AWK(1) AWK(1)
NAME
awk - pattern scanning and processing language
SYNOPSIS
awk [ -Fc ] [ prog ] [ file ] ...
DESCRIPTION
Awk scans each input file for lines that match any of a set
of patterns specified in prog. With each pattern in prog
there can be an associated action that will be performed
when a line of a file matches the pattern. The set of pat-
terns may appear literally as prog, or in a file specified
as -f file.
Files are read in order; if there are no files, the standard
input is read. The file name `-' means the standard input.
Each line 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, ... ; $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.
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
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
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,
AWK(1) AWK(1)
and are built using the operators +, -, *, /, %, and con-
catenation (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 con-
stants are 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 expres-
sion list according to the format (see printf(3)).
The built-in function length returns the length of its argu-
ment 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.
Patterns are arbitrary Boolean combinations (!, oror, &&, and
parentheses) of regular expressions and relational expres-
sions. Regular expressions must be surrounded by slashes
and are as in egrep. Isolated regular expressions in a pat-
tern apply to the entire line. Regular expressions may also
occur in relational expressions.
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.
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
AWK(1) AWK(1)
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 num-
ber 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").
EXAMPLES
Print lines longer than 72 characters:
length > 72
Print first two fields in opposite order:
{ print $2, $1 }
Add up first column, print sum and average:
{ s += $1 }
END { print "sum is", s, " average is", s/NR }
Print fields in reverse order:
{ for (i = NF; i > 0; --i) print $i }
Print all lines between start/stop pairs:
/start/, /stop/
Print all lines whose first field is different from previous
one:
$1 != prev { print; prev = $1 }
SEE ALSO
lex(1), sed(1)
A. V. Aho, B. W. Kernighan, P. J. Weinberger, Awk - a pat-
tern 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 concate-
nate "" to it.