The term runlevel refers to a mode of operation in one of the computer operating systems that implement Unix System V-style initialization. Conventionally, seven runlevels exist, numbered from zero to six; though up to ten, from zero to nine, may be used. S is sometimes used as a synonym for one of the levels.
In standard practice, when a computer enters runlevel zero, it halts, and when it enters runlevel six, it reboots. The intermediate runlevels (1-5) differ in terms of which drives are mounted, and which network services are started. Lower run levels are useful for maintenance or emergency repairs, since they usually don't offer any network services at all. The particular details of runlevel configuration differ widely among operating systems, and slightly among system administrators.
The runlevel system replaced the traditional /etc/rc script used in Version 7 Unix.
| ID | Name | Description |
|---|---|---|
| 0 | Halt | Shuts down the system. |
| S | Single-User Mode | Does not configure network interfaces or start daemons. |
| 6 | Reboot | Reboots the system. |
^ = Almost all systems use runlevel 1 for this purpose. This mode is intended to provide a safe environment to perform system maintenance. Originally this runlevel provided a single terminal (console) interface running a root login shell. The increasing trend towards physical access to the computer during the boot process has led to changes in this area.
說穿了,不過是設定不同的裝置罷了,Init這隻程式的部份功能就是Halt以及Reboot系統！
