Here's a simple program that registers two trap handlers and then displays them with trap -p. Then it does the same thing, but in a child background process.
Why does the background process ignore the SIGINT trap?
#!/bin/bash
echo "Traps on startup:"
trap -p
echo ""
trap 'echo "Received INT"' INT
trap 'echo "Received TERM"' TERM
echo "Traps set on parent:"
trap -p
echo ""
(
echo "Child traps on startup:"
trap -p
echo ""
trap 'echo "Child received INT"' INT
trap 'echo "Child received TERM"' TERM
echo "Traps set on child:"
trap -p
echo ""
) &
child_pid=$!
wait $child_pid
Output:
$ ./show-traps.sh
Traps on startup:
Traps set on parent:
trap -- 'echo "Received INT"' SIGINT
trap -- 'echo "Received TERM"' SIGTERM
Child traps on startup:
Traps set on child:
trap -- 'echo "Child received TERM"' SIGTERM
SIGINT and SIGQUIT are ignored in backgrounded processes (unless they're backgrounded with set -m on). It's a (weird) POSIX requirement (see
2. Shell Command Language or my SO question Why do shells ignore SIGINT and SIGQUIT in backgrounded processes? for more details).
Additionally, POSIX requires that:
When a subshell is entered, traps that are not being ignored shall be set to the default actions, except in the case of a command substitution containing only a single trap command ..
However, even if you set the INT handler in the subshell again after it was reset, the subshell won't be able to receive it because it's ignored (you can try it or you can inspect the signal ignore mask using ps, for example).