When running a C program, there is usually a way to pass command-line arguments or parameters to the program. When main function is invoked, it is called with two arguments.

• argument count: The first argument is the number if command-line arguments the program was invoked with.
• argument vector: The second argument is a pointer to an array of character strings that contain the actual arguments passed at command-line.

Let’s look at the following program that implements the unix command-line utility echo, which simply prints its argument to the screen. Example:

echo Hello World!

prints

Hello World!

Here is the implementation:

/* echo.c */
#include <stdio.h>

int main(int argc, char *argv[])
{
  int i;
  for (i = 1; i < argc; i++)
    printf("%s%s", argv[i], i != argc - 1 ? " " : "");
  printf("\n");
  return 0;
}

To run this code:

cc echo.c
./a.out Hello World!

Output:

Hello World!