System and Process Management
System and Process Management covers how Linux starts, schedules, and supervises work. The exam tests process inspection and signalling, systemd service control, boot targets, logging, and job scheduling.
Processes and signals
- Each process has a PID, PPID, state (R, S, D, Z), priority (nice), and owner. ps aux, ps -ef, pgrep, and top or htop show them.
- Signals: SIGTERM (15) asks for graceful shutdown; SIGKILL (9) cannot be caught; SIGHUP (1) often reloads configuration; SIGINT (2) is Ctrl-C; SIGSTOP and SIGCONT pause and resume.
- Zombies have exited but their parent has not reaped them; you cannot kill a zombie, so fix or restart the parent.
systemd and scheduling
- systemctl start, stop, restart, and status manage units; enable creates boot-time symlinks; disable removes them; mask prevents any start.
- After editing a unit file, run systemctl daemon-reload.
- journalctl -u sshd -b shows this boot's logs for a unit; -f follows output.
- cron uses /etc/crontab and /etc/cron.d; systemd timers use OnCalendar and are inspected with systemctl list-timers.
Worked example
A web service stops responding. Check systemctl status nginx; if active, find the worker PIDs with pgrep -a nginx. Send SIGTERM with kill -TERM , then watch with ps. If it does not exit after a reasonable wait, use kill -KILL. Then check journalctl -u nginx --since '10 min ago' for the cause. Reasoning: graceful termination allows connection draining; SIGKILL risks corruption and should be the last resort.
Traps
- Using kill -9 first.
- Confusing start with enable.
- Forgetting daemon-reload after unit edits.
- Reading load average as CPU percentage; it counts runnable and uninterruptible tasks.