Ubuntu and Fedora Replace init with Upstart

init is the program which is used on most Linuxes to generate all other processes on bootup. The BSD style (which runs from the /etc/rc script) is slightly different from the SysV style: SySV is probably the one you’re familiar with, as it’s used by Debian, SuSE, and until recently, Redhat. With this setup, the system checks /etc/inittab to find a default runlevel. This file also defines what to do for various runlevels. A runlevel is a way of specifying a particular set of processes to run.

The important point is that init is level-driven. This means that you allocate each service to a runlevel, and services are started up in blocks based on which runlevel you boot into (or move into once booted). You can of course manually start and stop services using the same scripts, but the basic idea is one of sets of services which correspond to particular usage situations.

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