getopt(3C) (man pages section 3: Basic Library Functions)

man pages section 3: Basic Library Functions

getopt(3C)

NAME

getopt– get option letter from argument vector

SYNOPSIS

SVID3, XPG3

#include <stdio.h>

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

argc

argv

optstring

POSIX.2, XPG4, SUS, SUSv2

#include <unistd.h>

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

argc

argv

optstring

DESCRIPTION

The getopt() function returns the next option letter in argv that matches a letter in optstring. It supports all the rules of the command syntax standard (see intro(1)). Since all new commands are intended to adhere to the command syntax standard, they should use getopts(1), getopt(3C), or getsubopt(3C) to parse positional parameters and check for options that are legal for that command.

The optstring argument must contain the option letters the command using getopt() will recognize; if a letter is followed by a colon, the option is expected to have an argument, or group of arguments, which may be separated from it by white space. The optarg argument is set to point to the start of the option argument on return from getopt().

The getopt() function places in optind the argv index of the next argument to be processed. optind is external and is initialized to 1 before the first call to getopt(). When all options have been processed (that is, up to the first non-option argument), getopt() returns -1. The special option “--” (two hyphens) may be used to delimit the end of the options; when it is encountered, -1 is returned and “--”' is skipped. This is useful in delimiting non-option arguments that begin with “-” (hyphen).

RETURN VALUES

The getopt() function returns the next option character specified on the command line.

A colon (':') is returned if getopt() detects a missing argument and the first character of optstring was a colon (':').

The getopt() function outputs an error message to standard error and returns a question mark ('?') when it encounters an option letter not included in optstring or no argument after an option that expects one. This error message can be disabled by setting opterr to 0. The value of the character that caused the error is in optopt.

Otherwise, getopt() returns -1 when all command line options are parsed.

ERRORS

No errors are defined.

EXAMPLES

Example 1 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 <unistd.h>

int
main(int argc, char *argv[ ])
{
    int c;
    int bflg, aflg, errflg;
    char *ifile;
    char *ofile;
    extern char *optarg;
    extern int optind, optopt;
    . . .
    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++;
                bproc();
            }
            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

Example 2 Check Options and Arguments.

The following example parses a set of command line options and prints messages to standard output for each option and argument that it encounters.

#include unistd.h>
#include <stdio.h>
...
int c;
char *filename;
extern char *optarg;
extern int optind, optopt, opterr;
...
while ((c = getopt(argc, argv, ":abf:")) != -1) {
    switch(c) {
    case 'a':
         printf("a is set\n");
         break;
    case 'b':
         printf("b is set\n");
         break;
    case 'f':
         filename = optarg;
         printf("filename is %s\n", filename);
         break;
    case ':':
         printf("-%c without filename\n", optopt);
         break;  
    case '?':
         printf("unknown arg %c\n", optopt);
         break;   
    } 
}

Example 3 Select 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>
...
char *Options = "hdbtl";
...
int dbtype, i;
char c;
char *st;
...
dbtype = 0;
while ((c = getopt(argc, argv, Options)) != -1) {
    if ((st = strchr(Options, c)) != NULL) {
        dbtype = st - Options;
        break;
    }
}

ENVIRONMENT VARIABLES

See environ(5) for descriptions of the following environment variables that affect the execution of getopt(): LANG, LC_ALL, and LC_MESSAGES.

LC_CTYPE: Determine the locale for the interpretation of sequences of bytes as characters in optstring.

USAGE

The getopt() function does not fully check for mandatory arguments; that is, given an option string a:b and the input -a -b, getopt() assumes that -b is the mandatory argument to the -a option and not that -a is missing a mandatory argument.

It is a violation of the command syntax standard (see intro(1)) for options with arguments to be grouped with other options, as in cmd -abo filename , where a and b are options, o is an option that requires an argument, and filename is the argument to o. Although this syntax is permitted in the current implementation, it should not be used because it may not be supported in future releases. The correct syntax to use is:

cmd- ab -o filename

ATTRIBUTES

See attributes(5) for descriptions of the following attributes:

ATTRIBUTE TYPE	ATTRIBUTE VALUE
Interface Stability	Standard
MT-Level	Unsafe