Sending a signal to a parent process

Question

I am trying to implement a timeout for a big script which calls a lot of external process.

I start a separate process as a watchdog that checks every second if the timeout is reached

(

  PARENT_PID=$$

  ((t = 2 ))
  while ((t > 0)); do
    sleep 1
    ((t -= 1))
  done

  kill -s ALRM ${PARENT_PID} 2> /dev/null

) &

And then I trap the ALRM signal in the main script

trap critical ALRM

and finally in the critical function I print out some text and exit the program cleanly

Now the problem

• if the timeout occurs while $PARENT_PID is running everything works as expected
• if the timeout occurs while a child process is running (e.g., I call openssl and openssl is running) nothing happens. If I terminate the running child process (e.g. by pressing CRTL+C) the parent process trap is activated and everything continues as expected

How can I fix the problem? So that the trap in the parent process is called and I can terminate the whole?

Update

If the watchdog kills with -KILL the parent process, the process with all the child processed are terminated as expected

The problem is that I would like to intercept the timeout and generate some output. For this reason I use the ALRM signal.

The KILL signal cannot be ignored while the ALRM one can.

My idea at the moment is the following:

• the parent process has a trap for the alarm
• the child process ignores the alarm signal
• when the signal is sent to the parent the child process is running and the parent is in the background
• my process is waken up when the child is terminated

If this is correct my idea will never work.

To make it work I should for each child process

• start it in the background
• wait for it and continue

So that the parent process will be able to trap the signal

Am I correct?

Update 2

A reduced example

critical() {
    printf "whathever\n"
    WATCHDOG_PID=$!
    kill -TERM $WATCHDOG_PID
}

(

    PARENT_PID=$$

    ((t = "${TIMEOUT}" ))
    while ((t > 0)); do
        sleep 1
        ((t -= 1))
    done

    kill -s ALRM ${PARENT_PID} 2> /dev/null

) &

trap critical ALRM

# an example of a long running command
(echo 'Q' | openssl s_client -connect corti.li:imap -servername corti.li -verify 6 ) &
wait

WATCHDOG_PID=$!
kill -TERM $WATCHDOG_PID

If the ALRM signal is sent while openssl is running nothing happens. Even if the openssl is in the background the trap is never called

Answer 1

A full answer needs the code from your parent, but lacking that, here is a hint of what is probably happening:

Open Group Base Specifications Issue 6 - Section 2.11

When a signal for which a trap has been set is received while the shell is waiting for the completion of a utility executing a foreground command, the trap associated with that signal shall not be executed until after the foreground command has completed.

When the shell is waiting, by means of the wait utility, for asynchronous commands to complete, the reception of a signal for which a trap has been set shall cause the wait utility to return immediately with an exit status >128, immediately after which the trap associated with that signal shall be taken.

(formatting mine)

What that means is that if your parent is waiting with anything but the wait command, signal handling is delayed until the end of whatever it is doing.

It's up to you to make your parent wait with wait, so it can receive your SIGALRM right away.