这是个很久的知识点了，只是自己一直都迷迷糊糊的，今天在翻出来好好理解下。。
0:停机
1：单用户形式，只root进行维护
2：多用户，不能使用net file system
3：完全多用户
5：图形化
4：安全模式
6：重启
其实，可以通过查看/etc/rc.d/中的rc*.d的文件来对比理解。。
init 0，对应的系统会运行，/etc/rc.d/rc0.d里指定的程序。我们来看下名称。
[root@localhost ～]# ls/etc/rc.d/rc0.d
K01dnsmasq K15ksmtuned K35nmb K60crond K74lm_sensors K83portreserve K85rpcgssd K88iscsi K90network S00killall
K10cups K16ksm K35smb K66gpsd K75netfs K84NetworkManager K85rpcidmapd K88rsyslog K92ip6tables S01halt
K10saslauthd K20nfs K36mysqld K69rpcsvcgssd K75udev-post K84wpa_supplicant K86nfslock K89iscsid K92iptables
K10xfs K25sshd K50haldaemon K70vboxdrv K76openvpn K85mdmonitor K87alsasound K89netplugd K98qemu
K15gpm K30sendmail K50netconsole K74acpid K83bluetooth K85messagebus K87rpcbind K89rdisc K99lvm2-monitor
依照上述红色字体，开机会执行的两个进程是killall和halt，这两个都表示为终止进程。故init0是用于表示关机的。
init 1，对应的系统会运行，/etc/rc.d/rc1.d里指定的程序。
[root@localhost ～] # ls/etc/rc.d/rc1.d
K01dnsmasq K15ksmtuned K35nmb K60crond K74lm_sensors K84NetworkManager K85rpcidmapd K88rsyslog K92ip6tables S99single
K10cups K16ksm K35smb K66gpsd K75netfs K84wpa_supplicant K86nfslock K89iscsid K92iptables
K10saslauthd K20nfs K36mysqld K69rpcsvcgssd K76openvpn K85mdmonitor K87alsasound K89netplugd K98qemu
K10xfs K25sshd K50haldaemon K70vboxdrv K83bluetooth K85messagebus K87rpcbind K89rdisc S02lvm2-monitor
K15gpm K30sendmail K50netconsole K74acpid K83portreserve K85rpcgssd K88iscsi K90network S26udev-post
这个级别启动的服务有三个，udev、lvm相关的和single(单用户模式的服务)。故此级别是单用户模式，只有root能用，不支持其他用户。
init 2，对应的系统会运行，/etc/rc.d/rc2.d里指定的程序。
[root@localhost ~ ]# ls/etc/rc.d/rc2.d/
K01dnsmasq K20nfs K36mysqld K74lm_sensors K85rpcgssd K89netplugd S08iptables S23NetworkManager S30vboxdrv S99local
K10saslauthd K25sshd K50haldaemon K75netfs K85rpcidmapd K89rdisc S12rsyslog S24portreserve S35qemu
K10xfs K30sendmail K50netconsole K76openvpn K86nfslock K90network S13rpcbind S25cups S85gpm
K15ksmtuned K35nmb K66gpsd K83bluetooth K88iscsi S02lvm2-monitor S15mdmonitor S26acpid S90crond
K16ksm K35smb K69rpcsvcgssd K84wpa_supplicant K89iscsid S08ip6tables S22messagebus S26udev-post S99alsasound
这个级别启动的服务多了，NetworkManager/iptables/acpid/alsa都已经开启，但是nfs,smb,openvpn相关服务没有开启，这个级别不支持nfs。
init 3 ， 对应的系统运行/etc/rc.d/rc3.d
[root@localhost ~] # ls/etc/rc.d/rc3.d/
K01dnsmasq K30sendmail K74lm_sensors K89rdisc S08iptables S18rpcidmapd S25cups S35qemu S85ksmtuned S99local
K10saslauthd K36mysqld K76openvpn K90network S12rsyslog S19rpcgssd S25netfs S50bluetooth S90crond
K10xfs K50netconsole K84wpa_supplicant K99lvm2-monitor S13iscsi S22messagebus S26acpid S50haldaemon S91nmb
K20nfs K66gpsd K85mdmonitor S07iscsid S13rpcbind S23NetworkManager S26udev-post S84ksm S91smb
K25sshd K69rpcsvcgssd K89netplugd S08ip6tables S14nfslock S24portreserve S30vboxdrv S85gpm S99alsasound
这个级别nfs服务是开启的，被成为完全多用户模式。
init 4
[root@localhost ~ ]# ls/etc/rc.d/rc4.d/
K01dnsmasq K30sendmail K66gpsd K85mdmonitor S07iscsid S13rpcbind S23NetworkManager S26udev-post S84ksm S99local
K10saslauthd K35nmb K69rpcsvcgssd K89netplugd S08ip6tables S14nfslock S24portreserve S30vboxdrv S85gpm
K10xfs K35smb K74lm_sensors K89rdisc S08iptables S18rpcidmapd S25cups S35qemu S85ksmtuned
K20nfs K36mysqld K76openvpn K90network S12rsyslog S19rpcgssd S25netfs S50bluetooth S90crond
K25sshd K50netconsole K84wpa_supplicant K99lvm2-monitor S13iscsi S22messagebus S26acpid S50haldaemon S99alsasound
此模式被称为安全模式。
init 5
[root@localhost ~ ]# ls/etc/rc.d/rc5.d/
K01dnsmasq K25sshd K66gpsd K84wpa_supplicant K87rpcbind K90network S22messagebus S26udev-post S84ksm S99local
K10saslauthd K30sendmail K69rpcsvcgssd K85mdmonitor K88iscsi K99lvm2-monitor S23NetworkManager S30vboxdrv S85ksmtuned
K10xfs K36mysqld K74lm_sensors K85rpcgssd K89iscsid S08ip6tables S25cups S35qemu S91nmb
K15gpm K50netconsole K76openvpn K85rpcidmapd K89netplugd S08iptables S25netfs S50bluetooth S91smb
K20nfs K60crond K83portreserve K86nfslock K89rdisc S12rsyslog S26acpid S50haldaemon S99alsasound
完整的图形模式
init 6
[root@localhost ~ ]# ls/etc/rc.d/rc6.d/
K01dnsmasq K15ksmtuned K35nmb K60crond K74lm_sensors K83portreserve K85rpcgssd K88iscsi K90network S00killall
K10cups K16ksm K35smb K66gpsd K75netfs K84NetworkManager K85rpcidmapd K88rsyslog K92ip6tables S01reboot
K10saslauthd K20nfs K36mysqld K69rpcsvcgssd K75udev-post K84wpa_supplicant K86nfslock K89iscsid K92iptables
K10xfs K25sshd K50haldaemon K70vboxdrv K76openvpn K85mdmonitor K87alsasound K89netplugd K98qemu
K15gpm K30sendmail K50netconsole K74acpid K83bluetooth K85messagebus K87rpcbind K89rdisc K99lvm2-monitor
这个级别里，只有两个服务，一个为killall，一个是reboot，即，关闭现在的系统，重启。故此级别是重启。
不同的系统版本，可能里面的文件会不同，如果要查看，可以通过ll来看，其实他们都是软连接。
