AWK(C) UNIX System V
Name
awk - pattern scanning and processing language
Syntax
awk [ -F re ] [ parameter... ] [ 'prog' ] [ -f progfile ]
[ file... ]
Description
The -F re option defines the input field separator to be the
regular expression re.
Parameters, in the form x=... y=... may be passed to awk,
where x and y are awk built-in variables (see list below).
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.
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. Each input 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 normally made up of fields separated by
white space. (This default can be changed by using the FS
built-in variable or the -F re option.) The fields are
denoted $1, $2, ...; $0 refers to the entire line.
A pattern-action statement has the form:
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.
Patterns are arbitrary Boolean combinations ( !, ||, &&,
and parentheses) of rational expressions and regular
expressions. A relational expression is one of the
following:
expression relop expression
expression matchop regular expression
where a relop is any of the six relational operators in C,
and a matchop is either ~ (contains) or ! ~ (does not
contain). A conditional is an arithmetic expression, a
relational expression, the special expression
var in array,
or a Boolean combination of these.
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.
Regular expressions are as in egrep (see grep(C)). In
patterns they must be surrounded by slashes. Isolated
regular expressions in a pattern 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
next occurrence of the second pattern.
A regular expression may be used to separate fields by using
the -F re 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. 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
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)
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)
An action is a sequence of statements. A statement may be
one of the following:
if ( conditional ) statement [ else statement ]
while ( conditional ) statement
do statement while ( conditional )
for ( expression ; conditional ; expression ) statement
for ( var in array ) statement
delete array[subscript]
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 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 or zero. Array subscripts 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 >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 printf(S) in
the Programmer's Reference).
awk has a variety of built-in functions: arithmetic,
string, input/output, and general.
The arithmetic functions are: atan2, cos, exp, int, log,
rand, sin, sqrt, and srand. int truncates its argument to
an integer. rand returns a random number between 0 and 1.
srand ( expr ) sets the seed value for rand to expr or uses
the time of day if expr is omitted.
The string functions are:
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.
length(s) returns the length 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.
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.
sprintf(fmt, expr, expr,...)
formats the expressions according to the
printf(S) 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.
The input/output and general 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 var sets variable var instead.
getline var <file
sets var from the next record of file.
system(cmd) executes cmd and returns to 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 }
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:
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 the 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(C):
BEGIN {
for (i = 1; i < ARGC; i++)
printf "%s", ARGV[i]
printf "\n"
exit
}
Print file, filling in page numbers starting at 5:
/Page/ { $2 = n++; }
{ print }
command line: awk -f program n=5 input
See Also
grep(C), sed(C), lex(CP), printf(S)
Notes
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.
Standards Conformance
awk is conformant with:
AT&T SVID Issue 2, Select Code 307-127;
and The X/Open Portability Guide II of January 1987.
(printed 8/28/89) AWK(C)