getopt.c

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/**
 * getopt.c
 *
 * Demonstrates using the getopt() function to parse command line options.
 *
 * Try running this with a combination of command line options, e.g.:
 * cc -Wall getopt.c -o getopt
 *
 * ./getopt -a test1 test2
 * ./getopt -s 'different string' test1 test2
 * ./getopt -s hi -a testing 1 2 3
 *
 * And observe the behavior.
 */

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <stdbool.h>
#include <ctype.h>

int main(int argc, char *argv[])
{
    char *str_option = "default string";
    bool some_boolean = false;

    int c;
    opterr = 0;
    // notice how we support 'a' and 's' options, and -s requires an argument so
    // it has a trailing colon character ':'
    while ((c = getopt(argc, argv, "as:")) != -1) {
        switch (c) {
            case 'a':
                // the -a flag will turn on some_boolean
                some_boolean = true;
                break;
            case 's':
                // the -s flag will change what str_option points at. Notice how
                // we use 'optarg' to get whatever came after -s, e.g., if the
                // command line included -s test then optarg points at 'test'.
                str_option = optarg;
                break;
            case '?':
                if (optopt == 's') {
                    // if -s was passed with no argument, that's a problem.
                    fprintf(stderr,
                            "Option -%c requires an argument.\n", optopt);
                } else if (isprint(optopt)) {
                    fprintf(stderr, "Unknown option '-%c'.\n", optopt);
                } else {
                    fprintf(stderr,
                            "Unknown option character `\\x%x'.\n", optopt);
                }
                return 1;
            default:
                abort();
        }
    }

    printf("Done parsing options. some_boolean = %s, str_option = %s\n",
            some_boolean ? "true" : "false",
            str_option);

    // 'optind' is the beginning of the non-option command line arguments.
    for (int i = optind; i < argc; i++) {
        printf("Non-option argument %s\n", argv[i]);
    }

    return 0;
}