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. These arguments provide additional flexibility and let users control the program's behavior without changing the code.

In C, command-line arguments are handled through the main function, which takes two parameters:

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

Here’s what these parameters mean:

1. argc (Argument Count): This integer represents the number of command-line arguments, including the program name.
2. argv (Argument Vector): This array of strings (char *argv[]) holds each command-line argument as a string. argv[0] is always the program's name, and the remaining elements are the actual arguments passed by the user.

Example: Printing Command-Line Arguments

Let’s start with a basic example where we simply print all 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;

}

Explanation:

1. Printing Argument Count: argc gives the count of arguments, which we print.
2. Printing Each Argument: We loop through argv, printing each argument. argv[0] is the program's name, and argv[1], argv[2], etc., are the arguments provided by the user.

Running the Program

If we compile this program as example, we could run it with arguments like this:

./example hello world 123

Expected Output:

Number of arguments: 4

Argument 0: ./example

Argument 1: hello

Argument 2: world

Argument 3: 123

Important Points

1. Data Type: Command-line arguments are passed as strings, so you may need to convert them (e.g., atoi, atof).
2. Error Handling: Always check if the right number of arguments is provided to avoid errors.
3. Applications: Command-line arguments are useful for tasks like specifying filenames, setting configuration options, or providing user inputs.

Command-line arguments give users control and make programs more versatile.

Sample Program to add two numbers passed as 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]));

  }

}

Note:if the default executable is used then use ./a.out 2 3 for adding two numbers

The function atoi() will convert string to integer

Program to display contents of several  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”,argv[i]);

p=fopen(argv[i],”r”);

if(fp==NULL)

{

printf(“cannot open file %s”,argv[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;

}

Simulation of copy command where the two file names are passed as arguments

#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;

}