System V init

This init program was inspired by the one from Unix System V and so dates back to the mid-1980s. The version most often found in Linux distributions was written initially by Miquel van Smoorenburg. Until recently, it was the init daemon for almost all desktop and server distributions and a fair number of embedded systems as well. However, in recent years, it has been replaced by systemd, which I will describe in the next section.

The BusyBox init daemon I have just described is just a trimmed-down version of System V init. Compared to BusyBox init, System V init has two advantages:

• Firstly, the boot scripts are written in a well-known, modular format, making it easy to add new packages at build time or runtime.
• Secondly, it has the concept of runlevels, which allow a collection of programs to be started or stopped in one go when switching from one runlevel to another.

There are eight runlevels, numbered from 0 to 6, plus S:

• S: Runs startup...