Signals Deep Dive and watch

The signals that matter most

• SIGTERM (15) — "please exit." Default of kill.
• SIGINT (2) — "the user pressed Ctrl+C."
• SIGHUP (1) — "your terminal closed," or, by convention, "reload your config."
• SIGKILL (9) — "die now." Cannot be caught.
• SIGSTOP (17) / SIGCONT (19) — pause and resume. Like Ctrl+Z then fg.
• SIGUSR1 / SIGUSR2 — application-defined. nginx uses USR1 for log rotation; Postgres uses USR1 for crash recovery.
• SIGPIPE (13) — "the pipe you were writing to closed." The cause when commands die mid-pipeline.

Catching signals in scripts: trap

trap 'echo "caught Ctrl+C"; cleanup; exit 130' INT
trap 'rm -f /tmp/lock' EXIT
trap '' HUP            # ignore SIGHUP for this script

Trap on EXIT is the canonical pattern for cleanup — it runs regardless of how the script ends. We dig deeper in the scripting track.

watch — repeat a command

watch -n 1 'ls -lah | head' reruns every 1 second and displays the latest output, highlighting changes. Perfect for monitoring queue size, log line count, file growth. macOS doesn't ship watch — brew install watch.

watch flags

• -n 5 — interval in seconds.
• -d — highlight differences from the previous output.
• -c — interpret ANSI colors in the output.
• -x — execute command verbatim instead of via shell (needed for some quoting cases).

The minimum-viable monitor

watch -n 2 -d 'ls -lh /var/log/build.log'

Run during a long build. Size and timestamp tick; you know it's alive without flooding your screen with tail -f.