我们在前面的课程中多次提到Linux系统的运行级别，runleve 那么 runlevel 是什么呢？Linux默认有7个运行级，从运行级0到运行级6，每一个运行级对应的含义如下：
- 运行级0：关机
- 运行级1：单用户模式，系统出现问题时，可以通过这种模式进行维护，典型的应用场景就是忘记 root 密码时可以进入此模式修改 root 密码。
- 运行级2：多用户模式，但是没有网络连接。
- 运行级3：完全多用户模式，这也是 linux系统最常见的运行级。
- 运行级4：保留未使用。
- 运行级5：窗口模式，支持多用户，支持网络。
- 运行级6：重启。
任何时候，Linux 只能在一种 runlevel 下运行。
系统在启动的过程中，会根据在 /etc/inittab 中的设定读取 runlevel 的数值X，并相应地读取和运行 /etc/rcX.d/ ( X 代表0～6 ) 下所有脚本。
在我们的实验环境下，运行 ll /etc/rc3.d 可以看到，我们的系统在3 这个运行级下面的启动脚本。
root@ubuntu:~# ll /etc/rc3.d/
total 20
drwxr-xr-x 2 root root 4096 Sep 25 00:02 ./
drwxr-xr-x 130 root root 12288 Oct 4 23:08 ../
-rw-r--r-- 1 root root 677 Feb 16 2016 README
lrwxrwxrwx 1 root root 20 Sep 9 01:25 S20kerneloops -> ../init.d/kerneloops*
lrwxrwxrwx 1 root root 15 Sep 9 01:25 S20rsync -> ../init.d/rsync*
lrwxrwxrwx 1 root root 24 Sep 25 00:02 S20screen-cleanup -> ../init.d/screen-cleanup*
lrwxrwxrwx 1 root root 27 Sep 9 01:25 S20speech-dispatcher -> ../init.d/speech-dispatcher*
lrwxrwxrwx 1 root root 15 Sep 9 01:25 S50saned -> ../init.d/saned*
lrwxrwxrwx 1 root root 19 Sep 9 01:25 S70dns-clean -> ../init.d/dns-clean*
lrwxrwxrwx 1 root root 18 Sep 9 01:25 S70pppd-dns -> ../init.d/pppd-dns*
lrwxrwxrwx 1 root root 21 Sep 9 01:25 S99grub-common -> ../init.d/grub-common*
lrwxrwxrwx 1 root root 18 Sep 9 01:25 S99ondemand -> ../init.d/ondemand*
lrwxrwxrwx 1 root root 18 Sep 9 01:25 S99rc.local -> ../init.d/rc.local*
可以通过查看 README 文件来查看关于文件夹下面脚本的说明：
root@ubuntu:~# cat /etc/rc3.d/README
The scripts in this directory are executed each time the system enters
this runlevel.
The scripts are all symbolic links whose targets are located in
/etc/init.d/ .
To disable a service in this runlevel, rename its script in this
directory so that the new name begins with a 'K' and a two-digit
number, and run 'update-rc.d script defaults' to reorder the scripts
according to dependencies. A warning about the current runlevels
being enabled not matching the LSB header in the init.d script will be
printed. To re-enable the service, rename the script back to its
original name beginning with 'S' and run update-rc.d again.
For a more information see /etc/init.d/README.
- 首先，所有可以执行的程序都存储在
/etc/init.d/，当前文件夹下面的都是到init.d 下面可执行文件的连接。 - 其次，系统首先运行
K开头，并跟着两个数字再跟着服务名称的连接脚本。其次是S开头的脚本，K和S 的意思分别是 kill 和 start。 - 再次，在同一个字母开头的脚本中，会按照数字小的先运行，数字大的后运行的顺序进行。
eg ： 运行级1 和运行级3 的对比：
root@ubuntu:~# ll /etc/rc1.d/
total 20
drwxr-xr-x 2 root root 4096 Sep 25 00:02 ./
drwxr-xr-x 130 root root 12288 Oct 4 23:45 ../
lrwxrwxrwx 1 root root 20 Sep 9 01:25 K20kerneloops -> ../init.d/kerneloops*
lrwxrwxrwx 1 root root 15 Sep 9 01:25 K20rsync -> ../init.d/rsync*
lrwxrwxrwx 1 root root 15 Sep 9 01:25 K20saned -> ../init.d/saned*
lrwxrwxrwx 1 root root 24 Sep 25 00:02 K20screen-cleanup -> ../init.d/screen-cleanup*
lrwxrwxrwx 1 root root 27 Sep 9 01:25 K20speech-dispatcher -> ../init.d/speech-dispatcher*
-rw-r--r-- 1 root root 369 Mar 12 2014 README
lrwxrwxrwx 1 root root 19 Sep 9 01:25 S30killprocs -> ../init.d/killprocs*
lrwxrwxrwx 1 root root 19 Sep 9 01:25 S70dns-clean -> ../init.d/dns-clean*
lrwxrwxrwx 1 root root 18 Sep 9 01:25 S70pppd-dns -> ../init.d/pppd-dns*
lrwxrwxrwx 1 root root 16 Sep 9 01:25 S90single -> ../init.d/single*
root@ubuntu:~# ll /etc/rc3.d/
total 20
drwxr-xr-x 2 root root 4096 Sep 25 00:02 ./
drwxr-xr-x 130 root root 12288 Oct 4 23:45 ../
-rw-r--r-- 1 root root 677 Feb 16 2016 README
lrwxrwxrwx 1 root root 20 Sep 9 01:25 S20kerneloops -> ../init.d/kerneloops*
lrwxrwxrwx 1 root root 15 Sep 9 01:25 S20rsync -> ../init.d/rsync*
lrwxrwxrwx 1 root root 24 Sep 25 00:02 S20screen-cleanup -> ../init.d/screen-cleanup*
lrwxrwxrwx 1 root root 27 Sep 9 01:25 S20speech-dispatcher -> ../init.d/speech-dispatcher*
lrwxrwxrwx 1 root root 15 Sep 9 01:25 S50saned -> ../init.d/saned*
lrwxrwxrwx 1 root root 19 Sep 9 01:25 S70dns-clean -> ../init.d/dns-clean*
lrwxrwxrwx 1 root root 18 Sep 9 01:25 S70pppd-dns -> ../init.d/pppd-dns*
lrwxrwxrwx 1 root root 21 Sep 9 01:25 S99grub-common -> ../init.d/grub-common*
lrwxrwxrwx 1 root root 18 Sep 9 01:25 S99ondemand -> ../init.d/ondemand*
lrwxrwxrwx 1 root root 18 Sep 9 01:25 S99rc.local -> ../init.d/rc.local*
