Identity

PID: 1 — systemd is the first process started by the kernel; all other processes are its descendants
System unit dirs: /etc/systemd/system/ (admin-created, highest priority), /lib/systemd/system/ (package-installed, do not edit), /run/systemd/system/ (runtime, ephemeral)
User unit dir: ~/.config/systemd/user/ (no root needed; started at login via --user instance)
Drop-in dir: /etc/systemd/system/<unit>.d/*.conf — fragments merged on top of the base unit
Logs: journalctl -u <unit> (system), journalctl --user -u <unit> (user instance)

Key Operations

Operation	Command
Show status + recent log	`systemctl status <unit>`
Start	`sudo systemctl start <unit>`
Stop	`sudo systemctl stop <unit>`
Restart (stop + start)	`sudo systemctl restart <unit>`
Reload (SIGHUP, no downtime)	`sudo systemctl reload <unit>`
Enable at boot	`sudo systemctl enable <unit>`
Enable and start now	`sudo systemctl enable --now <unit>`