Killing and signaling processes in Linux

Signals Are Messages Sent to Processes

A signal is a notification sent to a process. The process can choose to handle or ignore most signals. SIGKILL is the exception — it cannot be caught or ignored, and the kernel terminates the process immediately.

Common Signals

SIGTERM (15) requests graceful shutdown — the process cleans up and exits. This is the default for kill. SIGKILL (9) forces immediate termination. SIGHUP (1) traditionally reloads config without restarting. SIGINT (2) is what Ctrl+C sends.

# Graceful shutdown (SIGTERM is default)
kill 1234

# Force kill (use only when graceful fails)
kill -9 1234

# Kill by name (kills all matching processes)
pkill nginx

# Kill with SIGHUP to reload config
kill -HUP $(pgrep nginx)

# Kill all processes by name (be careful)
killall python3

trap — Catch Signals in Scripts

Use trap to run cleanup code when your script receives a signal or exits.

#!/bin/bash

cleanup() {
  echo "Cleaning up temp files..."
  rm -f /tmp/myapp_*
  exit 0
}

# Run cleanup on Ctrl+C or normal exit
trap cleanup SIGINT SIGTERM EXIT

echo "Working..."
sleep 100

Always add a trap for cleanup in scripts that create temp files or hold locks.