C Command-Line Arguments

Command-line arguments are values given to a program when it starts. In C, they let a program receive filenames, options, numbers, or other text without asking for input later.

To receive command-line arguments, write main with two parameters: argc and argv.

The main Function Parameters

The usual form is:

int main(int argc, char *argv[])

argc is the argument count. It tells you how many strings are in argv. argv is an array of strings. argv[0] is normally the program name, and the values typed after it are stored in argv[1], argv[2], and so on.

Expression	Meaning
argc	The number of command-line strings.
argv[0]	The program name or path used to run it.
argv[1]	The first argument after the program name.
argv[argc - 1]	The last argument.

Print All Arguments

This program loops through every command-line string and prints its index and value.

#include <stdio.h>

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

    printf("Argument count: %d\n", argc);

    for (i = 0; i < argc; i++) {
        printf("argv[%d] = %s\n", i, argv[i]);
    }

    return 0;
}

If the compiled program is run like this:

./args red blue

Output:

Argument count: 3
argv[0] = ./args
argv[1] = red
argv[2] = blue

The count is 3 because the program name counts as the first string. The two words after it become argv[1] and argv[2].

Check The Number Of Arguments

Always check argc before reading from argv. If your program expects one argument, then argc should be 2: one for the program name and one for the user-provided value.

#include <stdio.h>

int main(int argc, char *argv[])
{
    if (argc != 2) {
        printf("Usage: ./showfile filename\n");
        return 1;
    }

    printf("You asked for: %s\n", argv[1]);

    return 0;
}

If the compiled program is run like this:

./showfile notes.txt

Output:

You asked for: notes.txt

This program does not open the file yet; it only shows how to receive and validate the filename. If the user forgets the argument, the program prints a short usage message instead of reading argv[1] when it does not exist.

Important Details

• argv values are strings, even when the user types numbers.
• Use functions such as atoi, strtol, or strtod when you need to convert argument text to numbers.
• Arguments with spaces usually need quotes in the shell, such as ./program "hello world".
• Do not modify string data from argv unless you have a specific reason and understand the consequences.

The key idea is that argc tells you how many command-line strings are available, and argv lets you read each one safely by index.