Linux awk command

On Unix-like operating systems, the awk command runs AWK, a text-processing programming language.

This page covers the GNU/Linux version of AWK, gawk. On Linux systems, the awk command is generally a symbolic link to the executable file /usr/bin/gawk.

Description

Short for "Aho, Weinberger, and Kernighan," AWK is an interpreted programming language which focuses on processing text.

AWK was developed in the 1970s at Bell Labs by Alfred Aho, Peter Weinberger, and Brian Kernighan. It was designed to execute complex pattern-matching operations on streams of textual data. It makes heavy use of strings, associative arrays, and regular expressions, and is immensely useful for parsing system data and generating automatic reports.

AWK is a direct predecessor of Perl, and is still very useful in modern systems. The GNU free software project distributes an open-source version of AWK called gawk.

Syntax

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

Arguments

Overview

Awk scans each input file for lines that match any of a set of patterns specified literally in prog or in one or more files specified as -f progfile. With each pattern, there is an associated action that will be performed when a line of a file matches the pattern. Each line is matched against the pattern portion of every pattern-action statement; the associated action is performed for each matched pattern. The file name "-" (a dash) instructs awk to read from the standard input. The option -v followed by var=value is an assignment to be done before prog is executed; any number of -v options may be present. The -F fs option defines the input field separator to be the regular expression fs.

An input line is normally made up of fields separated by white space, or by regular expression fs. The fields are denoted $1, $2, ..., while $0 refers to the entire line. If fs is null, the input line is split into one field per character.

A pattern-action statement has the form

pattern { action }

A missing { action } means print the line; if no pattern is specified, it always matches. Pattern-action statements are separated by newlines 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

print [ expression-list ] [ > expression ]

printf format [ , expression-list ] [ > expression ]

return [ expression ]

next

(skips remaining patterns on this input line)

nextfile

(skips rest of this file, open next, start at top)

delete array[ expression ]

(deletes an array element)

delete array

(deletes all elements of array)

exit [ expression ]

(exits immediately; exit status is the evaluation of 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 white space).

operators !, ++, --, +=, -=, *=, /=, %=, ^=, >, >=, <, <=, ==, !=, and ?: are also available in expressions. Variables may be scalars, array elements (denoted x[i]) or fields. Variables are initialized to the null string. Array subscripts may 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 may 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). The built-in function close(expr) closes the file or pipe expr. The built-in function fflush(expr) flushes any buffered output for the file or pipe expr.

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

The 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 are arbitrary Boolean combinations (with ! || &&) of regular expressions and relational expressions. Regular expressions are as defined in re_format. Isolated regular expressions in a pattern apply to the entire line. Regular expressions may also occur in relational expressions, using the operators ~ and !~. /re/ is a constant regular expression; any string (constant or variable) may be used as a regular expression, except in the position of an isolated regular expression in a pattern.

A pattern may consist of two patterns separated by a comma. Here, 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 these.

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

Variable names with special meanings:

Functions may be defined (at the position of a pattern-action statement) like this:

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

Parameters are passed by value (if scalar) or by reference (if array name); functions may be called recursively. Parameters are local to the function; all other variables are global. Thus local variables may be created by providing excess parameters in the function definition.

Examples

awk 'length($0) > 72' text.txt

Print only lines of the file text.txt that are longer than 72 characters.

awk '{ print $2, $1 }' data.txt

Print first two fields of data in opposite order. For example, if the file data.txt contains the lines:

red apple blue berry green thumb

...then the output of the above awk command would be:

apple red berry blue thumb green

Most awk programs are too long to specify on the command line. If the program is saved in the file prog.awk, the command below.

awk -f prog.awk file.txt

The above command executes the awk program in prog.awk to process the contents of file file.txt.

The remainder of the examples are the awk programs themselves.

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

This program adds up first column of its input file, and print the sum and average of the values.

/start/, /stop/

This program prints all lines of text found between "start" and "stop".

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

This awk program simulates the echo command.

Related commands

gawk — GNU's version of AWK.