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<A NAME="308"> </A><H2><A NAME="SECTION00331000000000000000"><tt>init</tt></A></H2>
<P>
The single most important service in a UNIX system is provided
by <tt>init</tt><A NAME="310"> </A>. <tt>init</tt><A NAME="312"> </A> is started as the first process of
every UNIX system, as the last thing the kernel does when it
boots. When <tt>init</tt><A NAME="314"> </A> starts, it continues the boot process
by doing various startup chores (checking and mounting filesystems,
starting daemons, etc).
<A NAME="166"> </A>
<A NAME="167"> </A>
<A NAME="168"> </A>
<P>
The exact list of things that <tt>init</tt><A NAME="316"> </A>
does depends on which flavor it is; there are several to choose
from. <tt>init</tt><A NAME="318"> </A>
usually provides the concept of <b>single user mode</b>, in which
no one can log in and <tt>root</tt><A NAME="321"> </A> uses a shell at the console; the usual
mode is called <b>multiuser mode</b>.
Some flavors generalize this as <b>run levels</b>; single and
multiuser modes are considered to be two run levels, and there
can be additional ones as well, for example, to run X on the
console.
<A NAME="175"> </A>
<A NAME="176"> </A>
<A NAME="177"> </A>
<A NAME="178"> </A>
<P>
In normal operation, <tt>init</tt><A NAME="325"> </A> makes sure <tt>getty</tt><A NAME="327"> </A>s are
working (to allow users to log in), and to
adopt orphan processes (processes whose parent has died; in
UNIX <em>all</em> processes <em>must</em> be in a single tree,
so orphans must be adopted).
<P>
When the system is shut down, it is <tt>init</tt><A NAME="329"> </A> that is in charge
of killing all
other processes, unmounting all filesystems and stopping the processor,
along with anything else it has been configured to do.
<A NAME="184"> </A>
<A NAME="185"> </A>
<A NAME="186"> </A>
<P>
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<P><ADDRESS>
<I>Lars Wirzenius <BR>
Sat Nov 15 02:32:11 EET 1997</I>
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