If I do:

sleep 1 


sleep 1 & wait $!

will there be any difference in terms of CPU usage for spawning a foreground process versus a background process? Or will the performance of both lines be exactly equal?

2 Answers 2


With both commands, the shell will 1/ create a parallel subprocess which executes sleep, 2/tell the OS… put the shell process on pause until the sleep process ends. Then the performances are equal.



On my system the first takes 3.0 ms and the second takes 3.3 ms CPU time.

In practice I would never worry about the 0.3 ms CPU time if you are sleeping a full second.

The 0.3 ms is probably caused by the extra fork that is needed to put sleep in the background. In other words: It is a one-time cost, not a 10% extra cost for running jobs in the background.

  • 1
    I added that sleep command just as an example for the script, I ment the question more in general. Is your 0.3 ms difference caused purely by the extra wait command, or are there other reasons?
    – Maestro
    Apr 19, 2023 at 21:44
  • Can you explain the extra fork. I don't think that is obvious. Can you also describe how you measured the difference? Apr 20, 2023 at 7:10

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