Command Line Arguments

# CHAPTER 21

Command Line Arguments #

1. Introduction #

2. Learning Objectives #

• Understand the parameters of the main() function.

• Explain the purpose of argc and argv.

• Access and parse arguments passed from the terminal.

• Convert string arguments into integers.

3. Modifying the main() Function #

int main(int argc, char *argv[]) {
    // Code goes here
    return 0;
}

4. What are argc and argv? #

• argc (Argument Count): An integer representing the total number of arguments passed. Note: The name of the program itself counts as the first argument (so argc is always at least 1).

• argv (Argument Vector): An array of character pointers (an array of strings). It holds the actual arguments.

• argv[0] holds the name of the program.

• argv[1] holds the first argument, argv[2] holds the second, and so on.

5. Example: Printing Arguments #

#include <stdio.h>

int main(int argc, char *argv[]) {
    printf("Program Name: %s\n", argv[0]);
    printf("Total Arguments (including program name): %d\n\n", argc);

    for (int i = 1; i < argc; i++) {
        printf("Argument %d: %s\n", i, argv[i]);
    }

    return 0;
}

Running it in the terminal:

$ gcc echo.c -o echo
$ ./echo hello world 123

Output:

Program Name: ./echo
Total Arguments: 4

Argument 1: hello
Argument 2: world
Argument 3: 123

6. Converting String Arguments to Integers #

#include <stdio.h>
#include <stdlib.h> // Required for atoi()

int main(int argc, char *argv[]) {
    if (argc != 3) {
        printf("Usage: ./add num1 num2\n");
        return 1; // Exit with error
    }

    // Convert string arguments to integers
    int num1 = atoi(argv[1]);
    int num2 = atoi(argv[2]);

    printf("Sum: %d\n", num1 + num2);
    return 0;
}

Running it:

$ ./add 15 20
Sum: 35

7. Common Mistakes #

• Assuming argv[1] exists: If the user runs the program without arguments, accessing argv[1] will crash the program (Segmentation Fault). Always check argc first!

• Doing math on strings: Attempting argv[1] + argv[2] will add their memory addresses, not their numeric values. Use atoi() or atof().

8. Exercises #

1. 1. Write a program that takes one string argument (a name) and prints "Welcome, [name]!". If no name is provided, print "Welcome, Guest!".

1. 2. Write a command-line calculator that takes three arguments: ./calc 10 + 5. (Hint: check argv[2][0] for the operator).

9. MCQ Quiz with Answers #

10. Interview Questions #

• Q: Explain the purpose and data types of argc and argv.

• Q: How do you protect your program from crashing if a user forgets to provide command-line arguments?

• Q: What is the difference between argv being declared as char *argv[] and char argv? (Answer: They are identical in this context).