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, ... ; $0 refers to the entire line.
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 newlines or semicolons.
An action is a sequence of statements. A statement can be one of the following:
if ( conditional ) statement [ else statement ]
while ( conditional ) statement
for ( expression ; conditional ; expression ) statement
for ( var in array ) statement
do statement while ( conditional )
break
continue
{ [ statement ] ... }
expression # commonly variable = expression
print [ expression-list ] [ >expression ]
printf format [ , expression-list ] [ >expression ]
next # skip remaining patterns on this input line
exit [expr] # skip the rest of the input; exit status is expr
Statements are terminated by semicolons, newlines or right braces. An empty expression-list stands for the whole line. Expressions take on string or numeric values as appropriate, and are built using the operators +, -, *, /, %, ^ (exponentiation), and concatenation (indicated by a blank). The 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 are quoted "...", with the usual C escapes recognized within.
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 \(or\^cmd 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 function close closes the file or pipe named as its argument.
The built-in function length returns the length of its argument taken as a string, or of the whole line if no argument. Other built-in functions are exp, log, sqrt, sin, cos, atan2, rand (returns a random number on (0,1)), srand (sets seed for rand), and int (truncates its argument to an integer). substr(s,m,n) returns the n-character substring of s that begins at position m. index(s, returns the position in s where t occurs, or 0 if it does not. The function split(s, 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. FS may be a regular expression.
The function sub(r, substitutes t for the first occurrence of the regular expression r in the string s. If s is not given, $0 is used. The function gsub is the same except that all occurrences of the regular expression are replaced. Sub and gsub return the number of replacements.
The function match(s, returns the position in s where re occurs, or 0 if it does not. The variables RSTART and RLENGTH are set to the beginning and length of the matched string.
The function sprintf(fmt,expr,expr,...) formats the expressions according to the printf(3) format given by fmt and returns the resulting string.
The function system(cmd) 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\(orgetline 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 (!, \(or\(or, &&, and parentheses) of regular expressions and relational expressions. Regular expressions are as in egrep(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; in addition, 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 between an occurrence of the first pattern and the next 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 ~ (for contains) or !~ (for does not contain). A conditional is an arithmetic expression, a relational expression, or a Boolean combination of these.expression matchop regular-expression expression relop expression
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.
A regular expression r may be used to separate fields, by assigning to the variable FS or by means of the -Fs 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; FNR, the ordinal number of the current record in the current file; FILENAME, the name of the current input file; RS, the input record separator (default newline); OFS, the output field separator (default blank); ORS, the output record separator (default newline); OFMT, the output format for numbers (default "%.6g"); ARGC, the argument count; and ARGV, the argument array. ARGC and the ARGV array may be altered; non-null members are taken as filenames.
Functions may be defined (at the position of a pattern-action statement) as
func foo(a, b, c) {...}
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.
The
return
statement may be used to return a value.
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]*\(or[ \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(1):
BEGIN {
for (i = 1; i < ARGC; i++)
printf "%s ", ARGV[i]
printf "\n"
exit
}
A. V. Aho, B. W. Kernighan, P. J. Weinberger, Awk - a pattern scanning and processing language: user's manual
The scope rules for variables in functions are a botch.