System V init
This init program was inspired by the one from Unix System V and 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 However, in recent years it has been replaced by systemd, which we will describe in the next section.
The BusyBox init daemon is just a trimmed-down version of System V init. System V init has two advantages compared to BusyBox init:
- 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 tasks0: Halts...
