gawk.1

早期freebsd实现

.ds PX \s-1POSIX\s+1
.ds UX \s-1UNIX\s+1
.ds AN \s-1ANSI\s+1
.TH AWK 1 "April 15 1993" "Free Software Foundation" "Utility Commands"
.SH NAME
awk \- pattern scanning and processing language
.SH SYNOPSIS
.B awk
[ POSIX or GNU style options ]
.B \-f
.I program-file
[
.B \-\^\-
] file .\^.\^.
.br
.B awk
[ 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.
In the 4.4BSD distribution, it is installed as
.IR awk .
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 defined in the System V Release 4 version
of \*(UX
.IR awk .
.I Gawk
also provides 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 OPTIONS
.PP
.I Gawk
options may be either the traditional \*(PX one letter options,
or the GNU style long options.  \*(PX 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 \*(PX mandated features.  Other implementations of the AWK language
are likely to only accept the traditional one letter options.
.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, 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
.B \-W
option.
Each
.B \-W
option has a corresponding GNU style long option, as detailed below.
.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 \w'\fB\-\^\-copyright\fR'u+1n
.PD 0
.B "\-W compat"
.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.
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 error output.
.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 error output.
.TP
.PD 0
.B "\-W lint"
.TP
.PD 0
.B \-\^\-lint
Provide warnings about constructs that are
dubious or non-portable to other AWK implementations.
.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
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 ^= .
.RE
.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 size AWK programs used
in shell scripts.
.sp .5
The
.B "\-W source="
form of this option uses the rest of the command line argument for
.IR program-text ;
no other options to
.B \-W
will be recognized in the same argument.
.TP
.PD 0
.B "\-W version"
.TP
.PD
.B \-\^\-version
Print version information for this particular copy of
.I gawk
on the standard error 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.
.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
Any other options are flagged as illegal, but are otherwise ignored.
.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, or from the first non-option argument on the command line.
The
.B \-f
option may be used multiple times on the command line.
.I Gawk
will read the program text as if all the
.IR program-file s
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.  To use a library function in a file from a
program typed in on the command line, specify
.B /dev/tty
as one of the
.IR program-file s,
type your program, and end it with a
.B ^D
(control-d).
.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/lib/awk:/usr/local/lib/awk"\fR.
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,
.I gawk
compiles the program into an internal form.
Next, all variable assignments specified via the
.B \-v
option are performed.  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 line in the input,
.I gawk
tests to see if it matches any
.I pattern
in the AWK program.
For each pattern that the line 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 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; multiply dimensioned arrays may be simulated.
Several pre-defined variables are set as a program
runs; these will be described as needed and summarized below.
.SS Fields
.PP
As each input line is read,
.I gawk
splits the line 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.
Otherwise,
.B FS
is expected to be a full regular expression.
In the special case that
.B FS
is a single blank, fields are separated
by runs of blanks and/or tabs.
Note that the value of
.B IGNORECASE
(see below) will also affect how fields are split when
.B FS
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 line may be referenced by its position,
.BR $1 ,
.BR $2 ,
and so on.
.B $0
is the whole line. 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 line.
The variable
.B NF
is set to the total number of fields in the input line.
.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 .
.SS Built-in Variables
.PP
AWK'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 /u/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; expect the
semantics to 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 ``\-''.
.TP
.B FNR
The input record number in the current input file.
.TP
.B FS
The input field separator, a blank by default.
.TP
.B IGNORECASE
Controls the case-sensitivity of all regular expression operations. If
.B IGNORECASE
has a non-zero value, then pattern matching in rules,
field splitting with
.BR FS ,
regular expression
matching with
.B ~
and
.BR !~ ,
and the
.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 operations are normally case-sensitive.
.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 blank by default.
.TP
.B ORS
The output record separator, a newline by default.
.TP
.B RS
The input record separator, a newline by default.
.B 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
.IR gawk .)
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, then the newline character always acts as
a field separator, in addition to whatever value
.B FS
may have.
.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, \fB"\e034"\fR by default.
.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.