Signals & Killing Processes
Signals
Signals are asynchronous messages sent to processes.
| Signal | Number | Meaning | Catchable? |
|---|
| SIGHUP | 1 | Hang up / reload config | Yes |
| SIGINT | 2 | Interrupt (Ctrl+C) | Yes |
| SIGKILL | 9 | Force kill | No |
| SIGTERM | 15 | Graceful terminate (default) | Yes |
| SIGSTOP | 19 | Pause | No |
| SIGCONT | 18 | Resume | Yes |
| Command | Action |
|---|
kill <pid> | Send SIGTERM |
kill -9 <pid> | Send SIGKILL |
kill -HUP <pid> | Reload |
killall nginx | By name |
pkill -u alice | By user |
Exam tip: Always try SIGTERM (15) first; use SIGKILL (9) only when a process won't exit. SIGKILL and SIGSTOP cannot be trapped or ignored.