CentOS 7/8修改系統運行級別

CentOS 7/8修改系統運行級別

CentOS 5、 CentOS 6可以通過修改組態檔/etc/inittab 引數，從而修改系統運行級別，切換系統的runlevel，但是從CentOS 7開始，不能用這個命令了，只能由systemd統一管理，并且減少到了兩個等級（當然這里不包含poweroff.target、reboot.target等），/etc/inittab中也明確inittab不再使用了，

CentOS 7下的inittab檔案

注意：”systemd uses 'targets' instead of runlevels. By default, there are two main targets“這條資訊

[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檔案

注意：”systemd uses 'targets' instead of runlevels. By default, there are two main targets“這條資訊

[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

系統運行級別都定義在/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

從上圖可以看到runlevel2.target、runlevel3.target、runlevel4.target 這些軟連接都是指向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

通過命令來查看運行級別：

# systemctl get-default  # 查看默認運行級別

# runlevel

通過命令來設定運行級別：

systemd命令修改：

#systemctl set-default TARGET.target

修改軟連接方式：

# ln -sf /lib/systemd/system/multi-user.target /etc/systemd/system/default.target

測驗例子如下：

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

# 可設定的運行級別

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

修改后需要重啟服務器才能生效，否則只對當前連接生效，

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