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I need to run tail -f against a log file, but only for specific amount of time, for example 20 seconds, and then exit. What would be an elegant way to achieve this?

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55

With GNU timeout:

timeout 20 tail -f /path/to/file
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  • If the timeout is larger than typical file rotation you might also want to use the follow=name (-F) option of GNU Tail. – eckes Sep 20 '18 at 11:28
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For completeness sake, without timeout, you can do this:

#!/bin/sh
tail -f /var/log/syslog &
me=$!
trap "kill $me" INT TERM HUP QUIT EXIT
sleep 20

The trap line ensures that when the script or parent shell is terminated (we reach end of script (EXIT), Ctrl-C (INT), sending a SIGTERM via kill, logging out of the shell (HUP), etc) then the tail is killed.

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  • I'd need to test it, but I'm pretty sure the last line is not necessary as the shell should trap when exiting? – Olivier Dulac Sep 18 '18 at 14:07
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    Or you could trap EXIT along with the others. – user Sep 18 '18 at 15:02
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    @OlivierDulac Only if he trapped EXIT, I believe. – JoL Sep 18 '18 at 15:26
  • Good points all. On testing, a final kill is needed else tail is orphaned off on script exit. Additionally I've modified the script to make trap catch exit and remove the last kill, which looks a more elegant solution. A further shortening could be done by removing the me=$! line and referencing $! in the trap kill directly, but I leave this as an exercise for the consumer :-) – Chris J Sep 19 '18 at 7:35

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