Use getopt() function in C/C++ to parse command line arguments

The getopt() function is a builtin function in C and is used to parse command line arguments.
Just include <unistd.h>

Usage 1

Let's suppose you want to pass -h, -v, -f filename parameters to the program.

#include <stdio.h>  
#include <unistd.h>  
  
int main(int argc, char *argv[])  
{ 
    int opt; 
      
    while((opt = getopt(argc, argv, "hvf:")) != -1)  
    {  
        switch(opt)  
        {  
            case 'h':  
                printf("option: h is set\n");  
                break;  
            case 'v':  
                printf("option: v is set\n");  
                break;  
            case 'f':  
                printf("option h filename: %s\n", optarg);  
                break;  
        }  
    }  
      
    return 0; 
} 

Options that require additional parameters such as file name, add a colon after the option letter like this "f:"
If you want to pass 2 parameters(ex: -f filename1 -g filename2) those requireadditional parameters, use like this "f:g:"
If you just want to set or unset the flag, just add the letter like this "hv".
If you combine these two conditions, the result optstring whould be "hvf:" .
Let's compile and test the source(arg1.cpp).

root@spypiggy-ubuntu:/usr/local/src/study/cpp/arg# g++ arg1.cpp
root@spypiggy-ubuntu:/usr/local/src/study/cpp/arg# ./a.out -h
option: h is set
root@spypiggy-ubuntu:/usr/local/src/study/cpp/arg# ./a.out -h -v
option: h is set
option: v is set
root@spypiggy-ubuntu:/usr/local/src/study/cpp/arg# ./a.out -h -v -f Hello
option: h is set
option: v is set
option h filename: Hello

Usage 2

There is another function similar to the getopt function. This function is getopt_long.
The getopt () function is relatively easy to use. Normal command-line utilities support long-name options that make the options more obvious. The getopt_long function is used for this purpose.

For using getopt_long, you need to include additional header file getopt.h

#include <stdio.h>
#include <stdlib.h>
#include <getopt.h>

int
main (int argc, char **argv)
{
  int c;

  while (1)
    {
      static struct option long_options[] =
        {
          {"verbose",     no_argument,  0, 'v'},
          {"help,  no_argument,         0, 'h'},
          {"file",  required_argument,  0, 'f'},
          {0, 0, 0, 0}
        };
      /* getopt_long stores the option index here. */
      int option_index = 0;

      c = getopt_long (argc, argv, "vhf:", long_options, &option_index);

      if (c == -1)
        break;

      switch (c)
        {
        case 'v':
          printf ("option --verbose (-v) is set \n");
          break;

        case 'h':
          printf ("option --help (-h) is set \n");
          break;

        case 'f':
          printf ("option --file (-f) with value `%s'\n", optarg);
          break;

        default:
          abort ();
        }
    }
    return 0;
}

Let's compile and test the source(arg2.cpp).

root@spypiggy-ubuntu:/usr/local/src/study/cpp/arg# g++ argc2.cpp
root@spypiggy-ubuntu:/usr/local/src/study/cpp/arg# ./a.out --verbose --help
option --verbose (-v) is set
option --help (-h) is set
root@spypiggy-ubuntu:/usr/local/src/study/cpp/arg# ./a.out --verbose --help --file Hello
option --verbose (-v) is set
option --help (-h) is set
option --file (-f) with value `Hello'
root@spypiggy-ubuntu:/usr/local/src/study/cpp/arg# ./a.out -v -h --file Hello
option --verbose (-v) is set
option --help (-h) is set
option --file (-f) with value `Hello'

As you can see, both --option and --option are available when using getopt_long.