Linux / Unix Command: getopt



NAME

getopt - Parse command line options  

SYNOPSIS

#include <unistd.h>

int getopt(int argc, char * const argv[],
const char *optstring);

extern char *optarg;
extern int optind, opterr, optopt;

#define _GNU_SOURCE

#include <getopt.h>

int getopt_long(int argc, char * const argv[],
const char *optstring,
const struct option *longopts, int *longindex);

int getopt_long_only(int argc, char * const argv[],
const char *optstring,
const struct option *longopts, int *longindex);

 

DESCRIPTION

The getopt() function parses the command line arguments. Its arguments
argc and argv are the argument count and array as passed to the
main() function on program invocation. An element of argv that starts with `-' (and is not exactly "-" or "--") is an option element. The characters of this element (aside from the initial `-') are option characters. If
getopt() is called repeatedly, it returns successively each of the option characters from each of the option elements.
If getopt() finds another option character, it returns that character, updating the external variable
optind and a static variable nextchar so that the next call to
getopt() can resume the scan with the following option character or
argv-element.

If there are no more option characters, getopt() returns -1. Then
optind is the index in argv of the first argv-element that is not an option.

optstring is a string containing the legitimate option characters. If such a character is followed by a colon, the option requires an argument, so
getopt places a pointer to the following text in the same argv-element, or the text of the following
argv-element, in optarg. Two colons mean an option takes an optional arg; if there is text in the current
argv-element, it is returned in optarg, otherwise optarg is set to zero. This is a GNU extension. If
optstring contains W followed by a semicolon, then
-W foo is treated as the long option --foo. (The
-W option is reserved by POSIX.2 for implementation extensions.) This behaviour is a GNU extension, not available with libraries before GNU libc 2.

By default, getopt() permutes the contents of argv as it scans, so that eventually all the non-options are at the end. Two other modes are also implemented. If the first character of
optstring is `+' or the environment variable POSIXLY_CORRECT is set, then option processing stops as soon as a non-option argument is encountered. If the first character of
optstring is `-', then each non-option argv-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
argv-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.

If getopt() does not recognize an option character, it prints an error message to stderr, stores the character in
optopt, and returns `?'. The calling program may prevent the error message by setting
opterr to 0.

If getopt() finds an option character in argv that was not included in
optstring, or if it detects a missing option argument, it returns `?' and sets the external variable
optopt to the actual option character. If the first character of optstring is a colon (`:'), then
getopt() returns `:' instead of `?' to indicate a missing option argument. If an error was detected, and the first character of
optstring is not a colon, and the external variable opterr is nonzero (which is the default),
getopt() prints an error message.

The getopt_long() function works like getopt() except that it also accepts long options, started out by two dashes. 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=param or
--arg param.

longopts is a pointer to the first element of an array of struct option 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, or
optional_argument (or 2) if the option takes an optional argument. flag specifies how results are returned for a long option. If flag is
NULL, then getopt_long() returns val. (For example, the calling program may set
val to the equivalent short option character.) Otherwise, getopt_long() returns 0, and
flag points to a variable which is set to val 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 by flag.

The last element of the array has to be filled with zeroes.

If longindex is not NULL, it points to a variable which is set to the index of the long option relative to
longopts.

getopt_long_only() is like getopt_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.  

RETURN VALUE

The getopt() function returns the option character if the option was found successfully, `:' if there was a missing parameter for one of the options, `?' for an unknown option character, or -1 for the end of the option list.

getopt_long() and getopt_long_only() also return the option character when a short option is recognized. For a long option, they return
val if flag is NULL, and 0 otherwise. Error and -1 returns are the same as for
getopt(), plus `?' for an ambiguous match or an extraneous parameter.
 

Important: Use the
man command (% man) to see how a command is used on your particular computer.