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:
time df |
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).
More precisely:
- 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
https://stackoverflow.com/questions/556405/what-do-real-user-and-sys-mean-in-the-output-of-time1
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