SIGCHLD not delivered when a process exits

Question

I need to detect when one of my background processes exits. Hence, I installed a trap. run_gui and run_ai1 are simple exec functions.

run_gui & gui_pid=$!
run_ai1 & ai1_pid=$!
trap 'echo foo' SIGCHLD
while true; do
    echo "Started the loop"
    while true; do
        read -u $ai1_outfd line || echo "Nothing read"
        if [[ $line ]]; then
            : # Handle this
        fi
    done
    while true; do
        read -u $gui_outfd line || echo "nothing read"
        if [[ $line ]]; then
            : # Handle this
        fi
    done
done

When I close the GUI, nothing happens. The echo foo command is executed only if I press ctrl+c.

Why do I miss the SIGCHLD?

This doesn't change anything. Does it matter that the `read` is waiting for some input...? — marmistrz, May 31 '16 at 18:20

Cyrus · Accepted Answer · 2016-05-31T18:41:31.257

5

Non-interactive shells don't enable job control by default. See if putting 'set -o monitor' at the beginning of the script produces the results you want.

Source: Chet Ramey, GNU Bash's maintainer

edited May 31 '16 at 18:41

answered May 31 '16 at 18:36

Cyrus

84,225
14
89
153

My shabang is `#!/bin/bash -m`. So this shouldn't be the case – marmistrz May 31 '16 at 19:39
I suggest to test it. – Cyrus May 31 '16 at 19:54
The result is exactly the same. – marmistrz May 31 '16 at 20:20
This simple example works with GNU bash 4.2.8: `set -o monitor; trap 'echo hello' SIGCHLD; sleep 3 &` – Cyrus May 31 '16 at 20:30
After commenting out the `read` line, the problems seems gone away. It seems that `read` is blocking everything, the trap included. I read from a fd initialized as read-write in the main script and given to the `run_gui` internals – marmistrz May 31 '16 at 20:36

SIGCHLD not delivered when a process exits

1 Answers1