CentOS 7/8修改系统运行级别

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

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

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。