/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:5:initdefault:
# 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:5:initdefault:
各个运行级的详细解释：
0
为停机，机器关闭。
1 为单用户模式，就像Win9x下的安全模式类似。
2 为多用户模式，但是没有NFS支持。
3
为完整的多用户模式，是标准的运行级。
4 一般不用，在一些特殊情况下可以用它来做一些事情。例如在笔记本电脑的电池用尽时，可以切换到这个模式来做一些设置。
5 就是X11，进到X Window系统了。
6 为重启，运行init 6机器就会重启。
原文：http://www.cnblogs.com/jonnyCoder/p/3639123.html