Comment by patrec | Hacker News Reader

Comment by patrec | Hacker News Reader

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> Using `shellcheck --shell sh` is a good start :) > If adhering to POSIX it should run the same in bash, ash and dash.

Not so. Shellcheck will tell you about syntactically invalid bashisms, but will not warn you about the zillions of things that are under or unspecified in posix. Pertinent minimal example:

   $ echo $'#!/bin/sh\ntrap "echo exiting" EXIT\nsleep 1h' > exit.sh 
   $ shellcheck --shell sh exit.sh && echo "ALL GOOD!"
   $ /bin/dash exit.sh
   ^C
   $ /usr/bin/zsh exit.sh
   ^C
   $ /bin/bash exit.sh
   ^Cexiting

Good luck finding a person who can use trap in a way that actually works as intended across different posix conform shells without consulting stack overflow first.

Indeed, it's such a messy job coding for shell..

Comes with a lot trial and error.

The above will work as (tested in ash/dash/bash):

  #!/usr/bin/env sh
  trap "echo exiting" INT
  sleep 1h

But to your point, yes, how to actually know that without doing trial n' error & stackoverflow.

Thanks for the `sleep 1h`, didn't know that you could do hour as unit :)

(edited: turned out INT is enough to trap)

Nope, INT is (generally) too specific :)

Sometimes of course, you just want to run something on SIGINT and not on normal exit, SIGHUP or whatever. But maybe the main use case for trap is implementing try/finally logic and you really want it to fire whenever the shell exits, not matter how (to clean up resources like temporary files). Bash's `trap EXIT` mostly does what you want ("run this when the shell exits"), but writing something that behaves this way across shells (and doesn't fire multiple times etc). is amazingly painful.

Ok, I though it was to catch CTRL-c :)

trap EXIT, does work for me in ash/dash/bash, though, when just letting the code flow exit with and without errors.

Not sure your use case, but I can for sure concur that traps are very painful to get right, especially when getting into child/grandchild processes land, taking terminal session, etc in regard.

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