It proves sometimes useful to know the execution time of a task. The time command makes it possible to determine the duration of a process, it uses the clock system, so it depends on your processor and on its request: in other words, if you watch a movie and you compile a C++ program, the time compilation will last longer that if you compile it without watching the movie.

To time a task, we use the time command plus the task. Here, we want to list (ls -al) the whole current directory recursively(R), i.e. we also consider subdirectories.

nadir@ipowerht:~ $ time ls -aRl

real    0m1.541s
user    0m0.047s
sys     0m0.046s
  • real corresponds to the real time of the task. .
  • user corresponds to the user time, i.e. the CPU time used by the user’s program
  • sys defines time system, that corresponds to the time used by the system to manage the execution of the task.

Consequently, we easily determine time CPU of the task, I recall you that time CPU is defines as follows: CPU = user time + sys time. Then do not waste time, and test this command!