Lesson 9.3: Signals: kill, killall and pkill

What a signal is, the important ones (SIGTERM, SIGKILL, SIGINT, SIGHUP, SIGSTOP, SIGCONT) and their numbers, ending a process politely with kill and by force with kill -9, why some processes ignore SIGTERM, permissions when signalling other users' processes, killing by name with killall and pkill, and making a daemon reload its configuration with SIGHUP.

20 min

You will learn to

Explain what a signal is and name the common ones
End a process with SIGTERM and, only if needed, SIGKILL
Explain why SIGKILL is the last resort
Signal other users' processes with sudo
Kill processes by name with killall and pkill
Reload a daemon with SIGHUP

Key terms

Signal: A small numbered message the kernel delivers to a process. The process can run a handler for it, ignore it, or take the default action (usually: end).
SIGTERM (15): 'Please terminate.' The default of kill. Can be caught, so programs can save state and exit cleanly.
SIGKILL (9): 'Stop now.' Can't be caught or ignored; the kernel removes the process immediately. Last resort.
SIGINT (2): Interrupt: what Ctrl+C sends to the foreground program.
SIGHUP (1): 'Hang up': originally the terminal line was disconnected. Daemons use it as 'reload your configuration'.
SIGSTOP (19) and SIGCONT (18): Pause a process (uncatchable, like SIGKILL) and let it continue. Ctrl+Z sends the catchable SIGTSTP (20).
kill, killall, pkill: kill sends a signal to PIDs; killall to processes with an exact name; pkill to processes matching a pattern. Despite the name, all of them can send any signal.

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