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
#
CentOS 8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@kerrydb ~]# cat /etc/inittab
# inittab is no longer used.
#
# 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
系统运行级别都定义在/lib/systemd/system下:
[root@db-server ~]# ll /usr/lib/systemd/system/runlevel*
lrwxrwxrwx. 1 root root 15 Aug 2 00:11 /usr/lib/systemd/system/runlevel0.target -> poweroff.target
lrwxrwxrwx. 1 root root 13 Aug 2 00:11 /usr/lib/systemd/system/runlevel1.target -> rescue.target
lrwxrwxrwx. 1 root root 17 Aug 2 00:11 /usr/lib/systemd/system/runlevel2.target -> multi-user.target
lrwxrwxrwx. 1 root root 17 Aug 2 00:11 /usr/lib/systemd/system/runlevel3.target -> multi-user.target
lrwxrwxrwx. 1 root root 17 Aug 2 00:11 /usr/lib/systemd/system/runlevel4.target -> multi-user.target
lrwxrwxrwx. 1 root root 16 Aug 2 00:11 /usr/lib/systemd/system/runlevel5.target -> graphical.target
lrwxrwxrwx. 1 root root 13 Aug 2 00:11 /usr/lib/systemd/system/runlevel6.target -> reboot.target
/usr/lib/systemd/system/runlevel1.target.wants:
total 0
lrwxrwxrwx. 1 root root 39 Aug 2 00:11 systemd-update-utmp-runlevel.service -> ../systemd-update-utmp-runlevel.service
/usr/lib/systemd/system/runlevel2.target.wants:
total 0
lrwxrwxrwx. 1 root root 39 Aug 2 00:11 systemd-update-utmp-runlevel.service -> ../systemd-update-utmp-runlevel.service
/usr/lib/systemd/system/runlevel3.target.wants:
total 0
lrwxrwxrwx. 1 root root 39 Aug 2 00:11 systemd-update-utmp-runlevel.service -> ../systemd-update-utmp-runlevel.service
/usr/lib/systemd/system/runlevel4.target.wants:
total 0
lrwxrwxrwx. 1 root root 39 Aug 2 00:11 systemd-update-utmp-runlevel.service -> ../systemd-update-utmp-runlevel.service
/usr/lib/systemd/system/runlevel5.target.wants:
total 0
lrwxrwxrwx. 1 root root 39 Aug 2 00:11 systemd-update-utmp-runlevel.service -> ../systemd-update-utmp-runlevel.service
![clip_image001[6]](http://img.e-com-net.com/image/info8/e1d124067d5f40168d95ac27a700afdd.png "clip_image001[6]")
从上图可以看到runlevel2.target、runlevel3.target、runlevel4.target 这些软连接都是指向multi-user.target
[root@kerrydb ~]# ll /usr/lib/systemd/system/runlevel*
lrwxrwxrwx. 1 root root 15 Apr 24 11:53 /usr/lib/systemd/system/runlevel0.target -> poweroff.target
lrwxrwxrwx. 1 root root 13 Apr 24 11:53 /usr/lib/systemd/system/runlevel1.target -> rescue.target
lrwxrwxrwx. 1 root root 17 Apr 24 11:53 /usr/lib/systemd/system/runlevel2.target -> multi-user.target
lrwxrwxrwx. 1 root root 17 Apr 24 11:53 /usr/lib/systemd/system/runlevel3.target -> multi-user.target
lrwxrwxrwx. 1 root root 17 Apr 24 11:53 /usr/lib/systemd/system/runlevel4.target -> multi-user.target
lrwxrwxrwx. 1 root root 16 Apr 24 11:53 /usr/lib/systemd/system/runlevel5.target -> graphical.target
lrwxrwxrwx. 1 root root 13 Apr 24 11:53 /usr/lib/systemd/system/runlevel6.target -> reboot.target
/usr/lib/systemd/system/runlevel1.target.wants:
total 0
/usr/lib/systemd/system/runlevel2.target.wants:
total 0
/usr/lib/systemd/system/runlevel3.target.wants:
total 0
/usr/lib/systemd/system/runlevel4.target.wants:
total 0
/usr/lib/systemd/system/runlevel5.target.wants:
total 0
![clip_image002[6]](http://img.e-com-net.com/image/info8/282051f56a364d98983116a6f340ec3c.png "clip_image002[6]")
通过命令来查看运行级别：
# systemctl get-default # 查看默认运行级别
# runlevel
通过命令来设置运行级别：
systemd命令修改：
#systemctl set-default TARGET.target
修改软连接方式：
# ln -sf /lib/systemd/system/multi-user.target /etc/systemd/system/default.target
测试例子如下：
例1：
[root@db-server tmp]# systemctl set-default graphical.target
Removed symlink /etc/systemd/system/default.target.
Created symlink from /etc/systemd/system/default.target to /usr/lib/systemd/system/graphical.target.
[root@db-server tmp]#
例2：
[root@kerrydb ~]# systemctl get-default
graphical.target
[root@kerrydb ~]# runlevel
N 5
[root@kerrydb ~]# systemctl set-default multi-user.target
Removed /etc/systemd/system/default.target.
Created symlink /etc/systemd/system/default.target → /usr/lib/systemd/system/multi-user.target.
[root@kerrydb ~]# systemctl get-default
multi-user.target
[root@kerrydb ~]#
例3：
[root@kerrydb ~]# systemctl get-default
multi-user.target
[root@kerrydb ~]# ln -sf /lib/systemd/system/multi-user.target /etc/systemd/system/default.target
[root@kerrydb ~]# systemctl get-default
multi-user.target
[root@kerrydb ~]# ln -sf /lib/systemd/system/runlevel3.target /etc/systemd/system/default.target
[root@kerrydb ~]# systemctl get-default
multi-user.target
# 可设置的运行级别
runlevel0.target -> poweroff.target
runlevel1.target -> rescue.target
runlevel2.target -> multi-user.target
runlevel3.target -> multi-user.target
runlevel4.target -> multi-user.target
runlevel5.target -> graphical.target
runlevel6.target -> reboot.target
修改后需要重启服务器才能生效。否则只对当前连接生效。