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GAWK(1) USER COMMANDS GAWK(1)
NAME
gawk - pattern scanning and processing language
SYNOPSIS
gawk [ -Ffs ] -f program-file [ -f program-file ... ] [ -- ]
file ...
gawk [ -Ffs ] [ -- ] program-text file ...
DESCRIPTION
Gawk is the GNU Project's implementation of the AWK program-
ming language. It conforms to the definition and descrip-
tion of the language in The AWK Programming Language, by
Aho, Kernighan, and Weinberger, with the additional features
defined in the System V Release 4 version of UNIX awk.
The command line consists of options to gawk itself, the AWK
program text (if not supplied via the -f option), and values
to be made available in the ARGC and ARGV pre-defined AWK
variables.
The options that gawk accepts are:
-Ffs Use fs for the input field separator (the value of the
FS predefined variable).
-f program-file
Read the AWK program source from the file program-file,
instead of from the first command line argument.
-- Signal the end of options. This is useful to allow
further arguments to the AWK program itself to start
with a ``-''. This is mainly for consistency with the
argument parsing convention used by most other System V
programs.
Any other options are flagged as illegal, but are otherwise
ignored. (However, see the GNU EXTENSIONS section, below.)
An AWK program consists of a sequence of pattern-action
statements and optional function definitions.
pattern { action statements }
function name(parameter list) { statements }
Gawk first reads the program source from the program-file(s)
if specified, or from the first non-option argument on the
command line. The -f option may be used multiple times on
the command line. Gawk will read the program text as if all
the program-files had been concatenated together. This is
useful for building libraries of AWK functions, without hav-
ing to include them in each new AWK program that uses them.
To use a library function in a file from a program typed in
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GAWK(1) USER COMMANDS GAWK(1)
on the command line, specify /dev/tty as one of the
program-files, type your program, and end it with a ^D
(control-d).
The environment variable AWKPATH specifies a search path to
use when finding source files named with the -f option. If
this variable does not exist, the default path is
".:/usr/lib/awk:/usr/local/lib/awk". If a file name given
to the -f option contains a ``/'' character, no path search
is performed.
Gawk compiles the program into an internal form, and then
proceeds to read each file named in the ARGV array. If
there are no files named on the command line, gawk reads the
standard input.
If a ``file'' named on the command line has the form var=val
it is treated as a variable assignment. The variable var
will be assigned the value val. This is most useful for
dynamically assigning values to the variables AWK uses to
control how input is broken into fields and records. It is
also useful for controlling state if multiple passes are
needed over a single data file.
For each line in the input, gawk tests to see if it matches
any pattern in the AWK program. For each pattern that the
line matches, the associated action is executed.
VARIABLES AND FIELDS
AWK variables are dynamic; they come into existence when
they are first used. Their values are either floating-point
numbers or strings, depending upon how they are used. AWK
also has single dimension arrays; multiply dimensioned
arrays may be simulated. There are several pre-defined
variables that AWK sets as a program runs; these will be
described as needed and summarized below.
As each input line is read, gawk splits the line into
fields, using the value of the FS variable as the field
separator. If FS is a single character, fields are
separated by that character. Otherwise, FS is expected to
be a full regular expression. In the special case that FS
is a single blank, fields are separated by runs of blanks
and/or tabs. Note that the value of IGNORECASE (see below)
will also affect how fields are split when FS is a regular
expression.
Each field in the input line may be referenced by its posi-
tion, $1, $2, and so on. $0 is the whole line. The value of
a field may be assigned to as well. Fields need not be
referenced by constants:
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GAWK(1) USER COMMANDS GAWK(1)
n = 5
print $n
prints the fifth field in the input line. The variable NF
is set to the total number of fields in the input line.
References to non-existent fields (i.e. fields after $NF),
produce the null-string. However, assigning to a non-
existent field (e.g., $(NF+2) = 5) will increase the value
of NF, create any intervening fields with the null string as
their value, and cause the value of $0 to be recomputed,
with the fields being separated by the value of OFS.
Built-in Variables
AWK's built-in variables are:
ARGC the number of command line arguments (does not
include options to gawk, or the program source).
ARGV array of command line arguments. The array is
indexed from 0 to ARGC - 1. Dynamically changing
the contents of ARGV can control the files used
for data.
ENVIRON
An array containing the values of the current
environment. The array is indexed by the environ-
ment variables, each element being the value of
that variable (e.g., ENVIRON["HOME"] might be
/u/arnold). Changing this array does not affect
the environment seen by programs which gawk spawns
via redirection or the system function.
FILENAME
the name of the current input file. If no files
are specified on the command line, the value of
FILENAME is ``-''.
FNR the input record number in the current input file.
FS the input field separator, a blank by default.
IGNORECASE
Controls the case-sensitivity of all regular
expression operations. If IGNORECASE has a non-
zero value, then pattern matching in rules, field
splitting with FS, regular expression matching
with ~ and !~, and the gsub(), match(), split(),
and sub() pre-defined functions will all ignore
case when doing regular expression operations.
Thus, if IGNORECASE is not equal to zero, /aB/
matches all of the strings "ab", "aB", "Ab", and
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GAWK(1) USER COMMANDS GAWK(1)
"AB". As with all AWK variables, the initial
value of IGNORECASE is zero, so all regular
expression operations are normally case-sensitive.
NF the number of fields in the current input record.
NR the total number of input records seen so far.
OFMT the output format for numbers, %.6g by default.
OFS the output field separator, a blank by default.
ORS the output record separator, by default a newline.
RS the input record separator, by default a newline.
RS is exceptional in that only the first character
of its string value is used for separating
records. If RS is set to the null string, then
records are separated by blank lines. When RS is
set to the null string, then the newline character
always acts as a field separator, in addition to
whatever value FS may have.
RSTART
the index of the first character matched by
match(); 0 if no match.
RLENGTH
the length of the string matched by match(); -1 if
no match.
SUBSEP
the character used to separate multiple subscripts
in array elements, by default "\034".
Arrays
Arrays are subscripted with an expression between square
brackets ([ and ]). If the expression is an expression list
(expr, expr ...) then the array subscript is a string con-
sisting of the concatenation of the (string) value of each
expression, separated by the value of the SUBSEP variable.
This facility is used to simulate multiply dimensioned
arrays. For example:
i = "A" ; j = "B" ; k = "C"
x[i,j,k] = "hello, world\n"
assigns the string "hello, world\n" to the element of the
array x which is indexed by the string "A\034B\034C". All
arrays in AWK are associative, i.e. indexed by string
values.
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GAWK(1) USER COMMANDS GAWK(1)
The special operator in may be used in an if or while state-
ment to see if an array has an index consisting of a partic-
ular value.
if (val in array)
print array[val]
If the array has multiple subscripts, use (i, j) in array.
The in construct may also be used in a for loop to iterate
over all the elements of an array.
An element may be deleted from an array using the delete
statement.
Variable Typing
Variables and fields may be (floating point) numbers, or
strings, or both. How the value of a variable is interpreted
depends upon its context. If used in a numeric expression,
it will be treated as a number, if used as a string it will
be treated as a string.
To force a variable to be treated as a number, add 0 to it;
to force it to be treated as a string, concatenate it with
the null string.
The AWK language defines comparisons as being done numeri-
cally if possible, otherwise one or both operands are con-
verted to strings and a string comparison is performed.
Uninitialized variables have the numeric value 0 and the
string value "" (the null, or empty, string).
PATTERNS AND ACTIONS
AWK is a line oriented language. The pattern comes first,
and then the action. Action statements are enclosed in { and
}. Either the pattern may be missing, or the action may be
missing, but, of course, not both. If the pattern is miss-
ing, the action will be executed for every single line of
input. A missing action is equivalent to
{ print }
which prints the entire line.
Comments begin with the ``#'' character, and continue until
the end of the line. Blank lines may be used to separate
statements. Normally, a statement ends with a newline, how-
ever, this is not the case for lines ending in a ``,'',
``{'', ``?'', ``:'', ``&&'', or ``||''. Lines ending in do
or else also have their statements automatically continued
on the following line. In other cases, a line can be
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GAWK(1) USER COMMANDS GAWK(1)
continued by ending it with a ``\'', in which case the new-
line will be ignored.
Multiple statements may be put on one line by separating
them with a ``;''. This applies to both the statements
within the action part of a pattern-action pair (the usual
case), and to the pattern-action statements themselves.
Patterns
AWK patterns may be one of the following:
BEGIN
END
/regular expression/
relational expression
pattern && pattern
pattern || pattern
pattern ? pattern : pattern
(pattern)
! pattern
pattern1, pattern2
BEGIN and END are two special kinds of patterns which are
not tested against the input. The action parts of all BEGIN
patterns are merged as if all the statements had been writ-
ten in a single BEGIN block. They are executed before any of
the input is read. Similarly, all the END blocks are merged,
and executed when all the input is exhausted (or when an
exit statement is executed). BEGIN and END patterns cannot
be combined with other patterns in pattern expressions.
BEGIN and END patterns cannot have missing action parts.
For /regular expression/ patterns, the associated statement
is executed for each input line that matches the regular
expression. Regular expressions are the same as those in
egrep(1), and are summarized below.
A relational expression may use any of the operators defined
below in the section on actions. These generally test
whether certain fields match certain regular expressions.
The &&, ||, and ! operators are logical AND, logical OR, and
logical NOT, respectively, as in C. They do short-circuit
evaluation, also as in C, and are used for combining more
primitive pattern expressions. As in most languages,
parentheses may be used to change the order of evaluation.
The ?: operator is like the same operator in C. If the first
pattern is true then the pattern used for testing is the
second pattern, otherwise it is the third. Only one of the
second and third patterns is evaluated.
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GAWK(1) USER COMMANDS GAWK(1)
The pattern1, pattern2 form of an expression is called a
range pattern. It matches all input lines starting with a
line that matches pattern1, and continuing until a line that
matches pattern2, inclusive. It does not combine with any
other sort of pattern expression.
Regular Expressions
Regular expressions are the extended kind found in egrep.
They are composed of characters as follows:
c matches the non-metacharacter c.
\c matches the literal character c.
. matches any character except newline.
^ matches the beginning of a line or a string.
$ matches the end of a line or a string.
[abc...]
character class, matches any of the characters
abc....
[^abc...]
negated character class, matches any character
except abc... and newline.
r1|r2
alternation: matches either r1 or r2.
r1r2 concatenation: matches r1, and then r2.
r+ matches one or more r's.
r* matches zero or more r's.
r? matches zero or one r's.
(r) grouping: matches r.
Actions
Action statements are enclosed in braces, { and }. Action
statements consist of the usual assignment, conditional, and
looping statements found in most languages. The operators,
control statements, and input/output statements available
are patterned after those in C.
The operators in AWK, in order of increasing precedence, are
= += -= *= /= %= ^=
Assignment. Both absolute assignment (var = value)
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GAWK(1) USER COMMANDS GAWK(1)
and operator-assignment (the other forms) are sup-
ported.
?: The C conditional expression. This has the form
expr1 ? expr2 : expr3. If expr1 is true, the value
of the expression is expr2, otherwise it is expr3.
Only one of expr2 and expr3 is evaluated.
|| logical OR.
&& logical AND.
~ !~ regular expression match, negated match.
< <= > >= != ==
the regular relational operators.
blank
string concatenation.
+ - addition and subtraction.
* / %
multiplication, division, and modulus.
+ - !
unary plus, unary minus, and logical negation.
^ exponentiation (** may also be used, and **= for
the assignment operator).
++ --
increment and decrement, both prefix and postfix.
$ field reference.
The control statements are as follows:
if (condition) statement [ else statement ]
while (condition) statement
do statement while (condition)
for (expr1; expr2; expr3) statement
for (var in array) statement
break
continue
delete array[index]
exit [ expression ]
{ statements }
The input/output statements are as follows:
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GAWK(1) USER COMMANDS GAWK(1)
close(filename)
close file (or pipe, see below).
getline
set $0 from next input record; set NF, NR, FNR.
getline <file
set $0 from next record of file; set NF.
getline var
set var from next input record; set NF, FNR.
getline var <file
set var from next record of file.
next Stop processing the current input record. The next
input record is read and processing starts over
with the first pattern in the AWK program. If the
end of the input data is reached, the END
block(s), if any, are executed.
print
prints the current record.
print expr-list
prints expressions.
print expr-list >file
prints expressions on file.
printf fmt, expr-list
format and print.
printf fmt, expr-list >file
format and print on file.
system(cmd-line)
execute the command cmd-line, and return the exit
status. (This may not be available on systems
besides UNIX and GNU.)
Other input/output redirections are also allowed. For print
and printf, >>file appends output to the file, while | com-
mand writes on a pipe. In a similar fashion, command | get-
line pipes into getline. Getline will return 0 on end of
file, and -1 on an error.
The AWK versions of the printf and sprintf (see below) func-
tions accept the following conversion specification formats:
%c An ASCII character. If the argument used for %c
is numeric, it is treated as a character and
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GAWK(1) USER COMMANDS GAWK(1)
printed. Otherwise, the argument is assumed to be
a string, and the only first character of that
string is printed.
%d A decimal number (the integer part).
%e A floating point number of the form
[-]d.ddddddE[+-]dd.
%f A floating point number of the form [-]ddd.dddddd.
%g Use e or f conversion, whichever is shorter, with
nonsignificant zeros suppressed.
%o An unsigned octal number (again, an integer).
%s A character string.
%x An unsigned hexadecimal number (an integer).
%% A single % character; no argument is converted.
There are optional, additional parameters that may lie
between the % and the control letter:
- The expression should be left-justified within its
field.
width
The field should be padded to this width. If the
number has a leading zero, then the field will be
padded with zeros. Otherwise it is padded with
blanks.
.prec
A number indicating the maximum width of strings
or digits to the right of the decimal point.
The dynamic width and prec capabilities of the C library
printf routines are not supported. However, they may be
simulated by using the AWK concatenation operation to build
up a format specification dynamically.
When doing I/O redirection from either print or printf into
a file, or via getline from a file, gawk recognizes certain
special filenames internally. These filenames allow access
to open file descriptors inherited from gawk's parent pro-
cess (usually the shell). The filenames are:
/dev/stdin
The standard input.
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GAWK(1) USER COMMANDS GAWK(1)
/dev/stdout
The standard output.
/dev/stderr
The standard error output.
/dev/fd/n
The file denoted by the open file descriptor n.
These are particularly useful for error messages. For exam-
ple:
print "You blew it!" > "/dev/stderr"
whereas you would otherwise have to use
print "You blew it!" | "cat 1>&2"
These file names may also be used on the command line to
name data files.
AWK has the following pre-defined arithmetic functions:
atan2(y, x)
returns the arctangent of y/x in radians.
cos(expr)
returns the cosine in radians.
exp(expr)
the exponential function.
int(expr)
truncates to integer.
log(expr)
the natural logarithm function.
rand()
returns a random number between 0 and 1.
sin(expr)
returns the sine in radians.
sqrt(expr)
the square root function.
srand(expr)
use expr as a new seed for the random number gen-
erator. If no expr is provided, the time of day
will be used. The return value is the previous
seed for the random number generator.
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GAWK(1) USER COMMANDS GAWK(1)
AWK has the following pre-defined string functions:
gsub(r, s, t)
for each substring matching the regular expression
r in the string t, substitute the string s, and
return the number of substitutions. If t is not
supplied, use $0.
index(s, t)
returns the index of the string t in the string s,
or 0 if t is not present.
length(s)
returns the length of the string s.
match(s, r)
returns the position in s where the regular
expression r occurs, or 0 if r is not present, and
sets the values of RSTART and RLENGTH.
split(s, a, r)
splits the string s into the array a on the regu-
lar expression r, and returns the number of
fields. If r is omitted, FS is used instead.
sprintf(fmt, expr-list)
prints expr-list according to fmt, and returns the
resulting string.
sub(r, s, t)
this is just like gsub, but only the first match-
ing substring is replaced.
substr(s, i, n)
returns the n-character substring of s starting at
i. If n is omitted, the rest of s is used.
tolower(str)
returns a copy of the string str, with all the
upper-case characters in str translated to their
corresponding lower-case counterparts. Non-
alphabetic characters are left unchanged.
toupper(str)
returns a copy of the string str, with all the
lower-case characters in str translated to their
corresponding upper-case counterparts. Non-
alphabetic characters are left unchanged.
String constants in AWK are sequences of characters enclosed
between double quotes ("). Within strings, certain escape
sequences are recognized, as in C. These are:
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GAWK(1) USER COMMANDS GAWK(1)
\\ A literal backslash.
\a The ``alert'' character; usually the ASCII BEL
character.
\b backspace.
\f form-feed.
\n new line.
\r carriage return.
\t horizontal tab.
\v vertical tab.
\xhex digits
The character represented by the string of hexade-
cimal digits following the \x. As in ANSI C, all
following hexadecimal digits are considered part
of the escape sequence. (This feature should tell
us something about language design by committee.)
E.g., "\x1B" is the ASCII ESC (escape) character.
\ddd The character represented by the 1-, 2-, or 3-
digit sequence of octal digits. E.g. "\033" is the
ASCII ESC (escape) character.
\c The literal character c.
The escape sequences may also be used inside constant regu-
lar expressions (e.g., /[ \t\f\n\r\v]/ matches whitespace
characters).
FUNCTIONS
Functions in AWK are defined as follows:
function name(parameter list) { statements }
Functions are executed when called from within the action
parts of regular pattern-action statements. Actual parame-
ters supplied in the function call are used to instantiate
the formal parameters declared in the function. Arrays are
passed by reference, other variables are passed by value.
Since functions were not originally part of the AWK
language, the provision for local variables is rather
clumsy: they are declared as extra parameters in the parame-
ter list. The convention is to separate local variables from
real parameters by extra spaces in the parameter list. For
example:
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GAWK(1) USER COMMANDS GAWK(1)
function f(p, q, a, b) { # a & b are local
..... }
/abc/ { ... ; f(1, 2) ; ... }
The left parenthesis in a function call is required to
immediately follow the function name, without any interven-
ing white space. This is to avoid a syntactic ambiguity
with the concatenation operator. This restriction does not
apply to the built-in functions listed above.
Functions may call each other and may be recursive. Func-
tion parameters used as local variables are initialized to
the null string and the number zero upon function invoca-
tion.
The word func may be used in place of function.
EXAMPLES
Print and sort the login names of all users:
BEGIN { FS = ":" }
{ print $1 | "sort" }
Count lines in a file:
{ nlines++ }
END { print nlines }
Precede each line by its number in the file:
{ print FNR, $0 }
Concatenate and line number (a variation on a theme):
{ print NR, $0 }
SEE ALSO
The AWK Programming Language, Alfred V. Aho, Brian W. Ker-
nighan, Peter J. Weinberger, Addison-Wesley, 1988. ISBN 0-
201-07981-X.
SYSTEM V RELEASE 4 COMPATIBILITY
A primary goal for gawk is compatibility with the latest
version of UNIX awk. To this end, gawk incorporates the
following user visible features which are not described in
the AWK book, but are part of awk in System V Release 4.
When processing arguments, gawk uses the special option
``--'' to signal the end of arguments, and warns about, but
otherwise ignores, undefined options.
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GAWK(1) USER COMMANDS GAWK(1)
The AWK book does not define the return value of srand().
The System V Release 4 version of UNIX awk has it return the
seed it was using, to allow keeping track of random number
sequences. Therefore srand() in gawk also returns its
current seed.
The use of multiple -f options is a new feature, as is the
ENVIRON array.
GNU EXTENSIONS
Gawk has some extensions to System V awk. They are
described in this section. All the extensions described
here can be disabled by compiling gawk with -DSTRICT, or by
invoking gawk with the name awk. If the underlying operat-
ing system supports the /dev/fd directory and corresponding
files, then gawk can be compiled with -DNO_DEV_FD to disable
the special filename processing.
The following features of gawk are not available in System V
awk.
o The \a, \v, or \x escape sequences are not recog-
nized.
o The special file names available for I/O redirec-
tion are not recognized.
o The tolower and toupper built-in string functions
are not available.
o The IGNORECASE variable and its side-effects are
not available.
o No path search is performed for files named via
the -f option. Therefore the AWKPATH environment
variable is not special.
The AWK book does not define the return value of the close
function. Gawk's close returns the value from fclose(3), or
pclose(3), when closing a file or pipe, respectively.
When gawk is invoked as awk, if the fs argument to the -F
option is ``t'', then FS will be set to the tab character.
Since this is a rather ugly special case, it is not the
default behavior.
The rest of the features described in this section may
change at some time in the future, or may go away entirely.
You should not write programs that depend upon them.
Gawk accepts the following additional options:
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GAWK(1) USER COMMANDS GAWK(1)
-v Print version information for this particular copy of
gawk on the error output. This is useful mainly for
knowing if the current copy of gawk on your system is
up to date with respect to whatever the Free Software
Foundation is distributing.
-V Print the GNU copyright information message on the
error output.
BUGS
The -F option is not necessary given the command line vari-
able assignment feature; it remains only for backwards com-
patibility.
AUTHORS
The original version of UNIX awk was designed and imple-
mented by Alfred Aho, Peter Weinberger, and Brian Kernighan
of AT&T Bell Labs. Brian Kernighan continues to maintain and
enhance it.
Paul Rubin and Jay Fenlason, of the Free Software Founda-
tion, wrote gawk, to be compatible with the original version
of awk distributed in Seventh Edition UNIX. John Woods con-
tributed a number of bug fixes. David Trueman of Dalhousie
University, with contributions from Arnold Robbins at Emory
University, made gawk compatible with the new version of
UNIX awk.
ACKNOWLEDGEMENTS
Brian Kernighan of Bell Labs provided valuable assistance
during testing and debugging. We thank him.
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