A service control and switching run level
1, the service control
common service control type
start start
stop stop
restart restart (caution)
reload reload, without stopping the service [common]
status to view the status of
the boot-level view of the system 
common systemd power management command
2, the switching operation level
1), run-level view of the system
command: runlevel 
run level 2), the switching system
command: systemctl isolate multi-user.target or init 3 (terminal to switch the character Interface) 
command: systemctl isolate graphical.target or init 5 (graphical user interface switching)
We modify the default login screen for the character interface
2, services that are loaded automatically optimize boot
1), modified by the method of pseudo-terminal
order: ntsysv
2), modify the command
command: systemctl disable sshd.service (Prohibition start running sshd)
systemctl enable sshd.service (start running sshd)