
		Order of scripts run in /etc/rc?.d
		==================================

0. Overview.

   All scripts executed by the init system are located in /etc/init.d.
   The directories /etc/rc?.d (? = S, 0 .. 6) contain relative links to
   those scripts. These links are named S<2-digit-number><original-name>
   or K<2-digit-number><original-name>.

   When ``init`` changes runlevel first the targets of the links whose
   names start with a ``K`` are executed, each with the single argument
   ``stop``, followed by the scripts prefixed with an ``S``, each with the
   single argument ``start``. (The links are those in the ``/etc/rcn.d``
   directory corresponding to the new runlevel.) The ``K`` links are
   responsible for killing services and the ``S`` link for starting
   services upon entering the runlevel.

   The two-digit number mm is used to determine the order in which to run
   the scripts: low-numbered links have their scripts run first. For
   example, the ``K20`` scripts will be executed before the ``K30``
   scripts. This is used when a certain service must be started before
   another. For example, the name server ``bind`` might need to be started
   before the news server ``inn`` so that ``inn`` can set up its access
   lists. In this case, the script that starts ``bind`` would have a lower
   number than the script that starts ``inn`` so that it runs first:

   The two runlevels 0 (halt) and 6 (reboot) are slightly different. In
   these runlevels, the links with an ``S`` prefix are still called after
   those with a ``K`` prefix, but they too are called with the single
   argument ``stop``.

   If a scripts has the ".sh" suffix it is a bourne shell script and
   MAY be handled in an optimized manner. The behaviour of executing the
   script in an optimized way will not differ in any way from it being
   forked and executed in the regular way.

   The following runlevels are defined:

   N       System bootup (NONE).
   S       Single user mode (not to be switched to directly)
   0       halt
   1       single user mode
   2 .. 5  multi user mode
   6       reboot

1. Boot.

   When the systems boots, the /etc/init.d/rcS script is executed. It
   in turn executes all the S* scripts in /etc/rcS.d in alphabetical
   (and thus numerical) order. The first argument passed to the
   executed scripts is "start". The runlevel at this point is "N" (none).

   Only things that need to be run once to get the system in a consistent
   state are to be run. The rcS.d directory is NOT meant to replace rc.local.
   One should not start daemons in this runlevel unless absolutely
   necessary. Eg, NFS might need the portmapper, so it is OK to start it
   early in the bootprocess. But this is not the time to start the
   squid proxy server.

2. Going multiuser.

   After the rcS.d scripts have been executed, init switches to the
   default runlevel as specified in /etc/inittab, usually "2".

   Init then executes the /etc/init.d/rc script which takes care of
   starting the services in /etc/rc2.d.

   Because the previous runlevel is "N" (none) the /etc/rc2.d/KXXxxxx
   scripts will NOT be executed - there is nothing to stop yet,
   the system is busy coming up.

   If for example there is a service that wants to run in runlevel 4
   and ONLY in that level, it will place a KXXxxxx script in
   /etc/rc{2,3,5}.d to stop the service when switching out of runlevel 4.
   We do not need to run that script at this point.
   
   The /etc.rc2.d/SXXxxxx scripts will be executed in alphabetical
   order, with the first argument set to "start".

3. Switching runlevels.

   When one switches from (for example) runlevel 2 to runlevel 3,
   /etc/init.d/rc will first execute in alphabetical order all K
   scripts for runlevel 3 (/etc/rc3.d/KXXxxxx) with as first argument
   "stop" and then all S scripts for runlevel 3 (/etc/rc3.d/SXXxxxx)
   with as first argument "start".

   As an optimization, a check is made for each "service" to see if
   it was already running in the previous runlevel. If it was, and there
   is no K (stop) script present for it in the new runlevel, there is
   no need to start it a second time so that will not be done.

   On the other hand, if there was a K script present, it is assumed the
   service was stopped on purpose first and so needs to be restarted.

   We MIGHT make the same optimization for stop scripts as well-
   if no S script was present in the previous runlevel, we can assume
   that service was not running and we don't need to stop it either.
   In that case we can remove the "coming from level N" special case
   mentioned above in 2). But right now that has not been implemented.

4. Single user mode.

   Switching to single user mode is done by switching to runlevel 1.
   That will cause all services to be stopped (assuming they all have
   a K script in /etc/rc1.d). The runlevel 1 scripts will then switch
   to runlevel "S" which has no scripts - all it does is spawn
   a shell directly on /dev/console for maintenance.

5. Halt/reboot

   Going to runlevel 0 or 6 will cause the system to be halted or rebooted,
   respectively. For example, if we go to runlevel 6 (reboot) first
   all /etc/rc6.d/KXXxxxx scripts will be executed alphabetically with
   "stop" as the first argument.

   Then the /etc/rc6.d/SXXxxxx scripts will be executed alphabetically
   with "stop" as the first argument as well. The reason is that there
   is nothing to start anymore at this point - all scripts that are
   run are meant to bring the system down.

   In the future, the /etc/rc6.d/SXXxxxx scripts MIGHT be moved to
   /etc/rc6.d/K1XXxxxx for clarity.
