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:

-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:

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: 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: 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: or by converting the initial portion of the string to type double representation as follows: 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:

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:

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: shall be equivalent to the value returned by the ISO C standard function call: The expression: shall be equivalent to the ISO C standard expression: except that lvalue shall be evaluated only once. The value of the expression: shall be equivalent to the value returned by the ISO C standard function call: The expression: shall be equivalent to the ISO C standard expression: except that lvalue shall be evaluated only once. Variables and fields shall be set by the assignment statement: 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: 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.

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.

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 by the longest sequence of one, two, or three octal-digit characters (01234567). If all of the digits are 0 (that is, representation of the NUL character), the behavior is undefined.	The character whose encoding is represented by the one, two, or three-digit octal integer. Multi-byte characters require multiple, concatenated escape sequences of this type, including the leading <backslash> for each byte.
\c	A <backslash> character followed 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' ).	Undefined

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: 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:

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: 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: 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: 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 expressions 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).

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).

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: 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. 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:

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.

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 break continue

delete do else

END exit for

function getline if

in next print

printf return while

11.

The following words are names of built-in functions and shall be recognized as the token BUILTIN_FUNC_NAME:

atan2 close cos exp

gsub index int length

log match rand sin

split sprintf sqrt srand

sub substr system tolower

toupper

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:

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: On implementations where the "==" operator checks if strings are identical, applications needing to check whether strings collate equally can use:

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: 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:

2.

Write every tenth line:

3.

Write any line with a substring matching the regular expression:

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:

5.

Write any line in which the second field matches the regular expression and the fourth field does not:

6.

Write any line in which the second field contains a <backslash>:

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:

8.

Write the second to the last and the last field in each line. Separate the fields by a <colon>:

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:

10.

Write lines longer than 72 characters:

11.

Write the first two fields in opposite order separated by OFS:

12.

Same, with input fields separated by a <comma> or <space> and <tab> characters, or both:

13.

Add up the first column, print sum, and average:

14.

Write fields in reverse order, one per line (many lines out for each line in):

15.

Write all lines between occurrences of the strings start and stop:

16.

Write all lines whose first field is different from the previous one:

17.

Simulate echo:

18.

Write the path prefixes contained in the PATH environment variable, one per line:

19.

If there is a file named input containing page headers of the form: Page #

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:

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: 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: 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: 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: 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: 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: is parsed as: although many would argue that the intent was that the file ab should be read. However: parses as: Similar problems occur with the | version of getline, particularly in combination with $. For example: (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: 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: 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).

SEE ALSO

Section 1.3, Grammar Conventions, grep, lex, sed The Base Definitions volume of POSIX.1‐2017, Chapter 5, File Format Notation, Section 6.1, Portable Character Set, Chapter 8, Environment Variables, Chapter 9, Regular Expressions, Section 12.2, Utility Syntax Guidelines The System Interfaces volume of POSIX.1‐2017, atof(), exec, isspace(), popen(), setlocale(), strtod()

COPYRIGHT

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html . Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .