I am unable to trap a signal when running in a child / background process.
Here is my simple bash script :
#!/bin/bash
echo "in child"
trap "got_signal" SIGINT
function got_signal {
echo "trapped"
exit 0
}
while [ true ]; do
sleep 2
done
When running this and later do
kill -SIGINT (pid)
Everything works as expected, it prints 'trapped' and exits.
Now, if I start the same script from a parent script like this :
#!/bin/bash
echo "starting the child"
./child.sh &
Then the child does not trap the signal anymore.... ?
After changing to use SIGTERM instead of SIGINT, it seems to be working correctly... ?
The bash
manpage on OSX (but it should be the same in other versions) has this to say about signal handling:
Non-builtin commands run by bash
have signal handlers set to the values
inherited by the shell from its parent. When job control is not in
effect, asynchronous commands ignore SIGINT
and SIGQUIT
in addition to
these inherited handlers.
and further on, under the trap
command:
Signals ignored upon entry to the shell cannot
be trapped or reset.
Since scripts don't use job control by default, this means the case you're talking about.
Per your note:
Signals ignored upon entry to the shell cannot be trapped or reset.
I have noticed that ZSH does not ignore the signals sent back and forth between parent and child process, but bash does. Here's the question I posted myself:
Trapping CHLD signal - ZSH works but ksh/bash/sh don't?