System V init
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 considered the way to boot Linux, obviously including embedded systems, and BusyBox
init is a trimmed down version of System V
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, by switching from one runlevel to another.
There are 8 runlevels numbered from ...