Signal trap from background job without pressing enter

Question

In Bash 5 I ran into a situation where I want to do the following:

trap 'echo trapped!' USR1
while true; do kill -SIGUSR1 $$; sleep 1; done &

But I have to press enter on keyboard every time the signal is sent to make the trap code continue.

Actually I expect the trap to be executed without any keyboard interaction.

EDIT:

Finally I got it working like this:

trap 'echo trapped!' USR1
while true; do kill -USR1 $$; kill -INT $$; sleep 1; done &

But I consider the solution as workaround and not as answer.

Answer 1

Someone seems to have a very similar problem here.
Long story short if it's OK to execute the command from the trap built-in in a subprocess ('echo trapped!' in this case). You can put your commands in a file (for example a.sh):

trap 'echo trapped!' USR1
while true; do kill -USR1 $$; sleep 1; done

and execute the file as a background process with bash a.sh &
If it's not okay to do it in a subprocess there is no good solution since the interactive shell does a blocking read operation while waiting for the user to enter a command and it doesn't handle most signals while that is happening (as you have noticed it does handle SIGINT). The solution above works since the trap is in a non-interactive shell (thus it is not in the middle of a blocking read when it gets the signal).