Reading Command Line Arguments

[Guest]

This is a simple tutorial that will show you how to read command line arguments and how to parse them with getopt.

To read command line arguments, you must declare main in a special way.
Here is how you declare main:

int main(int argc, char **argv)

Their are two variables, argc and argv. argc stands for argument count, so it is how many arguments are passed on the command line. argv is a pointer to a pointer, it is the set of actual arguments. This may seem confusing at first.

About argv:
A string is a pointer to a bunch of characters, right? Well argv is a pointer to a bunch of strings. For example, argv[0] is a pointer to the first argument, stored as a string.

Here is an example program that prints out all of the command line arguments you provide:

#include <stdio.h>

int main(int argc, char **argv)
{
	int i; //counter
	for (i=0;i<argc;i++)
		printf("%s\n", argv[i]);
	return 0;
}

Seems easy enough, right? Now that is only half of it. Most programs accept arguments using dashes, for example:

ls -a

In Unix would list the directory, and -a means list all files. To recognize and parse these options, programs use a function called getopt.
In order to use getopt, you must include this line:

#include <unistd.h>

Note: This file is part of the C POSIX library, and will not work on Windows unless compiled with Mingw or Cygwin.

Using getopt:
getopt is almost always used in a loop. When there are no more arguments to parse, getopt will return -1. You use getopt like this:

c=getopt(argc, argv, "abc");

argc and argv are the arguments from main. "abc" is the set of characters that are acceptable arguments. c is simply a character that is the argument returned by getopt.

Arguments that take options:
Some arguments need to take an option. For example, many programs use the argument "-f file" to provide file access. When arguments use options, you must follow the acceptable argument by a colon. For example, "a:bc" will accept arguments a, b, and c, and a will have an extra option. The extra option is stored in *optarg.

Here is an example program that can take two arguments, -a and -x. It will tell you if -a is provided, and if -x is provided. It will also tell you the option that -x is provided with.

#include <unistd.h>
#include <stdio.h>

int main (int argc, char **argv)
{
	int c;
	while ((c=getopt(argc, argv, "ax:")) != -1) {
		if(c=='a')
			printf("-a was provided\n");
		if(c=='x')
			printf("-x was provided with %s\n", optarg);
	}
	return 0;
}

There are actually more things you can do with getopt, but I will not cover those things here. Any feedback is welcome. Please post here if you have any questions, or improvements. +rep is appreciated a lot!

[WingedPanther]

Nice intro +rep

[ZekeDragon]

Taking in command line arguments is a common topic of confusion among newer C programmers, and you explained it well. +rep 4 u!

[dargueta]

I would use a switch statement instead of a block of ifs, but yes, this is a good explanation! Do you know how to use getopt_long?

EDIT: Can't +rep you for some reason...

[Guest]

Yes, I know how to use getopt_long, but like I said I didn't explain some things here. Plus that is exclusive to GNU and not part of the standard POSIX library.

[dargueta]

Well that's no fun.

[Guest]

Maybe I will have a part two that includes GNU extensions.

[dargueta]

Multipart tutorials are always fun - especially when they get long and you skip numbers. Almost did that once.

[Egz0N]

i like it .. +rep

[Jordan]

Since you begged for the rep... And since you did an excellent job, +rep!