#include <stdio.h>

int main(int argc, char *argv[])
{
	int a,b, result;
	char opr;
	
	if(argc!=4)
	{
		printf("Invalid arguments...\n");
		return -1;
	}
	
	//get values
	a = atoi(argv[1]);
	b = atoi(argv[3]);
	
	//get operator
	opr=argv[2][0];
	
	//calculate according to operator
	switch(opr)
	{
		case '+':
			result=a+b;
			break;
		case '-':
			result=a-b;
			break;
		case '*':
			result=a*b;
			break;
		default:
			result=0;
			break;
	}
	
	if(opr=='+' || opr=='-' || opr=='*')
		printf("Result: %d %c %d = %d\n",a,opr,b,result);
	else
		printf("Undefined Operator...\n");
	
	return 0;
}

Output

First run:
sh-4.2$ ./main 10 + 20
Result: 10 + 20 = 30

Second run:
sh-4.2$ ./main 10 - 20
Result: 10 - 20 = -10 

Third run:
sh-4.2$ ./main 10 / 20
Undefined Operator... 

What is atoi()?

atoi() is a library function that converts string to integer, when program gets the input from command line, string values transfer in the program, we have to convert them to integers (in this program). atoi() is used to return the integer of the string arguments.