term% cat index.txt AWK(1) General Commands Manual 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 associ‐
ated action that will be performed when a line of a file matches the
pattern. The set of patterns 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 de‐
fault 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, and are built using the opera‐
tors +, -, *, /, %, and concatenation (indicated by a blank). The C
operators ++, --, +=, -=, *=, /=, and %= are also available in expres‐
sions. Variables may be scalars, array elements (denoted x[i]) or
fields. Variables are initialized to the null string. Array sub‐
scripts may be any string, not necessarily numeric; this allows for a
form of associative memory. String constants 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 sep‐
arator, 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 ar‐
gument to an integer. substr(s, m, n) returns the n-character sub‐
string 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 (!, ||, &&, and parenthe‐
ses) of regular expressions and relational expressions. Regular ex‐
pressions must be surrounded by slashes and are as in egrep. Isolated
regular expressions in a pattern apply to the entire line. Regular ex‐
pressions 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 be‐
fore 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 (de‐
fault 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 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.
AWK(1)