系统启动时，进入/etc/inittab中initdefault所定义的运行级别(deafult 3)各运行级别的定义：
run level:
0, (halt, shotdown, stop+a,sync,go )
1, (single user, boot -s, init s )
2, (multiuser, unshare net)
3, (multiuser, deafult)
5, (halt, power off )
6, (reboot to level 3 )
S (init 1, boot -s)
改变系统运行级别命令:
# init 0, halt, shotdown, stop+a,sync,go
# init 1, single user, boot -s, init s
# init 2, multiuser, unshare net
# init 3, multiuser, deafult
# init 5, halt, power off
# init 6, reboot to level 3
# init s, init 1, boot -s
# init *,shutdown,halt,poweroff, reboot -- -r, reboot -- -s
ok> boot -s, boot -sra | |
