Signals

Messages like SIGTERM and SIGKILL sent to processes.

Backend Engineer · Mid-level Data Engineer · Mid-level

Also known as: signals, unix signals, SIGTERM SIGKILL

A signal is a short notification the operating system delivers to a process to tell it something happened: someone asked it to stop, a child exited, a timer fired, the terminal hung up. It’s a limited, asynchronous message — just a number, not data — and it’s how the outside world interrupts a running program. Signals are Unix’s process-level counterpart to hardware interrupts.

A few matter constantly:

SIGTERM — “please terminate”. The polite shutdown request; a well-behaved program handles it and stops cleanly (see graceful shutdown).
SIGKILL — “die now”. Cannot be caught, blocked or ignored; the kernel ends the process immediately, with no chance to clean up.
SIGINT — the Ctrl-C interrupt, typically stopping a foreground program.
SIGHUP — historically “the terminal hung up”; often repurposed to mean “reload your configuration”.
SIGSTOP / SIGCONT — pause and resume.

kill -TERM 1234      # politely ask PID 1234 to stop
kill -KILL 1234      # force it

For each signal a process can have a default action (usually terminate, sometimes ignore or stop) or install its own handler. Handlers are how servers clean up on SIGTERM, or reload config on SIGHUP.

The classic mistakes:

Using SIGKILL first. Killing with -9 gives no chance to drain or clean up, so in-flight work is lost and resources may be left inconsistent. Send SIGTERM, wait, and only escalate if needed.
Assuming SIGTERM always works. A process that ignores it, or is stuck in uninterruptible I/O, won’t stop. That’s when SIGKILL is necessary — but understand you’re skipping cleanup.
Signal handlers doing too much. Handlers run in a restricted context; heavy work there is unsafe. Set a flag and handle it in the main flow.
Not handling SIGTERM in a service. The default action kills it abruptly, so deploys drop requests. Handle it and shut down gracefully.
Confusing signals with system calls or interrupts. A signal notifies a process; a system call is a program requesting kernel work; an interrupt is a CPU-level event. They interconnect but are distinct layers.

Signals are small and sharp: a handful of them drive shutdown, reload and control for every Unix program. Handling SIGTERM cleanly is the single most common signal task, and it’s what makes restarts and deploys safe.