Is it necessary to specify traps other than EXIT?

bash shell ksh zsh sh
William Pursell picture William Pursell · Nov 14, 2011 · Viewed 18.4k times · Source

I see a lot of shell scripts that do:

trap cmd 0 1 2 3 13 15 # EXIT HUP INT QUIT PIPE TERM

In every shell I have access to at the moment, all the traps other than 0 are redundant, and cmd will be executed upon receipt of a signal if the trap is simply specified:

trap cmd 0

Is the latter specification sufficient, or do some shells require the other signals to be specified?

Answer

carlo picture carlo · Feb 11, 2013

To make sure the EXIT signal handler will not be executed twice (which is almost always not what you want) it should always set to be ignored or reset within the definition of the EXIT signal handler itself.

The same goes for signals that have more than one signal handler defined for them in a program.

# reset
trap 'excode=$?; cmd; trap - EXIT; echo $excode' EXIT HUP INT QUIT PIPE TERM

# ignore
trap 'excode=$?; trap "" EXIT; cmd; echo $excode' EXIT HUP INT QUIT PIPE TERM

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