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AWK(1) DragonFly General Commands Manual AWK(1)
NAME
awk - pattern-directed scanning and processing language
SYNOPSIS
awk [ -F fs | --csv ] [ -v var=value ] [ 'prog' | -f progfile ] [ file
... ]
DESCRIPTION
Awk scans each input file for lines that match any of a set of patterns
specified literally in prog or in one or more files specified as -f
progfile. With each pattern there can be an associated action that
will be performed when a line of a file matches the pattern. Each line
is matched against the pattern portion of every pattern-action
statement; the associated action is performed for each matched pattern.
The file name - means the standard input. 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. 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. The -F fs option
defines the input field separator to be the regular expression fs. The
--csv option causes awk to process records using (more or less)
standard comma-separated values (CSV) format.
An input line is normally made up of fields separated by white space,
or by the regular expression FS. The fields are denoted $1, $2, ...,
while $0 refers to the entire line. If FS is null, the input line is
split into one field per character.
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( expression ) statement [ else statement ]
while( expression ) statement
for( expression ; expression ; expression ) statement
for( var in array ) statement
do statement while( expression )
break
continue
{ [ statement ... ] }
expression # commonly var = expression
print [ expression-list ] [ > expression ]
printf format [ , expression-list ] [ > expression ]
return [ expression ]
next # skip remaining patterns on this input line
nextfile # skip rest of this file, open next, start at top
delete array[ expression ]# delete an array element
delete array # delete all elements of array
exit [ expression ] # exit immediately; status is expression
Statements are terminated by semicolons, newlines or right braces. An
empty expression-list stands for $0. String constants are quoted " ",
with the usual C escapes recognized within. Expressions take on string
or numeric values as appropriate, and are built using the operators * -
* / % ^ (exponentiation), and concatenation (indicated by white space).
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. 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.
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 | cmd is
present), separated by the current output field separator, and
terminated by the output record separator. file and cmd may be literal
names or parenthesized expressions; identical string values in
different statements denote the same open file. The printf statement
formats its expression list according to the format (see printf(3)).
The built-in function close(expr) closes the file or pipe expr. The
built-in function fflush(expr) flushes any buffered output for the file
or pipe expr.
The mathematical functions atan2, cos, exp, log, sin, and sqrt are
built in. Other built-in functions:
length([v])
the length of its argument taken as a string, number of elements
in an array for an array argument, or length of $0 if no argument.
rand()
random number on [0,1).
srand([s])
sets seed for rand and returns the previous seed.
int(x)
truncates to an integer value.
substr(s, m [, n])
the n-character substring of s that begins at position m counted
from 1. If no n, use the rest of the string.
index(s, t)
the position in s where the string t occurs, or 0 if it does not.
match(s, r)
the position in s where the regular expression r occurs, or 0 if
it does not. The variables RSTART and RLENGTH are set to the
position and 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. An empty
string as field separator splits the string into one array element
per character.
sub(r, t [, s])
substitutes t for the first occurrence of the regular expression r
in the string s. If s is not given, $0 is used.
gsub(r, t [, s])
same as sub except that all occurrences of the regular expression
are replaced; sub and gsub return the number of replacements.
sprintf(fmt, expr, ...)
the string resulting from formatting expr ... according to the
printf(3) format fmt.
system(cmd)
executes cmd and returns its exit status. This will be -1 upon
error, cmd's exit status upon a normal exit, 256 + sig upon death-
by-signal, where sig is the number of the murdering signal, or 512
+ sig if there was a core dump.
tolower(str)
returns a copy of str with all upper-case characters translated to
their corresponding lower-case equivalents.
toupper(str)
returns a copy of str with all lower-case characters translated to
their corresponding upper-case equivalents.
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 | getline 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 (with ! || &&) of regular
expressions and relational expressions. Regular expressions are as in
egrep; see grep(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; 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 from an occurrence of the
first pattern through an occurrence of the second, inclusive.
A relational expression is one of the following:
expression matchop regular-expression
expression relop expression
expression in array-name
(expr,expr,...) in array-name
where a relop is any of the six relational operators in C, and a
matchop is either ~ (matches) or !~ (does not match). 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 and END
do not combine with other patterns. They may appear multiple times in
a program and execute in the order they are read by awk.
Variable names with special meanings:
ARGC argument count, assignable.
ARGV argument array, assignable; non-null members are taken as
filenames.
CONVFMT
conversion format used when converting numbers (default %.6g).
ENVIRON
array of environment variables; subscripts are names.
FILENAME
the name of the current input file.
FNR ordinal number of the current record in the current file.
FS regular expression used to separate fields; also settable by
option -Ffs.
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 space).
ORS output record separator (default newline).
RLENGTH
the length of a string matched by match.
RS input record separator (default newline). If empty, blank lines
separate records. If more than one character long, RS is treated
as a regular expression, and records are separated by text
matching the expression.
RSTART
the start position of a string matched by match.
SUBSEP
separates multiple subscripts (default 034).
Functions may be defined (at the position of a pattern-action
statement) thus:
function 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. Thus local variables may be
created by providing excess parameters in the function definition.
ENVIRONMENT VARIABLES
If POSIXLY_CORRECT is set in the environment, then awk follows the
POSIX rules for sub and gsub with respect to consecutive backslashes
and ampersands.
EXAMPLES
length($0) > 72
Print lines longer than 72 characters.
{ print $2, $1 }
Print first two fields in opposite order.
BEGIN { FS = ",[ \t]*|[ \t]+" }
{ print $2, $1 }
Same, with input fields separated by comma and/or spaces and
tabs.
{ s += $1 }
END { print "sum is", s, " average is", s/NR }
Add up first column, print sum and average.
/start/, /stop/
Print all lines between start/stop pairs.
BEGIN { # Simulate echo(1)
for (i = 1; i < ARGC; i++) printf "%s ", ARGV[i]
printf "\n"
exit }
SEE ALSO
grep(1), lex(1), sed(1)
A. V. Aho, B. W. Kernighan, P. J. Weinberger, The AWK Programming
Language, Second Edition, Addison-Wesley, 2024. ISBN
978-0-13-826972-2, 0-13-826972-6.
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.
The scope rules for variables in functions are a botch; the syntax is
worse.
Input is expected to be UTF-8 encoded. Other multibyte character sets
are not handled. However, in eight-bit locales, awk treats each input
byte as a separate character.
UNUSUAL FLOATING-POINT VALUES
Awk was designed before IEEE 754 arithmetic defined Not-A-Number (NaN)
and Infinity values, which are supported by all modern floating-point
hardware.
Because awk uses strtod(3) and atof(3) to convert string values to
double-precision floating-point values, modern C libraries also convert
strings starting with inf and nan into infinity and NaN values
respectively. This led to strange results, with something like this:
echo nancy | awk '{ print $1 + 0 }'
printing nan instead of zero.
Awk now follows GNU AWK, and prefilters string values before attempting
to convert them to numbers, as follows:
Hexadecimal values
Hexadecimal values (allowed since C99) convert to zero, as they
did prior to C99.
NaN values
The two strings +nan and -nan (case independent) convert to NaN.
No others do. (NaNs can have signs.)
Infinity values
The two strings +inf and -inf (case independent) convert to
positive and negative infinity, respectively. No others do.
AWK(1)