systemd

systemd is the init system and service manager on most modern Linux distros. systemctl controls units (services); journalctl reads their logs. Most commands need sudo to change state.

Control Services

Command Action
systemctl start nginx start a service now
systemctl stop nginx stop it now
systemctl restart nginx stop then start
systemctl reload nginx reload config without a full restart
systemctl enable nginx start automatically at boot
systemctl disable nginx don't start at boot
systemctl enable --now nginx enable at boot and start immediately

Check Status

Command Shows
systemctl status nginx state, recent logs, and PID
systemctl is-active nginx active / inactive (scriptable)
systemctl is-enabled nginx whether it starts at boot
systemctl --failed every unit that failed to start
systemctl list-units --type=service all loaded services
systemctl list-unit-files all installed units + their state

Read Logs (journalctl)

Command Shows
journalctl -u nginx all logs for one service
journalctl -u nginx -f follow (tail) live
journalctl -b logs since the last boot
journalctl -e jump to the end
journalctl --since '1 hour ago' time-filtered logs
journalctl -p err errors and worse only
journalctl -k kernel messages (like dmesg)
journalctl --disk-usage how much space the journal uses

Unit Files

Command Does
systemctl cat nginx print the unit file and any overrides
systemctl edit nginx create a drop-in override (safe)
systemctl edit --full nginx edit a full copy of the unit
systemctl daemon-reload reload after editing unit files
systemctl show nginx dump every property

Always daemon-reload after editing. systemd caches unit files. Change one and forget sudo systemctl daemon-reload and it keeps running the old definition — a classic "why isn't my change working" trap.

System State

Command Does
systemctl reboot restart the machine
systemctl poweroff shut down
systemctl suspend sleep
systemctl get-default current boot target (e.g. graphical)
systemd-analyze blame what slowed down the last boot