CentOS 7 之前的版本是通过 /etc/inittab 文件来定义系统运行级别:
[sywu@wusuyuan ~]$ cat /etc/centos-release CentOS release 6.4 (Final)[sywu@wusuyuan ~]$ cat /etc/inittab # inittab is only used by upstart for the default runlevel.## ADDING OTHER CONFIGURATION HERE WILL HAVE NO EFFECT ON YOUR SYSTEM.## System initialization is started by /etc/init/rcS.conf## Individual runlevels are started by /etc/init/rc.conf## Ctrl-Alt-Delete is handled by /etc/init/control-alt-delete.conf## Terminal gettys are handled by /etc/init/tty.conf and /etc/init/serial.conf,# with configuration in /etc/sysconfig/init.## For information on how to write upstart event handlers, or how# upstart works, see init(5), init(8), and initctl(8).## Default runlevel. The runlevels used are:# 0 - halt (Do NOT set initdefault to this)# 1 - Single user mode# 2 - Multiuser, without NFS (The same as 3, if you do not have networking)# 3 - Full multiuser mode# 4 - unused# 5 - X11# 6 - reboot (Do NOT set initdefault to this)# id:3:initdefault:[sywu@wusuyuan ~]$
CentOS 7 版本不再使用该文件定义系统运行级别,相关运行级别设置无效:
[sywu@c1-11r2-rac1 etc]$ cat /etc/centos-release CentOS Linux release 7.0.1406 (Core) [sywu@c1-11r2-rac1 etc]$ cat /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 /etc/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 set a default target, run:## ln -sf /lib/systemd/system/.target /etc/systemd/system/default.target#id:3:initdefault: #无效[sywu@c1-11r2-rac1 etc]$
新版本的运行级别都定义在 /lib/systemd/system下:
[sywu@c1-11r2-rac1 system]$ ls -ltr /lib/systemd/system/runlevel*lrwxrwxrwx. 1 root root 13 7月 30 14:22 /lib/systemd/system/runlevel1.target -> rescue.targetlrwxrwxrwx. 1 root root 15 7月 30 14:22 /lib/systemd/system/runlevel0.target -> poweroff.targetlrwxrwxrwx. 1 root root 17 7月 30 14:22 /lib/systemd/system/runlevel2.target -> multi-user.targetlrwxrwxrwx. 1 root root 17 7月 30 14:22 /lib/systemd/system/runlevel3.target -> multi-user.targetlrwxrwxrwx. 1 root root 17 7月 30 14:22 /lib/systemd/system/runlevel4.target -> multi-user.targetlrwxrwxrwx. 1 root root 16 7月 30 14:22 /lib/systemd/system/runlevel5.target -> graphical.targetlrwxrwxrwx. 1 root root 13 7月 30 14:22 /lib/systemd/system/runlevel6.target -> reboot.target
可以针对不同需要设置不同的运行级别:
如设置命令行级别(init 3)方法:
ln -sf /lib/systemd/system/runlevel3.target /etc/systemd/system/default.target或ln -sf /lib/systemd/system/multi-user.target /etc/systemd/system/default.target或systemctl set-default multi-user.target
设置窗口级别(init 5)方法:
ln -sf /lib/systemd/system/runlevel5.target /etc/systemd/system/default.target或ln -sf /lib/systemd/system/graphical.target /etc/systemd/system/default.target或systemctl set-default graphical.target
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修改系统运行级别：
1、systemd使用比sysvinit的运行级更为自由的target替代。第3运行级用multi-user.target替代。第5运行级用graphical.target替代。runlevel3.target和runlevel5.target分别是指向 multi-user.target和graphical.target的符号链接。可以使用下面的命令切换到“运行级别3 ”：systemctl isolate multi-user.target或systemctl isolate runlevel3.target可以使用下面的命令切换到“运行级别5 ”：
systemctl isolate graphical.target或systemctl isolate runlevel5.target2、如何改变默认运行级别？systemd使用链接来指向默认的运行级别。在创建新的链接前，可以通过下面命令删除存在的链接： rm /etc/systemd/system/default.target默认启动运行级别3 ：ln -sf /lib/systemd/system/multi-user.target /etc/systemd/system/default.target默认启动运行级别5 ：
ln -sf /lib/systemd/system/graphical.target/etc/systemd/system/default.targetsystemd不使用/etc/inittab文件。
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如何查看当前运行级别？
旧的runlevel命令在systemd下仍然可以使用。可以继续使用它，尽管systemd使用 ‘target’ 概念(多个的 ‘target’ 可以同时激活)替换了之前系统的runlevel。等价的systemd命令是systemctl list-units –type=target