awk(1)

NAME

awk — pattern-directed scanning and processing language

SYNOPSIS

awk [-F fs] [-v var=value ] [program | -f progfile ...] [file ...]

DESCRIPTION

awk scans each input file for lines that match any of a set of patterns specified literally in program or in one or more files specified as -f progfile. With each pattern there can be an associated action that is to be performed when a line in a file matches the pattern. Each line is matched against the pattern portion of every pattern-action statement, and the associated action is performed for each matched pattern. The file name - means the standard input. Any file of the form var=value is treated as an assignment, not a filename. An assignment is evaluated at the time it would have been opened if it were a filename, unless the -v option is used.

An input line is made up of fields separated by white space, or by regular expression FS. The fields are denoted $1, $2, ...; $0 refers to the entire line.

Options

awk recognizes the following options and arguments:

-F fs: Specify regular expression used to separate fields. The default is to recognize space and tab characters, and to discard leading spaces and tabs. If the -F option is used, leading input field separators are no longer discarded.
-f progfile: Specify an awk program file. Up to 100 program files can be specified. The pattern-action statements in these files are executed in the same order as the files were specified.
-v var=value: Cause var=value assignment to occur before the BEGIN action (if it exists) is executed.

Statements

A pattern-action statement has the form:

pattern { action }

A missing { action } means print the line; a missing pattern always matches. Pattern-action statements are separated by new-lines or semicolons.

An action is a sequence of statements. A statement can be one of the following:

if(expression) statement [ else statement ]
while(expression) statement
for(expression;expression;expression) statement
for(var in array) statement
do statement while(expression)
break
continue
{[statement  ...]}
expression          # commonly  var = expression
print [expression-list] [ > expression]
printf format [, expression-list] [ > expression]
return [expression]
next                # skip remaining patterns on this input line.
delete array [expression]   # delete an array element.
exit [expression]   # exit immediately; status is expression.

Statements are terminated by semicolons, newlines or right braces. An empty expression-list stands for $0. String constants are quoted (""), with the usual C escapes recognized within. Expressions take on string or numeric values as appropriate, and are built using the operators +, -, *, /, %, ^ (exponentiation), and concatenation (indicated by a blank). The operators ++, --, +=, -=, *=, /=, %=, ^=, **=, >, >=, <, <=, ==, !=, "" (double quotes, string conversion operator), and ?: are also available in expressions. Variables can be scalars, array elements (denoted x[i] ) or fields. Variables are initialized to the null string. Array subscripts can be any string, not necessarily numeric (this allows for a form of associative memory). Multiple subscripts such as [i, j,k] are permitted. The constituents are concatenated, separated by the value of SUBSEP.

The print statement prints its arguments on the standard output (or on a file if >file or >>file is present or on a pipe if |cmd is present), separated by the current output field separator, and terminated by the output record separator. file and cmd can be literal names or parenthesized expressions. Identical string values in different statements denote the same open file. The printf statement formats its expression list according to the format (see printf(3S)).

Built-In Functions

The built-in function close(expr) closes the file or pipe expr opened by a print or printf statement or a call to getline with the same string-valued expr. This function returns zero if successful, otherwise, it returns non-zero.

The customary functions exp, log, sqrt, sin, cos, atan2 are built in. Other built-in functions are:

blength[([s])]: Length of its associated argument (in bytes) taken as a string, or of $0 if no argument.
length[([s])]: Length of its associated argument (in characters) taken as a string, or of $0 if no argument.
rand(): Returns a random number between zero and one.
srand([expr]): Sets the seed value for rand, and returns the previous seed value. If no argument is given, the time of day is used as the seed value; otherwise, expr is used.
int(x): Truncates to an integer value
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, the substring is limited by the length of string s.
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.
match(s, ere): Return the position, in characters, numbering from 1, in string s where the extended regular expression ere occurs, or 0 if it does not. The variables RSTART and RLENGTH are set to the position and length of the matched string.
split(s, a[, fs]): Splits the string s into array elements a[1] , a[2] , ..., a[n] , and returns n. The separation is done with the regular expression fs, or with the field separator FS if fs is not given.
sub(ere, repl [, in]): Substitutes repl for the first occurrence of the extended regular expression ere in the string in. If in is not given, $0 is used.
gsub: Same as sub except that all occurrences of the regular expression are replaced; sub and gsub return the number of replacements.
sprintf(fmt, expr, ...): String resulting from formatting expr ... according to the printf(3S) format fmt
system(cmd): Executes cmd and returns its exit status
toupper(s): Converts the argument string s to uppercase and returns the result.
tolower(s): Converts the argument string s to lowercase and returns the result.

The built-in function getline sets $0 to the next input record from the current input file; getline < file sets $0 to the next record from file. getline x sets variable x instead. Finally, cmd | getline pipes the output of cmd into getline; each call of getline returns the next line of output from cmd. In all cases, getline returns 1 for a successful input, 0 for end of file, and -1 for an error.

Patterns

Patterns are arbitrary Boolean combinations (with ! || &&) of regular expressions and relational expressions. awk supports Extended Regular Expressions as described in regexp(5). Isolated regular expressions in a pattern apply to the entire line. Regular expressions can also occur in relational expressions, using the operators ~ and !~. /re/ is a constant regular expression; any string (constant or variable) can be used as a regular expression, except in the position of an isolated regular expression in a pattern.

A pattern can consist of two patterns separated by a comma; in this case, the action is performed for all lines from an occurrence of the first pattern though an occurrence of the second.

A relational expression is one of the following:

expression matchop regular-expression
expression relop expression
expression in array-name
(expr,expr,...) in array-name

where a relop is any of the six relational operators in C, and a matchop is either ~ (matches) or !~ (does not match). A conditional is an arithmetic expression, a relational expression, or a Boolean combination of the two.

The special patterns BEGIN and END can be used to capture control before the first input line is read and after the last. BEGIN and END do not combine with other patterns.

Special Characters

The following special escape sequences are recognized by awk in both regular expressions and strings:

Escape: Meaning
\a: alert character
\b: backspace character
\f: form-feed character
\n: new-line character
\r: carriage-return character
\t: tab character
\v: vertical-tab character
\nnn: 1- to 3-digit octal value nnn
\xhhh: 1- to n-digit hexadecimal number

Variable Names

Variable names with special meanings are:

FS

Input field separator regular expression; a space character by default; also settable by option -Ffs.

NF

The number of fields in the current record.

NR

The ordinal number of the current record from the start of input. Inside a BEGIN action the value is zero. Inside an END action the value is the number of the last record processed.

FNR

The ordinal number of the current record in the current file. Inside a BEGIN action the value is zero. Inside an END action the value is the number of the last record processed in the last file processed.

FILENAME

A pathname of the current input file.

RS

The input record separator; a newline character by default.

OFS

The print statement output field separator; a space character by default.

ORS

The print statement output record separator; a newline character by default.

OFMT

Output format for numbers (default %.6g). If the value of OFMT is not a floating-point format specification, the results are unspecified.

CONVFMT

Internal conversion format for numbers (default %.6g). If the value of CONVFMT is not a floating-point format specification, the results are unspecified.

Under the UNIX Standard environment (see standards(5)) if CONVFMT is not specified, %d is used as the internal conversion format for numbers by default.

SUBSEP

The subscript separator string for multi-dimensional arrays; the default value is "\034"

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 will 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 will 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 will be treated as an assignment rather than a file argument.

ENVIRON

Array of environment variables; subscripts are names. For example, if environment variable V=thing, ENVIRON["V"] produces thing.

RSTART

The starting position of the string matched by the match function, numbering from 1. This is always equivalent to the return value of the match function.

RLENGTH

The length of the string matched by the match function.

Functions can be defined (at the position of a pattern-action statement) as follows:

function foo(a, b, c) { ...; return x } 

Parameters are passed by value if scalar, and by reference if array name. Functions can be called recursively. Parameters are local to the function; all other variables are global.

Note that if pattern-action statements are used in an HP-UX command line as an argument to the awk command, the pattern-action statement must be enclosed in single quotes to protect it from the shell. For example, to print lines longer than 72 characters, the pattern-action statement as used in a script (-f progfile command form) is:

length > 72 

The same pattern action statement used as an argument to the awk command is quoted in this manner:

awk 'length > 72' 

EXTERNAL INFLUENCES

For information about the UNIX standard environment, see standards(5).

Environment Variables

LANG: Provides a default value for the internationalization variables that are unset or null. If LANG is unset or null, the default value of "C" (see lang(5)) is used. If any of the internationalization variables contains an invalid setting, awk will behave as if all internationalization variables are set to "C". See environ(5).
LC_ALL: If set to a non-empty string value, overrides the values of all the other internationalization variables.
LC_CTYPE: Determines the interpretation of text as single and/or multi-byte characters, the classification of characters as printable, and the characters matched by character class expressions in regular expressions.
LC_NUMERIC: Determines the radix character used when interpreting numeric input, performing conversion 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).
LC_COLLATE: Determines the locale for the behavior of ranges, equivalence classes and multi-character collating elements within regular expressions.
LC_MESSAGES: Determines the locale that should be used to affect the format and contents of diagnostic messages written to standard error and informative messages written to standard output.
NLSPATH: Determines the location of message catalogues for the processing of LC_MESSAGES.
PATH: Determines the search path when looking for commands executed by system(cmd), or input and output pipes.

In addition, all environment variables will be visible via the awk variable ENVIRON.

International Code Set Support

Single- and multi-byte character code sets are supported except that variable names must contain only ASCII characters and regular expressions must contain only valid characters.

DIAGNOSTICS

awk supports up to 199 fields ($1, $2, ..., $199) per record.

EXAMPLES

Print lines longer than 72 characters:

length > 72 

Print first two fields in opposite order:

{ print $2, $1 } 

Same, with input fields separated by comma and/or blanks and tabs:

BEGIN { FS = ",[ \t]*|[ \t]+" }
      { print $2, $1 }

Add up first column, print sum and average:

        { s += $1 }"
END     { print "sum is", s, " average is", s/NR }

Print all lines between start/stop pairs:

/start/, /stop/ 

Simulate echo command (see echo(1)):

BEGIN   {                             # Simulate echo(1)
        for (i = 1; i < ARGC; i++) printf "%s ", ARGV[i]
        printf "\n"
        exit }

AUTHOR

awk was developed by AT&T, IBM, OSF, and HP.

STANDARDS CONFORMANCE

awk: SVID2, SVID3, XPG2, XPG3, XPG4, POSIX.2