path: root/man3/getopt.3

'\" t
.\" Copyright (c) 1993 by Thomas Koenig (ig25@rz.uni-karlsruhe.de)
.\" and Copyright 2006-2008, Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" Modified Sat Jul 24 19:27:50 1993 by Rik Faith (faith@cs.unc.edu)
.\" Modified Mon Aug 30 22:02:34 1995 by Jim Van Zandt <jrv@vanzandt.mv.com>
.\"  longindex is a pointer, has_arg can take 3 values, using consistent
.\"  names for optstring and longindex, "\n" in formats fixed.  Documenting
.\"  opterr and getopt_long_only.  Clarified explanations (borrowing heavily
.\"  from the source code).
.\" Modified 8 May 1998 by Joseph S. Myers (jsm28@cam.ac.uk)
.\" Modified 990715, aeb: changed `EOF' into `-1' since that is what POSIX
.\"  says; moreover, EOF is not defined in <unistd.h>.
.\" Modified 2002-02-16, joey: added information about nonexistent
.\"  option character and colon as first option character
.\" Modified 2004-07-28, Michael Kerrisk <mtk.manpages@gmail.com>
.\"	Added text to explain how to order both '[-+]' and ':' at
.\"		the start of optstring
.\" Modified 2006-12-15, mtk, Added getopt() example program.
.\"
.TH getopt 3 2023-07-20 "Linux man-pages 6.05.01"
.SH NAME
getopt, getopt_long, getopt_long_only,
optarg, optind, opterr, optopt \- Parse command-line options
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.PP
.BI "int getopt(int " argc ", char *" argv [],
.BI "           const char *" optstring );
.PP
.BI "extern char *" optarg ;
.BI "extern int " optind ", " opterr ", " optopt ;
.PP
.B #include <getopt.h>
.PP
.BI "int getopt_long(int " argc ", char *" argv [],
.BI "           const char *" optstring ,
.BI "           const struct option *" longopts ", int *" longindex );
.BI "int getopt_long_only(int " argc ", char *" argv [],
.BI "           const char *" optstring ,
.BI "           const struct option *" longopts ", int *" longindex );
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR getopt ():
.nf
    _POSIX_C_SOURCE >= 2 || _XOPEN_SOURCE
.fi
.PP
.BR getopt_long (),
.BR getopt_long_only ():
.nf
    _GNU_SOURCE
.fi
.SH DESCRIPTION
The
.BR getopt ()
function parses the command-line arguments.
Its arguments
.I argc
and
.I argv
are the argument count and array as passed to the
.IR main ()
function on program invocation.
An element of \fIargv\fP that starts with \[aq]\-\[aq]
(and is not exactly "\-" or "\-\-")
is an option element.
The characters of this element
(aside from the initial \[aq]\-\[aq]) are option characters.
If
.BR getopt ()
is called repeatedly, it returns successively each of the option characters
from each of the option elements.
.PP
The variable
.I optind
is the index of the next element to be processed in
.IR argv .
The system initializes this value to 1.
The caller can reset it to 1 to restart scanning of the same
.IR argv ,
or when scanning a new argument vector.
.PP
If
.BR getopt ()
finds another option character, it returns that
character, updating the external variable \fIoptind\fP and a static
variable \fInextchar\fP so that the next call to
.BR getopt ()
can
resume the scan with the following option character or
\fIargv\fP-element.
.PP
If there are no more option characters,
.BR getopt ()
returns \-1.
Then \fIoptind\fP is the index in \fIargv\fP of the first
\fIargv\fP-element that is not an option.
.PP
.I optstring
is a string containing the legitimate option characters.
A legitimate option character is any visible one byte
.BR ascii (7)
character (for which
.BR isgraph (3)
would return nonzero) that is not \[aq]\-\[aq], \[aq]:\[aq], or \[aq];\[aq].
If such a
character is followed by a colon, the option requires an argument, so
.BR getopt ()
places a pointer to the following text in the same
\fIargv\fP-element, or the text of the following \fIargv\fP-element, in
.IR optarg .
Two colons mean an option takes
an optional arg; if there is text in the current \fIargv\fP-element
(i.e., in the same word as the option name itself, for example, "\-oarg"),
then it is returned in \fIoptarg\fP, otherwise \fIoptarg\fP is set to zero.
This is a GNU extension.
If
.I optstring
contains
.B W
followed by a semicolon, then
.B \-W foo
is treated as the long option
.BR \-\-foo .
(The
.B \-W
option is reserved by POSIX.2 for implementation extensions.)
This behavior is a GNU extension, not available with libraries before
glibc 2.
.PP
By default,
.BR getopt ()
permutes the contents of \fIargv\fP as it
scans, so that eventually all the nonoptions are at the end.
Two other scanning modes are also implemented.
If the first character of
\fIoptstring\fP is \[aq]+\[aq] or the environment variable
.B POSIXLY_CORRECT
is set, then option processing stops as soon as a nonoption argument is
encountered.
If \[aq]+\[aq] is not the first character of
.IR optstring ,
it is treated as a normal option.
If
.B POSIXLY_CORRECT
behaviour is required in this case
.I optstring
will contain two \[aq]+\[aq] symbols.
If the first character of \fIoptstring\fP is \[aq]\-\[aq], then
each nonoption \fIargv\fP-element is handled as if it were the argument of
an option with character code 1.
(This is used by programs that were
written to expect options and other \fIargv\fP-elements in any order
and that care about the ordering of the two.)
The special argument "\-\-" forces an end of option-scanning regardless
of the scanning mode.
.PP
While processing the option list,
.BR getopt ()
can detect two kinds of errors:
(1) an option character that was not specified in
.I optstring
and (2) a missing option argument
(i.e., an option at the end of the command line without an expected argument).
Such errors are handled and reported as follows:
.IP \[bu] 3
By default,
.BR getopt ()
prints an error message on standard error,
places the erroneous option character in
.IR optopt ,
and returns \[aq]?\[aq] as the function result.
.IP \[bu]
If the caller has set the global variable
.I opterr
to zero, then
.BR getopt ()
does not print an error message.
The caller can determine that there was an error by testing whether
the function return value is \[aq]?\[aq].
(By default,
.I opterr
has a nonzero value.)
.IP \[bu]
If the first character
(following any optional \[aq]+\[aq] or \[aq]\-\[aq] described above)
of \fIoptstring\fP
is a colon (\[aq]:\[aq]), then
.BR getopt ()
likewise does not print an error message.
In addition, it returns \[aq]:\[aq] instead of \[aq]?\[aq] to
indicate a missing option argument.
This allows the caller to distinguish the two different types of errors.
.\"
.SS getopt_long() and getopt_long_only()
The
.BR getopt_long ()
function works like
.BR getopt ()
except that it also accepts long options, started with two dashes.
(If the program accepts only long options, then
.I optstring
should be specified as an empty string (""), not NULL.)
Long option names may be abbreviated if the abbreviation is
unique or is an exact match for some defined option.
A long option
may take a parameter, of the form
.B \-\-arg=param
or
.BR "\-\-arg param" .
.PP
.I longopts
is a pointer to the first element of an array of
.I struct option
declared in
.I <getopt.h>
as
.PP
.in +4n
.EX
struct option {
    const char *name;
    int         has_arg;
    int        *flag;
    int         val;
};
.EE
.in
.PP
The meanings of the different fields are:
.TP
.I name
is the name of the long option.
.TP
.I has_arg
is:
\fBno_argument\fP (or 0) if the option does not take an argument;
\fBrequired_argument\fP (or 1) if the option requires an argument; or
\fBoptional_argument\fP (or 2) if the option takes an optional argument.
.TP
.I flag
specifies how results are returned for a long option.
If \fIflag\fP
is NULL, then
.BR getopt_long ()
returns \fIval\fP.
(For example, the calling program may set \fIval\fP to the equivalent short
option character.)
Otherwise,
.BR getopt_long ()
returns 0, and
\fIflag\fP points to a variable which is set to \fIval\fP if the
option is found, but left unchanged if the option is not found.
.TP
\fIval\fP
is the value to return, or to load into the variable pointed
to by \fIflag\fP.
.PP
The last element of the array has to be filled with zeros.
.PP
If \fIlongindex\fP is not NULL, it
points to a variable which is set to the index of the long option relative to
.IR longopts .
.PP
.BR getopt_long_only ()
is like
.BR getopt_long (),
but \[aq]\-\[aq] as well
as "\-\-" can indicate a long option.
If an option that starts with \[aq]\-\[aq]
(not "\-\-") doesn't match a long option, but does match a short option,
it is parsed as a short option instead.
.SH RETURN VALUE
If an option was successfully found, then
.BR getopt ()
returns the option character.
If all command-line options have been parsed, then
.BR getopt ()
returns \-1.
If
.BR getopt ()
encounters an option character that was not in
.IR optstring ,
then \[aq]?\[aq] is returned.
If
.BR getopt ()
encounters an option with a missing argument,
then the return value depends on the first character in
.IR optstring :
if it is \[aq]:\[aq], then \[aq]:\[aq] is returned;
otherwise \[aq]?\[aq] is returned.
.PP
.BR getopt_long ()
and
.BR getopt_long_only ()
also return the option
character when a short option is recognized.
For a long option, they
return \fIval\fP if \fIflag\fP is NULL, and 0 otherwise.
Error and \-1 returns are the same as for
.BR getopt (),
plus \[aq]?\[aq] for an
ambiguous match or an extraneous parameter.
.SH ENVIRONMENT
.TP
.B POSIXLY_CORRECT
If this is set, then option processing stops as soon as a nonoption
argument is encountered.
.TP
.B _<PID>_GNU_nonoption_argv_flags_
This variable was used by
.BR bash (1)
2.0 to communicate to glibc which arguments are the results of
wildcard expansion and so should not be considered as options.
This behavior was removed in
.BR bash (1)
2.01, but the support remains in glibc.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lb lb lbx
l l l.
Interface	Attribute	Value
T{
.na
.nh
.BR getopt (),
.BR getopt_long (),
.BR getopt_long_only ()
T}	Thread safety	T{
.na
.nh
MT-Unsafe race:getopt env
T}
.TE
.sp 1
.SH VERSIONS
POSIX specifies that the
.I argv
array argument should be
.IR const ,
but these functions permute its elements
unless the environment variable
.B POSIXLY_CORRECT
is set.
.I const
is used in the actual prototype to be compatible with other systems;
however, this page doesn't show the qualifier,
to avoid confusing readers.
.SH STANDARDS
.TP
.BR getopt ()
POSIX.1-2008.
.TP
.BR getopt_long ()
.TQ
.BR getopt_long_only ()
GNU.
.IP
The use of \[aq]+\[aq] and \[aq]\-\[aq] in
.I optstring
is a GNU extension.
.SH HISTORY
.TP
.BR getopt ()
POSIX.1-2001, and POSIX.2.
.PP
On some older implementations,
.BR getopt ()
was declared in
.IR <stdio.h> .
SUSv1 permitted the declaration to appear in either
.I <unistd.h>
or
.IR <stdio.h> .
POSIX.1-1996 marked the use of
.I <stdio.h>
for this purpose as LEGACY.
POSIX.1-2001 does not require the declaration to appear in
.IR <stdio.h> .
.SH NOTES
A program that scans multiple argument vectors,
or rescans the same vector more than once,
and wants to make use of GNU extensions such as \[aq]+\[aq]
and \[aq]\-\[aq] at the start of
.IR optstring ,
or changes the value of
.B POSIXLY_CORRECT
between scans,
must reinitialize
.BR getopt ()
by resetting
.I optind
to 0, rather than the traditional value of 1.
(Resetting to 0 forces the invocation of an internal initialization
routine that rechecks
.B POSIXLY_CORRECT
and checks for GNU extensions in
.IR optstring .)
.PP
Command-line arguments are parsed in strict order
meaning that an option requiring an argument will consume the next argument,
regardless of whether that argument is the correctly specified option argument
or simply the next option
(in the scenario the user mis-specifies the command line).
For example, if
.I optstring
is specified as "1n:"
and the user specifies the command line arguments incorrectly as
.IR "prog\ \-n\ \-1" ,
the
.I \-n
option will be given the
.B optarg
value "\-1", and the
.I \-1
option will be considered to have not been specified.
.SH EXAMPLES
.SS getopt()
The following trivial example program uses
.BR getopt ()
to handle two program options:
.IR \-n ,
with no associated value; and
.IR "\-t val" ,
which expects an associated value.
.PP
.\" SRC BEGIN (getopt.c)
.EX
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
\&
int
main(int argc, char *argv[])
{
    int flags, opt;
    int nsecs, tfnd;
\&
    nsecs = 0;
    tfnd = 0;
    flags = 0;
    while ((opt = getopt(argc, argv, "nt:")) != \-1) {
        switch (opt) {
        case \[aq]n\[aq]:
            flags = 1;
            break;
        case \[aq]t\[aq]:
            nsecs = atoi(optarg);
            tfnd = 1;
            break;
        default: /* \[aq]?\[aq] */
            fprintf(stderr, "Usage: %s [\-t nsecs] [\-n] name\en",
                    argv[0]);
            exit(EXIT_FAILURE);
        }
    }
\&
    printf("flags=%d; tfnd=%d; nsecs=%d; optind=%d\en",
           flags, tfnd, nsecs, optind);
\&
    if (optind >= argc) {
        fprintf(stderr, "Expected argument after options\en");
        exit(EXIT_FAILURE);
    }
\&
    printf("name argument = %s\en", argv[optind]);
\&
    /* Other code omitted */
\&
    exit(EXIT_SUCCESS);
}
.EE
.\" SRC END
.SS getopt_long()
The following example program illustrates the use of
.BR getopt_long ()
with most of its features.
.PP
.\" SRC BEGIN (getopt_long.c)
.EX
#include <getopt.h>
#include <stdio.h>     /* for printf */
#include <stdlib.h>    /* for exit */
\&
int
main(int argc, char *argv[])
{
    int c;
    int digit_optind = 0;
\&
    while (1) {
        int this_option_optind = optind ? optind : 1;
        int option_index = 0;
        static struct option long_options[] = {
            {"add",     required_argument, 0,  0 },
            {"append",  no_argument,       0,  0 },
            {"delete",  required_argument, 0,  0 },
            {"verbose", no_argument,       0,  0 },
            {"create",  required_argument, 0, \[aq]c\[aq]},
            {"file",    required_argument, 0,  0 },
            {0,         0,                 0,  0 }
        };
\&
        c = getopt_long(argc, argv, "abc:d:012",
                        long_options, &option_index);
        if (c == \-1)
            break;
\&
        switch (c) {
        case 0:
            printf("option %s", long_options[option_index].name);
            if (optarg)
                printf(" with arg %s", optarg);
            printf("\en");
            break;
\&
        case \[aq]0\[aq]:
        case \[aq]1\[aq]:
        case \[aq]2\[aq]:
            if (digit_optind != 0 && digit_optind != this_option_optind)
              printf("digits occur in two different argv\-elements.\en");
            digit_optind = this_option_optind;
            printf("option %c\en", c);
            break;
\&
        case \[aq]a\[aq]:
            printf("option a\en");
            break;
\&
        case \[aq]b\[aq]:
            printf("option b\en");
            break;
\&
        case \[aq]c\[aq]:
            printf("option c with value \[aq]%s\[aq]\en", optarg);
            break;
\&
        case \[aq]d\[aq]:
            printf("option d with value \[aq]%s\[aq]\en", optarg);
            break;
\&
        case \[aq]?\[aq]:
            break;
\&
        default:
            printf("?? getopt returned character code 0%o ??\en", c);
        }
    }
\&
    if (optind < argc) {
        printf("non\-option ARGV\-elements: ");
        while (optind < argc)
            printf("%s ", argv[optind++]);
        printf("\en");
    }
\&
    exit(EXIT_SUCCESS);
}
.EE
.\" SRC END
.SH SEE ALSO
.BR getopt (1),
.BR getsubopt (3)