awk(1) awk(1)
NAME
awk - pattern scanning and processing language
SYNOPSIS
awk [-f progfile | 'prog'] [-Ffieldsep] [-v var=value] [file . . .]
DESCRIPTION
awk scans each input file for lines that match any of a set of
patterns specified in prog. The prog string must be enclosed
in single quotes (') to protect it from the shell. Patterns
are arbitrary Boolean combinations of extended regular
expressions (see grep(1))and relational expressions. For each
pattern in prog there may be an associated action performed
when a line of a file matches the pattern. The set of
pattern-action statements may appear literally as prog or in a
file specified with the -f progfile option. Input files are
read in order; if there are no files, the standard input is
read. The file name - means the standard input.
awk processes supplementary code set characters in pattern-
action statements and comments, and recognizes supplementary
code set characters as field separators (see below) according
to the locale specified in the LC_CTYPE environment variable
[see LANG on environ(5)]. In regular expressions, pattern
searches are performed on characters, not bytes, as described
on grep(1).
Each input line is matched against the pattern portion of
every pattern-action statement; the associated action is
performed for each matched pattern. Any file of the form
var=value is treated as an assignment, not a filename, and is
executed at the time it would have been opened if it were a
filename, and is executed at the time it would have been
opened if it were a filename. 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.
An input line is normally made up of fields separated by white
space. (This default can be changed by using the FS built-in
variable or the -Ffieldsep option.) The fields are denoted
$1, $2, ...; $0 refers to the entire line.
A pattern-action statement has the form:
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awk(1) awk(1)
pattern { action }
Either pattern or action may be omitted. If there is no
action with a pattern, the matching line is printed. If there
is no pattern with an action, the action is performed on every
input line. Pattern-action statements are separated by
newlines or semicolons.
As noted, patterns are arbitrary Boolean combinations ( !, ||,
&&, and parentheses) of relational expressions and extended
regular expressions. A relational expression is one of the
following:
expression relop expression
expression matchop regular_expression
expression in array-name
(expression,expression,
...
) in array-name
where a relop is any of the six relational operators in C, and
a matchop is either ~ (contains) or !~ (does not contain). An
expression is an arithmetic expression, a relational
expression, the special expression
var in array
or a Boolean combination of these.
In patterns extended regular expressions must be surrounded by
slashes. Isolated regular expressions in a pattern apply to
the entire line. Extended 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 pattern.
The special patterns BEGIN and END may be used to capture
control before the first input line has been read and after
the last input line has been read respectively. These
keywords do not combine with any other patterns.
An extended regular expression may be used to separate fields
by using the -F fieldsep option or by assigning the expression
to the built-in variable FS. The default is to ignore leading
blanks and to separate fields by blanks and/or tab characters.
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awk(1) awk(1)
However, if FS is assigned a value, leading blanks are no
longer ignored.
Other built-in variables include:
ARGC command line argument count
ARGV command line argument array
ENVIRON array of environment variables;
subscripts are names
FILENAME name of the current input file
FNR ordinal number of the current record in
the current file
FS input field separator regular expression
(default blank and tab)
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 new-
line)
RS input record separator (default new-
line)
SUBSEP separates multiple subscripts (default
is 034)
The field separators specified with the -F option or with the
variables OFS, ORS, and FS may be supplementary code set
characters.
An action is a sequence of statements. A statement may be one
of the following:
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awk(1) awk(1)
if ( expression ) statement [else statement]
while ( expression ) statement
do statement while ( expression )
for ( expression ; expression ; expression ) statement
for ( var in array ) statement
delete array[subscript] #delete an array element
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
return [expr]
Statements are terminated by semicolons, new-lines, or right
braces. An empty expression-list stands for the whole input
line. Expressions take on string or numeric values as
appropriate, and are built using the operators +, -, *, /, %,
^ 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 or zero. 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. 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 >expression is present, or on a pipe
if | cmd is present. The arguments are 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 fprintf(3S)]. The built-in
function close(expr) closes the file or pipe expr.
The mathematical functions: atan2, cos, exp, log, sin, sqrt,
are built-in.
Other built-in functions include:
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awk(1) awk(1)
gsub(for, repl, in)
behaves like sub (see below), except that it
replaces successive occurrences of the regular
expression (like the ed global substitute command).
index(s, t)
returns the position in string s where string t
first occurs, or 0 if it does not occur at all.
int truncates to an integer value.
length(s) returns the length in bytes of its argument taken as
a string, or of the whole line if there is no
argument.
match(s, re)
returns the position in string s where the regular
expression re occurs, or 0 if it does not occur at
all. RSTART is set to the starting position (which
is the same as the returned value), and RLENGTH is
set to the length of the matched string.
rand random number on (0, 1).
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.
srand sets the seed for rand
sprintf(fmt, expr, expr,...)
formats the expressions according to the fprintf(3S)
format given by fmt and returns the resulting
string.
sub(for, repl, in)
substitutes the string repl in place of the first
instance of the regular expression for in string in
and returns the number of substitutions. If in is
omitted, awk substitutes in the current record ($0).
substr(s, m, n)
returns the n-character substring of s that begins
at position m.
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awk(1) awk(1)
The input/output built-in functions are:
close(filename)
closes the file or pipe named filename.
cmd | getline
pipes the output of cmd into getline; each
successive call to getline returns the next line of
output from cmd.
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.
getline x <file
sets x from the next record of file.
system(cmd)
executes cmd and returns its exit status.
All forms of getline return 1 for successful input, 0 for end
of file, and -1 for an error.
awk also provides user-defined functions. Such functions may
be defined (in the pattern position of a pattern-action
statement) as
function name(args,...) { stmts }
or
func name(args,...) { stmts }
Function arguments are passed by value if scalar and by
reference if array name. Argument names are local to the
function; all other variable names are global. Function calls
may be nested and functions may be recursive. The return
statement may be used to return a value.
EXAMPLES
Print lines longer than 72 characters:
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awk(1) awk(1)
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 }
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 }
Simulate echo(1):
BEGIN {
for (i = 1; i < ARGC; i++)
printf "%s", ARGV[i]
printf "\n"
exit
}
Print a file, filling in page numbers starting at 5:
/Page/ { $2 = n++; }
{ print }
Assuming this program is in a file named prog, the following
command line prints the file input numbering its pages
starting at 5: awk -f prog n=5 input.
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awk(1) awk(1)
FILES
/usr/lib/locale/locale/LC_MESSAGES/uxawk
language-specific message file [See LANG on environ(5).]
REFERENCES
fprintf(3S), grep(1), lex(1), sed(1)
A. V. Aho, B. W. Kernighan, P. J. Weinberger, The awk
Programming Language Addison-Wesley, 1988
NOTICES
nawk is equivalent to awk.
Input white space is not preserved on output if fields are
involved.
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 the null
string ("") to it.
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