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NAME |LIBRARY |SYNOPSIS |DESCRIPTION |RETURN VALUE |ENVIRONMENT |ATTRIBUTES |VERSIONS |STANDARDS |HISTORY |NOTES |EXAMPLES |SEE ALSO |COLOPHON | |
getopt(3) Library Functions Manualgetopt(3)getopt, getopt_long, getopt_long_only, optarg, optind, opterr, optopt - Parse command-line options
Standard C library (libc,-lc)
#include <unistd.h>int getopt(intargc, char *argv[],const char *optstring);extern char *optarg;extern intoptind,opterr,optopt;#include <getopt.h>int getopt_long(intargc, char *argv[],const char *optstring,const struct option *longopts, int *longindex);int getopt_long_only(intargc, char *argv[],const char *optstring,const struct option *longopts, int *longindex); Feature Test Macro Requirements for glibc (seefeature_test_macros(7)):getopt(): _POSIX_C_SOURCE >= 2 || _XOPEN_SOURCEgetopt_long(),getopt_long_only(): _GNU_SOURCE
Thegetopt() function parses the command-line arguments. Its argumentsargc andargv are the argument count and array as passed to themain() function on program invocation. An element ofargv that starts with '-' (and is not exactly "-" or "--") is an option element. The characters of this element (aside from the initial '-') are option characters. Ifgetopt() is called repeatedly, it returns successively each of the option characters from each of the option elements. The variableoptind is the index of the next element to be processed inargv. The system initializes this value to 1. The caller can reset it to 1 to restart scanning of the sameargv, or when scanning a new argument vector. Ifgetopt() finds another option character, it returns that character, updating the external variableoptind and a static variablenextchar so that the next call togetopt() can resume the scan with the following option character orargv-element. If there are no more option characters,getopt() returns -1. Thenoptind is the index inargv of the firstargv-element that is not an option.optstring is a string containing the legitimate option characters. A legitimate option character is any visible one byteascii(7) character (for whichisgraph(3) would return nonzero) that is not '-', ':', or ';'. If such a character is followed by a colon, the option requires an argument, sogetopt() places a pointer to the following text in the sameargv-element, or the text of the followingargv-element, inoptarg. Two colons mean an option takes an optional arg; if there is text in the currentargv- element (i.e., in the same word as the option name itself, for example, "-oarg"), then it is returned inoptarg, otherwiseoptarg is set to zero. This is a GNU extension. Ifoptstring containsW followed by a semicolon, then-W foois treated as the long option--foo. (The-Woption is reserved by POSIX.2 for implementation extensions.) This behavior is a GNU extension, not available with libraries before glibc 2. By default,getopt() permutes the contents ofargv as it scans, so that eventually all the nonoptions are at the end. Two other scanning modes are also implemented. If the first character ofoptstring is '+' or the environment variablePOSIXLY_CORRECTis set, then option processing stops as soon as a nonoption argument is encountered. If '+' is not the first character ofoptstring, it is treated as a normal option. IfPOSIXLY_CORRECTbehaviour is required in this caseoptstring will contain two '+' symbols. If the first character ofoptstring is '-', then each nonoptionargv- 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 otherargv-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. While processing the option list,getopt() can detect two kinds of errors: (1) an option character that was not specified inoptstring 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: • By default,getopt() prints an error message on standard error, places the erroneous option character inoptopt, and returns '?' as the function result. • If the caller has set the global variableopterr to zero, thengetopt() does not print an error message. The caller can determine that there was an error by testing whether the function return value is '?'. (By default,opterr has a nonzero value.) • If the first character (following any optional '+' or '-' described above) ofoptstring is a colon (':'), thengetopt() likewise does not print an error message. In addition, it returns ':' instead of '?' to indicate a missing option argument. This allows the caller to distinguish the two different types of errors.getopt_long() and getopt_long_only() Thegetopt_long() function works likegetopt() except that it also accepts long options, started with two dashes. (If the program accepts only long options, thenoptstring 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--arg=paramor--arg param.longopts is a pointer to the first element of an array ofstructoption declared in<getopt.h> as struct option { const char *name; int has_arg; int *flag; int val; }; The meanings of the different fields are:name is the name of the long option.has_arg is:no_argument(or 0) if the option does not take an argument;required_argument(or 1) if the option requires an argument; oroptional_argument(or 2) if the option takes an optional argument.flag specifies how results are returned for a long option. Ifflag is NULL, thengetopt_long() returnsval. (For example, the calling program may setval to the equivalent short option character.) Otherwise,getopt_long() returns 0, andflag points to a variable which is set toval if the option is found, but left unchanged if the option is not found.val is the value to return, or to load into the variable pointed to byflag. The last element of the array has to be filled with zeros. Iflongindex is not NULL, it points to a variable which is set to the index of the long option relative tolongopts.getopt_long_only() is likegetopt_long(), but '-' as well as "--" can indicate a long option. If an option that starts with '-' (not "--") doesn't match a long option, but does match a short option, it is parsed as a short option instead.If an option was successfully found, thengetopt() returns the option character. If all command-line options have been parsed, thengetopt() returns -1. Ifgetopt() encounters an option character that was not inoptstring, then '?' is returned. Ifgetopt() encounters an option with a missing argument, then the return value depends on the first character inoptstring: if it is ':', then ':' is returned; otherwise '?' is returned.getopt_long() andgetopt_long_only() also return the option character when a short option is recognized. For a long option, they returnval ifflag is NULL, and 0 otherwise. Error and -1 returns are the same as forgetopt(), plus '?' for an ambiguous match or an extraneous parameter.
POSIXLY_CORRECT If this is set, then option processing stops as soon as a nonoption argument is encountered.
For an explanation of the terms used in this section, seeattributes(7). ┌────────────────────┬───────────────┬───────────────────────────┐ │Interface│Attribute│Value│ ├────────────────────┼───────────────┼───────────────────────────┤ │getopt(), │ Thread safety │ MT-Unsafe race:getopt env │ │getopt_long(), │ │ │ │getopt_long_only() │ │ │ └────────────────────┴───────────────┴───────────────────────────┘
POSIX specifies that theargv array argument should beconst, but these functions permute its elements unless the environment variablePOSIXLY_CORRECTis set.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.
getopt() POSIX.1-2008.getopt_long()getopt_long_only() GNU. The use of '+' and '-' inoptstring is a GNU extension.
getopt() POSIX.1-2001, and POSIX.2. On some older implementations,getopt() was declared in<stdio.h>. SUSv1 permitted the declaration to appear in either<unistd.h> or<stdio.h>. POSIX.1-1996 marked the use of<stdio.h> for this purpose as LEGACY. POSIX.1-2001 does not require the declaration to appear in<stdio.h>. Very old versions of glibc were affected by a_PID_GNU_nonoption_argv_flags_environment variable ⟨https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=bf079e19f50d64aa5e05⟩.
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 '+' and '-' at the start ofoptstring, or changes the value ofPOSIXLY_CORRECTbetween scans, must reinitializegetopt() by resettingoptind to 0, rather than the traditional value of 1. (Resetting to 0 forces the invocation of an internal initialization routine that rechecksPOSIXLY_CORRECT and checks for GNU extensions inoptstring.) 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, ifoptstring is specified as "1n:" and the user specifies the command line arguments incorrectly asprog -n -1, the-n option will be given theoptargvalue "-1", and the-1 option will be considered to have not been specified.
getopt() The following trivial example program usesgetopt() to handle two program options:-n, with no associated value; and-t val, which expects an associated value. #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 'n': flags = 1; break; case 't': nsecs = atoi(optarg); tfnd = 1; break; default: /* '?' */ fprintf(stderr, "Usage: %s [-t nsecs] [-n] name\n", argv[0]); exit(EXIT_FAILURE); } } printf("flags=%d; tfnd=%d; nsecs=%d; optind=%d\n", flags, tfnd, nsecs, optind); if (optind >= argc) { fprintf(stderr, "Expected argument after options\n"); exit(EXIT_FAILURE); } printf("name argument = %s\n", argv[optind]); /* Other code omitted */ exit(EXIT_SUCCESS); }getopt_long() The following example program illustrates the use ofgetopt_long() with most of its features. #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, 'c'}, {"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("\n"); break; case '0': case '1': case '2': if (digit_optind != 0 && digit_optind != this_option_optind) printf("digits occur in two different argv-elements.\n"); digit_optind = this_option_optind; printf("option %c\n", c); break; case 'a': printf("option a\n"); break; case 'b': printf("option b\n"); break; case 'c': printf("option c with value '%s'\n", optarg); break; case 'd': printf("option d with value '%s'\n", optarg); break; case '?': break; default: printf("?? getopt returned character code 0%o ??\n", c); } } if (optind < argc) { printf("non-option ARGV-elements: "); while (optind < argc) printf("%s ", argv[optind++]); printf("\n"); } exit(EXIT_SUCCESS); }getopt(1),getsubopt(3)
This page is part of theman-pages (Linux kernel and C library user-space interface documentation) project. Information about the project can be found at ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report for this manual page, see ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩. This page was obtained from the tarball man-pages-6.15.tar.gz fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2025-08-11. If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which isnot part of the original manual page), send a mail to man-pages@man7.orgLinux man-pages 6.15 2025-05-17getopt(3)Pages that refer to this page:getopt(1), pipesz(1), execve(2), getsubopt(3), pmdagetoptions(3), pmgetoptions(3), rpmlua(8)
HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. | |