,
           higher precedence operators shall be evaluated before lower
           precedence operators. In this table _expr_ , _expr1_ , _expr2_ , and _expr3_
           represent any expression, while lvalue represents any entity that
           can be assigned to (that is, on the left side of an assignment
           operator).  The precise syntax of expressions is given in _Grammar_.
    
                 **Table 4-1: Expressions in Decreasing Precedence in** _awk_
    ┌──────────────────────┬─────────────────────────┬────────────────┬──────────────┐
    │        **Syntax** │          **Name** │ **Type of Result** │**Associativity** │
    ├──────────────────────┼─────────────────────────┼────────────────┼──────────────┤
    │ ( _expr_ )             │Grouping                 │Type of _expr_    │N/A           │
    ├──────────────────────┼─────────────────────────┼────────────────┼──────────────┤
    │ $_expr_                │Field reference          │String          │N/A           │
    ├──────────────────────┼─────────────────────────┼────────────────┼──────────────┤
    │ lvalue ++            │Post-increment           │Numeric         │N/A           │
    │ lvalue --            │Post-decrement           │Numeric         │N/A           │
    ├──────────────────────┼─────────────────────────┼────────────────┼──────────────┤
    │ ++ lvalue            │Pre-increment            │Numeric         │N/A           │
    │ -- lvalue            │Pre-decrement            │Numeric         │N/A           │
    ├──────────────────────┼─────────────────────────┼────────────────┼──────────────┤
    │ _expr_ ^ _expr_          │Exponentiation           │Numeric         │Right         │
    ├──────────────────────┼─────────────────────────┼────────────────┼──────────────┤
    │ ! _expr_               │Logical not              │Numeric         │N/A           │
    │ + _expr_               │Unary plus               │Numeric         │N/A           │
    │ - _expr_               │Unary minus              │Numeric         │N/A           │
    ├──────────────────────┼─────────────────────────┼────────────────┼──────────────┤
    │ _expr_ * _expr_          │Multiplication           │Numeric         │Left          │
    │ _expr_ / _expr_          │Division                 │Numeric         │Left          │
    │ _expr_ % _expr_          │Modulus                  │Numeric         │Left          │
    ├──────────────────────┼─────────────────────────┼────────────────┼──────────────┤
    │ _expr_ + _expr_          │Addition                 │Numeric         │Left          │
    │ _expr_ - _expr_          │Subtraction              │Numeric         │Left          │
    ├──────────────────────┼─────────────────────────┼────────────────┼──────────────┤
    │ _expr expr_            │String concatenation     │String          │Left          │
    ├──────────────────────┼─────────────────────────┼────────────────┼──────────────┤
    │ _expr_ < _expr_          │Less than                │Numeric         │None          │
    │ _expr_ <= _expr_         │Less than or equal to    │Numeric         │None          │
    │ _expr_ != _expr_         │Not equal to             │Numeric         │None          │
    │ _expr_ == _expr_         │Equal to                 │Numeric         │None          │
    │ _expr_ > _expr_          │Greater than             │Numeric         │None          │
    │ _expr_ >= _expr_         │Greater than or equal to │Numeric         │None          │
    ├──────────────────────┼─────────────────────────┼────────────────┼──────────────┤
    │ _expr_ ~ _expr_          │ERE match                │Numeric         │None          │
    │ _expr_ !~ _expr_         │ERE non-match            │Numeric         │None          │
    ├──────────────────────┼─────────────────────────┼────────────────┼──────────────┤
    │ _expr_ in array        │Array membership         │Numeric         │Left          │
    │ ( _index_ ) in _array_   │Multi-dimension array    │Numeric         │Left          │
    │                      │membership               │                │              │
    ├──────────────────────┼─────────────────────────┼────────────────┼──────────────┤
    │ _expr_ && _expr_         │Logical AND              │Numeric         │Left          │
    ├──────────────────────┼─────────────────────────┼────────────────┼──────────────┤
    │ _expr_ || _expr_         │Logical OR               │Numeric         │Left          │
    ├──────────────────────┼─────────────────────────┼────────────────┼──────────────┤
    │ _expr1_ ? _expr2_ : _expr3_ │Conditional expression   │Type of selected│Right         │
    │                      │                         │ _expr2_ or _expr3_  │              │
    ├──────────────────────┼─────────────────────────┼────────────────┼──────────────┤
    │ lvalue ^= _expr_       │Exponentiation assignment│Numeric         │Right         │
    │ lvalue %= _expr_       │Modulus assignment       │Numeric         │Right         │
    │ lvalue *= _expr_       │Multiplication assignment│Numeric         │Right         │
    │ lvalue /= _expr_       │Division assignment      │Numeric         │Right         │
    │ lvalue += _expr_       │Addition assignment      │Numeric         │Right         │
    │ lvalue -= _expr_       │Subtraction assignment   │Numeric         │Right         │
    │ lvalue = _expr_        │Assignment               │Type of _expr_    │Right         │
    └──────────────────────┴─────────────────────────┴────────────────┴──────────────┘
    
           Each expression shall have either a string value, a numeric value,
           or both. Except as stated for specific contexts, the value of an
           expression shall be implicitly converted to the type needed for
           the context in which it is used. A string value shall be converted
           to a numeric value either by the equivalent of the following calls
           to functions defined by the ISO C standard:
    
               setlocale(LC_NUMERIC, "");
               _numeric_value_ = atof(_string_value_);
    
           or by converting the initial portion of the string to type **double**
           representation as follows:
    
                  The input string is decomposed into two parts: an initial,
                  possibly empty, sequence of white-space characters (as
                  specified by _isspace_()) and a subject sequence interpreted
                  as a floating-point constant.
    
                  The expected form of the subject sequence is an optional
                  **'+'** or **'-'** sign, then a non-empty sequence of digits
                  optionally containing a <period>, then an optional exponent
                  part. An exponent part consists of **'e'** or **'E'** , followed by
                  an optional sign, followed by one or more decimal digits.
    
                  The sequence starting with the first digit or the <period>
                  (whichever occurs first) is interpreted as a floating
                  constant of the C language, and if neither an exponent part
                  nor a <period> appears, a <period> is assumed to follow the
                  last digit in the string. If the subject sequence begins
                  with a <hyphen-minus>, the value resulting from the
                  conversion is negated.
    
           A numeric value that is exactly equal to the value of an integer
           (see _Section 1.1.2_ , _Concepts Derived from the ISO C Standard_)
           shall be converted to a string by the equivalent of a call to the
           **sprintf** function (see _String Functions_) with the string **"%d"** as
           the _fmt_ argument and the numeric value being converted as the
           first and only _expr_ argument. Any other numeric value shall be
           converted to a string by the equivalent of a call to the **sprintf**
           function with the value of the variable **CONVFMT** as the _fmt_
           argument and the numeric value being converted as the first and
           only _expr_ argument. The result of the conversion is unspecified if
           the value of **CONVFMT** is not a floating-point format specification.
           This volume of POSIX.1‐2017 specifies no explicit conversions
           between numbers and strings. An application can force an
           expression to be treated as a number by adding zero to it, or can
           force it to be treated as a string by concatenating the null
           string (**""**) to it.
    
           A string value shall be considered a _numeric string_ if it comes
           from one of the following:
    
            1. Field variables
    
            2. Input from the _getline_() function
    
            3. **FILENAME**
    
            4. **ARGV** array elements
    
            5. **ENVIRON** array elements
    
            6. Array elements created by the _split_() function
    
            7. A command line variable assignment
    
            8. Variable assignment from another numeric string variable
    
           and an implementation-dependent condition corresponding to either
           case (a) or (b) below is met.
    
            a. After the equivalent of the following calls to functions
               defined by the ISO C standard, _string_value_end_ would differ
               from _string_value_ , and any characters before the terminating
               null character in _string_value_end_ would be <blank>
               characters:
    
                   char *string_value_end;
                   setlocale(LC_NUMERIC, "");
                   numeric_value = strtod (string_value, &string_value_end);
    
            b. After all the following conversions have been applied, the
               resulting string would lexically be recognized as a **NUMBER**
               token as described by the lexical conventions in _Grammar_ :
    
               --  All leading and trailing <blank> characters are discarded.
    
               --  If the first non-<blank> is **'+'** or **'-'** , it is discarded.
    
               --  Each occurrence of the decimal point character from the
                   current locale is changed to a <period>.
           In case (a) the numeric value of the _numeric string_ shall be the
           value that would be returned by the _strtod_() call. In case (b) if
           the first non-<blank> is **'-'** , the numeric value of the _numeric_
           _string_ shall be the negation of the numeric value of the
           recognized **NUMBER** token; otherwise, the numeric value of the
           _numeric string_ shall be the numeric value of the recognized **NUMBER**
           token. Whether or not a string is a _numeric string_ shall be
           relevant only in contexts where that term is used in this section.
    
           When an expression is used in a Boolean context, if it has a
           numeric value, a value of zero shall be treated as false and any
           other value shall be treated as true. Otherwise, a string value of
           the null string shall be treated as false and any other value
           shall be treated as true.  A Boolean context shall be one of the
           following:
    
            *  The first subexpression of a conditional expression
    
            *  An expression operated on by logical NOT, logical AND, or
               logical OR
    
            *  The second expression of a **for** statement
    
            *  The expression of an **if** statement
    
            *  The expression of the **while** clause in either a **while** or
               **do**...**while** statement
    
            *  An expression used as a pattern (as in Overall Program
               Structure)
    
           All arithmetic shall follow the semantics of floating-point
           arithmetic as specified by the ISO C standard (see _Section 1.1.2_ ,
           _Concepts Derived from the ISO C Standard_).
    
           The value of the expression:
    
               _expr1_ ^ _expr2_
    
           shall be equivalent to the value returned by the ISO C standard
           function call:
    
               pow(_expr1_ , _expr2_)
    
           The expression:
    
               lvalue ^= _expr_
    
           shall be equivalent to the ISO C standard expression:
    
               lvalue = pow(lvalue, _expr_)
    
           except that lvalue shall be evaluated only once. The value of the
           expression:
    
               _expr1_ % _expr2_
    
           shall be equivalent to the value returned by the ISO C standard
           function call:
    
               fmod(_expr1_ , _expr2_)
    
           The expression:
    
               lvalue %= _expr_
    
           shall be equivalent to the ISO C standard expression:
    
               lvalue = fmod(lvalue, _expr_)
    
           except that lvalue shall be evaluated only once.
    
           Variables and fields shall be set by the assignment statement:
    
               lvalue = _expression_
    
           and the type of _expression_ shall determine the resulting variable
           type. The assignment includes the arithmetic assignments (**"+="** ,
           **"-="** , **"*="** , **"/="** , **"%="** , **"^="** , **"++"** , **"--"**) all of which shall
           produce a numeric result. The left-hand side of an assignment and
           the target of increment and decrement operators can be one of a
           variable, an array with index, or a field selector.
    
           The _awk_ language supplies arrays that are used for storing numbers
           or strings.  Arrays need not be declared. They shall initially be
           empty, and their sizes shall change dynamically. The subscripts,
           or element identifiers, are strings, providing a type of
           associative array capability. An array name followed by a
           subscript within square brackets can be used as an lvalue and thus
           as an expression, as described in the grammar; see _Grammar_.
           Unsubscripted array names can be used in only the following
           contexts:
    
            *  A parameter in a function definition or function call
    
            *  The **NAME** token following any use of the keyword **in** as
               specified in the grammar (see _Grammar_); if the name used in
               this context is not an array name, the behavior is undefined
    
           A valid array _index_ shall consist of one or more <comma>-separated
           expressions, similar to the way in which multi-dimensional arrays
           are indexed in some programming languages. Because _awk_ arrays are
           really one-dimensional, such a <comma>-separated list shall be
           converted to a single string by concatenating the string values of
           the separate expressions, each separated from the other by the
           value of the **SUBSEP** variable. Thus, the following two index
           operations shall be equivalent:
    
               _var_**[**_expr1_ , _expr2_ , ... _exprn_**]**
    
               _var_**[**_expr1_ SUBSEP _expr2_ SUBSEP ... SUBSEP _exprn_**]**
    
           The application shall ensure that a multi-dimensioned _index_ used
           with the **in** operator is parenthesized. The **in** operator, which
           tests for the existence of a particular array element, shall not
           cause that element to exist. Any other reference to a nonexistent
           array element shall automatically create it.
    
           Comparisons (with the **' <'**, **" <="**, **"!="** , **"=="** , **' >'**, and **" >="**
           operators) shall be made numerically if both operands are numeric,
           if one is numeric and the other has a string value that is a
           numeric string, or if one is numeric and the other has the
           uninitialized value.  Otherwise, operands shall be converted to
           strings as required and a string comparison shall be made as
           follows:
    
            *  For the **"!="** and **"=="** operators, the strings should be
               compared to check if they are identical but may be compared
               using the locale-specific collation sequence to check if they
               collate equally.
    
            *  For the other operators, the strings shall be compared using
               the locale-specific collation sequence.
    
           The value of the comparison expression shall be 1 if the relation
           is true, or 0 if the relation is false.
    
       **Variables and Special Variables**
           Variables can be used in an _awk_ program by referencing them. With
           the exception of function parameters (see _User-Defined Functions_),
           they are not explicitly declared. Function parameter names shall
           be local to the function; all other variable names shall be
           global. The same name shall not be used as both a function
           parameter name and as the name of a function or a special _awk_
           variable. The same name shall not be used both as a variable name
           with global scope and as the name of a function. The same name
           shall not be used within the same scope both as a scalar variable
           and as an array.  Uninitialized variables, including scalar
           variables, array elements, and field variables, shall have an
           uninitialized value. An uninitialized value shall have both a
           numeric value of zero and a string value of the empty string.
           Evaluation of variables with an uninitialized value, to either
           string or numeric, shall be determined by the context in which
           they are used.
    
           Field variables shall be designated by a **'$'** followed by a number
           or numerical expression. The effect of the field number _expression_
           evaluating to anything other than a non-negative integer is
           unspecified; uninitialized variables or string values need not be
           converted to numeric values in this context. New field variables
           can be created by assigning a value to them. References to
           nonexistent fields (that is, fields after $**NF**), shall evaluate to
           the uninitialized value. Such references shall not create new
           fields. However, assigning to a nonexistent field (for example,
           $(**NF** +2)=5) shall increase the value of **NF** ; create any intervening
           fields with the uninitialized value; and cause the value of $0 to
           be recomputed, with the fields being separated by the value of
           **OFS**.  Each field variable shall have a string value or an
           uninitialized value when created. Field variables shall have the
           uninitialized value when created from $0 using **FS** and the variable
           does not contain any characters. If appropriate, the field
           variable shall be considered a numeric string (see _Expressions in_
           _awk_).
    
           Implementations shall support the following other special
           variables that are set by _awk_ :
    
           **ARGC** The number of elements in the **ARGV** array.
    
           **ARGV** An array of command line arguments, excluding options
                     and the _program_ argument, numbered from zero to **ARGC** -1.
    
                     The arguments in **ARGV** can be modified or added to; **ARGC**
                     can be altered. As each input file ends, _awk_ shall treat
                     the next non-null element of **ARGV** , up to the current
                     value of **ARGC** -1, inclusive, as the name of the next
                     input file. Thus, setting an element of **ARGV** to null
                     means that it shall not be treated as an input file. The
                     name **'-'** indicates the standard input. If an argument
                     matches the format of an _assignment_ operand, this
                     argument shall be treated as an _assignment_ rather than a
                     _file_ argument.
    
           **CONVFMT** The **printf** format for converting numbers to strings
                     (except for output statements, where **OFMT** is used);
                     **"%.6g"** by default.
    
           **ENVIRON** An array representing the value of the environment, as
                     described in the _exec_ functions defined in the System
                     Interfaces volume of POSIX.1‐2017. The indices of the
                     array shall be strings consisting of the names of the
                     environment variables, and the value of each array
                     element shall be a string consisting of the value of
                     that variable. If appropriate, the environment variable
                     shall be considered a _numeric string_ (see _Expressions in_
                     _awk_); the array element shall also have its numeric
                     value.
    
                     In all cases where the behavior of _awk_ is affected by
                     environment variables (including the environment of any
                     commands that _awk_ executes via the **system** function or
                     via pipeline redirections with the **print** statement, the
                     **printf** statement, or the **getline** function), the
                     environment used shall be the environment at the time
                     _awk_ began executing; it is implementation-defined
                     whether any modification of **ENVIRON** affects this
                     environment.
    
           **FILENAME** A pathname of the current input file. Inside a **BEGIN**
                     action the value is undefined. Inside an **END** action the
                     value shall be the name of the last input file
                     processed.
    
           **FNR** The ordinal number of the current record in the current
                     file. Inside a **BEGIN** action the value shall be zero.
                     Inside an **END** action the value shall be the number of
                     the last record processed in the last file processed.
    
           **FS** Input field separator regular expression; a <space> by
                     default.
    
           **NF** The number of fields in the current record. Inside a
                     **BEGIN** action, the use of **NF** is undefined unless a
                     **getline** function without a _var_ argument is executed
                     previously. Inside an **END** action, **NF** shall retain the
                     value it had for the last record read, unless a
                     subsequent, redirected, **getline** function without a _var_
                     argument is performed prior to entering the **END** action.
    
           **NR** The ordinal number of the current record from the start
                     of input.  Inside a **BEGIN** action the value shall be
                     zero. Inside an **END** action the value shall be the number
                     of the last record processed.
    
           **OFMT** The **printf** format for converting numbers to strings in
                     output statements (see _Output Statements_); **"%.6g"** by
                     default. The result of the conversion is unspecified if
                     the value of **OFMT** is not a floating-point format
                     specification.
    
           **OFS** The **print** statement output field separator; <space> by
                     default.
    
           **ORS** The **print** statement output record separator; a <newline>
                     by default.
    
           **RLENGTH** The length of the string matched by the **match** function.
    
           **RS** The first character of the string value of **RS** shall be
                     the input record separator; a <newline> by default. If
                     **RS** contains more than one character, the results are
                     unspecified. If **RS** is null, then records are separated
                     by sequences consisting of a <newline> plus one or more
                     blank lines, leading or trailing blank lines shall not
                     result in empty records at the beginning or end of the
                     input, and a <newline> shall always be a field
                     separator, no matter what the value of **FS** is.
    
           **RSTART** The starting position of the string matched by the **match**
                     function, numbering from 1. This shall always be
                     equivalent to the return value of the **match** function.
    
           **SUBSEP** The subscript separator string for multi-dimensional
                     arrays; the default value is implementation-defined.
    
       **Regular Expressions**
           The _awk_ utility shall make use of the extended regular expression
           notation (see the Base Definitions volume of POSIX.1‐2017, _Section_
           _9.4_ , _Extended Regular Expressions_) except that it shall allow the
           use of C-language conventions for escaping special characters
           within the EREs, as specified in the table in the Base Definitions
           volume of POSIX.1‐2017, _Chapter 5_ , _File Format Notation_ (**'\\'** ,
           **'\a'** , **'\b'** , **'\f'** , **'\n'** , **'\r'** , **'\t'** , **'\v'**) and the following table;
           these escape sequences shall be recognized both inside and outside
           bracket expressions. Note that records need not be separated by
           <newline> characters and string constants can contain <newline>
           characters, so even the **"\n"** sequence is valid in _awk_ EREs. Using
           a <slash> character within an ERE requires the escaping shown in
           the following table.
    
                           **Table 4-2: Escape Sequences in** _awk_
    ┌──────────┬────────────────────────────────────┬────────────────────────────────────┐
    │  **Escape** │                                    │                                    │
    │ **Sequence** │            **Description** │              **Meaning** │
    ├──────────┼────────────────────────────────────┼────────────────────────────────────┤
    │ \"       │ <backslash> <quotation-mark>       │ <quotation-mark> character         │
    ├──────────┼────────────────────────────────────┼────────────────────────────────────┤
    │ \/       │ <backslash> <slash>                │ <slash> character                  │
    ├──────────┼────────────────────────────────────┼────────────────────────────────────┤
    │ \ddd     │ A <backslash> character followed   │ The character whose encoding is    │
    │          │ by the longest sequence of one,    │ represented by the one, two, or    │
    │          │ two, or three octal-digit          │ three-digit octal integer. Multi-  │
    │          │ characters (01234567). If all of   │ byte characters require multiple,  │
    │          │ the digits are 0 (that is,         │ concatenated escape sequences of   │
    │          │ representation of the NUL          │ this type, including the leading   │
    │          │ character), the behavior is        │ <backslash> for each byte.         │
    │          │ undefined.                         │                                    │
    ├──────────┼────────────────────────────────────┼────────────────────────────────────┤
    │ \c       │ A <backslash> character followed   │ Undefined                          │
    │          │ by any character not described in  │                                    │
    │          │ this table or in the table in the  │                                    │
    │          │ Base Definitions volume of         │                                    │
    │          │ POSIX.1‐2017, _Chapter 5_ , _File_      │                                    │
    │          │ _Format Notation_ (**'\\'** , **'\a'** , **'\b'** , │                                    │
    │          │ **'\f'** , **'\n'** , **'\r'** , **'\t'** , **'\v'**).     │                                    │
    └──────────┴────────────────────────────────────┴────────────────────────────────────┘
    
           A  regular  expression  can be matched against a specific field or
           string by  using  one  of  the  two  regular  expression  matching
           operators,  **'~'** and  **"!~"**.  These operators shall interpret their
           right-hand operand as a regular  expression  and  their  left-hand
           operand as a string. If the regular expression matches the string,
           the  **'~'** expression  shall evaluate to a value of 1, and the **"!~"**
           expression shall evaluate to a value of 0. (The regular expression
           matching operation is as defined by the term matched in  the  Base
           Definitions   volume   of   POSIX.1‐2017,   _Section_  _9.1_ ,  _Regular_
           _Expression Definitions_ , where a match occurs on any  part  of  the
           string   unless   the  regular  expression  is  limited  with  the
           <circumflex> or <dollar-sign> special characters.) If the  regular
           expression  does  not  match  the string, the **'~'** expression shall
           evaluate to a value of 0, and the **"!~"** expression  shall  evaluate
           to a value of 1. If the right-hand operand is any expression other
           than  the  lexical  token  **ERE** , the string value of the expression
           shall be interpreted as an extended regular expression,  including
           the  escape  conventions  described  above.   Note that these same
           escape conventions shall also be applied in determining the  value
           of  a string literal (the lexical token **STRING**), and thus shall be
           applied a second time when  a  string  literal  is  used  in  this
           context.
    
           When  an  **ERE** token appears as an expression in any context other
           than as the right-hand of the **'~'** or **"!~"** operator or  as  one  of
           the  built-in function arguments described below, the value of the
           resulting expression shall be the equivalent of:
    
               $0 ~ /_ere_ /
    
           The _ere_ argument to the **gsub** , **match** , **sub** functions,  and  the  _fs_
           argument  to  the  **split** function (see _String Functions_) shall be
           interpreted as extended regular expressions. These can  be  either
           **ERE** tokens  or arbitrary expressions, and shall be interpreted in
           the same manner  as  the  right-hand  side  of  the  **'~'** or  **"!~"**
           operator.
    
           An  extended  regular expression can be used to separate fields by
           assigning a string  containing  the  expression  to  the  built-in
           variable  **FS** ,  either directly or as a consequence of using the **-F**
           _sepstring_ option.  The default value of the **FS** variable shall be a
           single <space>.  The following describes **FS** behavior:
    
            1. If **FS** is a null string, the behavior is unspecified.
    
            2. If **FS** is a single character:
    
                a. If **FS** is <space>, skip leading and trailing <blank> and
                   <newline> characters; fields shall be delimited by sets of
                   one or more <blank> or <newline> characters.
    
                b. Otherwise, if **FS** is any other character _c_ , fields shall be
                   delimited by each single occurrence of _c_.
    
            3. Otherwise, the string value of **FS** shall be considered to be an
               extended regular expression. Each occurrence of a sequence
               matching the extended regular expression shall delimit fields.
    
           Except for the **'~'** and **"!~"** operators, and in the **gsub** , **match** ,
           **split** , and **sub** built-in functions, ERE matching shall be based on
           input records; that is, record separator characters (the first
           character of the value of the variable **RS** , <newline> by default)
           cannot be embedded in the expression, and no expression shall
           match the record separator character. If the record separator is
           not <newline>, <newline> characters embedded in the expression can
           be matched. For the **'~'** and **"!~"** operators, and in those four
           built-in functions, ERE matching shall be based on text strings;
           that is, any character (including <newline> and the record
           separator) can be embedded in the pattern, and an appropriate
           pattern shall match any character. However, in all _awk_ ERE
           matching, the use of one or more NUL characters in the pattern,
           input record, or text string produces undefined results.
    
       **Patterns**
           A _pattern_ is any valid _expression_ , a range specified by two
           expressions separated by a comma, or one of the two special
           patterns **BEGIN** or **END**.
    
       **Special Patterns**
           The _awk_ utility shall recognize two special patterns, **BEGIN** and
           **END**.  Each **BEGIN** pattern shall be matched once and its associated
           action executed before the first record of input is read—except
           possibly by use of the **getline** function (see _Input/Output and_
           _General Functions_) in a prior **BEGIN** action—and before command line
           assignment is done. Each **END** pattern shall be matched once and its
           associated action executed after the last record of input has been
           read. These two patterns shall have associated actions.
    
           **BEGIN** and **END** shall not combine with other patterns. Multiple
           **BEGIN** and **END** patterns shall be allowed. The actions associated
           with the **BEGIN** patterns shall be executed in the order specified
           in the program, as are the **END** actions. An **END** pattern can precede
           a **BEGIN** pattern in a program.
    
           If an _awk_ program consists of only actions with the pattern **BEGIN** ,
           and the **BEGIN** action contains no **getline** function, _awk_ shall exit
           without reading its input when the last statement in the last
           **BEGIN** action is executed. If an _awk_ program consists of only
           actions with the pattern **END** or only actions with the patterns
           **BEGIN** and **END** , the input shall be read before the statements in
           the **END** actions are executed.
    
       **Expression Patterns**
           An expression pattern shall be evaluated as if it were an
           expression in a Boolean context. If the result is true, the
           pattern shall be considered to match, and the associated action
           (if any) shall be executed. If the result is false, the action
           shall not be executed.
    
       **Pattern Ranges**
           A pattern range consists of two expressions separated by a comma;
           in this case, the action shall be performed for all records
           between a match of the first expression and the following match of
           the second expression, inclusive. At this point, the pattern range
           can be repeated starting at input records subsequent to the end of
           the matched range.
    
       **Actions**
           An action is a sequence of statements as shown in the grammar in
           _Grammar_.  Any single statement can be replaced by a statement list
           enclosed in curly braces. The application shall ensure that
           statements in a statement list are separated by <newline> or
           <semicolon> characters. Statements in a statement list shall be
           executed sequentially in the order that they appear.
    
           The _expression_ acting as the conditional in an **if** statement shall
           be evaluated and if it is non-zero or non-null, the following
           statement shall be executed; otherwise, if **else** is present, the
           statement following the **else** shall be executed.
    
           The **if** , **while** , **do**...**while** , **for** , **break** , and **continue** statements are
           based on the ISO C standard (see _Section 1.1.2_ , _Concepts Derived_
           _from the ISO C Standard_), except that the Boolean expressions
           shall be treated as described in _Expressions in awk_ , and except in
           the case of:
    
               for (_variable_ in _array_)
    
           which shall iterate, assigning each _index_ of _array_ to _variable_ in
           an unspecified order. The results of adding new elements to _array_
           within such a **for** loop are undefined. If a **break** or **continue**
           statement occurs outside of a loop, the behavior is undefined.
    
           The **delete** statement shall remove an individual array element.
           Thus, the following code deletes an entire array:
    
               for (index in array)
                   delete array[index]
    
           The **next** statement shall cause all further processing of the
           current input record to be abandoned. The behavior is undefined if
           a **next** statement appears or is invoked in a **BEGIN** or **END** action.
    
           The **exit** statement shall invoke all **END** actions in the order in
           which they occur in the program source and then terminate the
           program without reading further input. An **exit** statement inside an
           **END** action shall terminate the program without further execution
           of **END** actions. If an expression is specified in an **exit**
           statement, its numeric value shall be the exit status of _awk_ ,
           unless subsequent errors are encountered or a subsequent **exit**
           statement with an expression is executed.
    
       **Output Statements**
           Both **print** and **printf** statements shall write to standard output by
           default. The output shall be written to the location specified by
           _output_redirection_ if one is supplied, as follows:
    
               > _expression_
               >> _expression_
               | _expression_
    
           In all cases, the _expression_ shall be evaluated to produce a
           string that is used as a pathname into which to write (for **' >' **or
           **" >>"**) or as a command to be executed (for **'|'**).  Using the first
           two forms, if the file of that name is not currently open, it
           shall be opened, creating it if necessary and using the first
           form, truncating the file. The output then shall be appended to
           the file. As long as the file remains open, subsequent calls in
           which _expression_ evaluates to the same string value shall simply
           append output to the file. The file remains open until the **close**
           function (see _Input/Output and General Functions_) is called with
           an expression that evaluates to the same string value.
    
           The third form shall write output onto a stream piped to the input
           of a command. The stream shall be created if no stream is
           currently open with the value of _expression_ as its command name.
           The stream created shall be equivalent to one created by a call to
           the _popen_() function defined in the System Interfaces volume of
           POSIX.1‐2017 with the value of _expression_ as the _command_ argument
           and a value of _w_ as the _mode_ argument. As long as the stream
           remains open, subsequent calls in which _expression_ evaluates to
           the same string value shall write output to the existing stream.
           The stream shall remain open until the **close** function (see
           _Input/Output and General Functions_) is called with an expression
           that evaluates to the same string value.  At that time, the stream
           shall be closed as if by a call to the _pclose_() function defined
           in the System Interfaces volume of POSIX.1‐2017.
    
           As described in detail by the grammar in _Grammar_ , these output
           statements shall take a <comma>-separated list of _expression_ s
           referred to in the grammar by the non-terminal symbols **expr_list** ,
           **print_expr_list** , or **print_expr_list_opt**.  This list is referred to
           here as the _expression list_ , and each member is referred to as an
           _expression argument_.
    
           The **print** statement shall write the value of each expression
           argument onto the indicated output stream separated by the current
           output field separator (see variable **OFS** above), and terminated by
           the output record separator (see variable **ORS** above). All
           expression arguments shall be taken as strings, being converted if
           necessary; this conversion shall be as described in _Expressions in_
           _awk_ , with the exception that the **printf** format in **OFMT** shall be
           used instead of the value in **CONVFMT**.  An empty expression list
           shall stand for the whole input record ($0).
    
           The **printf** statement shall produce output based on a notation
           similar to the File Format Notation used to describe file formats
           in this volume of POSIX.1‐2017 (see the Base Definitions volume of
           POSIX.1‐2017, _Chapter 5_ , _File Format Notation_).  Output shall be
           produced as specified with the first _expression_ argument as the
           string _format_ and subsequent _expression_ arguments as the strings
           _arg1_ to _argn_ , inclusive, with the following exceptions:
    
            1. The _format_ shall be an actual character string rather than a
               graphical representation. Therefore, it cannot contain empty
               character positions. The <space> in the _format_ string, in any
               context other than a _flag_ of a conversion specification, shall
               be treated as an ordinary character that is copied to the
               output.
    
            2. If the character set contains a **'** ' character and that
               character appears in the _format_ string, it shall be treated as
               an ordinary character that is copied to the output.
    
            3. The _escape sequences_ beginning with a <backslash> character
               shall be treated as sequences of ordinary characters that are
               copied to the output. Note that these same sequences shall be
               interpreted lexically by _awk_ when they appear in literal
               strings, but they shall not be treated specially by the **printf**
               statement.
    
            4. A _field width_ or _precision_ can be specified as the **'*'**
               character instead of a digit string. In this case the next
               argument from the expression list shall be fetched and its
               numeric value taken as the field width or precision.
    
            5. The implementation shall not precede or follow output from the
               **d** or **u** conversion specifier characters with <blank> characters
               not specified by the _format_ string.
    
            6. The implementation shall not precede output from the **o**
               conversion specifier character with leading zeros not
               specified by the _format_ string.
    
            7. For the **c** conversion specifier character: if the argument has
               a numeric value, the character whose encoding is that value
               shall be output. If the value is zero or is not the encoding
               of any character in the character set, the behavior is
               undefined. If the argument does not have a numeric value, the
               first character of the string value shall be output; if the
               string does not contain any characters, the behavior is
               undefined.
    
            8. For each conversion specification that consumes an argument,
               the next expression argument shall be evaluated. With the
               exception of the **c** conversion specifier character, the value
               shall be converted (according to the rules specified in
               _Expressions in awk_) to the appropriate type for the conversion
               specification.
    
            9. If there are insufficient expression arguments to satisfy all
               the conversion specifications in the _format_ string, the
               behavior is undefined.
    
           10. If any character sequence in the _format_ string begins with a
               **'%'** character, but does not form a valid conversion
               specification, the behavior is unspecified.
    
           Both **print** and **printf** can output at least {LINE_MAX} bytes.
    
       **Functions**
           The _awk_ language has a variety of built-in functions: arithmetic,
           string, input/output, and general.
    
       **Arithmetic Functions**
           The arithmetic functions, except for **int** , shall be based on the
           ISO C standard (see _Section 1.1.2_ , _Concepts Derived from the ISO C_
           _Standard_).  The behavior is undefined in cases where the ISO C
           standard specifies that an error be returned or that the behavior
           is undefined. Although the grammar (see _Grammar_) permits built-in
           functions to appear with no arguments or parentheses, unless the
           argument or parentheses are indicated as optional in the following
           list (by displaying them within the **"[]"** brackets), such use is
           undefined.
    
           **atan2**(_y_ ,_x_)
                     Return arctangent of _y_ /_x_ in radians in the range [-π,π].
    
           **cos**(_x_)    Return cosine of _x_ , where _x_ is in radians.
    
           **sin**(_x_)    Return sine of _x_ , where _x_ is in radians.
    
           **exp**(_x_)    Return the exponential function of _x_.
    
           **log**(_x_)    Return the natural logarithm of _x_.
    
           **sqrt**(_x_)   Return the square root of _x_.
    
           **int**(_x_)    Return the argument truncated to an integer. Truncation
                     shall be toward 0 when _x_ >0.
    
           **rand**()    Return a random number _n_ , such that 0≤ _n_ <1.
    
           **srand**(**[**_expr_**]**)
                     Set the seed value for _rand_ to _expr_ or use the time of
                     day if _expr_ is omitted. The previous seed value shall be
                     returned.
    
       **String Functions**
           The string functions in the following list shall be supported.
           Although the grammar (see _Grammar_) permits built-in functions to
           appear with no arguments or parentheses, unless the argument or
           parentheses are indicated as optional in the following list (by
           displaying them within the **"[]"** brackets), such use is undefined.
    
           **gsub**(_ere_ , _repl_**[** , _in_**]**)
                     Behave like **sub**(see below), except that it shall
                     replace all occurrences of the regular expression (like
                     the _ed_ utility global substitute) in $0 or in the _in_
                     argument, when specified.
    
           **index**(_s_ , _t_)
                     Return the position, in characters, numbering from 1, in
                     string _s_ where string _t_ first occurs, or zero if it does
                     not occur at all.
    
           **length[**(**[**_s_**]**)**]**
                     Return the length, in characters, of its argument taken
                     as a string, or of the whole record, $0, if there is no
                     argument.
    
           **match**(_s_ , _ere_)
                     Return the position, in characters, numbering from 1, in
                     string _s_ where the extended regular expression _ere_
                     occurs, or zero if it does not occur at all. RSTART
                     shall be set to the starting position (which is the same
                     as the returned value), zero if no match is found;
                     RLENGTH shall be set to the length of the matched
                     string, -1 if no match is found.
    
           **split**(_s_ , _a_**[** , _fs_ **]**)
                     Split the string _s_ into array elements _a_[1], _a_[2], ...,
                     _a_[_n_], and return _n_.  All elements of the array shall be
                     deleted before the split is performed. The separation
                     shall be done with the ERE _fs_ or with the field
                     separator **FS** if _fs_ is not given. Each array element
                     shall have a string value when created and, if
                     appropriate, the array element shall be considered a
                     numeric string (see _Expressions in awk_).  The effect of
                     a null string as the value of _fs_ is unspecified.
    
           **sprintf**(_fmt_ , _expr_ , _expr_ , ...)
                     Format the expressions according to the **printf** format
                     given by _fmt_ and return the resulting string.
    
           **sub(**_ere_ , _repl_**[** , _in_ **]**)
                     Substitute the string _repl_ in place of the first
                     instance of the extended regular expression _ERE_ in
                     string _in_ and return the number of substitutions. An
                     <ampersand> (**' &'**) appearing in the string _repl_ shall be
                     replaced by the string from _in_ that matches the ERE. An
                     <ampersand> preceded with a <backslash> shall be
                     interpreted as the literal <ampersand> character. An
                     occurrence of two consecutive <backslash> characters
                     shall be interpreted as just a single literal
                     <backslash> character. Any other occurrence of a
                     <backslash> (for example, preceding any other character)
                     shall be treated as a literal <backslash> character.
                     Note that if _repl_ is a string literal (the lexical token
                     **STRING** ; see _Grammar_), the handling of the <ampersand>
                     character occurs after any lexical processing, including
                     any lexical <backslash>-escape sequence processing. If
                     _in_ is specified and it is not an lvalue (see _Expressions_
                     _in awk_), the behavior is undefined. If _in_ is omitted,
                     _awk_ shall use the current record ($0) in its place.
    
           **substr**(_s_ , _m_**[** , _n_ **]**)
                     Return the at most _n_ -character substring of _s_ that
                     begins at position _m_ , numbering from 1. If _n_ is omitted,
                     or if _n_ specifies more characters than are left in the
                     string, the length of the substring shall be limited by
                     the length of the string _s_.
    
           **tolower**(_s_)
                     Return a string based on the string _s_.  Each character
                     in _s_ that is an uppercase letter specified to have a
                     **tolower** mapping by the _LC_CTYPE_ category of the current
                     locale shall be replaced in the returned string by the
                     lowercase letter specified by the mapping. Other
                     characters in _s_ shall be unchanged in the returned
                     string.
    
           **toupper**(_s_)
                     Return a string based on the string _s_.  Each character
                     in _s_ that is a lowercase letter specified to have a
                     **toupper** mapping by the _LC_CTYPE_ category of the current
                     locale is replaced in the returned string by the
                     uppercase letter specified by the mapping. Other
                     characters in _s_ are unchanged in the returned string.
    
           All of the preceding functions that take _ERE_ as a parameter expect
           a pattern or a string valued expression that is a regular
           expression as defined in _Regular Expressions_.
    
       **Input/Output and General Functions**
           The input/output and general functions are:
    
           **close**(_expression_)
                     Close the file or pipe opened by a **print** or **printf**
                     statement or a call to **getline** with the same string-
                     valued _expression_.  The limit on the number of open
                     _expression_ arguments is implementation-defined. If the
                     close was successful, the function shall return zero;
                     otherwise, it shall return non-zero.
    
           _expression |_ **getline [**_var_**]**
                     Read a record of input from a stream piped from the
                     output of a command. The stream shall be created if no
                     stream is currently open with the value of _expression_ as
                     its command name. The stream created shall be equivalent
                     to one created by a call to the _popen_() function with
                     the value of _expression_ as the _command_ argument and a
                     value of _r_ as the _mode_ argument. As long as the stream
                     remains open, subsequent calls in which _expression_
                     evaluates to the same string value shall read subsequent
                     records from the stream. The stream shall remain open
                     until the **close** function is called with an expression
                     that evaluates to the same string value. At that time,
                     the stream shall be closed as if by a call to the
                     _pclose_() function. If _var_ is omitted, $0 and **NF** shall be
                     set; otherwise, _var_ shall be set and, if appropriate, it
                     shall be considered a numeric string (see _Expressions in_
                     _awk_).
    
                     The **getline** operator can form ambiguous constructs when
                     there are unparenthesized operators (including
                     concatenate) to the left of the **'|'**(to the beginning of
                     the expression containing **getline**).  In the context of
                     the **'$'** operator, **'|'** shall behave as if it had a lower
                     precedence than **'$'**.  The result of evaluating other
                     operators is unspecified, and conforming applications
                     shall parenthesize properly all such usages.
    
           **getline** Set $0 to the next input record from the current input
                     file. This form of **getline** shall set the **NF** , **NR** , and **FNR**
                     variables.
    
           **getline** _var_
                     Set variable _var_ to the next input record from the
                     current input file and, if appropriate, _var_ shall be
                     considered a numeric string (see _Expressions in awk_).
                     This form of **getline** shall set the **FNR** and **NR** variables.
    
           **getline [**_var_**]** < _expression_
                     Read the next record of input from a named file. The
                     _expression_ shall be evaluated to produce a string that
                     is used as a pathname.  If the file of that name is not
                     currently open, it shall be opened. As long as the
                     stream remains open, subsequent calls in which
                     _expression_ evaluates to the same string value shall read
                     subsequent records from the file. The file shall remain
                     open until the **close** function is called with an
                     expression that evaluates to the same string value. If
                     _var_ is omitted, $0 and **NF** shall be set; otherwise, _var_
                     shall be set and, if appropriate, it shall be considered
                     a numeric string (see _Expressions in awk_).
    
                     The **getline** operator can form ambiguous constructs when
                     there are unparenthesized binary operators (including
                     concatenate) to the right of the **' <' **(up to the end of
                     the expression containing the **getline**).  The result of
                     evaluating such a construct is unspecified, and
                     conforming applications shall parenthesize properly all
                     such usages.
    
           **system**(_expression_)
                     Execute the command given by _expression_ in a manner
                     equivalent to the _system_() function defined in the
                     System Interfaces volume of POSIX.1‐2017 and return the
                     exit status of the command.
    
           All forms of **getline** shall return 1 for successful input, zero for
           end-of-file, and -1 for an error.
    
           Where strings are used as the name of a file or pipeline, the
           application shall ensure that the strings are textually identical.
           The terminology ``same string value'' implies that ``equivalent
           strings'', even those that differ only by <space> characters,
           represent different files.
    
       **User-Defined Functions**
           The _awk_ language also provides user-defined functions. Such
           functions can be defined as:
    
               function _name_(**[**_parameter_ , ...**]**) { _statements_ }
    
           A function can be referred to anywhere in an _awk_ program; in
           particular, its use can precede its definition. The scope of a
           function is global.
    
           Function parameters, if present, can be either scalars or arrays;
           the behavior is undefined if an array name is passed as a
           parameter that the function uses as a scalar, or if a scalar
           expression is passed as a parameter that the function uses as an
           array. Function parameters shall be passed by value if scalar and
           by reference if array name.
    
           The number of parameters in the function definition need not match
           the number of parameters in the function call. Excess formal
           parameters can be used as local variables. If fewer arguments are
           supplied in a function call than are in the function definition,
           the extra parameters that are used in the function body as scalars
           shall evaluate to the uninitialized value until they are otherwise
           initialized, and the extra parameters that are used in the
           function body as arrays shall be treated as uninitialized arrays
           where each element evaluates to the uninitialized value until
           otherwise initialized.
    
           When invoking a function, no white space can be placed between the
           function name and the opening parenthesis. Function calls can be
           nested and recursive calls can be made upon functions. Upon return
           from any nested or recursive function call, the values of all of
           the calling function's parameters shall be unchanged, except for
           array parameters passed by reference. The **return** statement can be
           used to return a value. If a **return** statement appears outside of a
           function definition, the behavior is undefined.
    
           In the function definition, <newline> characters shall be optional
           before the opening brace and after the closing brace. Function
           definitions can appear anywhere in the program where a _pattern-_
           _action_ pair is allowed.
    
       **Grammar**
           The grammar in this section and the lexical conventions in the
           following section shall together describe the syntax for _awk_
           programs. The general conventions for this style of grammar are
           described in _Section 1.3_ , _Grammar Conventions_.  A valid program
           can be represented as the non-terminal symbol _program_ in the
           grammar. This formal syntax shall take precedence over the
           preceding text syntax description.
    
               %token NAME NUMBER STRING ERE
               %token FUNC_NAME   /* Name followed by '(' without white space. */
    
               /* Keywords */
               %token       Begin   End
               /*          'BEGIN' 'END'                            */
    
               %token       Break   Continue   Delete   Do   Else
               /*          'break' 'continue' 'delete' 'do' 'else'  */
    
               %token       Exit   For   Function   If   In
               /*          'exit' 'for' 'function' 'if' 'in'        */
    
               %token       Next   Print   Printf   Return   While
               /*          'next' 'print' 'printf' 'return' 'while' */
    
               /* Reserved function names */
               %token BUILTIN_FUNC_NAME
                           /* One token for the following:
                            * atan2 cos sin exp log sqrt int rand srand
                            * gsub index length match split sprintf sub
                            * substr tolower toupper close system
                            */
               %token GETLINE
                           /* Syntactically different from other built-ins. */
    
               /* Two-character tokens. */
               %token ADD_ASSIGN SUB_ASSIGN MUL_ASSIGN DIV_ASSIGN MOD_ASSIGN POW_ASSIGN
               /*     '+='       '-='       '*='       '/='       '%='       '^=' */
    
               %token OR   AND  NO_MATCH   EQ   LE   GE   NE   INCR  DECR  APPEND
               /*     '||' '&&' '!~' '==' '<=' '>=' '!=' '++'  '--'  '>>'   */
    
               /* One-character tokens. */
               %token '{' '}' '(' ')' '[' ']' ',' ';' NEWLINE
               %token '+' '-' '*' '%' '^' '!' '>' '<' '|' '?' ':' '~' '$' '='
    
               %start program
               %%
    
               program          : item_list
                                | item_list item
                                ;
    
               item_list        : /* empty */
                                | item_list item terminator
                                ;
    
               item             : action
                                | pattern action
                                | normal_pattern
                                | Function NAME      '(' param_list_opt ')'
                                      newline_opt action
                                | Function FUNC_NAME '(' param_list_opt ')'
                                      newline_opt action
                                ;
    
               param_list_opt   : /* empty */
                                | param_list
                                ;
    
               param_list       : NAME
                                | param_list ',' NAME
                                ;
    
               pattern          : normal_pattern
                                | special_pattern
                                ;
    
               normal_pattern   : expr
                                | expr ',' newline_opt expr
                                ;
    
               special_pattern  : Begin
                                | End
                                ;
    
               action           : '{' newline_opt                             '}'
                                | '{' newline_opt terminated_statement_list   '}'
                                | '{' newline_opt unterminated_statement_list '}'
                                ;
    
               terminator       : terminator NEWLINE
                                |            ';'
                                |            NEWLINE
                                ;
    
               terminated_statement_list : terminated_statement
                                | terminated_statement_list terminated_statement
                                ;
    
               unterminated_statement_list : unterminated_statement
                                | terminated_statement_list unterminated_statement
                                ;
    
               terminated_statement : action newline_opt
                                | If '(' expr ')' newline_opt terminated_statement
                                | If '(' expr ')' newline_opt terminated_statement
                                      Else newline_opt terminated_statement
                                | While '(' expr ')' newline_opt terminated_statement
                                | For '(' simple_statement_opt ';'
                                     expr_opt ';' simple_statement_opt ')' newline_opt
                                     terminated_statement
                                | For '(' NAME In NAME ')' newline_opt
                                     terminated_statement
                                | ';' newline_opt
                                | terminatable_statement NEWLINE newline_opt
                                | terminatable_statement ';'     newline_opt
                                ;
    
               unterminated_statement : terminatable_statement
                                | If '(' expr ')' newline_opt unterminated_statement
                                | If '(' expr ')' newline_opt terminated_statement
                                     Else newline_opt unterminated_statement
                                | While '(' expr ')' newline_opt unterminated_statement
                                | For '(' simple_statement_opt ';'
                                 expr_opt ';' simple_statement_opt ')' newline_opt
                                     unterminated_statement
                                | For '(' NAME In NAME ')' newline_opt
                                     unterminated_statement
                                ;
    
               terminatable_statement : simple_statement
                                | Break
                                | Continue
                                | Next
                                | Exit expr_opt
                                | Return expr_opt
                                | Do newline_opt terminated_statement While '(' expr ')'
                                ;
    
               simple_statement_opt : /* empty */
                                | simple_statement
                                ;
    
               simple_statement : Delete NAME '[' expr_list ']'
                                | expr
                                | print_statement
                                ;
    
               print_statement  : simple_print_statement
                                | simple_print_statement output_redirection
                                ;
    
               simple_print_statement : Print  print_expr_list_opt
                                | Print  '(' multiple_expr_list ')'
                                | Printf print_expr_list
                                | Printf '(' multiple_expr_list ')'
                                ;
    
               output_redirection : '>'    expr
                                | APPEND expr
                                | '|'    expr
                                ;
    
               expr_list_opt    : /* empty */
                                | expr_list
                                ;
    
               expr_list        : expr
                                | multiple_expr_list
                                ;
    
               multiple_expr_list : expr ',' newline_opt expr
                                | multiple_expr_list ',' newline_opt expr
                                ;
    
               expr_opt         : /* empty */
                                | expr
                                ;
    
               expr             : unary_expr
                                | non_unary_expr
                                ;
    
               unary_expr       : '+' expr
                                | '-' expr
                                | unary_expr '^'      expr
                                | unary_expr '*'      expr
                                | unary_expr '/'      expr
                                | unary_expr '%'      expr
                                | unary_expr '+'      expr
                                | unary_expr '-'      expr
                                | unary_expr          non_unary_expr
                                | unary_expr '<'      expr
                                | unary_expr LE       expr
                                | unary_expr NE       expr
                                | unary_expr EQ       expr
                                | unary_expr '>'      expr
                                | unary_expr GE       expr
                                | unary_expr '~'      expr
                                | unary_expr NO_MATCH expr
                                | unary_expr In NAME
                                | unary_expr AND newline_opt expr
                                | unary_expr OR  newline_opt expr
                                | unary_expr '?' expr ':' expr
                                | unary_input_function
                                ;
    
               non_unary_expr   : '(' expr ')'
                                | '!' expr
                                | non_unary_expr '^'      expr
                                | non_unary_expr '*'      expr
                                | non_unary_expr '/'      expr
                                | non_unary_expr '%'      expr
                                | non_unary_expr '+'      expr
                                | non_unary_expr '-'      expr
                                | non_unary_expr          non_unary_expr
                                | non_unary_expr '<'      expr
                                | non_unary_expr LE       expr
                                | non_unary_expr NE       expr
                                | non_unary_expr EQ       expr
                                | non_unary_expr '>'      expr
                                | non_unary_expr GE       expr
                                | non_unary_expr '~'      expr
                                | non_unary_expr NO_MATCH expr
                                | non_unary_expr In NAME
                                | '(' multiple_expr_list ')' In NAME
                                | non_unary_expr AND newline_opt expr
                                | non_unary_expr OR  newline_opt expr
                                | non_unary_expr '?' expr ':' expr
                                | NUMBER
                                | STRING
                                | lvalue
                                | ERE
                                | lvalue INCR
                                | lvalue DECR
                                | INCR lvalue
                                | DECR lvalue
                                | lvalue POW_ASSIGN expr
                                | lvalue MOD_ASSIGN expr
                                | lvalue MUL_ASSIGN expr
                                | lvalue DIV_ASSIGN expr
                                | lvalue ADD_ASSIGN expr
                                | lvalue SUB_ASSIGN expr
                                | lvalue '=' expr
                                | FUNC_NAME '(' expr_list_opt ')'
                                     /* no white space allowed before '(' */
                                | BUILTIN_FUNC_NAME '(' expr_list_opt ')'
                                | BUILTIN_FUNC_NAME
                                | non_unary_input_function
                                ;
    
               print_expr_list_opt : /* empty */
                                | print_expr_list
                                ;
    
               print_expr_list  : print_expr
                                | print_expr_list ',' newline_opt print_expr
                                ;
    
               print_expr       : unary_print_expr
                                | non_unary_print_expr
                                ;
    
               unary_print_expr : '+' print_expr
                                | '-' print_expr
                                | unary_print_expr '^'      print_expr
                                | unary_print_expr '*'      print_expr
                                | unary_print_expr '/'      print_expr
                                | unary_print_expr '%'      print_expr
                                | unary_print_expr '+'      print_expr
                                | unary_print_expr '-'      print_expr
                                | unary_print_expr          non_unary_print_expr
                                | unary_print_expr '~'      print_expr
                                | unary_print_expr NO_MATCH print_expr
                                | unary_print_expr In NAME
                                | unary_print_expr AND newline_opt print_expr
                                | unary_print_expr OR  newline_opt print_expr
                                | unary_print_expr '?' print_expr ':' print_expr
                                ;
    
               non_unary_print_expr : '(' expr ')'
                                | '!' print_expr
                                | non_unary_print_expr '^'      print_expr
                                | non_unary_print_expr '*'      print_expr
                                | non_unary_print_expr '/'      print_expr
                                | non_unary_print_expr '%'      print_expr
                                | non_unary_print_expr '+'      print_expr
                                | non_unary_print_expr '-'      print_expr
                                | non_unary_print_expr          non_unary_print_expr
                                | non_unary_print_expr '~'      print_expr
                                | non_unary_print_expr NO_MATCH print_expr
                                | non_unary_print_expr In NAME
                                | '(' multiple_expr_list ')' In NAME
                                | non_unary_print_expr AND newline_opt print_expr
                                | non_unary_print_expr OR  newline_opt print_expr
                                | non_unary_print_expr '?' print_expr ':' print_expr
                                | NUMBER
                                | STRING
                                | lvalue
                                | ERE
                                | lvalue INCR
                                | lvalue DECR
                                | INCR lvalue
                                | DECR lvalue
                                | lvalue POW_ASSIGN print_expr
                                | lvalue MOD_ASSIGN print_expr
                                | lvalue MUL_ASSIGN print_expr
                                | lvalue DIV_ASSIGN print_expr
                                | lvalue ADD_ASSIGN print_expr
                                | lvalue SUB_ASSIGN print_expr
                                | lvalue '=' print_expr
                                | FUNC_NAME '(' expr_list_opt ')'
                                    /* no white space allowed before '(' */
                                | BUILTIN_FUNC_NAME '(' expr_list_opt ')'
                                | BUILTIN_FUNC_NAME
                                ;
    
               lvalue           : NAME
                                | NAME '[' expr_list ']'
                                | '$' expr
                                ;
    
               non_unary_input_function : simple_get
                                | simple_get '<' expr
                                | non_unary_expr '|' simple_get
                                ;
    
               unary_input_function : unary_expr '|' simple_get
                                ;
    
               simple_get       : GETLINE
                                | GETLINE lvalue
                                ;
    
               newline_opt      : /* empty */
                                | newline_opt NEWLINE
                                ;
    
           This grammar has several ambiguities that shall be resolved as
           follows:
    
            *  Operator precedence and associativity shall be as described in
               _Table 4-1, Expressions in Decreasing Precedence in awk_.
    
            *  In case of ambiguity, an **else** shall be associated with the
               most immediately preceding **if** that would satisfy the grammar.
    
            *  In some contexts, a <slash> (**'/'**) that is used to surround an
               ERE could also be the division operator.  This shall be
               resolved in such a way that wherever the division operator
               could appear, a <slash> is assumed to be the division
               operator. (There is no unary division operator.)
    
           Each expression in an _awk_ program shall conform to the precedence
           and associativity rules, even when this is not needed to resolve
           an ambiguity. For example, because **'$'** has higher precedence than
           **'++'** , the string **"$x++--"** is not a valid _awk_ expression, even
           though it is unambiguously parsed by the grammar as **"$(x++)--"**.
    
           One convention that might not be obvious from the formal grammar
           is where <newline> characters are acceptable. There are several
           obvious placements such as terminating a statement, and a
           <backslash> can be used to escape <newline> characters between any
           lexical tokens. In addition, <newline> characters without
           <backslash> characters can follow a comma, an open brace, logical
           AND operator (**" &&"**), logical OR operator (**"||"**), the **do** keyword,
           the **else** keyword, and the closing parenthesis of an **if** , **for** , or
           **while** statement. For example:
    
               { print  $1,
                        $2 }
    
       **Lexical Conventions**
           The lexical conventions for _awk_ programs, with respect to the
           preceding grammar, shall be as follows:
    
            1. Except as noted, _awk_ shall recognize the longest possible
               token or delimiter beginning at a given point.
    
            2. A comment shall consist of any characters beginning with the
               <number-sign> character and terminated by, but excluding the
               next occurrence of, a <newline>.  Comments shall have no
               effect, except to delimit lexical tokens.
    
            3. The <newline> shall be recognized as the token **NEWLINE**.
    
            4. A <backslash> character immediately followed by a <newline>
               shall have no effect.
    
            5. The token **STRING** shall represent a string constant. A string
               constant shall begin with the character **'"'**.  Within a string
               constant, a <backslash> character shall be considered to begin
               an escape sequence as specified in the table in the Base
               Definitions volume of POSIX.1‐2017, _Chapter 5_ , _File Format_
               _Notation_ (**'\\'** , **'\a'** , **'\b'** , **'\f'** , **'\n'** , **'\r'** , **'\t'** , **'\v'**).  In
               addition, the escape sequences in _Table 4-2, Escape Sequences_
               _in awk_ shall be recognized. A <newline> shall not occur within
               a string constant. A string constant shall be terminated by
               the first unescaped occurrence of the character **'"'** after the
               one that begins the string constant. The value of the string
               shall be the sequence of all unescaped characters and values
               of escape sequences between, but not including, the two
               delimiting **'"'** characters.
    
            6. The token **ERE** represents an extended regular expression
               constant. An ERE constant shall begin with the <slash>
               character. Within an ERE constant, a <backslash> character
               shall be considered to begin an escape sequence as specified
               in the table in the Base Definitions volume of POSIX.1‐2017,
               _Chapter 5_ , _File Format Notation_.  In addition, the escape
               sequences in _Table 4-2, Escape Sequences in awk_ shall be
               recognized. The application shall ensure that a <newline> does
               not occur within an ERE constant. An ERE constant shall be
               terminated by the first unescaped occurrence of the <slash>
               character after the one that begins the ERE constant. The
               extended regular expression represented by the ERE constant
               shall be the sequence of all unescaped characters and values
               of escape sequences between, but not including, the two
               delimiting <slash> characters.
    
            7. A <blank> shall have no effect, except to delimit lexical
               tokens or within **STRING** or **ERE** tokens.
    
            8. The token **NUMBER** shall represent a numeric constant. Its form
               and numeric value shall either be equivalent to the **decimal-**
               **floating-constant** token as specified by the ISO C standard, or
               it shall be a sequence of decimal digits and shall be
               evaluated as an integer constant in decimal. In addition,
               implementations may accept numeric constants with the form and
               numeric value equivalent to the **hexadecimal-constant** and
               **hexadecimal-floating-constant** tokens as specified by the ISO C
               standard.
    
               If the value is too large or too small to be representable
               (see _Section 1.1.2_ , _Concepts Derived from the ISO C Standard_),
               the behavior is undefined.
    
            9. A sequence of underscores, digits, and alphabetics from the
               portable character set (see the Base Definitions volume of
               POSIX.1‐2017, _Section 6.1_ , _Portable Character Set_), beginning
               with an <underscore> or alphabetic character, shall be
               considered a word.
    
           10. The following words are keywords that shall be recognized as
               individual tokens; the name of the token is the same as the
               keyword:
               **BEGIN      delete     END        function   in         printf**
               **break      do         exit       getline    next       return**
               **continue   else       for        if         print      while**
    
           11. The following words are names of built-in functions and shall
               be recognized as the token **BUILTIN_FUNC_NAME** :
               **atan2     gsub      log       split     sub       toupper**
               **close     index     match     sprintf   substr**
               **cos       int       rand      sqrt      system**
               **exp       length    sin       srand     tolower**
    
               The above-listed keywords and names of built-in functions are
               considered reserved words.
    
           12. The token **NAME** shall consist of a word that is not a keyword
               or a name of a built-in function and is not followed
               immediately (without any delimiters) by the **'('** character.
    
           13. The token **FUNC_NAME** shall consist of a word that is not a
               keyword or a name of a built-in function, followed immediately
               (without any delimiters) by the **'('** character. The **'('**
               character shall not be included as part of the token.
    
           14. The following two-character sequences shall be recognized as
               the named tokens:
                     ┌────────────┬──────────┬────────────┬──────────┐
                     │ **Token Name** │ **Sequence** │ **Token Name** │ **Sequence** │
                     ├────────────┼──────────┼────────────┼──────────┤
                     │ **ADD_ASSIGN** │    +=    │ **NO_MATCH** │    !~    │
                     │ **SUB_ASSIGN** │    -=    │ **EQ** │    ==    │
                     │ **MUL_ASSIGN** │    *=    │ **LE** │    <=    │
                     │ **DIV_ASSIGN** │    /=    │ **GE** │    >=    │
                     │ **MOD_ASSIGN** │    %=    │ **NE** │    !=    │
                     │ **POW_ASSIGN** │    ^=    │ **INCR** │    ++    │
                     │ **OR** │    ||    │ **DECR** │    --    │
                     │ **AND** │    &&    │ **APPEND** │    >>    │
                     └────────────┴──────────┴────────────┴──────────┘
    
           15. The following single characters shall be recognized as tokens
               whose names are the character:
    
                   <newline> { } ( ) [ ] , ; + - * % ^ ! > < | ? : ~ $ =
    
           There is a lexical ambiguity between the token **ERE** and the tokens
           **'/'** and **DIV_ASSIGN**.  When an input sequence begins with a <slash>
           character in any syntactic context where the token **'/'** or
           **DIV_ASSIGN** could appear as the next token in a valid program, the
           longer of those two tokens that can be recognized shall be
           recognized. In any other syntactic context where the token **ERE**
           could appear as the next token in a valid program, the token **ERE**
           shall be recognized.
    


## EXIT STATUS



           The following exit values shall be returned:
    
            0    All input files were processed successfully.
    
           >0    An error occurred.
    
           The exit status can be altered within the program by using an **exit**
           expression.
    


## CONSEQUENCES OF ERRORS



           If any _file_ operand is specified and the named file cannot be
           accessed, _awk_ shall write a diagnostic message to standard error
           and terminate without any further action.
    
           If the program specified by either the _program_ operand or a
           _progfile_ operand is not a valid _awk_ program (as specified in the
           EXTENDED DESCRIPTION section), the behavior is undefined.
    
           _The following sections are informative._
    


## APPLICATION USAGE



           The **index** , **length** , **match** , and **substr** functions should not be
           confused with similar functions in the ISO C standard; the _awk_
           versions deal with characters, while the ISO C standard deals with
           bytes.
    
           Because the concatenation operation is represented by adjacent
           expressions rather than an explicit operator, it is often
           necessary to use parentheses to enforce the proper evaluation
           precedence.
    
           When using _awk_ to process pathnames, it is recommended that
           LC_ALL, or at least LC_CTYPE and LC_COLLATE, are set to POSIX or C
           in the environment, since pathnames can contain byte sequences
           that do not form valid characters in some locales, in which case
           the utility's behavior would be undefined. In the POSIX locale
           each byte is a valid single-byte character, and therefore this
           problem is avoided.
    
           On implementations where the **"=="** operator checks if strings
           collate equally, applications needing to check whether strings are
           identical can use:
    
               length(a) == length(b) && index(a,b) == 1
    
           On implementations where the **"=="** operator checks if strings are
           identical, applications needing to check whether strings collate
           equally can use:
    
               a <= b && a >= b
    


## EXAMPLES



           The _awk_ program specified in the command line is most easily
           specified within single-quotes (for example, '_program_ ') for
           applications using _sh_ , because _awk_ programs commonly contain
           characters that are special to the shell, including double-quotes.
           In the cases where an _awk_ program contains single-quote
           characters, it is usually easiest to specify most of the program
           as strings within single-quotes concatenated by the shell with
           quoted single-quote characters. For example:
    
               awk '/'\''/ { print "quote:", $0 }'
    
           prints all lines from the standard input containing a single-quote
           character, prefixed with _quote_ :.
    
           The following are examples of simple _awk_ programs:
    
            1. Write to the standard output all input lines for which field 3
               is greater than 5:
    
                   $3 > 5
    
            2. Write every tenth line:
    
                   (NR % 10) == 0
    
            3. Write any line with a substring matching the regular
               expression:
    
                   /(G|D)(2[0-9][[:alpha:]]*)/
    
            4. Print any line with a substring containing a **'G'** or **'D'** ,
               followed by a sequence of digits and characters. This example
               uses character classes **digit** and **alpha** to match language-
               independent digit and alphabetic characters respectively:
    
                   /(G|D)([[:digit:][:alpha:]]*)/
    
            5. Write any line in which the second field matches the regular
               expression and the fourth field does not:
    
                   $2 ~ /xyz/ && $4 !~ /xyz/
    
            6. Write any line in which the second field contains a
               <backslash>:
    
                   $2 ~ /\\/
    
            7. Write any line in which the second field contains a
               <backslash>.  Note that <backslash>-escapes are interpreted
               twice; once in lexical processing of the string and once in
               processing the regular expression:
    
                   $2 ~ "\\\\"
    
            8. Write the second to the last and the last field in each line.
               Separate the fields by a <colon>:
    
                   {OFS=":";print $(NF-1), $NF}
    
            9. Write the line number and number of fields in each line. The
               three strings representing the line number, the <colon>, and
               the number of fields are concatenated and that string is
               written to standard output:
    
                   {print NR ":" NF}
    
           10. Write lines longer than 72 characters:
    
                   length($0) > 72
    
           11. Write the first two fields in opposite order separated by **OFS** :
    
                   { print $2, $1 }
    
           12. Same, with input fields separated by a <comma> or <space> and
               <tab> characters, or both:
    
                   BEGIN { FS = ",[ \t]*|[ \t]+" }
                         { print $2, $1 }
    
           13. Add up the first column, print sum, and average:
    
                         {s += $1 }
                   END   {print "sum is ", s, " average is", s/NR}
    
           14. Write fields in reverse order, one per line (many lines out
               for each line in):
    
                   { for (i = NF; i > 0; --i) print $i }
    
           15. Write all lines between occurrences of the strings **start** and
               **stop** :
    
                   /start/, /stop/
    
           16. Write all lines whose first field is different from the
               previous one:
    
                   $1 != prev { print; prev = $1 }
    
           17. Simulate _echo_ :
    
                   BEGIN  {
                           for (i = 1; i < ARGC; ++i)
                           printf("%s%s", ARGV[i], i==ARGC-1?"\n":" ")
                   }
    
           18. Write the path prefixes contained in the _PATH_ environment
               variable, one per line:
    
                   BEGIN  {
                           n = split (ENVIRON["PATH"], path, ":")
                           for (i = 1; i <= n; ++i)
                           print path[i]
                   }
    
           19. If there is a file named **input** containing page headers of the
               form: Page #
    
               and a file named **program** that contains:
    
                   /Page/   { $2 = n++; }
                            { print }
    
               then the command line:
    
                   awk -f program n=5 input
    
               prints the file **input** , filling in page numbers starting at 5.
    


## RATIONALE



           This description is based on the new _awk_ , ``nawk'', (see the
           referenced _The AWK Programming Language_), which introduced a
           number of new features to the historical _awk_ :
    
            1. New keywords: **delete** , **do** , **function** , **return**
    
            2. New built-in functions: **atan2** , **close** , **cos** , **gsub** , **match** , **rand** ,
               **sin** , **srand** , **sub** , **system**
    
            3. New predefined variables: **FNR** , **ARGC** , **ARGV** , **RSTART** , **RLENGTH** ,
               **SUBSEP**
    
            4. New expression operators: **?** , **:** , **,** , **^**
    
            5. The **FS** variable and the third argument to **split** , now treated
               as extended regular expressions.
    
            6. The operator precedence, changed to more closely match the C
               language.  Two examples of code that operate differently are:
    
                   while ( n /= 10 > 1) ...
                   if (!"wk" ~ /bwk/) ...
    
           Several features have been added based on newer implementations of
           _awk_ :
    
            *  Multiple instances of **-f** _progfile_ are permitted.
    
            *  The new option **-v** _assignment._
    
            *  The new predefined variable **ENVIRON**.
    
            *  New built-in functions **toupper** and **tolower**.
    
            *  More formatting capabilities are added to **printf** to match the
               ISO C standard.
    
           Earlier versions of this standard required implementations to
           support multiple adjacent <semicolon>s, lines with one or more
           <semicolon> before a rule (_pattern-action_ pairs), and lines with
           only <semicolon>(s).  These are not required by this standard and
           are considered poor programming practice, but can be accepted by
           an implementation of _awk_ as an extension.
    
           The overall _awk_ syntax has always been based on the C language,
           with a few features from the shell command language and other
           sources. Because of this, it is not completely compatible with any
           other language, which has caused confusion for some users. It is
           not the intent of the standard developers to address such issues.
           A few relatively minor changes toward making the language more
           compatible with the ISO C standard were made; most of these
           changes are based on similar changes in recent implementations, as
           described above. There remain several C-language conventions that
           are not in _awk_.  One of the notable ones is the <comma> operator,
           which is commonly used to specify multiple expressions in the C
           language **for** statement. Also, there are various places where _awk_
           is more restrictive than the C language regarding the type of
           expression that can be used in a given context. These limitations
           are due to the different features that the _awk_ language does
           provide.
    
           Regular expressions in _awk_ have been extended somewhat from
           historical implementations to make them a pure superset of
           extended regular expressions, as defined by POSIX.1‐2008 (see the
           Base Definitions volume of POSIX.1‐2017, _Section 9.4_ , _Extended_
           _Regular Expressions_).  The main extensions are
           internationalization features and interval expressions. Historical
           implementations of _awk_ have long supported <backslash>-escape
           sequences as an extension to extended regular expressions, and
           this extension has been retained despite inconsistency with other
           utilities. The number of escape sequences recognized in both
           extended regular expressions and strings has varied (generally
           increasing with time) among implementations. The set specified by
           POSIX.1‐2008 includes most sequences known to be supported by
           popular implementations and by the ISO C standard. One sequence
           that is not supported is hexadecimal value escapes beginning with
           **'\x'**.  This would allow values expressed in more than 9 bits to be
           used within _awk_ as in the ISO C standard. However, because this
           syntax has a non-deterministic length, it does not permit the
           subsequent character to be a hexadecimal digit. This limitation
           can be dealt with in the C language by the use of lexical string
           concatenation. In the _awk_ language, concatenation could also be a
           solution for strings, but not for extended regular expressions
           (either lexical ERE tokens or strings used dynamically as regular
           expressions). Because of this limitation, the feature has not been
           added to POSIX.1‐2008.
    
           When a string variable is used in a context where an extended
           regular expression normally appears (where the lexical token ERE
           is used in the grammar) the string does not contain the literal
           <slash> characters.
    
           Some versions of _awk_ allow the form:
    
               func name(args, ... ) { statements }
    
           This has been deprecated by the authors of the language, who asked
           that it not be specified.
    
           Historical implementations of _awk_ produce an error if a **next**
           statement is executed in a **BEGIN** action, and cause _awk_ to
           terminate if a **next** statement is executed in an **END** action. This
           behavior has not been documented, and it was not believed that it
           was necessary to standardize it.
    
           The specification of conversions between string and numeric values
           is much more detailed than in the documentation of historical
           implementations or in the referenced _The AWK Programming Language_.
           Although most of the behavior is designed to be intuitive, the
           details are necessary to ensure compatible behavior from different
           implementations. This is especially important in relational
           expressions since the types of the operands determine whether a
           string or numeric comparison is performed. From the perspective of
           an application developer, it is usually sufficient to expect
           intuitive behavior and to force conversions (by adding zero or
           concatenating a null string) when the type of an expression does
           not obviously match what is needed. The intent has been to specify
           historical practice in almost all cases. The one exception is
           that, in historical implementations, variables and constants
           maintain both string and numeric values after their original value
           is converted by any use. This means that referencing a variable or
           constant can have unexpected side-effects. For example, with
           historical implementations the following program:
    
               {
                   a = "+2"
                   b = 2
                   if (NR % 2)
                       c = a + b
                   if (a == b)
                       print "numeric comparison"
                   else
                       print "string comparison"
               }
    
           would perform a numeric comparison (and output numeric comparison)
           for each odd-numbered line, but perform a string comparison (and
           output string comparison) for each even-numbered line.
           POSIX.1‐2008 ensures that comparisons will be numeric if
           necessary. With historical implementations, the following program:
    
               BEGIN {
                   OFMT = "%e"
                   print 3.14
                   OFMT = "%f"
                   print 3.14
               }
    
           would output **"3.140000e+00"** twice, because in the second **print**
           statement the constant **"3.14"** would have a string value from the
           previous conversion. POSIX.1‐2008 requires that the output of the
           second **print** statement be **"3.140000"**.  The behavior of historical
           implementations was seen as too unintuitive and unpredictable.
    
           It was pointed out that with the rules contained in early drafts,
           the following script would print nothing:
    
               BEGIN {
                   y[1.5] = 1
                   OFMT = "%e"
                   print y[1.5]
               }
    
           Therefore, a new variable, **CONVFMT** , was introduced. The **OFMT**
           variable is now restricted to affecting output conversions of
           numbers to strings and **CONVFMT** is used for internal conversions,
           such as comparisons or array indexing. The default value is the
           same as that for **OFMT** , so unless a program changes **CONVFMT**(which
           no historical program would do), it will receive the historical
           behavior associated with internal string conversions.
    
           The POSIX _awk_ lexical and syntactic conventions are specified more
           formally than in other sources. Again the intent has been to
           specify historical practice. One convention that may not be
           obvious from the formal grammar as in other verbal descriptions is
           where <newline> characters are acceptable. There are several
           obvious placements such as terminating a statement, and a
           <backslash> can be used to escape <newline> characters between any
           lexical tokens. In addition, <newline> characters without
           <backslash> characters can follow a comma, an open brace, a
           logical AND operator (**" &&"**), a logical OR operator (**"||"**), the **do**
           keyword, the **else** keyword, and the closing parenthesis of an **if** ,
           **for** , or **while** statement. For example:
    
               { print $1,
                       $2 }
    
           The requirement that _awk_ add a trailing <newline> to the program
           argument text is to simplify the grammar, making it match a text
           file in form. There is no way for an application or test suite to
           determine whether a literal <newline> is added or whether _awk_
           simply acts as if it did.
    
           POSIX.1‐2008 requires several changes from historical
           implementations in order to support internationalization. Probably
           the most subtle of these is the use of the decimal-point
           character, defined by the _LC_NUMERIC_ category of the locale, in
           representations of floating-point numbers.  This locale-specific
           character is used in recognizing numeric input, in converting
           between strings and numeric values, and in formatting output.
           However, regardless of locale, the <period> character (the
           decimal-point character of the POSIX locale) is the decimal-point
           character recognized in processing _awk_ programs (including
           assignments in command line arguments). This is essentially the
           same convention as the one used in the ISO C standard. The
           difference is that the C language includes the _setlocale_()
           function, which permits an application to modify its locale.
           Because of this capability, a C application begins executing with
           its locale set to the C locale, and only executes in the
           environment-specified locale after an explicit call to
           _setlocale_().  However, adding such an elaborate new feature to the
           _awk_ language was seen as inappropriate for POSIX.1‐2008. It is
           possible to execute an _awk_ program explicitly in any desired
           locale by setting the environment in the shell.
    
           The undefined behavior resulting from NULs in extended regular
           expressions allows future extensions for the GNU _gawk_ program to
           process binary data.
    
           The behavior in the case of invalid _awk_ programs (including
           lexical, syntactic, and semantic errors) is undefined because it
           was considered overly limiting on implementations to specify. In
           most cases such errors can be expected to produce a diagnostic and
           a non-zero exit status. However, some implementations may choose
           to extend the language in ways that make use of certain invalid
           constructs. Other invalid constructs might be deemed worthy of a
           warning, but otherwise cause some reasonable behavior. Still other
           constructs may be very difficult to detect in some
           implementations.  Also, different implementations might detect a
           given error during an initial parsing of the program (before
           reading any input files) while others might detect it when
           executing the program after reading some input. Implementors
           should be aware that diagnosing errors as early as possible and
           producing useful diagnostics can ease debugging of applications,
           and thus make an implementation more usable.
    
           The unspecified behavior from using multi-character **RS** values is
           to allow possible future extensions based on extended regular
           expressions used for record separators. Historical implementations
           take the first character of the string and ignore the others.
    
           Unspecified behavior when _split_(_string_ ,_array_ ,<null>) is used is to
           allow a proposed future extension that would split up a string
           into an array of individual characters.
    
           In the context of the **getline** function, equally good arguments for
           different precedences of the **|** and **< **operators can be made.
           Historical practice has been that:
    
               getline < "a" "b"
    
           is parsed as:
    
               ( getline < "a" ) "b"
    
           although many would argue that the intent was that the file **ab**
           should be read. However:
    
               getline < "x" + 1
    
           parses as:
    
               getline < ( "x" + 1 )
    
           Similar problems occur with the **|** version of **getline** , particularly
           in combination with **$**.  For example:
    
               $"echo hi" | getline
    
           (This situation is particularly problematic when used in a **print**
           statement, where the **|getline** part might be a redirection of the
           **print**.)
    
           Since in most cases such constructs are not (or at least should
           not) be used (because they have a natural ambiguity for which
           there is no conventional parsing), the meaning of these constructs
           has been made explicitly unspecified. (The effect is that a
           conforming application that runs into the problem must
           parenthesize to resolve the ambiguity.)  There appeared to be few
           if any actual uses of such constructs.
    
           Grammars can be written that would cause an error under these
           circumstances. Where backwards-compatibility is not a large
           consideration, implementors may wish to use such grammars.
    
           Some historical implementations have allowed some built-in
           functions to be called without an argument list, the result being
           a default argument list chosen in some ``reasonable'' way. Use of
           **length** as a synonym for **length($0)** is the only one of these forms
           that is thought to be widely known or widely used; this particular
           form is documented in various places (for example, most historical
           _awk_ reference pages, although not in the referenced _The AWK_
           _Programming Language_) as legitimate practice.  With this
           exception, default argument lists have always been undocumented
           and vaguely defined, and it is not at all clear how (or if) they
           should be generalized to user-defined functions. They add no
           useful functionality and preclude possible future extensions that
           might need to name functions without calling them. Not
           standardizing them seems the simplest course. The standard
           developers considered that **length** merited special treatment,
           however, since it has been documented in the past and sees
           possibly substantial use in historical programs.  Accordingly,
           this usage has been made legitimate, but Issue 5 removed the
           obsolescent marking for XSI-conforming implementations and many
           otherwise conforming applications depend on this feature.
    
           In **sub** and **gsub** , if _repl_ is a string literal (the lexical token
           **STRING**), then two consecutive <backslash> characters should be
           used in the string to ensure a single <backslash> will precede the
           <ampersand> when the resultant string is passed to the function.
           (For example, to specify one literal <ampersand> in the
           replacement string, use **gsub**(**ERE** , **"\\ &"**).)
    
           Historically, the only special character in the _repl_ argument of
           **sub** and **gsub** string functions was the <ampersand> (**' &'**) character
           and preceding it with the <backslash> character was used to turn
           off its special meaning.
    
           The description in the ISO POSIX‐2:1993 standard introduced
           behavior such that the <backslash> character was another special
           character and it was unspecified whether there were any other
           special characters. This description introduced several
           portability problems, some of which are described below, and so it
           has been replaced with the more historical description. Some of
           the problems include:
    
            *  Historically, to create the replacement string, a script could
               use **gsub**(**ERE** , **"\\ &"**), but with the ISO POSIX‐2:1993 standard
               wording, it was necessary to use **gsub**(**ERE** , **"\\\\ &"**).  The
               <backslash> characters are doubled here because all string
               literals are subject to lexical analysis, which would reduce
               each pair of <backslash> characters to a single <backslash>
               before being passed to **gsub**.
    
            *  Since it was unspecified what the special characters were, for
               portable scripts to guarantee that characters are printed
               literally, each character had to be preceded with a
               <backslash>.  (For example, a portable script had to use
               **gsub**(**ERE** , **"\\h\\i"**) to produce a replacement string of **"hi"**.)
    
           The description for comparisons in the ISO POSIX‐2:1993 standard
           did not properly describe historical practice because of the way
           numeric strings are compared as numbers. The current rules cause
           the following code:
    
               if (0 == "000")
                   print "strange, but true"
               else
                   print "not true"
    
           to do a numeric comparison, causing the **if** to succeed. It should
           be intuitively obvious that this is incorrect behavior, and
           indeed, no historical implementation of _awk_ actually behaves this
           way.
    
           To fix this problem, the definition of _numeric string_ was enhanced
           to include only those values obtained from specific circumstances
           (mostly external sources) where it is not possible to determine
           unambiguously whether the value is intended to be a string or a
           numeric.
    
           Variables that are assigned to a numeric string shall also be
           treated as a numeric string. (For example, the notion of a numeric
           string can be propagated across assignments.) In comparisons, all
           variables having the uninitialized value are to be treated as a
           numeric operand evaluating to the numeric value zero.
    
           Uninitialized variables include all types of variables including
           scalars, array elements, and fields. The definition of an
           uninitialized value in _Variables and Special Variables_ is
           necessary to describe the value placed on uninitialized variables
           and on fields that are valid (for example, **< $NF**) but have no
           characters in them and to describe how these variables are to be
           used in comparisons. A valid field, such as **$1** , that has no
           characters in it can be obtained from an input line of **"\t\t"** when
           **FS='\t'**.  Historically, the comparison (**$1 <**10) was done
           numerically after evaluating **$1** to the value zero.
    
           The phrase ``... also shall have the numeric value of the numeric
           string'' was removed from several sections of the ISO POSIX‐2:1993
           standard because is specifies an unnecessary implementation
           detail. It is not necessary for POSIX.1‐2008 to specify that these
           objects be assigned two different values.  It is only necessary to
           specify that these objects may evaluate to two different values
           depending on context.
    
           Historical implementations of _awk_ did not parse hexadecimal
           integer or floating constants like **"0xa"** and **"0xap0"**.  Due to an
           oversight, the 2001 through 2004 editions of this standard
           required support for hexadecimal floating constants. This was due
           to the reference to _atof_().  This version of the standard allows
           but does not require implementations to use _atof_() and includes a
           description of how floating-point numbers are recognized as an
           alternative to match historic behavior. The intent of this change
           is to allow implementations to recognize floating-point constants
           according to either the ISO/IEC 9899:1990 standard or
           ISO/IEC 9899:1999 standard, and to allow (but not require)
           implementations to recognize hexadecimal integer constants.
    
           Historical implementations of _awk_ did not support floating-point
           infinities and NaNs in _numeric strings_ ; e.g., **"-INF"** and **"NaN"**.
           However, implementations that use the _atof_() or _strtod_() functions
           to do the conversion picked up support for these values if they
           used a ISO/IEC 9899:1999 standard version of the function instead
           of a ISO/IEC 9899:1990 standard version. Due to an oversight, the
           2001 through 2004 editions of this standard did not allow support
           for infinities and NaNs, but in this revision support is allowed
           (but not required). This is a silent change to the behavior of _awk_
           programs; for example, in the POSIX locale the expression:
    
               ("-INF" + 0 < 0)
    
           formerly had the value 0 because **"-INF"** converted to 0, but now it
           may have the value 0 or 1.
    


## FUTURE DIRECTIONS



           A future version of this standard may require the **"!="** and **"=="**
           operators to perform string comparisons by checking if the strings
           are identical (and not by checking if they collate equally).
    


* * *