Bash – exit trap in dash vs ksh and bash

bashdashkshtrap:

Here's a simple script that sets up a temp dir in the current dir and a trap to delete it on exit.

#filename: script   
set -x  
trap 'rm -rf "$d"' exit
d=`TMPDIR=$PWD mktemp -d`
"$@"

If I do ksh script sleep 100 or bash script sleep 100 and interrupt it with, C-C, the trap gets executed and the directory is deleted.
It doesn't work with dash. Why? Is this a bug or intended behavior?

Best Answer

zsh, pdksh (though not recent versions of mksh derived from that), yash, the Bourne shell behave like dash.

Only bash, ksh88, ksh93 and mksh behave otherwise.

The POSIX spec is not clear on what should be the correct behaviour, but there's nothing in there that says that the shell is allowed to override the default handler for the SIGINT (or other) signal.

It says EXIT trap action should be evaluated when exit is invoked, but AFAICT, it doesn't even say for instance if it should be evaluated when the shell exits as the result of set -e or set -u or error conditions like syntax errors or failing special builtins.

To be able to run EXIT trap upon reception of a signal, the shell would need to install a handler on that signal.

That's what ksh, mksh and bash do, but the list of signals they handle is different between all three implementations. The only signals common between all 3 seem to be INT, QUIT, TERM, ALRM and HUP.

If you want the EXIT trap to be run upon some signals, the portable way would be to handle those signals yourself:

trap 'exit 1' INT HUP QUIT TERM ALRM USR1
trap 'cleanup' EXIT

That approach however doesn't work with zsh, which doesn't run EXIT trap if exit is called from a trap handler.

It also fails to report your death-by-signal to your parent.

So instead, you could do:

for sig in INT QUIT HUP TERM ALRM USR1; do
  trap "
    cleanup
    trap - $sig EXIT
    kill -s $sig "'"$$"' "$sig"
done
trap cleanup EXIT

Now, beware though that if more signals arrive while you're executing cleanup, cleanup may be run again. You may want to make sure your cleanup works correctly if invoked several times and/or ignore signals during its execution.

Related Solutions

Bash – How to delete a directory automatically when an executable is killed

Your last example is the most fail safe.

trap 'rm -rf "$dir"' EXIT

This will execute as long as the shell itself is still functional. Basically SIGKILL is the only thing that it won't handle since the shell is forcibly terminated.
(perhaps SIGSEGV too, didn't try, but it can be caught)

If you don't leave it up to the shell to clean up after itself, the only other possible alternative is to have the kernel do it. This is not normally a kernel feature, however there is one trick you can do, but it has it's own issues:

#!/bin/bash
mkdir /tmp/$$
mount -t tmpfs none /tmp/$$
cd /tmp/$$
umount -l /tmp/$$
rmdir /tmp/$$

do_stuff

Basically you create a tmpfs mount, and then lazy unmount it. Once the script is done it'll be removed.
The downside other than being overly complex, is that if the script dies for any reason before the unmount, you've not got a mount laying around.
This also uses tmpfs, which will consume memory. But you could make the process more complex and use a loop filesystem, and remove the file backing it after it's mounted.

Ultimately the trap is best as far as simplicity and safety, and unless you're script is regularly getting SIGKILLed, I'd stick with it.

Bash – Difference between ‘trap -⁠- EXIT’ and ‘trap – EXIT’

This is explained some in the trap(1P) man page and more in the Advanced Bash Scripting Guide, which explains that the -- is a Bash built-in, used to denote the "end of options" for a command. Since scripts using sh are designed to be portable between other systems with it, bash behaves as if it were executed with --posix, which changes some shell behavior to match the POSIX specification.

When a signal is "trapped", (e.g., EXIT), it binds the first argument after the command to the signal, eval-ing it when the signal is issued, and ignoring its normal behavior (except for EXIT). So when trap 'echo something' EXIT is run, then exit, the shell will eval 'echo something' prior to exiting. This would also work with a different signal, such as TERM, which could be bound to, for example, a graceful exit function in a script. When trap -- EXIT is run, the -- is interpreted as an "end of arguments", indicating to trap that the signal is to be bound to null ('') (since there were no flags prior to or after --), indicating to the shell to ignore the signal (however, this doesn't work with EXIT, but works with other signals). Running trap -- 'echo something' EXIT, however, will still eval 'echo something' and exit upon exit. As per the specification of trap, the command - by itself indicates that the shell will reset any traps for the signals specified, which is why it works in both sh and Bash.

Best Answer

Related Solutions

Bash – How to delete a directory automatically when an executable is killed

Bash – Difference between ‘trap -⁠- EXIT’ and ‘trap – EXIT’

Related Question