NAME

awk − pattern scanning and processing language

SYNOPSIS

awk [ −Fc ] [ pattern { action }  ] [ file ] ... 

DESCRIPTION

Awk scans each input file for lines that match any of a set of patterns specified in the pattern { action } program.  With each pattern in pattern { action } there can be an associated action that will be performed when a line of a file matches the pattern.  If no action is specified, the lines that qualify will be printed on the standard output. 

Patterns may be specified on the command line, or they may be taken from an awk command file used with the −f file option. 

Files to be examined are read in order; if there are no files named, the standard input is read.  The option ’−’ means to use the standard input. 

Each line from the files is matched against the pattern portion of every pattern-action statement; the associated action is performed for each matched pattern. 

An input line is made up of fields separated by white space.  (This default can be changed by using FS, vide infra.) The fields are denoted $1, $2, ... . In contrast to some other programs in which "0" is the first field, in awk $0 refers to the entire line. 

A pattern-action statement has the form

pattern { action }

The "pattern" should be enclosed in double quotation marks if it is a string, and 0 should also be added to the "pattern" to force it to be explicitly treated as a number. 

A missing { action } means print the line; a missing pattern always matches. 

Patterns may be arbitrary Boolean combinations (!, ||, &&, and parentheses) of regular expressions and relational expressions. 

Regular expressions must be surrounded by slashes, and the syntax and metacharacters (as well as the need to escape the metacharacters) follows the same general syntax as does egrep.

If the shell complains, also enclose the expressions in double quotation marks. 

Isolated regular expressions in a pattern apply to the entire line. 

A pattern may also consist of two patterns separated by a comma; in this case, the action is performed for all lines between an occurrence of the first pattern and the next occurrence of the second.  The action is performed recursively for all such /start/, /stop/ pairs in the file. 

Regular expressions may also be used in relational expressions. 

A relational expression is one of the following:

expression matchop regular-expression
expression relop expression

where a relop is any of the six relational operators in C, and a matchop is either ~ (for contains) or !~ (for does not contain).  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 must be the first pattern, END the last. 

A single character c may be used to separate the fields by starting the program with

BEGIN { FS = "c" }

or by using the −Fc option. 

Other variable names with special meanings include NF, the number of fields in the current record; NR, the ordinal number of the current record; FILENAME, the name of the current input file; OFS, the output field separator (default blank); ORS, the output record separator (default newline); and OFMT, the output format for numbers (default "%.6g"). 

An action is a sequence of statements.  The statements should be connected with a backslash before each newline, if they occupy more than one command line. 

A statement can be one of the following:

if ( conditional ) statement [ else statement ]
while ( conditional ) statement
for ( expression ; conditional ; expression ) statement
break
continue
{ [ statement ] ... }
variable = expression
print [ expression-list ] [ >expression ]
printf format [ , expression-list ] [ >expression ]
next# skip remaining patterns on this input line
exit# skip the rest of the input

Action statements are terminated by semicolons, newlines or right braces.  Be sure to escape the newline with a backslash immediately preceding it.  Beginning and ending curly braces should be escaped with single quotation marks, one before the opening brace and one immediately after the closing brace. (see EXAMPLES, below).  That is, enclose the entire action statement in single quotation marks ´{ action }’ in order not to be trapped by the shell. 

An empty expression-list stands for the whole line.  Expressions take on string or numeric values as appropriate, and are built using the operators +, −, *, /, %,  and concatenation (indicated by a blank).  The C 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.  String constants must be quoted "...". 

The print statement prints its arguments on the standard output (or on a file if >file is present), separated by the current output field separator, and terminated by the output record separator.  The printf statement formats its expression list according to the format (see printf(3)).

The built-in function length returns the length of its argument taken as a string, or of the whole line if no argument.  There are also built-in functions exp, log, sqrt, and int. The last truncates its argument to an integer. substr(s, m, n) returns the n-character substring of s that begins at position m. The function sprintf (fmt, expr, expr, ...)  formats the expressions according to the printf(3) format given by fmt and returns the resulting string. 

EXAMPLES

awk "length > 72" filea

would print lines longer than 72 characters on the standard output. 

awk ’{ print $2, $1 }’ filea

would print the first two fields of each line in opposite order. 

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

would add up the first column and print the sum and average. 

awk ’{ for (i = NF; i > 0; −−i) print $1 }’ filea

would print all the fields of each line in reverse order.  The output prints one field per line, beginning at the end of the file, unless otherwise directed. 

awk "/start/, /stop/" filea

would print all lines between start/stop pattern pairs, for every such pair in the file. 

SEE ALSO

egrep(1), lex(1), sed(1)
A. V. Aho, B. W. Kernighan, P. J. Weinberger, Awk − a pattern scanning and processing language

BUGS

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. 

7th Edition  —  2/2/82