SIGKILL are two signals that cannot be caught and handled by a process.
SIGTSTP is like
SIGSTOP except that it can be caught and handled.
SIGTSTP signals stop a process in its tracks, ready for
SIGCONT. When you send that process a
SIGTERM, the process isn't running and so it cannot run the code to exit.
(There are also
SIGTTOU, which are signals generated by the TTY layer when a backgrounded job tries to read or write to the terminal. They can be caught but will otherwise stop (suspend) the process, just like
SIGTSTP. But I'm now going to ignore those two for the remainder of this answer.)
Your CtrlZ sends the process a
SIGTSTP, which appears not to be handled specially in any way by
rsyslogd, so it simply suspends the process pending
The solution here is also to send
SIGCONT after your
SIGTERM so that the process can receive and handle the signal.
sleep 999 &
# Assume we got PID 456 for this process
kill -TSTP 456 # Suspend the process (nicely)
kill -TERM 456 # Terminate the process (nicely). Nothing happens
kill -CONT 456 # Continue the process so it can exit cleanly
The documentation for the GNU C Library explains this quite well, I think (my highlighting):
While a process is stopped, no more signals can be delivered to it until it is continued, except
SIGKILL signals and (obviously)
SIGCONT signals. The signals are marked as pending, but not delivered until the process is continued. The
SIGKILL signal always causes termination of the process and can’t be blocked, handled or ignored. You can ignore
SIGCONT, but it always causes the process to be continued anyway if it is stopped. Sending a
SIGCONT signal to a process causes any pending stop signals for that process to be discarded. Likewise, any pending
SIGCONT signals for a process are discarded when it receives a stop signal