Direct shell input as C program argument

Question

Let's say I have a C program whose function declaration is void square(int n), (it's also defined) all it does it printf the squared value of n. I want to be able to run it from bash shell like so: square 5, where 5 is the input to the C program.

How would I go about this? I've looked into using getopt, read, I've read the man pages several times and watched a few getopt tutorials, but I can't seem to figure out a way to do this. I can't find an example of getopt that doesn't use flags in the examples, so I don't know how to apply it to a simple integer input. Could anyone share with me how to do this? I would really appreciate it.

Answer 1

If you don't have any other command line options you need to handle, getopt is probably overkill. All you need is to read the value from argv:

int main(int argc, char *argv[])
{
    int n;

    // need "2 args" because the program's name counts as 1
    if (argc != 2)
    {
        fprintf(stderr, "usage: square <n>\n");
        return -1;
    }

    // convert the first argument, argv[1], from string to int;
    // see note below about using strtol() instead
    n = atoi(argv[1]);

    square(n);

    return 0;
}

A better solution will use strtol() instead of atoi() in order to check if the conversion was valid.