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><A
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>Run levels</A
></H1
><P
>A <I
CLASS="GLOSSTERM"
>run level</I
> is a state of
<B
CLASS="COMMAND"
>init</B
> and the whole system that defines what
system services are operating. Run levels are identified by
numbers, see <A
HREF="x1812.html#RUN-LEVELS"
>Table 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
><DIV
CLASS="TABLE"
><P
><B
><A
NAME="RUN-LEVELS"
>Table 7-1. Run level numbers</A
></B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
>0</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>Halt the system.</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
>1</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>Single-user mode (for special administration).</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
>2-5</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>Normal operation (user defined).</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
>6</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>Reboot.</TD
></TR
></TABLE
></DIV
><P
>Run levels are configured in <TT
CLASS="FILENAME"
>/etc/inittab</TT
> by lines like
the following:
<PRE
CLASS="SCREEN"
>l2:2:wait:/etc/init.d/rc 2</PRE
>
The first field is an arbitrary label, the second one means
that this applies for run level 2. The third field means
that <B
CLASS="COMMAND"
>init</B
> should run the command in the
fourth field once, when the run level is entered, and that
<B
CLASS="COMMAND"
>init</B
> should wait for it to complete. The
<TT
CLASS="FILENAME"
>/etc/init.d/rc</TT
> command runs whatever
commands are necessary to start and stop services to enter run
level 2.</P
><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
><P
>When <B
CLASS="COMMAND"
>init</B
> starts, it looks for a line
in <TT
CLASS="FILENAME"
>/etc/inittab</TT
> that specifies the default
run level:
<PRE
CLASS="SCREEN"
>id:2:initdefault:</PRE
>
You can ask <B
CLASS="COMMAND"
>init</B
> to go to a non-default run
level at startup by giving the kernel a command line argument
of <TT
CLASS="LITERAL"
>single</TT
> or <TT
CLASS="LITERAL"
>emergency</TT
>.
Kernel command line arguments can be given via LILO, for example.
This allows you to choose the single user mode (run level 1).</P
><P
>While the system is running, the <B
CLASS="COMMAND"
>telinit</B
>
command can change the run level. When the run level is
changed, <B
CLASS="COMMAND"
>init</B
> runs the relevant command from
<TT
CLASS="FILENAME"
>/etc/inittab</TT
>. </P
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> to start <B
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>: the <TT
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