SIGALRM Signal Explained (Signal 14)

SIGALRM (signal 14) is delivered when a timer set with alarm() or setitimer() expires. It is used for timeouts.

What it does

Default action: Terminate. It can be caught with a signal handler.

Typically sent by: the kernel when an alarm timer fires. List all signals with kill -l.

Signal number and delivery

Practical examples

Send it from the shell

kill -14 <PID>
kill -SIGALRM <PID>
pkill -14 nginx

Handle it in a script

trap 'echo got SIGALRM; cleanup; exit' SIGALRM
while true; do sleep 1; done

Handle it in code

import signal, sys
def handler(sig, frame):
    print('received', sig)
    sys.exit(0)
signal.signal(signal.SIGALRM, handler)

Common mistakes

• Using kill -9 before trying SIGTERM.
• Forgetting to find the right PID (ps aux | grep, pgrep).
• Not handling signals in long-running services.

Key takeaways

• SIGALRM is signal 14.
• Default: Terminate.
• Use kill -l to list all signals.

FAQ

How do I time-limit a command?

Use timeout 10 command, which relies on signals.

How do I list all signals?

Run kill -l.