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This manual page is part of the POSIX Programmer's Manual. The Linux implementation of
this interface may differ (consult the corresponding Linux manual page for details of
Linux behavior), or the interface may not be implemented on Linux.
NAME
getopt, optarg, opterr, optind, optopt — command option parsing
SYNOPSIS
#include <unistd.h>
int getopt(int argc, char * const argv[], const char *optstring);
extern char *optarg;
extern int opterr, optind, optopt;
DESCRIPTION
The getopt() function is a command-line parser that shall follow Utility Syntax Guidelines
3, 4, 5, 6, 7, 9, and 10 in the Base Definitions volume of POSIX.1‐2017, Section 12.2,
Utility Syntax Guidelines.
The parameters argc and argv are the argument count and argument array as passed to main()
(see exec()). The argument optstring is a string of recognized option characters; if a
character is followed by a <colon>, the option takes an argument. All option characters
allowed by Utility Syntax Guideline 3 are allowed in optstring. The implementation may
accept other characters as an extension.
The variable optind is the index of the next element of the argv[] vector to be processed.
It shall be initialized to 1 by the system, and getopt() shall update it when it finishes
with each element of argv[]. If the application sets optind to zero before calling
getopt(), the behavior is unspecified. When an element of argv[] contains multiple option
characters, it is unspecified how getopt() determines which options have already been
processed.
The getopt() function shall return the next option character (if one is found) from argv
that matches a character in optstring, if there is one that matches. If the option takes
an argument, getopt() shall set the variable optarg to point to the option-argument as
follows:
1. If the option was the last character in the string pointed to by an element of argv,
then optarg shall contain the next element of argv, and optind shall be incremented by
2. If the resulting value of optind is greater than argc, this indicates a missing
option-argument, and getopt() shall return an error indication.
2. Otherwise, optarg shall point to the string following the option character in that
element of argv, and optind shall be incremented by 1.
If, when getopt() is called:
argv[optind] is a null pointer
*argv[optind] is not the character -
argv[optind] points to the string "-"
getopt() shall return -1 without changing optind. If:
argv[optind] points to the string "--"
getopt() shall return -1 after incrementing optind.
If getopt() encounters an option character that is not contained in optstring, it shall
return the <question-mark> ('?') character. If it detects a missing option-argument, it
shall return the <colon> character (':') if the first character of optstring was a
<colon>, or a <question-mark> character ('?') otherwise. In either case, getopt() shall
set the variable optopt to the option character that caused the error. If the application
has not set the variable opterr to 0 and the first character of optstring is not a
<colon>, getopt() shall also print a diagnostic message to stderr in the format specified
for the getopts utility, unless the stderr stream has wide orientation, in which case the
behavior is undefined.
The getopt() function need not be thread-safe.
RETURN VALUE
The getopt() function shall return the next option character specified on the command
line.
A <colon> (':') shall be returned if getopt() detects a missing argument and the first
character of optstring was a <colon> (':').
A <question-mark> ('?') shall be returned if getopt() encounters an option character not
in optstring or detects a missing argument and the first character of optstring was not a
<colon> (':').
Otherwise, getopt() shall return -1 when all command line options are parsed.
ERRORS
If the application has not set the variable opterr to 0, the first character of optstring
is not a <colon>, and a write error occurs while getopt() is printing a diagnostic message
to stderr, then the error indicator for stderr shall be set; but getopt() shall still
succeed and the value of errno after getopt() is unspecified.
The following sections are informative.
EXAMPLES
Parsing Command Line Options
The following code fragment shows how you might process the arguments for a utility that
can take the mutually-exclusive options a and b and the options f and o, both of which
require arguments:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int
main(int argc, char *argv[ ])
{
int c;
int bflg = 0, aflg = 0, errflg = 0;
char *ifile;
char *ofile;
. . .
while ((c = getopt(argc, argv, ":abf:o:")) != -1) {
switch(c) {
case 'a':
if (bflg)
errflg++;
else
aflg++;
break;
case 'b':
if (aflg)
errflg++;
else
bflg++;
break;
case 'f':
ifile = optarg;
break;
case 'o':
ofile = optarg;
break;
case ':': /* -f or -o without operand */
fprintf(stderr,
"Option -%c requires an operand\n", optopt);
errflg++;
break;
case '?':
fprintf(stderr,
"Unrecognized option: '-%c'\n", optopt);
errflg++;
}
}
if (errflg) {
fprintf(stderr, "usage: . . . ");
exit(2);
}
for ( ; optind < argc; optind++) {
if (access(argv[optind], R_OK)) {
. . .
}
This code accepts any of the following as equivalent:
cmd -ao arg path path
cmd -a -o arg path path
cmd -o arg -a path path
cmd -a -o arg -- path path
cmd -a -oarg path path
cmd -aoarg path path
Selecting Options from the Command Line
The following example selects the type of database routines the user wants to use based on
the Options argument.
#include <unistd.h>
#include <string.h>
...
const char *Options = "hdbtl";
...
int dbtype, c;
char *st;
...
dbtype = 0;
while ((c = getopt(argc, argv, Options)) != -1) {
if ((st = strchr(Options, c)) != NULL) {
dbtype = st - Options;
break;
}
}
APPLICATION USAGE
The getopt() function is only required to support option characters included in Utility
Syntax Guideline 3. Many historical implementations of getopt() support other characters
as options. This is an allowed extension, but applications that use extensions are not
maximally portable. Note that support for multi-byte option characters is only possible
when such characters can be represented as type int.
Applications which use wide-character output functions with stderr should ensure that any
calls to getopt() do not write to stderr, either by setting opterr to 0 or by ensuring the
first character of optstring is always a <colon>.
While ferror(stderr) may be used to detect failures to write a diagnostic to stderr when
getopt() returns '?', the value of errno is unspecified in such a condition. Applications
desiring more control over handling write failures should set opterr to 0 and
independently perform output to stderr, rather than relying on getopt() to do the output.
RATIONALE
The optopt variable represents historical practice and allows the application to obtain
the identity of the invalid option.
The description has been written to make it clear that getopt(), like the getopts utility,
deals with option-arguments whether separated from the option by <blank> characters or
not. Note that the requirements on getopt() and getopts are more stringent than the
Utility Syntax Guidelines.
The getopt() function shall return -1, rather than EOF, so that <stdio.h> is not required.
The special significance of a <colon> as the first character of optstring makes getopt()
consistent with the getopts utility. It allows an application to make a distinction
between a missing argument and an incorrect option letter without having to examine the
option letter. It is true that a missing argument can only be detected in one case, but
that is a case that has to be considered.
FUTURE DIRECTIONS
None.
SEE ALSO
exec
The Base Definitions volume of POSIX.1‐2017, Section 12.2, Utility Syntax Guidelines,
<unistd.h>
The Shell and Utilities volume of POSIX.1‐2017, getopts
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1-2017, Standard for Information Technology -- Portable Operating System Interface
(POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by
the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and The Open Group
Standard, the original IEEE and The Open Group Standard is the referee document. The
original Standard can be obtained online at http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page are most likely to have
been introduced during the conversion of the source files to man page format. To report
such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .