安裝GNOME桌面後想設置默認啓動桌面環境，結果發現centos 7的/etc/inittab文件和centos 6不一樣了，特記錄centos 7設置開機啓動級別方法。
cat /etc/inittab
# inittab is no longer used when using systemd.
#
# ADDING CONFIGURATION HERE WILL HAVE NO EFFECT ON YOUR SYSTEM.
#
# Ctrl-Alt-Delete is handled by /usr/lib/systemd/system/ctrl-alt-del.target
#
# systemd uses 'targets' instead of runlevels. By default, there are two main targets:
#
# multi-user.target: analogous to runlevel 3
# graphical.target: analogous to runlevel 5
#
# To view current default target, run:
# systemctl get-default
#
# To set a default target, run:
# systemctl set-default TARGET.target通過該文件可以看出centos 7 用targets代替了啓動級別的設置
multi-user.target類似於 級別3
graphical.target類似於 級別5
獲取當前運行級別
systemctl get-default設置運行級別爲3
systemctl set-default multi-user.target設置運行級別爲5
systemctl set-default graphical.target