show system status

systemctl status

show running or failed units

systemctl list-units
systemctl
systemctl --failed

show installed unit files

systemctl list-unit-files
ls -l /usr/lib/systemd/system /etc/systemd/system

start, stop, restart or reload a unit

systemctl start unit
systemctl stop unit
systemctl restart unit
systemctl reload unit

show status of a unit

systemctl status unit

enable or disable a unit to be started on bootstrap

systemctl enable unit
systemctl disable unit

check whether a unit is enabled or not

systemctl is-enabled unit

mask or unmask a unit to make it impossible to start it

systemctl mask unit
systemctl unmask unit

reboot or poweroff the system

systemctl reboot
systemctl poweroff