Does bash have a hook to determine when child shell exits?

Question

In bash I spin up a t/csh shell, which updates the tty input (stty erase ^H) in the .cshrc, but when exit-ing and returning to bash the backspace key no longer works. This is because bash uses readline and I need to reset the erase (stty erase ^?).

If I understand trap correctly, it seems like I could trap the t/csh exit from within the t/csh process, and run commands after it exits. However, my preference would be for the trapping to occur in bash, which ideally would detect a shell was called/exited and then run a script to reset its configurations.

This is preferred because I could call any number of shells (e.g., dash, zsh), which may inadvertently modify bash-specific settings. Instead of maintaining multiple trap commands for each shell, it seems like it would be best to have one for bash.

---

Is my understanding of trap correct? If so, is it possible for bash to detect a child shell exit and then run a script?

Answer 1

You were on the right track with trap. What you want is the following:

trap "stty erase ^?" SIGCHLD

You can add that to .bashrc. Note that this will run when any subprocess ends.

This will only work in interactive sessions. For non-interactive sessions, bash will not enable job control by default, so you will need to run set -o monitor first. But I doubt you'd need backspaces in non-interactive scripts.

SIGCHLD is sent to the parent process whenever a subprocess exits.

---

An alternative method is to wrap your other shell in a script, such as:

#!/bin/sh
tcsh
stty erase ^?

Then, if you launch your shell through the script, the script will run the stty erase command after the shell exits. This is less likely to have side effects than a global trap handler, but of course it'll only work if you launch through the script every time (or create an alias to do so, e.g. alias 'tcsh' '~/launch-tcsh.sh').