systemctl's service storage directory
/lib/systemd/system
/usr/lib/systemd/system
/etc/systemd/system
systemctl is the main command of Systemd, which can be used to manage the system
List all services
systemctl list-units --type=service
systemctl --type=service
List all active (running or exiting) services
systemctl list-units --type=service --state=active
List all running services
systemctl list-units --type=service --state=running
List all running or failed services
systemctl list-units --type service --state running,failed
List all enabled services
systemctl list-unit-files --state=enabled