84

Many examples for trap use trap ... INT TERM EXIT for cleanup tasks. But is it really necessary to list all the three sigspecs?

The manual says:

If a SIGNAL_SPEC is EXIT (0) ARG is executed on exit from the shell.

which I believe applies whether the script finished normally or it finished because it received SIGINT or SIGTERM. An experiment also confirms my belief:

$ cat ./trap-exit
#!/bin/bash
trap 'echo TRAP' EXIT
sleep 3
$ ./trap-exit & sleep 1; kill -INT %1
[1] 759
TRAP
[1]+  Interrupt               ./trap-exit
$ ./trap-exit & sleep 1; kill -TERM %1
[1] 773
TRAP
[1]+  Terminated              ./trap-exit

Then why do so many examples list all of INT TERM EXIT? Or did I miss something and is there any case where a sole EXIT would miss?

1
  • 7
    Also keep in mind that with a spec like INT TERM EXIT the cleanup code is executed twice when SIGTERM or SIGINT is received. Oct 29, 2016 at 16:49

6 Answers 6

36

Yes, there is a difference.

This script will exit when you press Enter, or send it SIGINT or SIGTERM:

trap '' EXIT
echo ' --- press ENTER to close --- '
read response

This script will exit when you press Enter:

trap '' EXIT INT TERM
echo ' --- press ENTER to close --- '
read response

* Tested in sh, Bash, and Zsh.   (no longer works in sh when you add a command for trap to run)


There's also what @Shawn said: Ash and Dash don't trap signals with EXIT.

So, to handle signals robustly, it's best to avoid trapping EXIT altogether, and use something like this:

cleanup() {
    echo "Cleaning stuff up..."
    exit
}

trap cleanup INT TERM
echo ' --- press ENTER to close --- '
read var
cleanup
7
  • 1
    The solution with cleanup does the right thing - very elegant! It has become an idiom for my bash scripts with mktemp calls. Jan 6, 2016 at 13:20
  • 5
    This doesn't work if you have shellscript errors in your code that cause it to exit prematurely.
    – ijw
    Nov 10, 2016 at 3:30
  • 3
    @ijw: In Bash and Ksh, you can trap ERR to handle that, but it is not portable.
    – Zaz
    Feb 6, 2017 at 14:40
  • 6
    This solution isn't robust when another shell calls it. It doesn't handle wait on cooperative exit; you will want to trap - INT TERM; kill -2 $$ as the last line of cleanup, to tell the parent shell that it exited prematurely. If a parent shell foobar.sh calls your script (foo.sh), and then calls bar.sh, you don't want bar.sh to execute if INT/TERM is sent to your foo.sh. trap cleanup EXIT will handle this propagation automatically, so it is IMO the most robust. It also means you wouldn't have to call cleanup at the end of the script. Jul 20, 2018 at 20:47
  • 1
    Doing kill -2 $$ will propagate up the chain of non-interactive shells, killing each parent shell, and then stop at the first interactive shell, giving stdin back to the keyboard (Which is as excepted, ctrl+c should return to the interactive shell, not hang). Quite sad ash doesn't work with EXIT as bash makes it real easy (so nvm on EXIT being robust if you're talking compatibility), but the kill is important there. Jul 20, 2018 at 20:51
27

The POSIX spec doesn't say much about the conditions resulting in executing the EXIT trap, only about what its environment must look like when it is executed.

In Busybox's ash shell, your trap-exit test does not echo 'TRAP' before exiting due to either SIGINT or SIGTERM. I would suspect there are other shells in existance that may not work that way as well.

# /tmp/test.sh & sleep 1; kill -INT %1
# 
[1]+  Interrupt                  /tmp/test.sh
# 
# 
# /tmp/test.sh & sleep 1; kill -TERM %1
# 
[1]+  Terminated                 /tmp/test.sh
# 
3
  • 5
    dash also doesn't trap on just EXIT when it receives SIGINT/SIGTERM. Oct 29, 2016 at 16:50
  • 5
    zsh as well - thus, perhaps bash is the only shell where EXIT also does match signals. Oct 29, 2016 at 16:59
  • 3
    @maxschlepzig zsh doesn't trap on EXIT when it receives INT, but it does when it receives TERM. EDIT: I just noticed how old this was...
    – JoL
    May 15, 2018 at 15:57
13

Refining the last answer, because it has issues:

# Our general exit handler
cleanup() {
    err=$?
    echo "Cleaning stuff up..."
    trap '' EXIT INT TERM
    exit $err 
}
sig_cleanup() {
    trap '' EXIT # some shells will call EXIT after the INT handler
    false # sets $?
    cleanup
}
trap cleanup EXIT
trap sig_cleanup INT QUIT TERM

Points above:

INT and TERM handlers don't quit for me when I test - they handle the error then the shell returns to exiting (and this is not too surprising). So I ensure that the cleanup exits afterwards, and in the case of the signals always uses an error code (and in the other case of a normal exit, preserves the error code).

With bash, it seems that exiting in the INT handler also calls the EXIT handler, hence I untrap the exit handler and call it myself (which will work in any shell regardless of behaviour).

I trap exit because shell scripts can exit before they reach the bottom - syntax errors, set -e and a nonzero return, simply calling exit. You can't rely on a shellscript getting to the bottom.

SIGQUIT is Ctrl-\ if you've never tried it. Gets you a bonus coredump. So I think it's also worth trapping, even if it's a little obscure.

Past experience says if you (like me) always press Ctrl-C several times, you'll sometimes catch it half way through the cleanup part of your shell script, so this works but not always as perfectly as you'd like.

13
  • 2
    The caller would just get 1 as the exit code, no matter what signal caused the exit, while withour trap the caller would get 130 for SIGINT, 143 for SIGTERM, etc. So I would capture and pass the correct exit code as: sig_cleanup() { err=$?; trap '' EXIT; (exit $err); cleanup; }.
    – musiphil
    Nov 14, 2016 at 5:12
  • 2
    Can you clarify the purpose of trap '' EXIT INT TERM in the cleanup function? Is this to prevent accidental user interruption of cleanup that you mentioned in the last paragraph? Isn't the EXIT redundant?
    – Six
    Feb 3, 2017 at 8:33
  • 1
    With set -o errexit, calling false in sig_cleanup() impedes cleanup() from being called. In this case, trap cleanup EXIT seems sufficient.
    – Enno
    Jan 19, 2020 at 11:28
  • @musiphil - err=$? in sig_cleanup only gives 0 in dash, so apparently the original signal code is lost when trapping signals there, and that's why something like calling false is required. But as @Enno says, this would need a set +e in sig_cleanup to avoid exiting before cleanup is called.
    – mstorsjo
    Jan 29, 2020 at 8:52
  • @mstorsjo err=$? seems to work in dash 0.5.10.2-6; by which version did you try?
    – jarno
    Mar 24, 2020 at 13:42
2

This is how you can make the Bash script report its return code $?, while being able to catch the SIGINT and SIGTERM signals. I find this very useful for scripts running in a CI/CD pipeline:

notify() {
    [[ $1 = 0 ]] || echo ❌ EXIT $1
    # you can notify some external services here,
    # ie. Slack webhook, Github commit/PR etc.
}

trap '(exit 130)' INT
trap '(exit 143)' TERM
trap 'rc=$?; notify $rc; exit $rc' EXIT
1
  • Redirecting INT and TERM to the EXIT trap is an elegant solution, which prevents dual calls to the trap command. Thanks!
    – Noam Manos
    Jun 2, 2021 at 11:46
0

It depends on what you're trying to achieve, and which shells you're targeting. For bash it's probably okay to just use EXIT. But not all shells invoke the EXIT handler on SIGINT/SIGTERM.

For them you can try to set one handler for several signals (trap '...' INT EXIT), but then it may be invoked several times:

$ bash -c 'trap "echo trap" INT EXIT; sleep 3' & pid=$!; sleep 1; kill -INT $pid; wait
[1] 276923
trap
trap
[1]+  Done                    bash -c 'trap "echo trap" INT EXIT; sleep 3'

So either you write it with that in mind, or you can try to forward everything to the EXIT handler:

$ bash -c 'trap "exit 123" INT; trap "echo EXIT \$?" EXIT; sleep 3' & pid=$!; sleep 1; kill -INT $pid; wait
[1] 286229
EXIT 123
[1]+  Exit 123                bash -c 'trap "exit 123" INT; trap "echo EXIT \$?" EXIT; sleep 3'

But if you set up a handler for SIGINT, you generally want it to kill the script with SIGINT:

a.sh:

trap 'exit 123' INT
trap 'echo EXIT $?; trap - INT; kill -INT $$' EXIT
sleep 3
$ bash h.sh & pid=$!; sleep 1; kill -INT $pid; wait $pid
[1] 236263
EXIT 123
[1]+  Interrupt               bash h.sh

And under Debian < 10 (dash < 0.5.10) the signal that killed the script (if any) is not passed.

The solution I came up with:

set -eu
cleanup() {
    echo "cleanup ($1)"
    trap - INT TERM EXIT  # avoid reexecuting handlers
    if [ "$1" = 130 ]; then
        kill -INT $$
    elif [ "$1" = 143 ]; then
        kill -TERM $$
    else
        exit "$1"
    fi
}
trap 'cleanup 130' INT
trap 'cleanup 143' TERM
trap 'cleanup $?' EXIT

if [ "${1-}" = fail ]; then
    no-such-command
fi
sleep 3
$ bash f.sh; echo $?
cleanup (0)
0

$ bash f.sh fail; echo $?
f.sh: line 20: no-such-command: command not found
cleanup (127)
127

$ bash f.sh & pid=$!; sleep 1; kill -INT $pid; wait $pid
[1] 282422
cleanup (130)
[1]+  Interrupt               bash f.sh

$ bash f.sh & pid=$!; sleep 1; kill -TERM $pid; wait $pid
[1] 282458
cleanup (143)
[1]+  Terminated              bash f.sh

Tested in:

  • bash: 5.1.8
  • dash: 0.5.10, 0.5.8, 0.5.7
  • Alpine Linux 3.14 (busybox)
0

I found no 100% perfect answer, but @ijw answer and @musiphil comment were very close: "trap ... INT TERM EXIT" really necessary?

The problem with their answers:

  • if you press many times Ctrl^C, the cleanup function is repeated which is not what we would usually expect

Here's a proper code I tested under bash and POSIX sh

#!/bin/sh

# Script to test traps

# If interrupted:
# - SIGINT: Ctrl^C (kill -2), exit code 128+2=130
# - SIGTERM: kill -15, allows a graceful termination 
#            unlike kill -9 that cannot be trapped
#            exit code 128+15=143
# - SIGQUIT: Ctrl^\ or kill -3, like SIGINT but do a core dump
#            exit code 128+3=131
# - EXIT:    on any exit reason. On some shells, it is always called
#            even after SIGINT, SIGTERM...
#            this causes the clean function to be called twice,
#            unless we cancel the trap like in sig_cleanup()
#            we need to save the exit code when calling cleanup function
# If left to properly finish, simulate error exit 2 to test script exit code (Signal EXIT)

# Our test function to handle cleanup:
some_job() {
    echo "Working hard on some stuff..."
    for i in $(seq 1 5); do
        #printf "."
        printf '%s' "$i."
        sleep 1
    done

    echo ""
    echo "Job done, but we found some errors !"
    return 2 # to simulate script exit code 2
}

# Our clean temp files function
# - should not be interrupted
# - should not be called twice if interrupted
clean_tempfiles() {
    echo ""
    echo "Cleaning temp files, do not interrupt..."
    for i in $(seq 1 5); do
        printf "> "
        sleep 1
    done
    echo ""
}

# Called on signal EXIT, or indirectly on INT QUIT TERM
clean_exit() {
    # save the return code of the script
    err=$?

    # reset trap for all signals to not interrupt clean_tempfiles() on any next signal
    trap '' EXIT INT QUIT TERM

    clean_tempfiles
    exit $err # exit the script with saved $?
}

# Called on signals INT QUIT TERM
sig_cleanup() {
    # save error code (130 for SIGINT, 143 for SIGTERM, 131 for SIGQUIT)
    err=$?

    # some shells will call EXIT after the INT signal
    # causing EXIT trap to be executed, so we trap EXIT after INT
    trap '' EXIT 

    (exit $err) # execute in a subshell just to pass $? to clean_exit()
    clean_exit
}

trap clean_exit EXIT
trap sig_cleanup INT QUIT TERM

some_job
exit # remove to test curl interruption
curl -T test.sh "tftp://10.0.10.55"

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