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>Chapter 9. <B
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><H1
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><A
NAME="RUN-LEVELS-INTRO"
></A
>9.3. Run levels</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. 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
>The following table defines how most Linux Distributions
	define the different run levels.  However, run-levels 2 through 5 
	can be modified to suit your own tastes.</P
><DIV
CLASS="TABLE"
><A
NAME="RUN-LEVELS-TABLE"
></A
><P
><B
>Table 9-1. Run level numbers</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><TBODY
><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</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>Local Multiuser with Networking
			but without network service (like NFS)</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
>3</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>Full Multiuser with Networking 
			</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
>4</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>Not Used 
			</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
>5</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>Full Multiuser with Networking
			and X Windows(GUI)</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
>6</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>Reboot.</TD
></TR
></TBODY
></TABLE
></DIV
><P
>Services that get started at a certain runtime are determined
	by the contents of the various <TT
CLASS="FILENAME"
>rcN.d</TT
> directories. 
	Most distributions locate these directories either at
	<TT
CLASS="FILENAME"
>/etc/init.d/rcN.d</TT
> or 
	<TT
CLASS="FILENAME"
>/etc/rcN.d</TT
>. (Replace the N with the run-level
	number.)</P
><P
>		
	</P
><P
>In each run-level you will find a series of if links pointing 
	to start-up scripts located in <TT
CLASS="FILENAME"
>/etc/init.d</TT
>.  
	The names of these links all start as either K or S, followed by a 
	number.  If the name of the link starts with an S, then that indicates
	the service will be started when you go into that run level.  If the 
	name of the link starts with a K, the service will be killed (if 
	running).</P
><P
>The number following the K or S indicates the order the scripts
	will be run.  Here is a sample of what an 
	<TT
CLASS="FILENAME"
>/etc/init.d/rc3.d</TT
> may look like.

<TABLE
BORDER="1"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="SCREEN"
><TT
CLASS="PROMPT"
>#</TT
> <TT
CLASS="USERINPUT"
><B
>ls -l /etc/init.d/rc3.d</B
></TT
>
<TT
CLASS="COMPUTEROUTPUT"
>lrwxrwxrwx  1 root root 10 2004-11-29 22:09 K12nfsboot -&#62; ../nfsboot
lrwxrwxrwx  1 root root  6 2005-03-29 13:42 K15xdm -&#62; ../xdm
lrwxrwxrwx  1 root root  9 2004-11-29 22:08 S01pcmcia -&#62; ../pcmcia
lrwxrwxrwx  1 root root  9 2004-11-29 22:06 S01random -&#62; ../random
lrwxrwxrwx  1 root root 11 2005-03-01 11:56 S02firewall -&#62; ../firewall
lrwxrwxrwx  1 root root 10 2004-11-29 22:34 S05network -&#62; ../network
lrwxrwxrwx  1 root root  9 2004-11-29 22:07 S06syslog -&#62; ../syslog
lrwxrwxrwx  1 root root 10 2004-11-29 22:09 S08portmap -&#62; ../portmap
lrwxrwxrwx  1 root root  9 2004-11-29 22:07 S08resmgr -&#62; ../resmgr
lrwxrwxrwx  1 root root  6 2004-11-29 22:09 S10nfs -&#62; ../nfs
lrwxrwxrwx  1 root root 12 2004-11-29 22:40 S12alsasound -&#62; ../alsasound
lrwxrwxrwx  1 root root  8 2004-11-29 22:09 S12fbset -&#62; ../fbset
lrwxrwxrwx  1 root root  7 2004-11-29 22:10 S12sshd -&#62; ../sshd
lrwxrwxrwx  1 root root  8 2005-02-01 09:24 S12xntpd -&#62; ../xntpd
lrwxrwxrwx  1 root root  7 2004-12-02 20:34 S13cups -&#62; ../cups
lrwxrwxrwx  1 root root  6 2004-11-29 22:09 S13kbd -&#62; ../kbd
lrwxrwxrwx  1 root root 13 2004-11-29 22:10 S13powersaved -&#62; ../powersaved
lrwxrwxrwx  1 root root  9 2004-11-29 22:09 S14hwscan -&#62; ../hwscan
lrwxrwxrwx  1 root root  7 2004-11-29 22:10 S14nscd -&#62; ../nscd
lrwxrwxrwx  1 root root 10 2004-11-29 22:10 S14postfix -&#62; ../postfix
lrwxrwxrwx  1 root root  6 2005-02-04 13:27 S14smb -&#62; ../smb
lrwxrwxrwx  1 root root  7 2004-11-29 22:10 S15cron -&#62; ../cron
lrwxrwxrwx  1 root root  8 2004-12-22 20:35 S15smbfs -&#62; ../smbfs</TT
>
<TT
CLASS="PROMPT"
></TT
></PRE
></FONT
></TD
></TR
></TABLE
>

	</P
><P
>How run levels start are configured in 
	<TT
CLASS="FILENAME"
>/etc/inittab</TT
> by lines like the following:

<TABLE
BORDER="1"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="SCREEN"
>l2:2:wait:/etc/init.d/rc 2</PRE
></FONT
></TD
></TR
></TABLE
>

	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:

<TABLE
BORDER="1"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="SCREEN"
>id:2:initdefault:</PRE
></FONT
></TD
></TR
></TABLE
>

	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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