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init is the Unix program which spawns all other processes. The standard init system used is a descendant of System V init, however some distributions such as Slackware use a BSD-style init system. As well as starting the Unix system, init is also used to change runlevels after bootup.
As well as being the first thing that runs on a booted system, init is the last thing run by the kernel. After init is invoked, it examines the '/etc/inittab' file for an 'initdefault' entry, which tells init whether there is a default runlevel. If there is no default runlevel, then the user is dumped to a system console, where a runlevel must be entered manually.
Runlevels describe certain states of a machine, characterized by the processes run. There are generally 8 runlevels. These are the runlevels 0-6 and S or s, which are aliased to the same runlevel. Of these eight, 3 are so-called 'reserved' runlevels:
0. Runlevel 0 is used to halt the system
1. Used to reboot the machine
6. Drops the system into single user text mode.
The other somewhat-reserved runlevels are 3 and 5:
3. Much like 6, runlevel 3 drops the user to a console mode. However 3 is a multiuser environment. Runlevel 3 is the default on some Linux distributions, including Debian Linux.
5. Runlevel 5 is the most common runlevel, as it is the default on SuSe Linux, Mandrake Linux, and Yoper, among many others. Runlevel 5 is a multiuser graphical environment running X, usually with a display manager such as xdm, kdm or gdm running.
The runlevel is generally changed by root running the telinit command: 'telinit x', where x is a valid runlevel (0-6, s).
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