Question

I would like to do the following:

I want to link a process A to a file F, so:

• If F dissapears A crashes.
• F will only dissapear when A finishes.

Is this possible? Thank you very much.

Answer 1

You should not avoid PIDs. They are process identifiers, and meant to be used.

Bash automatically monitors child processes it starts. The most recent background process id is maintained in $!. Bash also supports job controls using ‘%n’ syntax.

You can trap child procs status changes with trap SIGCHLD, and you can “wait” for one or all child processes to complete with the wait command.

Here is a rough approximation of your two process monitoring, which consists of “job1” and “job2” being started the the sample script:

job1 &    # start job1 in background
j1pid=$!  # get its process id
job2 &    # start job2 in background
j2pid=$1  # get its process id

trap 'err=1' ERR  # trap all errors
err=
wait $j1pid  # wait for job1 to complete

# at this point job1 could have completed normally,
# or either process could have had an error

trap - ERR   # revert to "normal" handling of most errors

# kill the processes nicely, or abruptly
# kill -TERM sends the TERM signal to the process, which it can trap
# and do whatever pre-exit process is needed.
# kill -9 cannot be trapped.

for pid in $j1pid $j2pid ; do
  kill -TERM $pid 2>/dev/null || kill -9 $pid
done