Under Linux, the time command is quite convenient to get the elapsed time taken by a command call. It is very simple to use: just type your command preceded by the time command itself. For instance:
The output looks like
real 0m3.905s user 0m2.408s sys 0m1.238s
In brief, Real refers to actual elapsed time including other processes that may be running at the same time; User and Sys refer to CPU time used only by the process (here the df command).
- Real is wall clock time – time from start to finish of the call including time used by other processes and time the process spends blocked (for example if it is waiting for I/O to complete).
User is the actual CPU time used in executing the process. Other processes and time the process spends blocked do not count.
- Sys is the amount of CPU time spent in the kernel within the process.
So, User + Sys is the actual CPU time used by your process
For more details, you can consult this quite precise description