freebsd-dev/contrib/awk/doc/awk.1

.\" $FreeBSD$
.ds PX \s-1POSIX\s+1
.ds UX \s-1UNIX\s+1
.ds AN \s-1ANSI\s+1
.TH GAWK 1 "Apr 28 1999" "Free Software Foundation" "Utility Commands"
.SH NAME
gawk \- pattern scanning and processing language
.SH SYNOPSIS
.B gawk
[ POSIX or GNU style options ]
.B \-f
.I program-file
[
.B \-\^\-
] file .\^.\^.
.br
.B gawk
[ POSIX or GNU style options ]
[
.B \-\^\-
]
.I program-text
file .\^.\^.
.SH DESCRIPTION
.I Gawk
is the GNU Project's implementation of the AWK programming language.
It conforms to the definition of the language in
the \*(PX 1003.2 Command Language And Utilities Standard.
This version in turn is based on the description in
.IR "The AWK Programming Language" ,
by Aho, Kernighan, and Weinberger,
with the additional features found in the System V Release 4 version
of \*(UX
.IR awk .
.I Gawk
also provides more recent Bell Labs
.I awk
extensions, and some GNU-specific extensions.
.PP
The command line consists of options to
.I gawk
itself, the AWK program text (if not supplied via the
.B \-f
or
.B \-\^\-file
options), and values to be made
available in the
.B ARGC
and
.B ARGV
pre-defined AWK variables.
.SH OPTION FORMAT
.PP
.I Gawk
options may be either the traditional \*(PX one letter options,
or the GNU style long options.  \*(PX options start with a single ``\-'',
while long options start with ``\-\^\-''.
Long options are provided for both GNU-specific features and
for \*(PX mandated features.
.PP
Following the \*(PX standard,
.IR gawk -specific
options are supplied via arguments to the
.B \-W
option.  Multiple
.B \-W
options may be supplied
Each
.B \-W
option has a corresponding long option, as detailed below.
Arguments to long options are either joined with the option
by an
.B =
sign, with no intervening spaces, or they may be provided in the
next command line argument.
Long options may be abbreviated, as long as the abbreviation
remains unique.
.SH OPTIONS
.PP
.I Gawk
accepts the following options.
.TP
.PD 0
.BI \-F " fs"
.TP
.PD
.BI \-\^\-field-separator " fs"
Use
.I fs
for the input field separator (the value of the
.B FS
predefined
variable).
.TP
.PD 0
\fB\-v\fI var\fB\^=\^\fIval\fR
.TP
.PD
\fB\-\^\-assign \fIvar\fB\^=\^\fIval\fR
Assign the value
.IR val ,
to the variable
.IR var ,
before execution of the program begins.
Such variable values are available to the
.B BEGIN
block of an AWK program.
.TP
.PD 0
.BI \-f " program-file"
.TP
.PD
.BI \-\^\-file " program-file"
Read the AWK program source from the file
.IR program-file ,
instead of from the first command line argument.
Multiple
.B \-f
(or
.BR \-\^\-file )
options may be used.
.TP
.PD 0
.BI \-mf " NNN"
.TP
.PD
.BI \-mr " NNN"
Set various memory limits to the value
.IR NNN .
The
.B f
flag sets the maximum number of fields, and the
.B r
flag sets the maximum record size.  These two flags and the
.B \-m
option are from the Bell Labs research version of \*(UX
.IR awk .
They are ignored by
.IR gawk ,
since
.I gawk
has no pre-defined limits.
.TP
.PD 0
.B "\-W traditional"
.TP
.PD 0
.B "\-W compat"
.TP
.PD 0
.B \-\^\-traditional
.TP
.PD
.B \-\^\-compat
Run in
.I compatibility
mode.  In compatibility mode,
.I gawk
behaves identically to \*(UX
.IR awk ;
none of the GNU-specific extensions are recognized.
The use of
.B \-\^\-traditional
is preferred over the other forms of this option.
See
.BR "GNU EXTENSIONS" ,
below, for more information.
.TP
.PD 0
.B "\-W copyleft"
.TP
.PD 0
.B "\-W copyright"
.TP
.PD 0
.B \-\^\-copyleft
.TP
.PD
.B \-\^\-copyright
Print the short version of the GNU copyright information message on
the standard output, and exits successfully.
.TP
.PD 0
.B "\-W help"
.TP
.PD 0
.B "\-W usage"
.TP
.PD 0
.B \-\^\-help
.TP
.PD
.B \-\^\-usage
Print a relatively short summary of the available options on
the standard output.
(Per the
.IR "GNU Coding Standards" ,
these options cause an immediate, successful exit.)
.TP
.PD 0
.B "\-W lint"
.TP
.PD
.B \-\^\-lint
Provide warnings about constructs that are
dubious or non-portable to other AWK implementations.
.TP
.PD 0
.B "\-W lint\-old"
.TP
.PD
.B \-\^\-lint\-old
Provide warnings about constructs that are
not portable to the original version of Unix
.IR awk .
.ig
.\" This option is left undocumented, on purpose.
.TP
.PD 0
.B "\-W nostalgia"
.TP
.PD
.B \-\^\-nostalgia
Provide a moment of nostalgia for long time
.I awk
users.
..
.TP
.PD 0
.B "\-W posix"
.TP
.PD
.B \-\^\-posix
This turns on
.I compatibility 
mode, with the following additional restrictions:
.RS
.TP \w'\(bu'u+1n
\(bu
.B \ex
escape sequences are not recognized.
.TP
\(bu
Only space and tab act as field separators when
.B FS
is set to a single space, newline does not.
.TP
\(bu
The synonym
.B func
for the keyword
.B function
is not recognized.
.TP
\(bu
The operators
.B **
and
.B **=
cannot be used in place of
.B ^
and
.BR ^= .
.TP
\(bu
The
.B fflush()
function is not available.
.RE
.TP
.PD 0
.B "\-W re\-interval"
.TP
.PD
.B \-\^\-re\-interval
Enable the use of
.I "interval expressions"
in regular expression matching
(see
.BR "Regular Expressions" ,
below).
Interval expressions were not traditionally available in the
AWK language. The POSIX standard added them, to make
.I awk
and
.I egrep
consistent with each other.
However, their use is likely
to break old AWK programs, so
.I gawk
only provides them if they are requested with this option, or when
.B \-\^\-posix
is specified.
.TP
.PD 0
.BI "\-W source " program-text
.TP
.PD
.BI \-\^\-source " program-text"
Use
.I program-text
as AWK program source code.
This option allows the easy intermixing of library functions (used via the 
.B \-f
and
.B \-\^\-file
options) with source code entered on the command line.
It is intended primarily for medium to large AWK programs used
in shell scripts.
.TP
.PD 0
.B "\-W version"
.TP
.PD
.B \-\^\-version
Print version information for this particular copy of
.I gawk
on the standard output.
This is useful mainly for knowing if the current copy of
.I gawk
on your system
is up to date with respect to whatever the Free Software Foundation
is distributing.
This is also useful when reporting bugs.
(Per the
.IR "GNU Coding Standards" ,
these options cause an immediate, successful exit.)
.TP
.B \-\^\-
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 \*(PX programs.
.PP
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
.B ARGV
array for processing.  This is particularly useful for running AWK
programs via the ``#!'' executable interpreter mechanism.
.SH AWK PROGRAM EXECUTION
.PP
An AWK program consists of a sequence of pattern-action statements
and optional function definitions.
.RS
.PP
\fIpattern\fB	{ \fIaction statements\fB }\fR
.br
\fBfunction \fIname\fB(\fIparameter list\fB) { \fIstatements\fB }\fR
.RE
.PP
.I Gawk
first reads the program source from the
.IR program-file (s)
if specified,
from arguments to
.BR \-\^\-source ,
or from the first non-option argument on the command line.
The
.B \-f
and
.B \-\^\-source
options may be used multiple times on the command line.
.I Gawk
will read the program text as if all the
.IR program-file s
and command line source texts
had been concatenated together.  This is useful for building 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.
.PP
The environment variable
.B AWKPATH
specifies a search path to use when finding source files named with
the 
.B \-f
option.  If this variable does not exist, the default path is
\fB".:/usr/local/share/awk"\fR.
(The actual directory may vary, depending upon how
.I gawk
was built and installed.)
If a file name given to the
.B \-f
option contains a ``/'' character, no path search is performed.
.PP
.I Gawk
executes AWK programs in the following order.
First,
all variable assignments specified via the
.B \-v
option are performed.
Next,
.I gawk
compiles the program into an internal form.
Then,
.I gawk
executes the code in the
.B BEGIN
block(s) (if any),
and then proceeds to read
each file named in the
.B ARGV
array.
If there are no files named on the command line,
.I gawk
reads the standard input.
.PP
If a filename on the command line has the form
.IB var = val
it is treated as a variable assignment. The variable
.I var
will be assigned the value
.IR val .
(This happens after any
.B 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.
.PP
If the value of a particular element of
.B ARGV
is empty (\fB""\fR),
.I gawk
skips over it.
.PP
For each record in the input,
.I gawk
tests to see if it matches any
.I pattern
in the AWK program.
For each pattern that the record matches, the associated
.I action
is executed.
The patterns are tested in the order they occur in the program.
.PP
Finally, after all the input is exhausted,
.I gawk
executes the code in the
.B END
block(s) (if any).
.SH VARIABLES, RECORDS 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.
.SS Records
Normally, records are separated by newline characters. You can control how
records are separated by assigning values to the built-in variable
.BR RS .
If 
.B RS
is any single character, that character separates records.
Otherwise,
.B RS
is a regular expression.  Text in the input that matches this
regular expression will separate the record.
However, in compatibility mode,
only the first character of its string
value is used for separating records.
If
.B RS
is set to the null string, then records are separated by
blank lines.
When
.B RS
is set to the null string, the newline character always acts as
a field separator, in addition to whatever value
.B FS
may have.
.SS Fields
.PP
As each input record is read,
.I gawk
splits the record into
.IR fields ,
using the value of the
.B FS
variable as the field separator.
If
.B FS
is a single character, fields are separated by that character.
If
.B FS
is the null string, then each individual character becomes a
separate field.
Otherwise,
.B FS
is expected to be a full regular expression.
In the special case that
.B FS
is a single space, fields are separated
by runs of spaces and/or tabs and/or newlines.
(But see the discussion of
.BR \-\-posix ,
below).
Note that the value of
.B IGNORECASE
(see below) will also affect how fields are split when
.B FS
is a regular expression, and how records are separated when
.B RS
is a regular expression.
.PP
If the
.B FIELDWIDTHS
variable is set to a space separated list of numbers, each field is
expected to have fixed width, and
.I gawk
will split up the record using the specified widths.  The value of
.B FS
is ignored.
Assigning a new value to
.B FS
overrides the use of
.BR FIELDWIDTHS ,
and restores the default behavior.
.PP
Each field in the input record may be referenced by its position,
.BR $1 ,
.BR $2 ,
and so on.
.B $0
is the whole record. The value of a field may be assigned to as well.
Fields need not be referenced by constants:
.RS
.PP
.ft B
n = 5
.br
print $n
.ft R
.RE
.PP
prints the fifth field in the input record.
The variable
.B NF
is set to the total number of fields in the input record.
.PP
References to non-existent fields (i.e. fields after
.BR $NF )
produce the null-string. However, assigning to a non-existent field
(e.g., 
.BR "$(NF+2) = 5" )
will increase the value of
.BR NF ,
create any intervening fields with the null string as their value, and
cause the value of
.B $0
to be recomputed, with the fields being separated by the value of
.BR OFS .
References to negative numbered fields cause a fatal error.
Decrementing
.B NF
causes the values of fields past the new value to be lost, and the value of
.B $0
to be recomputed, with the fields being separated by the value of
.BR OFS .
.SS Built-in Variables
.PP
.IR Gawk 's
built-in variables are:
.PP
.TP \w'\fBFIELDWIDTHS\fR'u+1n
.B ARGC
The number of command line arguments (does not include options to
.IR gawk ,
or the program source).
.TP
.B ARGIND
The index in
.B ARGV
of the current file being processed.
.TP
.B ARGV
Array of command line arguments. The array is indexed from
0 to
.B ARGC
\- 1.
Dynamically changing the contents of
.B ARGV
can control the files used for data.
.TP
.B CONVFMT
The conversion format for numbers, \fB"%.6g"\fR, by default.
.TP
.B ENVIRON
An array containing the values of the current environment.
The array is indexed by the environment variables, each element being
the value of that variable (e.g., \fBENVIRON["HOME"]\fP might be
.BR /home/arnold ).
Changing this array does not affect the environment seen by programs which
.I gawk
spawns via redirection or the
.B system()
function.
(This may change in a future version of
.IR gawk .)
.\" but don't hold your breath...
.TP
.B ERRNO
If a system error occurs either doing a redirection for
.BR getline ,
during a read for
.BR getline ,
or during a
.BR close() ,
then
.B ERRNO
will contain
a string describing the error.
.TP
.B FIELDWIDTHS
A white-space separated list of fieldwidths.  When set,
.I gawk
parses the input into fields of fixed width, instead of using the
value of the
.B FS
variable as the field separator.
The fixed field width facility is still experimental; the
semantics may change as
.I gawk
evolves over time.
.TP
.B FILENAME
The name of the current input file.
If no files are specified on the command line, the value of
.B FILENAME
is ``\-''.
However,
.B FILENAME
is undefined inside the
.B BEGIN
block.
.TP
.B FNR
The input record number in the current input file.
.TP
.B FS
The input field separator, a space by default.  See
.BR Fields ,
above.
.TP
.B IGNORECASE
Controls the case-sensitivity of all regular expression 
and string operations. If
.B IGNORECASE
has a non-zero value, then string comparisons and
pattern matching in rules,
field splitting with
.BR FS ,
record separating with
.BR RS ,
regular expression
matching with
.B ~
and
.BR !~ ,
and the
.BR gensub() ,
.BR gsub() ,
.BR index() ,
.BR match() ,
.BR split() ,
and
.B sub()
pre-defined functions will all ignore case when doing regular expression
operations.  Thus, if
.B IGNORECASE
is not equal to zero,
.B /aB/
matches all of the strings \fB"ab"\fP, \fB"aB"\fP, \fB"Ab"\fP,
and \fB"AB"\fP.
As with all AWK variables, the initial value of
.B IGNORECASE
is zero, so all regular expression and string
operations are normally case-sensitive.
Under Unix, the full ISO 8859-1 Latin-1 character set is used
when ignoring case.
.B NOTE:
In versions of
.I gawk
prior to 3.0,
.B IGNORECASE
only affected regular expression operations. It now affects string
comparisons as well.
.TP
.B NF
The number of fields in the current input record.
.TP
.B NR
The total number of input records seen so far.
.TP
.B OFMT
The output format for numbers, \fB"%.6g"\fR, by default.
.TP
.B OFS
The output field separator, a space by default.
.TP
.B ORS
The output record separator, by default a newline.
.TP
.B RS
The input record separator, by default a newline.
.TP
.B RT
The record terminator.
.I Gawk
sets
.B RT
to the input text that matched the character or regular expression
specified by
.BR RS .
.TP
.B RSTART
The index of the first character matched by
.BR match() ;
0 if no match.
.TP
.B RLENGTH
The length of the string matched by
.BR match() ;
\-1 if no match.
.TP
.B SUBSEP
The character used to separate multiple subscripts in array
elements, by default \fB"\e034"\fR.
.SS Arrays
.PP
Arrays are subscripted with an expression between square brackets
.RB ( [ " and " ] ).
If the expression is an expression list
.RI ( 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
.B SUBSEP
variable.
This facility is used to simulate multiply dimensioned
arrays. For example:
.PP
.RS
.ft B
i = "A";\^ j = "B";\^ k = "C"
.br
x[i, j, k] = "hello, world\en"
.ft R
.RE
.PP
assigns the string \fB"hello, world\en"\fR to the element of the array
.B x
which is indexed by the string \fB"A\e034B\e034C"\fR. All arrays in AWK
are associative, i.e. indexed by string values.
.PP
The special operator
.B in
may be used in an
.B if
or
.B while
statement to see if an array has an index consisting of a particular
value.
.PP
.RS
.ft B
.nf
if (val in array)
	print array[val]
.fi
.ft
.RE
.PP
If the array has multiple subscripts, use
.BR "(i, j) in array" .
.PP
The
.B in
construct may also be used in a
.B for
loop to iterate over all the elements of an array.
.PP
An element may be deleted from an array using the
.B delete
statement.
The
.B delete
statement may also be used to delete the entire contents of an array,
just by specifying the array name without a subscript.
.SS Variable Typing And Conversion
.PP
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.
.PP
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.
.PP
When a string must be converted to a number, the conversion is accomplished
using
.IR atof (3).
A number is converted to a string by using the value of
.B CONVFMT
as a format string for
.IR 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
.I always
converted as integers.  Thus, given
.PP
.RS
.ft B
.nf
CONVFMT = "%2.2f"
a = 12
b = a ""
.fi
.ft R
.RE
.PP
the variable
.B b
has a string value of \fB"12"\fR and not \fB"12.00"\fR.
.PP
.I 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 \*(PX standard, even if two strings are
numeric strings, a numeric comparison is performed.  However, this is
clearly incorrect, and
.I gawk
does not do this.
.PP
Note that string constants, such as \fB"57"\fP, are
.I not
numeric strings, they are string constants.  The idea of ``numeric string''
only applies to fields,
.B getline
input,
.BR FILENAME ,
.B ARGV
elements,
.B ENVIRON
elements and the elements of an array created by
.B split()
that are numeric strings.
The basic idea is that
.IR "user input" ,
and only user input, that looks numeric,
should be treated that way.
.PP
Uninitialized variables have the numeric value 0 and the string value ""
(the null, or empty, string).
.SH PATTERNS AND ACTIONS
AWK is a line oriented language. The pattern comes first, and then the
action. Action statements are enclosed in
.B {
and
.BR } .
Either the pattern may be missing, or the action may be missing, but,
of course, not both. If the pattern is missing, the action will be
executed for every single record of input.
A missing action is equivalent to
.RS
.PP
.B "{ print }"
.RE
.PP
which prints the entire record.
.PP
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 ``,'',
.BR { ,
.BR ? ,
.BR : ,
.BR && ,
or
.BR || .
Lines ending in
.B do
or
.B else
also have their statements automatically continued on the following line.
In other cases, a line can be continued by ending it with a ``\e'',
in which case the newline will be ignored.
.PP
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.
.SS Patterns
AWK patterns may be one of the following:
.PP
.RS
.nf
.B BEGIN
.B END
.BI / "regular expression" /
.I "relational expression"
.IB pattern " && " pattern
.IB pattern " || " pattern
.IB pattern " ? " pattern " : " pattern
.BI ( pattern )
.BI ! " pattern"
.IB pattern1 ", " pattern2
.fi
.RE
.PP
.B BEGIN
and
.B END
are two special kinds of patterns which are not tested against
the input.
The action parts of all
.B BEGIN
patterns are merged as if all the statements had
been written in a single
.B BEGIN
block. They are executed before any
of the input is read. Similarly, all the
.B END
blocks are merged,
and executed when all the input is exhausted (or when an
.B exit
statement is executed).
.B BEGIN
and
.B END
patterns cannot be combined with other patterns in pattern expressions.
.B BEGIN
and
.B END
patterns cannot have missing action parts.
.PP
For
.BI / "regular expression" /
patterns, the associated statement is executed for each input record that matches
the regular expression.
Regular expressions are the same as those in
.IR egrep (1),
and are summarized below.
.PP
A
.I "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.
.PP
The
.BR && ,
.BR || ,
and
.B !
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.
.PP
The
.B ?\^:
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.
.PP
The 
.IB pattern1 ", " pattern2
form of an expression is called a
.IR "range pattern" .
It matches all input records starting with a record that matches
.IR pattern1 ,
and continuing until a record that matches
.IR pattern2 ,
inclusive. It does not combine with any other sort of pattern expression.
.SS Regular Expressions
Regular expressions are the extended kind found in
.IR egrep .
They are composed of characters as follows:
.TP \w'\fB[^\fIabc...\fB]\fR'u+2n
.I c
matches the non-metacharacter
.IR c .
.TP
.I \ec
matches the literal character
.IR c .
.TP
.B .
matches any character
.I including
newline.
.TP
.B ^
matches the beginning of a string.
.TP
.B $
matches the end of a string.
.TP
.BI [ abc... ]
character list, matches any of the characters
.IR abc... .
.TP
.BI [^ abc... ]
negated character list, matches any character except
.IR abc... .
.TP
.IB r1 | r2
alternation: matches either
.I r1
or
.IR r2 .
.TP
.I r1r2
concatenation: matches
.IR r1 ,
and then
.IR r2 .
.TP
.IB r +
matches one or more
.IR r 's. 
.TP
.IB r *
matches zero or more
.IR r 's. 
.TP
.IB r ?
matches zero or one
.IR r 's. 
.TP
.BI ( r )
grouping: matches
.IR r .
.TP
.PD 0
.IB r { n }
.TP
.PD 0
.IB r { n ,}
.TP
.PD
.IB r { n , m }
One or two numbers inside braces denote an
.IR "interval expression" .
If there is one number in the braces, the preceding regexp
.I r
is repeated
.I n
times.  If there are two numbers separated by a comma,
.I r
is repeated
.I n
to
.I m
times.
If there is one number followed by a comma, then
.I r
is repeated at least
.I n
times.
.sp .5
Interval expressions are only available if either
.B \-\^\-posix
or
.B \-\^\-re\-interval
is specified on the command line.
.TP
.B \ey
matches the empty string at either the beginning or the
end of a word.
.TP
.B \eB
matches the empty string within a word.
.TP
.B \e<
matches the empty string at the beginning of a word.
.TP
.B \e>
matches the empty string at the end of a word.
.TP
.B \ew
matches any word-constituent character (letter, digit, or underscore).
.TP
.B \eW
matches any character that is not word-constituent.
.TP
.B \e`
matches the empty string at the beginning of a buffer (string).
.TP
.B \e'
matches the empty string at the end of a buffer.
.PP
The escape sequences that are valid in string constants (see below)
are also legal in regular expressions.
.PP
.I "Character classes"
are a new feature introduced in the POSIX standard.
A character class is a special notation for describing
lists of characters that have a specific attribute, but where the 
actual characters themselves can vary from country to country and/or
from character set to character set.  For example, the notion of what
is an alphabetic character differs in the USA and in France.
.PP
A character class is only valid in a regexp
.I inside
the brackets of a character list.  Character classes consist of
.BR [: ,
a keyword denoting the class, and
.BR :] .
Here are the character
classes defined by the POSIX standard.
.TP
.B [:alnum:]
Alphanumeric characters.
.TP
.B [:alpha:]
Alphabetic characters.
.TP
.B [:blank:]
Space or tab characters.
.TP
.B [:cntrl:]
Control characters.
.TP
.B [:digit:]
Numeric characters.
.TP
.B [:graph:]
Characters that are both printable and visible.
(A space is printable, but not visible, while an
.B a
is both.)
.TP
.B [:lower:]
Lower-case alphabetic characters.
.TP
.B [:print:]
Printable characters (characters that are not control characters.)
.TP
.B [:punct:]
Punctuation characters (characters that are not letter, digits,
control characters, or space characters).
.TP
.B [:space:]
Space characters (such as space, tab, and formfeed, to name a few).
.TP
.B [:upper:]
Upper-case alphabetic characters.
.TP
.B [:xdigit:]
Characters that are hexadecimal digits.
.PP
For example, before the POSIX standard, to match alphanumeric
characters, you would have had to write
.BR /[A\-Za\-z0\-9]/ .
If your character set had other alphabetic characters in it, this would not
match them.  With the POSIX character classes, you can write
.BR /[[:alnum:]]/ ,
and this will match
.I all
the alphabetic and numeric characters in your character set.
.PP
Two additional special sequences can appear in character lists.
These apply to non-ASCII character sets, which can have single symbols
(called 
.IR "collating elements" )
that are represented with more than one
character, as well as several characters that are equivalent for
.IR collating ,
or sorting, purposes.  (E.g., in French, a plain ``e''
and a grave-accented e\` are equivalent.)
.TP
Collating Symbols
A collating symbols is a multi-character collating element enclosed in
.B [.
and
.BR .] .
For example, if
.B ch
is a collating element, then
.B [[.ch.]]
is a regexp that matches this collating element, while
.B [ch]
is a regexp that matches either
.B c
or
.BR h .
.TP
Equivalence Classes
An equivalence class is a locale-specific name for a list of
characters that are equivalent. The name is enclosed in
.B [=
and
.BR =] .
For example, the name
.B e
might be used to represent all of
``e,'' ``e\`,'' and ``e\`.''
In this case,
.B [[=e=]]
is a regexp
that matches any of
.BR e ,
.BR e\' ,
or
.BR e\` .
.PP
These features are very valuable in non-English speaking locales.
The library functions that
.I gawk
uses for regular expression matching
currently only recognize POSIX character classes; they do not recognize
collating symbols or equivalence classes.
.PP
The
.BR \ey ,
.BR \eB ,
.BR \e< ,
.BR \e> ,
.BR \ew ,
.BR \eW ,
.BR \e` ,
and
.B \e'
operators are specific to
.IR gawk ;
they are extensions based on facilities in the GNU regexp libraries.
.PP
The various command line options
control how
.I gawk
interprets characters in regexps.
.TP
No options
In the default case,
.I gawk
provide all the facilities of
POSIX regexps and the GNU regexp operators described above.
However, interval expressions are not supported.
.TP
.B \-\^\-posix
Only POSIX regexps are supported, the GNU operators are not special.
(E.g.,
.B \ew
matches a literal
.BR w ).
Interval expressions are allowed.
.TP
.B \-\^\-traditional
Traditional Unix
.I awk
regexps are matched. The GNU operators
are not special, interval expressions are not available, and neither
are the POSIX character classes
.RB ( [[:alnum:]]
and so on).
Characters described by octal and hexadecimal escape sequences are
treated literally, even if they represent regexp metacharacters.
.TP
.B \-\^\-re\-interval
Allow interval expressions in regexps, even if
.B \-\^\-traditional
has been provided.
.SS Actions
Action statements are enclosed in braces,
.B {
and
.BR } .
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.
.SS Operators
.PP
The operators in AWK, in order of decreasing precedence, are
.PP
.TP "\w'\fB*= /= %= ^=\fR'u+1n"
.BR ( \&... )
Grouping
.TP
.B $
Field reference.
.TP
.B "++ \-\^\-"
Increment and decrement, both prefix and postfix.
.TP
.B ^
Exponentiation (\fB**\fR may also be used, and \fB**=\fR for
the assignment operator).
.TP
.B "+ \- !"
Unary plus, unary minus, and logical negation.
.TP
.B "* / %"
Multiplication, division, and modulus.
.TP
.B "+ \-"
Addition and subtraction.
.TP
.I space
String concatenation.
.TP
.PD 0
.B "< >"
.TP
.PD 0
.B "<= >="
.TP
.PD
.B "!= =="
The regular relational operators.
.TP
.B "~ !~"
Regular expression match, negated match.
.B NOTE:
Do not use a constant regular expression
.RB ( /foo/ )
on the left-hand side of a
.B ~
or
.BR !~ .
Only use one on the right-hand side.  The expression
.BI "/foo/ ~ " exp
has the same meaning as \fB(($0 ~ /foo/) ~ \fIexp\fB)\fR.
This is usually
.I not
what was intended.
.TP
.B in
Array membership.
.TP
.B &&
Logical AND.
.TP
.B ||
Logical OR.
.TP
.B ?:
The C conditional expression. This has the form
.IB expr1 " ? " expr2 " : " expr3\c
\&. If
.I expr1
is true, the value of the expression is
.IR expr2 ,
otherwise it is
.IR expr3 .
Only one of
.I expr2
and
.I expr3
is evaluated.
.TP
.PD 0
.B "= += \-="
.TP
.PD
.B "*= /= %= ^="
Assignment. Both absolute assignment
.BI ( var " = " value )
and operator-assignment (the other forms) are supported.
.SS Control Statements
.PP
The control statements are
as follows:
.PP
.RS
.nf
\fBif (\fIcondition\fB) \fIstatement\fR [ \fBelse\fI statement \fR]
\fBwhile (\fIcondition\fB) \fIstatement \fR
\fBdo \fIstatement \fBwhile (\fIcondition\fB)\fR
\fBfor (\fIexpr1\fB; \fIexpr2\fB; \fIexpr3\fB) \fIstatement\fR
\fBfor (\fIvar \fBin\fI array\fB) \fIstatement\fR
\fBbreak\fR
\fBcontinue\fR
\fBdelete \fIarray\^\fB[\^\fIindex\^\fB]\fR
\fBdelete \fIarray\^\fR
\fBexit\fR [ \fIexpression\fR ]
\fB{ \fIstatements \fB}
.fi
.RE
.SS "I/O Statements"
.PP
The input/output statements are as follows:
.PP
.TP "\w'\fBprintf \fIfmt, expr-list\fR'u+1n"
.BI close( file )
Close file (or pipe, see below).
.TP
.B getline
Set
.B $0
from next input record; set
.BR NF ,
.BR NR ,
.BR FNR .
.TP
.BI "getline <" file
Set
.B $0
from next record of
.IR file ;
set
.BR NF .
.TP
.BI getline " var"
Set
.I var
from next input record; set
.BR NR ,
.BR FNR .
.TP
.BI getline " var" " <" file
Set
.I var
from next record of
.IR file .
.TP
.B 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
.B END
block(s), if any, are executed.
.TP
.B "nextfile"
Stop processing the current input file.  The next input record read
comes from the next input file.
.B FILENAME
and
.B ARGIND
are updated,
.B 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
.B END
block(s), if any, are executed.
.B NOTE:
Earlier versions of gawk used
.BR "next file" ,
as two words. While this usage is still recognized, it generates a
warning message and will eventually be removed.
.TP
.B print
Prints the current record.
The output record is terminated with the value of the
.B ORS
variable.
.TP
.BI print " expr-list"
Prints expressions.
Each expression is separated by the value of the
.B OFS
variable.
The output record is terminated with the value of the
.B ORS
variable.
.TP
.BI print " expr-list" " >" file
Prints expressions on
.IR file .
Each expression is separated by the value of the
.B OFS
variable. The output record is terminated with the value of the
.B ORS
variable.
.TP
.BI printf " fmt, expr-list"
Format and print.
.TP
.BI printf " fmt, expr-list" " >" file
Format and print on
.IR file .
.TP
.BI system( cmd-line )
Execute the command
.IR cmd-line ,
and return the exit status.
(This may not be available on non-\*(PX systems.)
.TP
\&\fBfflush(\fR[\fIfile\^\fR]\fB)\fR
Flush any buffers associated with the open output file or pipe
.IR file .
If
.I file
is missing, then standard output is flushed.
If
.I file
is the null string,
then all open output files and pipes
have their buffers flushed.
.PP
Other input/output redirections are also allowed. For
.B print
and
.BR printf ,
.BI >> file
appends output to the
.IR file ,
while
.BI | " command"
writes on a pipe.
In a similar fashion,
.IB command " | getline"
pipes into
.BR getline .
The
.BR getline
command will return 0 on end of file, and \-1 on an error.
.SS The \fIprintf\fP\^ Statement
.PP
The AWK versions of the
.B printf
statement and
.B sprintf()
function
(see below)
accept the following conversion specification formats:
.TP
.B %c
An \s-1ASCII\s+1 character.
If the argument used for
.B %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.
.TP
.PD 0
.B %d
.TP
.PD
.B %i
A decimal number (the integer part).
.TP
.PD 0
.B %e
.TP
.PD
.B %E
A floating point number of the form
.BR [\-]d.dddddde[+\^\-]dd .
The
.B %E
format uses
.B E
instead of
.BR e .
.TP
.B %f
A floating point number of the form
.BR [\-]ddd.dddddd .
.TP
.PD 0
.B %g
.TP
.PD
.B %G
Use
.B %e
or
.B %f
conversion, whichever is shorter, with nonsignificant zeros suppressed.
The
.B %G
format uses
.B %E
instead of
.BR %e .
.TP
.B %o
An unsigned octal number (again, an integer).
.TP
.B %s
A character string.
.TP
.PD 0
.B %x
.TP
.PD
.B %X
An unsigned hexadecimal number (an integer).
.The
.B %X
format uses
.B ABCDEF
instead of
.BR abcdef .
.TP
.B %%
A single
.B %
character; no argument is converted.
.PP
There are optional, additional parameters that may lie between the
.B %
and the control letter:
.TP
.B \-
The expression should be left-justified within its field.
.TP
.I space
For numeric conversions, prefix positive values with a space, and
negative values with a minus sign.
.TP
.B +
The plus sign, used before the width modifier (see below),
says to always supply a sign for numeric conversions, even if the data
to be formatted is positive. The
.B +
overrides the space modifier.
.TP
.B #
Use an ``alternate form'' for certain control letters.
For
.BR %o ,
supply a leading zero.
For
.BR %x ,
and
.BR %X ,
supply a leading
.BR 0x 
or
.BR 0X 
for
a nonzero result.
For
.BR %e ,
.BR %E ,
and
.BR %f ,
the result will always contain a
decimal point.
For
.BR %g ,
and
.BR %G ,
trailing zeros are not removed from the result.
.TP
.B 0
A leading
.B 0
(zero) acts as a flag, that indicates output should be
padded with zeroes instead of spaces.
This applies even to non-numeric output formats.
This flag only has an effect when the field width is wider than the
value to be printed.
.TP
.I width
The field should be padded to this width. The field is normally padded
with spaces.  If the
.B 0
flag has been used, it is padded with zeroes.
.TP
.BI \&. prec
A number that specifies the precision to use when printing.
For the
.BR %e ,
.BR %E ,
and
.BR %f 
formats, this specifies the
number of digits you want printed to the right of the decimal point.
For the
.BR %g ,
and
.B %G
formats, it specifies the maximum number
of significant digits.  For the
.BR %d ,
.BR %o ,
.BR %i ,
.BR %u ,
.BR %x ,
and
.B %X
formats, it specifies the minimum number of
digits to print.  For a string, it specifies the maximum number of
characters from the string that should be printed.
.PP
The dynamic
.I width
and
.I prec
capabilities of the \*(AN C
.B printf()
routines are supported.
A
.B *
in place of either the
.B width
or
.B prec
specifications will cause their values to be taken from
the argument list to
.B printf
or
.BR sprintf() .
.SS Special File Names
.PP
When doing I/O redirection from either
.B print
or
.B printf
into a file,
or via
.B getline
from a file,
.I gawk
recognizes certain special filenames internally.  These filenames
allow access to open file descriptors inherited from
.IR gawk 's
parent process (usually the shell).
Other special filenames provide access to information about the running
.B gawk
process.
The filenames are:
.TP \w'\fB/dev/stdout\fR'u+1n
.B /dev/pid
Reading this file returns the process ID of the current process,
in decimal, terminated with a newline.
.TP
.B /dev/ppid
Reading this file returns the parent process ID of the current process,
in decimal, terminated with a newline.
.TP
.B /dev/pgrpid
Reading this file returns the process group ID of the current process,
in decimal, terminated with a newline.
.TP
.B /dev/user
Reading this file returns a single record terminated with a newline.
The fields are separated with spaces.
.B $1
is the value of the
.IR getuid (2)
system call,
.B $2
is the value of the
.IR geteuid (2)
system call,
.B $3
is the value of the
.IR getgid (2)
system call, and
.B $4
is the value of the
.IR getegid (2)
system call.
If there are any additional fields, they are the group IDs returned by
.IR getgroups (2).
Multiple groups may not be supported on all systems.
.TP
.B /dev/stdin
The standard input.
.TP
.B /dev/stdout
The standard output.
.TP
.B /dev/stderr
The standard error output.
.TP
.BI /dev/fd/\^ n
The file associated with the open file descriptor
.IR n .
.PP
These are particularly useful for error messages. For example:
.PP
.RS
.ft B
print "You blew it!" > "/dev/stderr"
.ft R
.RE
.PP
whereas you would otherwise have to use
.PP
.RS
.ft B
print "You blew it!" | "cat 1>&2"
.ft R
.RE
.PP
These file names may also be used on the command line to name data files.
.SS Numeric Functions
.PP
AWK has the following pre-defined arithmetic functions:
.PP
.TP \w'\fBsrand(\fR[\fIexpr\^\fR]\fB)\fR'u+1n
.BI atan2( y , " x" )
returns the arctangent of
.I y/x
in radians.
.TP
.BI cos( expr )
returns the cosine of
.IR expr ,
which is in radians.
.TP
.BI exp( expr )
the exponential function.
.TP
.BI int( expr )
truncates to integer.
.TP
.BI log( expr )
the natural logarithm function.
.TP
.B rand()
returns a random number between 0 and 1.
.TP
.BI sin( expr )
returns the sine of
.IR expr ,
which is in radians.
.TP
.BI sqrt( expr )
the square root function.
.TP
\&\fBsrand(\fR[\fIexpr\^\fR]\fB)\fR
uses
.I expr
as a new seed for the random number generator. If no
.I expr
is provided, the time of day will be used.
The return value is the previous seed for the random
number generator.
.SS String Functions
.PP
.I Gawk
has the following pre-defined string functions:
.PP
.TP "\w'\fBsprintf(\^\fIfmt\fB\^, \fIexpr-list\^\fB)\fR'u+1n"
\fBgensub(\fIr\fB, \fIs\fB, \fIh \fR[\fB, \fIt\fR]\fB)\fR
search the target string
.I t
for matches of the regular expression
.IR r .
If
.I h
is a string beginning with
.B g
or
.BR G ,
then replace all matches of
.I r
with
.IR s .
Otherwise,
.I h
is a number indicating which match of
.I r
to replace.
If no
.I t
is supplied,
.B $0
is used instead.
Within the replacement text
.IR s ,
the sequence
.BI \e n\fR,
where
.I n
is a digit from 1 to 9, may be used to indicate just the text that
matched the
.IR n 'th
parenthesized subexpression. The sequence
.B \e0
represents the entire matched text, as does the character
.BR & .
Unlike
.B sub()
and
.BR gsub() ,
the modified string is returned as the result of the function,
and the original target string is
.I not
changed.
.TP "\w'\fBsprintf(\^\fIfmt\fB\^, \fIexpr-list\^\fB)\fR'u+1n"
\fBgsub(\fIr\fB, \fIs \fR[\fB, \fIt\fR]\fB)\fR
for each substring matching the regular expression
.I r
in the string
.IR t ,
substitute the string
.IR s ,
and return the number of substitutions.
If
.I t
is not supplied, use
.BR $0 .
An
.B &
in the replacement text is replaced with the text that was actually matched.
Use
.B \e&
to get a literal
.BR & .
See
.I "AWK Language Programming"
for a fuller discussion of the rules for
.BR &'s
and backslashes in the replacement text of
.BR sub() ,
.BR gsub() ,
and
.BR gensub() .
.TP
.BI index( s , " t" )
returns the index of the string
.I t
in the string
.IR s ,
or 0 if
.I t
is not present.
.TP
\fBlength(\fR[\fIs\fR]\fB)
returns the length of the string
.IR s ,
or the length of
.B $0
if
.I s
is not supplied.
.TP
.BI match( s , " r" )
returns the position in
.I s
where the regular expression
.I r
occurs, or 0 if
.I r
is not present, and sets the values of
.B RSTART
and
.BR RLENGTH .
.TP
\fBsplit(\fIs\fB, \fIa \fR[\fB, \fIr\fR]\fB)\fR
splits the string
.I s
into the array
.I a
on the regular expression
.IR r ,
and returns the number of fields. If
.I r
is omitted,
.B FS
is used instead.
The array
.I a
is cleared first.
Splitting behaves identically to field splitting, described above.
.TP
.BI sprintf( fmt , " expr-list" )
prints
.I expr-list
according to
.IR fmt ,
and returns the resulting string.
.TP
\fBsub(\fIr\fB, \fIs \fR[\fB, \fIt\fR]\fB)\fR
just like
.BR gsub() ,
but only the first matching substring is replaced.
.TP
\fBsubstr(\fIs\fB, \fIi \fR[\fB, \fIn\fR]\fB)\fR
returns the at most
.IR n -character
substring of
.I s
starting at
.IR i .
If
.I n
is omitted, the rest of
.I s
is used.
.TP
.BI tolower( str )
returns a copy of the string
.IR str ,
with all the upper-case characters in
.I str
translated to their corresponding lower-case counterparts.
Non-alphabetic characters are left unchanged.
.TP
.BI toupper( str )
returns a copy of the string
.IR str ,
with all the lower-case characters in
.I str
translated to their corresponding upper-case counterparts.
Non-alphabetic characters are left unchanged.
.SS Time Functions
.PP
Since one of the primary uses of AWK programs is processing log files
that contain time stamp information,
.I gawk
provides the following two functions for obtaining time stamps and
formatting them.
.PP
.TP "\w'\fBsystime()\fR'u+1n"
.B systime()
returns the current time of day as the number of seconds since the Epoch
(Midnight UTC, January 1, 1970 on \*(PX systems).
.TP
\fBstrftime(\fR[\fIformat \fR[\fB, \fItimestamp\fR]]\fB)\fR
formats
.I timestamp
according to the specification in
.IR format.
The
.I timestamp
should be of the same form as returned by
.BR systime() .
If
.I timestamp
is missing, the current time of day is used.
If
.I format
is missing, a default format equivalent to the output of
.IR date (1)
will be used.
See the specification for the
.B strftime()
function in \*(AN C for the format conversions that are
guaranteed to be available.
A public-domain version of
.IR strftime (3)
and a man page for it come with
.IR gawk ;
if that version was used to build
.IR gawk ,
then all of the conversions described in that man page are available to
.IR gawk.
.SS String Constants
.PP
String constants in AWK are sequences of characters enclosed
between double quotes (\fB"\fR). Within strings, certain
.I "escape sequences"
are recognized, as in C. These are:
.PP
.TP \w'\fB\e\^\fIddd\fR'u+1n
.B \e\e
A literal backslash.
.TP
.B \ea
The ``alert'' character; usually the \s-1ASCII\s+1 \s-1BEL\s+1 character.
.TP
.B \eb
backspace.
.TP
.B \ef
form-feed.
.TP
.B \en
newline.
.TP
.B \er
carriage return.
.TP
.B \et
horizontal tab.
.TP
.B \ev
vertical tab.
.TP
.BI \ex "\^hex digits"
The character represented by the string of hexadecimal digits following
the
.BR \ex .
As in \*(AN 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., \fB"\ex1B"\fR is the \s-1ASCII\s+1 \s-1ESC\s+1 (escape) character.
.TP
.BI \e ddd
The character represented by the 1-, 2-, or 3-digit sequence of octal
digits. E.g. \fB"\e033"\fR is the \s-1ASCII\s+1 \s-1ESC\s+1 (escape) character.
.TP
.BI \e c
The literal character
.IR c\^ .
.PP
The escape sequences may also be used inside constant regular expressions
(e.g.,
.B "/[\ \et\ef\en\er\ev]/"
matches whitespace characters).
.PP
In compatibility mode, the characters represented by octal and
hexadecimal escape sequences are treated literally when used in
regexp constants. Thus,
.B /a\e52b/
is equivalent to
.BR /a\e*b/ .
.SH FUNCTIONS
Functions in AWK are defined as follows:
.PP
.RS
\fBfunction \fIname\fB(\fIparameter list\fB) { \fIstatements \fB}\fR
.RE
.PP
Functions are executed when they are called from within expressions
in either patterns or actions.  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.
.PP
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:
.PP
.RS
.ft B
.nf
function  f(p, q,     a, b)	# a & b are local
{
	\&.....
}

/abc/	{ ... ; f(1, 2) ; ... }
.fi
.ft R
.RE
.PP
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.
.PP
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.
.PP
Use
.BI return " expr"
to return a value from a function. The return value is undefined if no
value is provided, or if the function returns by ``falling off'' the
end.
.PP
If
.B \-\^\-lint
has been provided,
.I gawk
will warn about calls to undefined functions at parse time,
instead of at run time.
Calling an undefined function at run time is a fatal error.
.PP
The word
.B func
may be used in place of
.BR function .
.SH EXAMPLES
.nf
Print and sort the login names of all users:

.ft B
	BEGIN	{ FS = ":" }
		{ print $1 | "sort" }

.ft R
Count lines in a file:

.ft B
		{ nlines++ }
	END	{ print nlines }

.ft R
Precede each line by its number in the file:

.ft B
	{ print FNR, $0 }

.ft R
Concatenate and line number (a variation on a theme):

.ft B
	{ print NR, $0 }
.ft R
.fi
.SH SEE ALSO
.IR egrep (1),
.IR getpid (2),
.IR getppid (2),
.IR getpgrp (2),
.IR getuid (2),
.IR geteuid (2),
.IR getgid (2),
.IR getegid (2),
.IR getgroups (2)
.PP
.IR "The AWK Programming Language" ,
Alfred V. Aho, Brian W. Kernighan, Peter J. Weinberger,
Addison-Wesley, 1988. ISBN 0-201-07981-X.
.PP
.IR "AWK Language Programming" ,
Edition 1.0, published by the Free Software Foundation, 1995.
.SH POSIX COMPATIBILITY
A primary goal for
.I gawk
is compatibility with the \*(PX standard, as well as with the
latest version of \*(UX
.IR awk .
To this end,
.I gawk
incorporates the following user visible
features which are not described in the AWK book,
but are part of the Bell Labs version of
.IR awk ,
and are in the \*(PX standard.
.PP
The
.B \-v
option for assigning variables before program execution starts is new.
The book indicates that command line variable assignment happens when
.I awk
would otherwise open the argument as a file, which is after the
.B BEGIN
block is executed.  However, in earlier implementations, when such an
assignment appeared before any file names, the assignment would happen
.I before
the
.B BEGIN
block was run.  Applications came to depend on this ``feature.''
When
.I 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.)
.PP
The
.B \-W
option for implementation specific features is from the \*(PX standard.
.PP
When processing arguments,
.I gawk
uses the special option ``\fB\-\^\-\fP'' to signal the end of
arguments.
In compatibility 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.
.PP
The AWK book does not define the return value of
.BR srand() .
The \*(PX standard
has it return the seed it was using, to allow keeping track
of random number sequences. Therefore
.B srand()
in
.I gawk
also returns its current seed.
.PP
Other new features are:
The use of multiple
.B \-f
options (from MKS
.IR awk );
the
.B ENVIRON
array; the
.BR \ea ,
and
.BR \ev
escape sequences (done originally in
.I gawk
and fed back into AT&T's); the
.B tolower()
and
.B toupper()
built-in functions (from AT&T); and the \*(AN C conversion specifications in
.B printf
(done first in AT&T's version).
.SH GNU EXTENSIONS
.I Gawk
has a number of extensions to \*(PX
.IR awk .
They are described in this section.  All the extensions described here
can be disabled by
invoking
.I gawk
with the
.B \-\^\-traditional
option.
.PP
The following features of
.I gawk
are not available in
\*(PX
.IR awk .
.RS
.TP \w'\(bu'u+1n
\(bu
The
.B \ex
escape sequence.
(Disabled with
.BR \-\^\-posix .)
.TP \w'\(bu'u+1n
\(bu
The
.B fflush()
function.
(Disabled with
.BR \-\^\-posix .)
.TP
\(bu
The
.BR systime(),
.BR strftime(),
and
.B gensub()
functions.
.TP
\(bu
The special file names available for I/O redirection are not recognized.
.TP
\(bu
The
.BR ARGIND ,
.BR ERRNO ,
and
.B RT
variables are not special.
.TP
\(bu
The
.B IGNORECASE
variable and its side-effects are not available.
.TP
\(bu
The
.B FIELDWIDTHS
variable and fixed-width field splitting.
.TP
\(bu
The use of
.B RS
as a regular expression.
.TP
\(bu
The ability to split out individual characters using the null string
as the value of
.BR FS ,
and as the third argument to
.BR split() .
.TP
\(bu
No path search is performed for files named via the
.B \-f
option.  Therefore the
.B AWKPATH
environment variable is not special.
.TP
\(bu
The use of
.B "nextfile"
to abandon processing of the current input file.
.TP
\(bu
The use of
.BI delete " array"
to delete the entire contents of an array.
.RE
.PP
The AWK book does not define the return value of the
.B close()
function.
.IR Gawk\^ 's
.B close()
returns the value from
.IR fclose (3),
or
.IR pclose (3),
when closing a file or pipe, respectively.
.PP
When
.I gawk
is invoked with the
.B \-\^\-traditional
option,
if the
.I fs
argument to the
.B \-F
option is ``t'', then
.B FS
will be set to the tab character.
Note that typing
.B "gawk \-F\et \&..."
simply causes the shell to quote the ``t,'', and does not pass
``\et'' to the
.B \-F
option.
Since this is a rather ugly special case, it is not the default behavior.
This behavior also does not occur if
.B \-\^\-posix
has been specified.
To really get a tab character as the field separator, it is best to use
quotes:
.BR "gawk \-F'\et' \&..." .
.ig
.PP
If
.I gawk
was compiled for debugging, it will
accept the following additional options:
.TP
.PD 0
.B \-Wparsedebug
.TP
.PD
.B \-\^\-parsedebug
Turn on
.IR yacc (1)
or
.IR bison (1)
debugging output during program parsing.
This option should only be of interest to the
.I gawk
maintainers, and may not even be compiled into
.IR gawk .
..
.SH HISTORICAL FEATURES
There are two features of historical AWK implementations that
.I gawk
supports.
First, it is possible to call the
.B length()
built-in function not only with no argument, but even without parentheses!
Thus,
.RS
.PP
.ft B
a = length	# Holy Algol 60, Batman!
.ft R
.RE
.PP
is the same as either of
.RS
.PP
.ft B
a = length()
.br
a = length($0)
.ft R
.RE
.PP
This feature is marked as ``deprecated'' in the \*(PX standard, and
.I gawk
will issue a warning about its use if
.B \-\^\-lint
is specified on the command line.
.PP
The other feature is the use of either the
.B continue
or the
.B break
statements outside the body of a
.BR while ,
.BR for ,
or
.B do
loop.  Traditional AWK implementations have treated such usage as
equivalent to the
.B next
statement.
.I Gawk
will support this usage if
.B \-\^\-traditional
has been specified.
.SH ENVIRONMENT
If
.B POSIXLY_CORRECT
exists in the environment, then
.I gawk
behaves exactly as if
.B \-\^\-posix
had been specified on the command line.
If
.B \-\^\-lint
has been specified,
.I gawk
will issue a warning message to this effect.
.PP
The
.B AWKPATH
environment variable can be used to provide a list of directories that
.I gawk
will search when looking for files named via the
.B \-f
and
.B \-\^\-file
options.
.SH BUGS
The
.B \-F
option is not necessary given the command line variable assignment feature;
it remains only for backwards compatibility.
.PP
If your system actually has support for
.B /dev/fd
and the associated
.BR /dev/stdin ,
.BR /dev/stdout ,
and
.B /dev/stderr
files, you may get different output from
.I gawk
than you would get on a system without those files.  When
.I gawk
interprets these files internally, it synchronizes output to the standard
output with output to
.BR /dev/stdout ,
while on a system with those files, the output is actually to different
open files.
Caveat Emptor.
.PP
Syntactically invalid single character programs tend to overflow
the parse stack, generating a rather unhelpful message.  Such programs
are surprisingly difficult to diagnose in the completely general case,
and the effort to do so really is not worth it.
.SH VERSION INFORMATION
This man page documents
.IR gawk ,
version 3.0.4.
.SH AUTHORS
The original version of \*(UX
.I 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.
.PP
Paul Rubin and Jay Fenlason,
of the Free Software Foundation, wrote
.IR gawk ,
to be compatible with the original version of
.I awk
distributed in Seventh Edition \*(UX.
John Woods contributed a number of bug fixes.
David Trueman, with contributions
from Arnold Robbins, made
.I gawk
compatible with the new version of \*(UX
.IR awk .
Arnold Robbins is the current maintainer.
.PP
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.  Fred Fish supplied support for the Amiga.
.SH BUG REPORTS
If you find a bug in
.IR gawk ,
please send electronic mail to
.BR bug-gnu-utils@gnu.org ,
.I with
a carbon copy to
.BR arnold@gnu.org .
Please include your operating system and its revision, the version of
.IR 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.
.PP
Before sending a bug report, please do two things. First, verify that
you have the latest version of
.IR gawk .
Many bugs (usually subtle ones) are fixed at each release, and if
yours 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.
.PP
Whatever you do, do
.B NOT
post a bug report in
.BR comp.lang.awk .
While the
.I gawk
developers occasionally read this newsgroup, posting bug reports there
is an unreliable way to report bugs. Instead, please use the electronic mail
addresses given above.
.SH ACKNOWLEDGEMENTS
Brian Kernighan of Bell Labs
provided valuable assistance during testing and debugging.
We thank him.
.SH COPYING PERMISSIONS
Copyright \(co) 1996,97,98,99 Free Software Foundation, Inc.
.PP
Permission is granted to make and distribute verbatim copies of
this manual page provided the copyright notice and this permission
notice are preserved on all copies.
.ig
Permission is granted to process this file through troff and print the
results, provided the printed document carries copying permission
notice identical to this one except for the removal of this paragraph
(this paragraph not being relevant to the printed manual page).
..
.PP
Permission is granted to copy and distribute modified versions of this
manual page under the conditions for verbatim copying, provided that
the entire resulting derived work is distributed under the terms of a
permission notice identical to this one.
.PP
Permission is granted to copy and distribute translations of this
manual page into another language, under the above conditions for
modified versions, except that this permission notice may be stated in
a translation approved by the Foundation.