Linux 使用init命令实现关机，重启，切换模式

# inittab    This file describes how the INIT process should set up  
#        the system in a certain run-level.  
#  
# Author:    Miquel van Smoorenburg, <miquels@drinkel.nl.mugnet.org>  
#        Modified for RHS Linux by Marc Ewing and Donnie Barnes  
#  
# Default runlevel. The runlevels used by RHS 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: