systemd Units & Services

systemd manages the system via units. Unit types include .service, .socket, .target, .mount, .timer.

Command	Action
`systemctl start nginx`	Start now
`systemctl stop nginx`	Stop now
`systemctl restart nginx`	Restart
`systemctl reload nginx`	Reload config (no downtime)
`systemctl enable nginx`	Start at boot
`systemctl disable nginx`	Don't start at boot
`systemctl enable --now nginx`	Enable and start
`systemctl status nginx`	Current state
`systemctl is-active nginx`	active/inactive
`systemctl list-units --type=service`	All services

Unit files live in /etc/systemd/system/ (admin) and /lib/systemd/system/ (packages). After editing, run systemctl daemon-reload.

$ systemctl enable --now sshd
Created symlink /etc/systemd/system/multi-user.target.wants/sshd.service.

Exam trap: start ≠ enable. start runs it now; enable makes it start on boot. You usually want both.