Command Line Arguments in C

The most important function of C is the main() function. It is mostly defined with a return type of int and without parameters.

int main() { 
    ... 
}

We can also give command-line arguments in C. Command-line arguments are the values given after the name of the program in the command-line shell of Operating Systems. Command-line arguments are handled by the main() function of a C program.

To pass command-line arguments, we typically define main() with two arguments: the first argument is the number of command-line arguments and the second is a list of command-line arguments.

Syntax

int main(int argc, char *argv[]) { /* ... */ }
            or
int main(int argc, char **argv) { /* ... */ }

Here,

• argc (ARGument Count) is an integer variable that stores the number of command-line arguments passed by the user including the name of the program. So if we pass a value to a program, the value of argc would be 2 (one for argument and one for program name)
• The value of argc should be non-negative.
• argv (ARGument Vector) is an array of character pointers listing all the arguments.
• If argc is greater than zero, the array elements from argv[0] to argv[argc-1] will contain pointers to strings.
• argv[0] is the name of the program , After that till argv[argc-1] every element is command -line arguments.

For better understanding run this code on your Linux machine.

Example

The below example illustrates the printing of command line arguments.

// C program named mainreturn.c to demonstrate the working
// of command line arguement
#include <stdio.h>

// defining main with arguments
int main(int argc, char* argv[])
{
    printf("You have entered %d arguments:\n", argc);

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

Output

You have entered 4 arguments:
./main
geeks
for
geeks

for Terminal Input

$ g++ mainreturn.cpp -o main $ ./main geeks for geeks

Note: Other platform-dependent formats are also allowed by the C standards; for example, Unix (though not POSIX.1) and Microsoft Visual C++ have a third argument giving the program’s environment, otherwise accessible through getenv in stdlib.h. Refer C program to print environment variables for details.

Properties of Command Line Arguments in C

1. They are passed to the main() function.
2. They are parameters/arguments supplied to the program when it is invoked.
3. They are used to control programs from outside instead of hard coding those values inside the code.
4. argv[argc] is a NULL pointer.
5. argv[0] holds the name of the program.
6. argv[1] points to the first command line argument and argv[argc-1] points to the last argument.

Note: You pass all the command line arguments separated by a space, but if the argument itself has a space, then you can pass such arguments by putting them inside double quotes "" or single quotes ''. 

Example

The below program demonstrates the working of command line arguments.

// C program to illustrate
// command line arguments
#include <stdio.h>

int main(int argc, char* argv[])
{
    printf("Program name is: %s", argv[0]);

    if (argc == 1)
        printf("\nNo Extra Command Line Argument Passed "
               "Other Than Program Name");

    if (argc >= 2) {
        printf("\nNumber Of Arguments Passed: %d", argc);
        printf("\n----Following Are The Command Line "
               "Arguments Passed----");
        for (int i = 0; i < argc; i++)
            printf("\nargv[%d]: %s", i, argv[i]);
    }
    return 0;
}

Output in different scenarios:

1. Without argument: When the above code is compiled and executed without passing any argument, it produces the following output.

Terminal Input

$ ./a.out

Output

Program Name Is: ./a.out
No Extra Command Line Argument Passed Other Than Program Name

2. Three arguments: When the above code is compiled and executed with three arguments, it produces the following output.

Terminal Input

$ ./a.out First Second Third

Output

Program Name Is: ./a.out
Number Of Arguments Passed: 4
----Following Are The Command Line Arguments Passed----
argv[0]: ./a.out
argv[1]: First
argv[2]: Second
argv[3]: Third

3. Single Argument: When the above code is compiled and executed with a single argument separated by space but inside double quotes, it produces the following output.

Terminal Input

$ ./a.out "First Second Third"

Output

Program Name Is: ./a.out
Number Of Arguments Passed: 2
----Following Are The Command Line Arguments Passed----
argv[0]: ./a.out
argv[1]: First Second Third

4. A single argument in quotes separated by space: When the above code is compiled and executed with a single argument separated by space but inside single quotes, it produces the following output.

Terminal Input

$ ./a.out 'First Second Third'

Output

Program Name Is: ./a.out
Number Of Arguments Passed: 2
----Following Are The Command Line Arguments Passed----
argv[0]: ./a.out
argv[1]: First Second Third