Textbook of Linux

systemctl is the primary command-line client for interacting with systemd. It inspects, starts, stops, enables, disables, and reloads units, and queries their state. Every systemd user learns it by heart.

systemctl status sshd                  # show state and recent logs
systemctl start sshd
systemctl stop sshd
systemctl restart sshd
systemctl reload sshd                  # if the unit supports it
systemctl enable sshd                  # start at boot
systemctl disable sshd                 # don't start at boot
systemctl enable --now sshd            # enable and start now
systemctl list-units --type=service     # currently loaded services
systemctl list-unit-files --state=enabled
systemctl daemon-reload                 # reload after editing unit files
systemctl cat sshd                      # show the unit file
systemctl edit sshd                     # override file
systemctl edit --full sshd              # edit the full unit
systemctl is-enabled sshd
systemctl isolate rescue.target         # switch to rescue mode
sudo systemctl poweroff / reboot

Unit-file overrides created with systemctl edit live in /etc/systemd/system/<unit>.d/override.conf, letting you tweak a distribution-provided unit without editing the original. This is the recommended way to customise service behaviour, because the original file will be overwritten by the package manager on upgrade.