centos 7 變換 runlevel

先確認目前 runlevel
systemctl get-default
graphical.target #預設式runlevel=5

確認有哪些模式可用
systemctl list-units –type=target

設定runlevel=3
systemctl set-default multi-user.target

切換運作層級至 GUI 模式（runlevel 5）
systemctl isolate graphical.target