Cannot rewrite “trap command” for SIGTSTP, SIGTTIN and SIGTTOU

Question

I want to trap the SIGTSTP signal, but I'm not able to. When I call just trap command without arguments, I'm getting this:

trap -- '' SIGTSTP
trap -- '' SIGTTIN
trap -- '' SIGTTOU

Obviously, my trap has some already pre-defined behaviour. The problem is, I'm not able to rewrite it, even when I'm logged as a root. When I use the trap command, it's simply ignored for these three signals. Otherwise it works normally.

I tried to rewrite it just by command, like that:

trap -- 'echo SIGTSTP' SIGTSTP

The other strange thing is, I'm able to use this signals and they're working normally, they're not ignored. I can simply use SIGTSTP to stop working some other process and make it continue with SIGCONT again.

So, do you know, how to rewrite those commands? And could you, please, explain this strange behaviour a little?

---

My OS is Lubuntu 15.04 and I'm using GNU bash, version 4.3.30(1)-release (x86_64-pc-linux-gnu).

Answer 1

With bash on Ubuntu 15.04, it seems to work as expected... how do you write your script?

#!/bin/bash
set -e
trap 'echo TSTP' TSTP
trap 'echo TTIN' TTIN
trap 'echo TTOU' TTOU
trap
while :; do read a; done # wait forever

Sample session (typed 3 times Ctrl+Z then Ctrl+C):

$ ./signal.sh
trap -- 'echo TSTP' SIGTSTP
trap -- 'echo TTIN' SIGTTIN
trap -- 'echo TTOU' SIGTTOU
^ZTSTP
^ZTSTP
^ZTSTP
^C

---

I found in the manual that bash ignores those signals when it's interactive and job control is enabled:

2. Job Control (see Job Control) is enabled by default. When job control is in effect, Bash ignores the keyboard-generated job control signals SIGTTIN, SIGTTOU, and SIGTSTP.

You can disable job cotnrol by set +m. However, bash won't allow to update handlers in interactive mode anyway:

$ bash -c trap
$ bash -c 'trap true TSTP && trap'
trap -- 'true' SIGTSTP
$ bash -i -c trap
trap -- '' SIGTSTP
trap -- '' SIGTTIN
trap -- '' SIGTTOU
$ bash -i -c 'trap true TSTP && trap'
trap -- '' SIGTSTP
trap -- '' SIGTTIN
trap -- '' SIGTTOU
$ bash -i -c 'set +m && trap true TSTP && trap'
trap -- '' SIGTSTP
trap -- '' SIGTTIN
trap -- '' SIGTTOU

Answer 2

You did not mention which shell you are using, so in case this is a bug in the shell, there is no way to help you.

Looking at the Bourne Shell man page: http://schillix.sourceforge.net/man/man1/bosh.1.html you see the following text:

     bolic  names. Any attempt to set a trap on a signal that
     was ignored on entry to the current  shell  is  ineffec-
     tive.  An  attempt  to  trap on signal 11 (memory fault)
     produces an error.code here

So everything may be OK but your shell was called with these signals ignored.

If you are on a modern OS, you may call

 `psig $$`

to get the signal disposition of your current shell and you may replace $$ by other process IDs to check other processes.