系统版本：CentOS Linux release 7.3.1611 (Core) 

不再使用inittab进行设置，systemd通过使用targets的概念取代以前的启动级别

通常有两个主要target：

multi-user.target，相当于以前的level 3

graphical.target，相当于以前的level 5

查询当前默认target：systemctl get-default

设置默认target：systemctl set-default TARGET.target

例如设置默认target为multi-user.target:systemctl set-default multi-user.target

附/etc/inittab原文内容：

# 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

CentOS 7.3 关于系统启动级别