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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 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!