Command-Line Arguments in C

Command-line arguments are a way to pass information or inputs to a C program when you run it from the command line or terminal.

They provide flexibility and let users control the program’s behavior without changing the code.

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Syntax in C

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

Parameters

• argc (Argument Count)

  • Integer representing the number of command-line arguments

  • Includes the program name

• argv (Argument Vector)

  • Array of strings containing the arguments

  • argv[0] → program name

  • argv[1], argv[2], ... → user arguments

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Example 1: Printing Command-Line Arguments

#include <stdio.h>

int main(int argc, char *argv[]) {
    printf("Number of arguments: %d\n", argc);
    
    for (int i = 0; i < argc; i++) {
        printf("Argument %d: %s\n", i, argv[i]);
    }

    return 0;
}

Running the Program

./example hello world 123

Expected Output

Number of arguments: 4
Argument 0: ./example
Argument 1: hello
Argument 2: world
Argument 3: 123

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Important Points

• Data Type: Arguments are passed as strings

• Conversion: Use functions like atoi(), atof() to convert values

• Error Handling: Always check argument count

• Applications:

  • Passing filenames

  • Configuration settings

  • User inputs

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Example 2: Add Two Numbers Using Command-Line Arguments

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

int main(int argc, char *argv[])
{
    if(argc != 3)
    {
        printf("Invalid number of arguments...\n");
        exit(0);
    }
    else
    {
        printf("Sum = %d", atoi(argv[1]) + atoi(argv[2]));
    }
}

Run

./a.out 2 3

Note: atoi() converts string to integer.

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Example 3: Display Contents of Files Passed as Arguments

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

int main(int argc, char *argv[])
{
    int i = 1;
    int c;
    int nargs = 0;
    FILE *fp;

    if(argc == 1)
    {
        printf("No input file to display");
        exit(1);
    }

    nargs = argc - 1;

    while(nargs > 0)
    {
        nargs--;
        printf("Displaying the content of file %s\n", argv[i]);

        fp = fopen(argv[i], "r");

        if(fp == NULL)
        {
            printf("Cannot open file %s\n", argv[i]);
            i++;
            continue;
        }

        c = getc(fp);
        while(c != EOF)
        {
            putchar(c);
            c = getc(fp);
        }

        fclose(fp);
        printf("\n\n************ End of file %s ************\n\n", argv[i]);
        i++;
    }

    return 0;
}

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Example 4: Simulation of Copy Command

(Copy contents from one file to another)

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

int main(int argc, char *argv[])
{
    FILE *in, *out;
    int c;

    if(argc != 3)
    {
        printf("Invalid number of arguments");
        exit(0);
    }

    in = fopen(argv[1], "r");
    if(in == NULL)
    {
        printf("Couldn't open the source file..");
        return 0;
    }

    out = fopen(argv[2], "w");
    if(out == NULL)
    {
        printf("Couldn't open the destination file..");
        return 0;
    }

    while(!feof(in))
    {
        c = fgetc(in);
        if(!feof(in))
            fputc(c, out);
    }

    fclose(in);
    fclose(out);

    return 0;
}