getopt example in C

Header file:

    unistd.h

Synopsis:

     int getopt (int argc, char **argv, const char *options)

Description:

     getopt is used to parse command line options.  We need to know the below information to get clear idea on getopt.

int opterr

If opterr is non-zero, then getopt will print error message for unknown options or options with missing required argument.  If opterr is zero, then getopt will return '?' for unknown options or options with missing required argument.

int optopt
If getopt finds an unknown option character or an option with a missing required argument, then it stores that option character in optint.

int optind

Holds the index of the next element in argv and it will be set by getopt.

char *optarg

Pointer to the value of the required option argument or optional argument and it will be set by getopt.

Argument options indicates option characters. If the option character followed by single colon(:), it indicates that the option takes required argument.  If the option character followed by double colon(::), it indicates that the option takes optional argument.  And hypen '-' is used to recognize options.

getopt function C example

  #include <stdio.h>
  #include <unistd.h>
  #include <stdlib.h>
  int main (int argc, char **argv) {
        int i, ch;
        opterr = 0;
        while ((ch = getopt(argc, argv, "ab::c:")) != -1) {
                switch (ch) {
                        case 'a':
                                printf("Option 'a\'\n");
                                break;
                        case 'b':
                                printf("Option 'b\'");
                                if (optarg)
                                        printf(" & its Optional arg-%s", optarg);
                                printf("\n");
                                break;
                        case 'c':
                                printf("Option 'c\'");
                                printf(" & its required arg - %s\n", optarg);
                                break;
                        case '?':
                                if (optopt == 'c')
                                        printf("For option 'c\', argument is mandatory\n");
                                else if (isprint (optopt))
                                        printf("U have given an unknown "
                                                          "option - %c\n", optopt);
                                else
                                        printf("Unknown Error-0x%08x\n", optopt);
                                break;
                        default:
                                exit(0);
                }
        }
        for (i = optind; i < argc; i++)
                printf("Redundant argument - %s\n", argv[i]);
        return 0;
  }

  Output:

  jp@jp-VirtualBox:$ ./a.out -boptionB -c optionC
  Option 'b' & its Optional arg-optionB
  Option 'c' & its required arg - optionC
  jp@jp-VirtualBox:$ ./a.out -a -c optionC optionD
  Option 'a'
  Option 'c' & its required arg - optionC
  Redundant argument - optionD
  jp@jp-VirtualBox:$ ./a.out -a -e
  Option 'a'
  U have given an unknown option - e

Notes:
Why optional argument is not parsed by getopt?
For optional argument, the argument should immediately follow the option character. There should not be any space between option and its argument.

Eg: 
./a.out -bhello
Here, option b will take the optional argument hello since we don't have space between the option and argument

./a.out -b hello
Here, option b won't take the optional argument hello since we have space between the option and argument.