1000 Ways to Die (`kill`)

kill, and its sister killall, are likely familiar to you already. They are simple tools to send signals to a process. Most commonly they are used to end processes with the TERM signal.

Signals we care about:

• TERM: Cleanly exit a process, like a CTL+C.

• HUP: 'Hang up,' many services such as databases use this signal as a notice to restart without terminating. Eg, for a configuration change.

• KILL: Harshly exit a process, without allowing graceful shutdown.

• STOP: Pause a process, like a CTL+Z.

• CONT: Resume a process.

kill requires a PID (such as from ps), while killall supports process names.

Terminating a process:

killall apache

Sending a specific signal:

killall -s HUP postgres

Pausing a process for several seconds:

killall -s STOP tikv-server
sleep `shuf -i 1000-10000 -n 1`
killall -s CONT tikv-server

Exercises

• Try sending STOP to a running apache2 server. Do requests still work? Now send CONT and test again, does it work again?

• Open a psql shell (sudo -u postgres psql), try sending an HUP to postgres using another shell, does your psql shell stop working? What about when you send KILL?