gawk
NAME
gawk - pattern scanning and processing language
SYNOPSIS
gawk [ POSIX or GNU style options ] -f program-file [ -- ]
file ...
gawk [ POSIX or GNU style options ] [ -- ] program-text file
...
DESCRIPTION
Gawk is the GNU Project's implementation of the AWK
programming language. It conforms to the definition of the
language in the POSIX 1003.2 Command Language And Utilities
Standard. This version in turn is based on the description
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. Gawk also provides
some GNU-specific extensions.
The command line consists of options to gawk itself, the AWK
program text (if not supplied via the -f or --file options),
and values to be made available in the ARGC and ARGV pre-
defined AWK variables.
OPTIONS
Gawk options may be either the traditional POSIX one letter
options, or the GNU style long options. POSIX style options
start with a single ``-'', while GNU long options start with
``--''. GNU style long options are provided for both GNU-
specific features and for POSIX mandated features. Other
implementations of the AWK language are likely to only
accept the traditional one letter options.
Following the POSIX standard, gawk-specific options are
supplied via arguments to the -W option. Multiple -W
options may be supplied, or multiple arguments may be
supplied together if they are separated by commas, or
enclosed in quotes and separated by white space. Case is
ignored in arguments to the -W option. Each -W option has a
corresponding GNU style long option, as detailed below.
Arguments to GNU style long options are either joined with
the option by an = sign, with no intervening spaces, or they
may be provided in the next command line argument.
Gawk accepts the following options.
-F fs
--field-separator=fs
Use fs for the input field separator (the value of the
FS predefined variable).
-v var=val
--assign=var=val
Assign the value val, to the variable var, before
execution of the program begins. Such variable values
are available to the BEGIN block of an AWK program.
-f program-file
--file=program-file
Read the AWK program source from the file program-file,
instead of from the first command line argument.
Multiple -f (or --file) options may be used.
-mf=NNN
-mr=NNN
Set various memory limits to the value NNN. The f flag
sets the maximum number of fields, and the r flag sets
the maximum record size. These two flags and the -m
option are from the AT&T Bell Labs research version of
UNIX awk. They are ignored by gawk, since gawk has no
pre-defined limits.
-W compat
--compat Run in compatibility mode. In compatibility
mode, gawk behaves identically to UNIX awk; none
of the GNU-specific extensions are recognized.
See GNU EXTENSIONS, below, for more information.
-W copyleft
-W copyright
--copyleft
--copyright Print the short version of the GNU copyright
information message on the error output.
-W help
-W usage
--help
--usage Print a relatively short summary of the
available options on the error output. Per the
GNU Coding Standards, these options cause an
immediate, successful exit.
-W lint
--lint Provide warnings about constructs that are
dubious or non-portable to other AWK
implementations.
-W posix
--posix This turns on compatibility mode, with the
following additional restrictions:
+ \x escape sequences are not recognized.
+ The synonym func for the keyword function is
not recognized.
+ The operators ** and **= cannot be used in
place of ^ and ^=.
-W source=program-text
--source=program-text
Use program-text as AWK program source code.
This option allows the easy intermixing of
library functions (used via the -f and --file
options) with source code entered on the command
line. It is intended primarily for medium to
large size AWK programs used in shell scripts.
9 The -W source= form of this option uses the rest
of the command line argument for program-text;
no other options to -W will be recognized in the
same argument.
-W version
--version 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. Per the GNU Coding Standards,
these options cause an immediate, successful
exit.
-- 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 POSIX programs.
In compatibility mode, any other options are flagged as
illegal, but are otherwise ignored. In normal operation, as
long as program text has been supplied, unknown options are
passed on to the AWK program in the ARGV array for
processing. This is particularly useful for running AWK
programs via the ``#!'' executable interpreter mechanism.
AWK PROGRAM EXECUTION
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, from arguments to -W source=, or from the
first non-option argument on the command line. The -f and
-W source= options may be used multiple times on the command
line. Gawk will read the program text as if all the
program-files and command line source texts had been
libraries of AWK functions, without having to include them
in each new AWK program that uses them. It also provides
the ability to mix library functions with command line
programs.
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 executes AWK programs in the following order. First,
all variable assignments specified via the -v option are
performed. Next, gawk compiles the program into an internal
form. Then, gawk executes the code in the BEGIN block(s)
(if any), 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 filename 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 happens after any BEGIN
block(s) have been run.) Command line variable assignment 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.
If the value of a particular element of ARGV is empty (""),
gawk skips over it.
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. The
patterns are tested in the order they occur in the program.
Finally, after all the input is exhausted, gawk executes the
code in the END block(s) (if any).
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, or both, depending upon how they are
used. AWK also has one dimensional arrays; arrays with
multiple dimensions may be simulated. Several pre-defined
variables are set as a program runs; these will be described
as needed and summarized below.
Fields
As each input line is read, gawk splits the line into
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.
If the FIELDWIDTHS variable is set to a space separated list
of numbers, each field is expected to have fixed width, and
gawk will split up the record using the specified widths.
The value of FS is ignored. Assigning a new value to FS
overrides the use of FIELDWIDTHS, and restores the default
behavior.
Each field in the input line may be referenced by its
position, $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:
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.
References to negative numbered fields cause a fatal error.
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).
ARGIND The index in ARGV of the current file being
processed.
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.
CONVFMT The conversion format for numbers, "%.6g", by
default.
environment. The array is indexed by the
environment 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. (This may change in a future
version of gawk.)
ERRNO If a system error occurs either doing a
redirection for getline, during a read for
getline, or during a close(), then ERRNO will
contain a string describing the error.
FIELDWIDTHS A white-space separated list of fieldwidths.
When set, gawk parses the input into fields of
fixed width, instead of using the value of the
FS variable as the field separator. The fixed
field width facility is still experimental;
expect the semantics to change as gawk evolves
over time.
FILENAME The name of the current input file. If no files
are specified on the command line, the value of
FILENAME is ``-''. However, FILENAME is
undefined inside the BEGIN block.
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(), index(),
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 "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.
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. (This will probably change
in a future release of gawk.) 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
consisting 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.
The special operator in may be used in an if or while
statement to see if an array has an index consisting of a
particular value.
if (val in array)
print array[val]
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. The delete statement may also be used to delete
the entire contents of an array.
Variable Typing And Conversion
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.
When a string must be converted to a number, the conversion
is accomplished using atof(3). A number is converted to a
string by using the value of CONVFMT as a format string for
sprintf(3), with the numeric value of the variable as the
argument. However, even though all numbers in AWK are
floating-point, integral values are always converted as
integers. Thus, given
CONVFMT = "%2.2f"
a = 12
b = a ""
the variable b has a string value of "12" and not "12.00".
Gawk performs comparisons as follows: If two variables are
numeric, they are compared numerically. If one value is
numeric and the other has a string value that is a ``numeric
string,'' then comparisons are also done numerically.
Otherwise, the numeric value is converted to a string and a
string comparison is performed. Two strings are compared,
of course, as strings. According to the POSIX standard,
even if two strings are numeric strings, a numeric
comparison is performed. However, this is clearly
incorrect, and gawk does not do this.
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, 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,
however, 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
continued by ending it with a ``\'', in which case the
newline 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
written 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.
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.
The pattern1, pattern2 form of an expression is called a
range pattern. It matches all input records starting with a
line that matches pattern1, and continuing until a record
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.
below) are also legal in regular expressions.
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.
Operators
The operators in AWK, in order of increasing precedence, are
= += -=
*= /= %= ^= Assignment. Both absolute assignment (var =
value)) and operator-assignment (the other forms)
are supported.
?: 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. NOTE:
Do not use a constant regular expression (/foo/)
on the left-hand side of a ~ or !~. Only use
one on the right-hand side. The expression
/foo/ ~ exp has the same meaning as (($0 ~
/foo/) ~ exp)). This is usually not what was
intended.
< >
<= >=
!= == 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
$ Field reference.
Control Statements
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]
delete array
exit [ expression ]
{ statements }
I/O Statements
The input/output statements are as follows:
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.
next file Stop processing the current input
file. The next input record read
comes from the next input file.
FILENAME is updated, FNR is reset to
1, 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 expr-list Prints expressions. Each expression
is separated by the value of the OFS
variable. The output record is
terminated with the value of the ORS
variable.
print expr-list >file Prints expressions on file. Each
expression is separated by the value
of the OFS variable. The output record
is terminated with the value of the
ORS variable.
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 non-POSIX systems.)
Other input/output redirections are also allowed. For print
and printf, >>file appends output to the file, while |
command writes on a pipe. In a similar fashion, command |
getline pipes into getline. The getline command will return
0 on end of file, and -1 on an error.
The printf Statement
The AWK versions of the printf statement and sprintf()
function (see below) 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 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).
%i Just like %d.
%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).
%x An unsigned hexadecimal number (an integer).
%X Like %x, but using ABCDEF instead of abcdef.
%% 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. This
applies even to the non-numeric output formats.
.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 ANSI C
printf() routines are supported. A * in place of either the
width or prec specifications will cause their values to be
taken from the argument list to printf or sprintf().
Special File Names
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
process (usually the shell). Other special filenames
provide access information about the running gawk process.
The filenames are:
/dev/pid Reading this file returns the process ID of the
current process, in decimal, terminated with a
newline.
/dev/ppid Reading this file returns the parent process ID
of the current process, in decimal, terminated
with a newline.
/dev/pgrpid Reading this file returns the process group ID
of the current process, in decimal, terminated
with a newline.
/dev/user Reading this file returns a single record
terminated with a newline. The fields are
getuid(2) system call, $2 is the value of the
geteuid(2) system call, $3 is the value of the
getgid(2) system call, and $4 is the value of
the getegid(2) system call. If there are any
additional fields, they are the group IDs
returned by getgroups(2). Multiple groups may
not be supported on all systems.
/dev/stdin The standard input.
/dev/stdout The standard output.
/dev/stderr The standard error output.
/dev/fd/n The file associated with the open file
descriptor n.
These are particularly useful for error messages. For
example:
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.
Numeric Functions
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
generator. If no expr is provided, the time of
day will be used. The return value is the
String Functions
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,
or the length of $0 if s is not
supplied.
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 regular expression r, and
returns the number of fields. If r
is omitted, FS is used instead. The
array a is cleared first.
sprintf(fmt, expr-list)) prints expr-list according to fmt,
and returns the resulting string.
sub(r, s, t) just like gsub(), but only the first
matching 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
Time Functions
Since one of the primary uses of AWK programs is processing
log files that contain time stamp information, gawk provides
the following two functions for obtaining time stamps and
formatting them.
systime() returns the current time of day as the number of
seconds since the Epoch (Midnight UTC, January 1,
1970 on POSIX systems).
strftime(format, timestamp))
formats timestamp according to the specification
in format. The timestamp should be of the same
form as returned by systime(). If timestamp is
missing, the current time of day is used. See the
specification for the strftime() function in ANSI
C for the format conversions that are guaranteed
to be available. A public-domain version of
strftime(3) and a man page for it are shipped with
gawk; if that version was used to build gawk, then
all of the conversions described in that man page
are available to gawk.
String Constants
String constants in AWK are sequences of characters enclosed
between double quotes ("). Within strings, certain escape
sequences are recognized, as in C. These are:
\\ 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 hexadecimal
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
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
regular 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
parameters 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
parameter list. The convention is to separate local
variables from real parameters by extra spaces in the
parameter list. For example:
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
intervening 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.
Function parameters used as local variables are initialized
to the null string and the number zero upon function
invocation.
The word func may be used in place of function.
EXAMPLES
Print and sort the login names of all users:
{ 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
egrep(1), getpid(2), getppid(2), getpgrp(2), getuid(2),
geteuid(2), getgid(2), getegid(2), getgroups(2)
The AWK Programming Language, Alfred V. Aho, Brian W.
Kernighan, Peter J. Weinberger, Addison-Wesley, 1988. ISBN
0-201-07981-X.
The GAWK Manual, Edition 0.15, published by the Free
Software Foundation, 1993.
POSIX COMPATIBILITY
A primary goal for gawk is compatibility with the POSIX
standard, as well as 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, and are in the POSIX
standard.
The -v option for assigning variables before program
execution starts is new. The book indicates that command
line variable assignment happens when awk would otherwise
open the argument as a file, which is after the BEGIN block
is executed. However, in earlier implementations, when such
an assignment appeared before any file names, the assignment
would happen before the BEGIN block was run. Applications
came to depend on this ``feature.'' When awk was changed to
match its documentation, this option was added to
accommodate applications that depended upon the old
behavior. (This feature was agreed upon by both the AT&T
and GNU developers.)
The -W option for implementation specific features is from
the POSIX standard.
When processing arguments, gawk uses the special option
mode, it will warn about, but otherwise ignore, undefined
options. In normal operation, such arguments are passed on
to the AWK program for it to process.
The AWK book does not define the return value of srand().
The System V Release 4 version of UNIX awk (and the POSIX
standard) 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.
Other new features are: The use of multiple -f options (from
MKS awk); the ENVIRON array; the \a, and \v escape sequences
(done originally in gawk and fed back into AT&T's); the
tolower() and toupper() built-in functions (from AT&T); and
the ANSI C conversion specifications in printf (done first
in AT&T's version).
GNU EXTENSIONS
Gawk has some extensions to POSIX awk. They are described
in this section. All the extensions described here can be
disabled by invoking gawk with the -W compat option.
The following features of gawk are not available in POSIX
awk.
+ The \x escape sequence.
+ The systime() and strftime() functions.
+ The special file names available for I/O redirection
are not recognized.
+ The ARGIND and ERRNO variables are not special.
+ The IGNORECASE variable and its side-effects are not
available.
+ The FIELDWIDTHS variable and fixed width field
splitting.
+ No path search is performed for files named via the
-f option. Therefore the AWKPATH environment
variable is not special.
+ The use of next file to abandon processing of the
current input file.
+ The use of delete array to delete the entire contents
of an array.
The AWK book does not define the return value of the close()
or pclose(3), when closing a file or pipe, respectively.
When gawk is invoked with the -W compat option, 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. This behavior also
does not occur if -W posix has been specified.
HISTORICAL FEATURES
There are two features of historical AWK implementations
that gawk supports. First, it is possible to call the
length() built-in function not only with no argument, but
even without parentheses! Thus,
a = length
is the same as either of
a = length()
a = length($0)
This feature is marked as ``deprecated'' in the POSIX
standard, and gawk will issue a warning about its use if -W
lint is specified on the command line.
The other feature is the use of either the continue or the
break statements outside the body of a while, for, or do
loop. Traditional AWK implementations have treated such
usage as equivalent to the next statement. Gawk will
support this usage if -W compat has been specified.
ENVIRONMENT VARIABLES
If POSIXLY_CORRECT exists in the environment, then gawk
behaves exactly as if --posix had been specified on the
command line. If --lint has been specified, gawk will issue
a warning message to this effect.
BUGS
The -F option is not necessary given the command line
variable assignment feature; it remains only for backwards
compatibility.
If your system actually has support for /dev/fd and the
associated /dev/stdin, /dev/stdout, and /dev/stderr files,
you may get different output from gawk than you would get on
a system without those files. When gawk interprets these
files internally, it synchronizes output to the standard
output with output to /dev/stdout, while on a system with
those files, the output is actually to different open files.
Caveat Emptor.
This man page documents gawk, version 2.15.
Starting with the 2.15 version of gawk, the -c, -V, -C, -a,
and -e options of the 2.11 version are no longer recognized.
This fact will not even be documented in the manual page for
the next major version.
AUTHORS
The original version of UNIX awk was designed and
implemented 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
Foundation, wrote gawk, to be compatible with the original
version of awk distributed in Seventh Edition UNIX. John
Woods contributed a number of bug fixes. David Trueman,
with contributions from Arnold Robbins, made gawk compatible
with the new version of UNIX awk. Arnold Robbins is the
current maintainer.
The initial DOS port was done by Conrad Kwok and Scott
Garfinkle. Scott Deifik is the current DOS maintainer. Pat
Rankin did the port to VMS, and Michal Jaegermann did the
port to the Atari ST. The port to OS/2 was done by Kai Uwe
Rommel, with contributions and help from Darrel Hankerson.
BUG REPORTS
If you find a bug in gawk, please send electronic mail to
bug-gnu-utils@prep.ai.mit.edu, with a carbon copy to
arnold@gnu.ai.mit.edu. Please include your operating system
and its revision, the version of gawk, what C compiler you
used to compile it, and a test program and data that are as
small as possible for reproducing the problem.
Before sending a bug report, please do two things. First,
verify that you have the latest version of gawk. Many bugs
(usually subtle ones) are fixed at each release, and if
your's is out of date, the problem may already have been
solved. Second, please read this man page and the reference
manual carefully to be sure that what you think is a bug
really is, instead of just a quirk in the language.
ACKNOWLEDGEMENTS
Brian Kernighan of Bell Labs provided valuable assistance
during testing and debugging. We thank him.