CentOS 7 运行级别切换

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本文详细介绍了CentOS系统在从inittab文件到systemd运行级别的过渡过程中，如何定义和设置不同运行级别的操作，包括如何在命令行级别和窗口级别进行设置。

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 name>.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.target
lrwxrwxrwx. 1 root root 15 7月  30 14:22 /lib/systemd/system/runlevel0.target -> poweroff.target
lrwxrwxrwx. 1 root root 17 7月  30 14:22 /lib/systemd/system/runlevel2.target -> multi-user.target
lrwxrwxrwx. 1 root root 17 7月  30 14:22 /lib/systemd/system/runlevel3.target -> multi-user.target
lrwxrwxrwx. 1 root root 17 7月  30 14:22 /lib/systemd/system/runlevel4.target -> multi-user.target
lrwxrwxrwx. 1 root root 16 7月  30 14:22 /lib/systemd/system/runlevel5.target -> graphical.target
lrwxrwxrwx. 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

设置窗口级别方法:

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