User1 User1 - 3 months ago 27
Linux Question

Bash: a sleep in a while loop gets its own pid

I have a bash script that does some parallel processing in a loop. I don't want the parallel process to spike the CPU, so I use a sleep command. Here's a simplified version.


(while true;do sleep 99999;done)&


So I execute the above line from a bash prompt and get something like:
[1] 12345


Where
[1]
is the job number and
12345
is the process ID (pid) of the while loop. I do a
kill 12345
and get:


[1]+ Terminated ( while true; do
sleep 99999;
done )


It looks like the entire script was terminated. However, I do a
ps aux|grep sleep
and find the sleep command is still going strong but with its own pid! I can kill the
sleep
and everything seems fine. However, if I were to kill the sleep first, the while loop starts a new
sleep
pid. This is such a surprise to me since the sleep is not parallel to the while loop. The loop itself is a single path of execution.

So I have two questions:


  1. Why did the sleep command get its own process ID?

  2. How do I easily kill the while loop and the sleep?


Answer
  1. Sleep gets its own PID because it is a process running and just waiting. Try which sleep to see where it is.
  2. You can use ps -uf to see the process tree on your system. From there you can determine what the PPID (parent PID) of the shell (the one running the loop) of the sleep is.
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