CentOS 7/8修改系统运行级别
CentOS 5、 CentOS 6可以通过修改配置文件/etc/inittab 参数，从而修改系统运行级别，切换系统的runlevel，但是从CentOS 7开始，不能用这个命令了，只能由systemd统一管理，并且减少到了两个等级（当然这里不包含poweroff.target、reboot.target等）。/etc/inittab中也明确inittab不再使用了。
CentOS 7下的inittab文件
注意：”systemd uses 'targets' instead of runlevels. By default, there are two main targets“这条信息
[root@db-server ~]# more /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
#