awk - pattern-directed scanning and processing language


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


The awk(1) utility 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 file name, and is executed at the time it would have been opened if it were a file name. 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 usually made up of fields separated by white space. (To change this default, use the FS built-in variable or the -F fs option.) The fields are denoted $1 $2 ..., while $0 refers to the entire line.

A pattern-action statement has the form:


A missing { 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, listed in approximately lexical order:

Begin comment; continues to end-of-line.
Leave innermost enclosing for, do, or while loop.
Skip to next iteration of innermost enclosing for, do, or while loop.
{ [ statement ... ] }
delete array[ expression ]
Delete an array element.
do statement while( expression )
Execute statement and repeat so long as expression is true.
exit [ expression ]
Move immediately to the END action; if in the END action, exit immediately. Exit status is expression.
Commonly var = expression
for ( expression1; expression2 ;expression3 ) statement
Conditional loop, executing statement so long as expression2 is true. This is equivalent to expression1; while (expression2) { statement; expression3 }
for ( var in array )statement
Loop iterating over array contents.
if ( expression ) statement1 [ else statement2 ]
Conditional statement: execute statement1 if expression is true; otherwise, execute statement2
Skip remaining patterns on this input line and start the next input line.
print [ expression-list ] [ > expression ]
Print output.
printf format [ , expression-list ] [ >expression ]
Print formatted output.
return [ expression ]
Return from function.
while( expression ) statement
Conditional loop: so long as expression is true, execute statement

Statements are terminated by semicolons, newlines or right braces. An empty expression-list stands for $0 . String constants are quoted " "; the usual C escapes are recognized within strings. Expressions take on string or numeric values as appropriate, and are built using the operators + - * / % ^ (exponentiation), and concatenation (indicated by a blank). 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 separated by the current output field separator and terminated by the output record separator. By default, it prints to standard output. It prints to a file if >file or >>file is present, or on a pipe if |cmd is present.

The file and cmd arguments 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 similar to printf(3). The built-in function close(expr) closes the file or pipe expr.

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

The length of its argument taken as a string, or of $0 if no argument.
Random number on (0,1).
Sets seed for rand() and returns the previous seed.
Truncates to an integer value.
substr(s, m, n)
The n-character substring of s that begins at position m counted from 1.
index(s, t)
The position in s where the string t occurs, or 0 if it does not.
match(s, r)
The position in s where the regular expression r 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(r, t, s)
Substitutes t for the first occurrence of the regular expression r in the string s. If s is not given, $0 is used.
gsub(r, t, s)
Same as sub(), except that all occurrences of the regular expression are replaced. sub() and gsub() return the number of replacements.
sprintf(fmt, expr, ...)
The string resulting from formatting expr... according to the printf(3) function format fmt.
Executes cmd and returns its exit status.

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 POSIX.2 Extended Regular Expressions (ERE). 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 though an occurrence of the second.

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.

The following are variable names with special meanings:

Variable Meaning
ARGC argument count, assignable
ARGV argument array, assignable; non-null members are taken as file names
ENVIRON array of environment variables; subscripts are names.
FILENAME the name of the current input file
FNR ordinal number of the current record in the current file
FS regular expression used to separate fields; also set by option -F fs
NF number of fields in the current record
NR ordinal number of the current record
OFMT output format for numbers (default "%.6g" )
OFS output field separator (default blank)
ORS output record separator (default newline)
RS input record separator (default newline)
SUBSEP separates multiple subscripts (default 034)

Functions may 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 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.


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 }


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.