| AWK(1) | General Commands Manual | AWK(1) |
awk - pattern-directed scanning and processing language
awk [ -F fs | --csv ] [ -v var=value ] [ 'prog' | -f progfile ] [ file ... ]
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 can be 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 - means the standard input. Any file of the form var=value is treated as an assignment, not a filename, and is executed at the time it would have been opened if it were a filename. 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. The --csv option causes awk to process records using (more or less) standard comma-separated values (CSV) format.
An input line is normally made up of fields separated by white space, or by the 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:
A missing { action } means print the line; a missing pattern 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 # commonly var = expression print [ expression-list ] [ > expression ] printf format [ , expression-list ] [ > expression ] return [ expression ] next # skip remaining patterns on this input line nextfile # skip rest of this file, open next, start at top delete array[ expression ] # delete an array element delete array # delete all elements of array 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 white space). The operators ! ++ -- += -= *= /= %= ^= > >= < <= == != ?: 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(3)). 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 atan2, cos, exp, log, sin, and sqrt 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 in egrep; see grep(1). 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; in this case, the action is performed for all lines from an occurrence of the first pattern through an occurrence of the second, inclusive.
A relational expression is one of the following:
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 may 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. They may appear multiple times in a program and execute in the order they are read by awk.
Variable names with special meanings:
Functions may be defined (at the position of a pattern-action statement) thus:
Parameters are passed by value if scalar and 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.
If POSIXLY_CORRECT is set in the environment, then awk follows the POSIX rules for sub and gsub with respect to consecutive backslashes and ampersands.
length($0) > 72
Print lines longer than 72 characters.
{ print $2, $1 }
Print first two fields in opposite order.
BEGIN { FS = ",[ \t]*|[ \t]+" }
{ print $2, $1 }
{ s += $1 } END { print "sum is", s, " average is", s/NR }
/start/, /stop/
Print all lines between start/stop pairs.
BEGIN { # Simulate echo(1) for (i = 1; i < ARGC; i++) printf "%s ", ARGV[i] printf "\n" exit }
grep(1), lex(1), sed(1)
A. V. Aho, B. W. Kernighan, P. J. Weinberger, The AWK Programming Language,
Second Edition, Addison-Wesley, 2024. ISBN 978-0-13-826972-2,
0-13-826972-6.
There are no explicit conversions between numbers and strings. To force an expression to be treated as a number add 0 to it; to force it to be treated as a string concatenate "" to it.
The scope rules for variables in functions are a botch; the syntax is worse.
Input is expected to be UTF-8 encoded. Other multibyte character sets are not handled. However, in eight-bit locales, awk treats each input byte as a separate character.
Awk was designed before IEEE 754 arithmetic defined Not-A-Number (NaN) and Infinity values, which are supported by all modern floating-point hardware.
Because awk uses strtod(3) and atof(3) to convert string values to double-precision floating-point values, modern C libraries also convert strings starting with inf and nan into infinity and NaN values respectively. This led to strange results, with something like this:
echo nancy | awk '{ print $1 + 0 }'
printing nan instead of zero.
Awk now follows GNU AWK, and prefilters string values before attempting to convert them to numbers, as follows:
| x86_64 |