arg/arg.c

/* 
 * arg - A simple C program by Bruce Hill for parsing command line arguments
 *   Licensed under the MIT license (see LICENSE for details).
 *
 *   Example: arg --foo a b --foo=blah --baz -xyz
 *     - Prints "blah" and exits with success
 *   Example: arg --nope a b --foo=blah --baz -xyz
 *     - Prints nothing and exits with failure
 *   Example: arg -y a b --foo=blah --baz -xyz
 *     - Prints nothing and exits with success
 */
#include <stdio.h>
#include <string.h>

#define EXIT_SUCCESS 0
#define EXIT_NO_MATCH 1
#define EXIT_BAD_USAGE -1

int main(int argc, char **argv)
{
    char *flag = argv[1];
    if (!flag) {
        fprintf(stderr, "Usage: arg <flag> ...\n");
        return EXIT_BAD_USAGE;
    }
    size_t flaglen = strlen(flag);
    // Look for: --flag=val, --flag val, --flag, -f=val, -f val, or -f
    for (int i = 2; i < argc; i++) {
        char *arg = argv[i];
        if (strncmp(arg, flag, flaglen) != 0) continue;
        if (arg[flaglen] == '\0') { // --flag ...
            if (argv[i+1] && argv[i+1][0] != '-') // --flag <value>
                puts(argv[i+1]); // value of the flag
            return EXIT_SUCCESS;
        } else if (arg[flaglen] == '=') { // --flag=...
            puts(&arg[flaglen+1]);
            return EXIT_SUCCESS;
        }
    }
    // If flag is single-character without a value, e.g. -f, look for it
    // among other single character flags like -xfy.
    if (flag[0] == '-' && flaglen == 2) {
        for (int i = 2; i < argc; i++) {
            char *arg = argv[i];
            if (arg[0] != '-' || arg[1] == '-')
                continue; // skip foo and --foo
            for (char *c = &arg[1]; *c; c++) {
                if (*c == flag[1])
                    return EXIT_SUCCESS;
            }
        }
    }
    return EXIT_NO_MATCH;
}