How to find time taken by a command/program on Linux Shell?

We have already discussed a way to find time taken by a function through C libraries. If we are on Linux, then it becomes very easy to find time taken by a program/command.

We can use time command for this purpose. The time taken is shown in three forms.
real: Total end to end time taken by program/command
user: Time taken in user mode.
sys: Time taken in kernel mode

A Command Example (Time taken by ls-l):

$ time ls -l

The above command runs "ls -l" and shows 
contents of current directory followed by
the time taken by command "ls -l".  

A program example (Time taken by fib(30)):
let us consider below program.

#include<stdio.h>
int fib(int n)
{
   if (n <= 1)
      return n;
   return fib(n-1) + fib(n-2);
}

int main ()
{
  printf("Fibonacci Number is %d", fib(30));
  return 0;
}

Let we save above program as fib.c.

// Compiling above program on shell
~$ gcc fib.c

// Running the generated executable with time
~$ time ./a.out
Fibonacci Number is 832040
real    0m0.017s
user    0m0.017s
sys    0m0.000s

Note: 0.017 seconds (shown with real) is total 
time taken by program.

This article is contributed Dheeraj Gupta.