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<H1><A NAME="SECTION00830000000000000000">Run levels</A></H1>
<P>
A <b>run level</b> is a state of <tt>init</tt><A NAME="2432"> </A> and the whole
system that defines what system services are operating. Run
levels are identified by numbers, see table <A HREF="node76.html#tabrunlevels">7.1</A>.
There is no consensus of how to use the user defined run levels
(2 through 5). Some system administrators use run levels to
define which subsystems are working, e.g., whether X is running,
whether the network is operational, and so on. Others have all
subsystems always running or start and stop them individually,
without changing run levels, since run levels are too coarse
for controlling their systems. You need to decide for yourself,
but it might be easiest to follow the way your Linux distribution
does things.
<P>
<P><A NAME="2242"> </A><A NAME="tabrunlevels"> </A><IMG WIDTH=381 HEIGHT=89 ALIGN=BOTTOM ALT="table2241" SRC="img12.gif"><BR>
<STRONG>Table 7.1:</STRONG> Run level numbers<BR>
<P>
<P>
Run levels are configured in <tt>/etc/inittab</tt><A NAME="2434"> </A> by lines like
the following:
<BLOCKQUOTE> <TT>
l2:2:wait:/etc/init.d/rc 2
</TT></BLOCKQUOTE>
The first field is an arbitrary label, the second one means
that this applies for run level 2. The third field means that
<tt>init</tt><A NAME="2436"> </A> should run the command in the fourth field once,
when the run level is entered, and that <tt>init</tt><A NAME="2438"> </A> should wait
for it to complete. The
<tt>/etc/init.d/rc</tt><A NAME="2440"> </A> command runs whatever commands are necessary
to start and stop services to enter run level 2.
<P>
The command in the fourth field does all the hard work of
setting up a run level. It starts services that aren't already
running, and stops services that shouldn't be running in the
new run level any more. Exactly what the command is, and how
run levels are configured, depends on the Linux distribution.
<P>
When <tt>init</tt><A NAME="2442"> </A> starts, it looks for a line in <tt>/etc/inittab</tt><A NAME="2444"> </A>
that specifies the default run level:
<BLOCKQUOTE> <TT>
id:2:initdefault:
</TT></BLOCKQUOTE>
You can ask <tt>init</tt><A NAME="2446"> </A> to go to a non-default
run level at startup by giving the kernel a command line
argument of <tt>single</tt> or <tt>emergency</tt>.<A NAME="tex2html36" HREF="footnode.html#2312"><IMG ALIGN=BOTTOM ALT="gif" SRC="./foot_motif.gif"></A> This allows you to choose
the single user mode (run level 1), which is described in
section <A HREF="node78.html#secsingleusermode">7.5</A>.
<P>
While the system is running, the <tt>telinit</tt><A NAME="2448"> </A> command can change
the run level. When the run level is changed, <tt>init</tt><A NAME="2450"> </A> runs
the relevant command from <tt>/etc/inittab</tt><A NAME="2452"> </A>.
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<P><ADDRESS>
<I>Lars Wirzenius <BR>
Sat Nov 15 02:32:11 EET 1997</I>
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