Signal handling in shell background processes

This article is about my learning experience in signal handling and monitoring sub processes.

Yesterday I got puzzled when a supposedly simple test program did not act as expected.
The shell script trap.sh below sets a trap to catch SIGINT (or signal 2) and exit upon receiving it. It works as expected when run standalone but when invoked as a background process in another test script trapWrapper.sh it failed.

trap.sh	trapWrapper.sh
#!/bin/sh # Catch signal 2 trap "echo trapped 2;I=1" 2 # Wait until var 'I' is set to something while : ; do sleep 1; [ -n "$I" ] && break done echo DONE	#!/bin/sh # Run trap.sh in the background ./trap.sh 2>&1 & pid=$! # Sleep 5 seconds sleep 5 # ... and then kill the background process kill -2 $pid # Wait wait $pid # Exit with exit code of background process exit $?
	Running trapWrapper.sh will wait forever and never end. When killing it with Ctrl-C it will go away but the trap.sh process will be left behind and needs to be killed manually.

(the bigger idea behind all this is to have a monitoring script which starts a number of background processes and kills them after a certain timeout period has passed).

So what's the difference when run in background?

The sh man page has the answer:

man sh
...
  Signals
     The INTERRUPT and QUIT signals for an  invoked  command  are
     ignored if the command is followed by &. Otherwise, signals
     have the values inherited by the shell from its parent, with
     the  exception  of  signal 11 (but see also the trap command
     below).

i.e. SIGINT in a background process is ignored (as well as SIGQUIT).

SIGTERM is not mentioned here so the next idea is to enhance trap.sh and adding a signal handler for it and changing trapWrapper.sh so that it sends SIGTERM to the background process.

trap.sh	trapWrapper.sh
#!/bin/sh # Catch signal 2 (INT) and 15 (TERM) trap "echo trapped 2;I=1" 2 trap "echo trapped 15;I=1" 15 # Wait until var 'I' is set to something while : ; do sleep 1; [ -n "$I" ] && break done echo DONE	#!/bin/sh # Run trap.sh in the background ./trap.sh 2>&1 & pid=$! # Sleep 5 seconds sleep 5 # ... and then kill the background process kill -15 $pid # Wait wait $pid # Exit with exit code of background process exit $?
	after 5 seconds this will result in what we wanted: trapped 15 DONE

Something to remember: the supposedly stronger kill with SIGINT (and SIGQUIT would be the same) does not work due to the ignored signal whereas SIGTERM works fine.

So if you write a script which should act upon SIGINT or SIGQUIT let is also act upon SIGTERM, just to be safe.

Note: this might be different in other shells. When you test this interactively you'll see the difference:

sh	csh
$ trap.sh& 8379 $ ptree 8379 8360 sh 8379 /bin/sh ./trap.sh 8385 sleep 1 $ kill -2 8379 $ ptree 8379 8360 sh 8379 /bin/sh ./trap.sh 8785 sleep 1 $ kill 8379 $ trapped 15 DONE	% trap.sh& [1] 8116 % ptree 8116 33465 -csh 8116 /bin/sh trap.sh 8207 sleep 1 % kill -2 8116 % trapped 2 DONE
the background process ignores the signal	the background process in csh accepts SIGINT and exits

In case you've wondered about ptree: this was tested on a Solaris box.