Runlevel笔记
Linux inittab和systemctl set-default
简介
运行级别（Runlevel）指的是Unix或者Linux等类Unix操作系统下不同的运行模式。运行级别通常分为 7 等，分别是从 0 到 6 ，但如果必要的话也可以更多。
例如在大多数Linux操作系统下一共有如下7个典型的运行级别：
- 0 停机，关机
- 1 单用户，无网络连接，不运行守护进程，不允许非超级用户登录
- 2 多用户，无网络连接，不运行守护进程
- 3 多用户，正常启动系统
- 4 用户自定义
- 5 多用户，带图形界面
- 6 重启
/etc/init.d 和 /etc/rc*.d 那些事
- 1. /etc/rc*.d ----> The scripts are all symbolic links whose targets are located in /etc/init.d/
- 2. /etc/rc0.d ---->
Generally it is not necessary to alter the scripts in this directory.
Their purpose is to stop all services and to make the system ready
for shutdown.
- 3. /etc/rc1.d ----->
Generally it is not necessary to alter the scripts in this directory.
Their purpose is to stop all services and thus to put the system in
single-user mode.
- 4. /etc/rc2/3/4/5.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.
- 5. /etc/rc6.d ----->
Generally it is not necessary to alter the scripts in this directory.
Their purpose is to stop all services and to make the system ready for reboot.
- 6. /etc/rcS.d ----->
The scripts in this directory whose names begin with an 'S' are
executed once when booting the system, even when booting directly into single user mode.
To disable a script in this directory, rename it so that it begins with a 'K' and run 'update-rc.d script defaults' to update the order using the script dependencies.
