From: Petter Reinholdtsen <pere@hungry.com>
Date: Thu, 4 Aug 2011 10:42:04 +0000 (+0000)
Subject: New post.
X-Git-Url: https://pere.pagekite.me/gitweb/homepage.git/commitdiff_plain/64abb61e99a4c628d3a63ded857d7c33a9cb3069?ds=inline

New post.
---

diff --git a/blog/data/2011-08-04-rcS.txt b/blog/data/2011-08-04-rcS.txt
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+Title: How is booting into runlevel 1 different from single user boots?
+Tags: english, debian, bootsystem
+Date: 2011-08-04 12:40
+
+<p>Wouter Verhelst have some
+<a href="http://grep.be/blog/en/retorts/pere_kubuntu_boot">interesting
+pcomments and opinions</a> on my blog post on
+<a href="http://people.skolelinux.org/pere/blog/What_should_start_from__etc_rcS_d__in_Debian____almost_nothing.html">the
+need to clean up /etc/rcS.d/ in Debian</a> and my blog post about
+<a href="http://people.skolelinux.org/pere/blog/What_is_missing_in_the_Debian_desktop__or_why_my_parents_use_Kubuntu.html">the
+default KDE desktop in Debian</a>.  I only have time to address one
+small piece of his comment now, and though it best to address the
+misunderstanding he bring forward:</p>
+
+<p><blockquote>
+Currently, a system admin has four options: [...] boot to a
+single-user system (by adding 'single' to the kernel command line;
+this runs rcS and rc1 scripts)
+</blockquote></p>
+
+<p>This make me believe Wouter believe booting into single user mode
+and booting into runlevel 1 is the same.  I am not surprised he
+believe this, because it would make sense and is a quite sensible
+thing to believe.  But because the boot in Debian is slightly broken,
+runlevel 1 do not work properly and it isn't the same as single user
+mode.  I'll try to explain what is actually happing, but it is a bit
+hard to explain.</p>
+
+<p>Single user mode is defined like this in /etc/inittab:
+"<tt>~~:S:wait:/sbin/sulogin</tt>".  This means the only thing that is
+executed in single user mode in sulogin.  Single user mode is a boot
+state "between" the runlevels, and when booting into single user mode,
+only the scripts in /etc/rcS.d/ are executed before the init process
+enters the single user state.  When switching to runlevel 1, the state
+is in fact not ending in runlevel 1, but it passes through runlevel 1
+and end up in the single user mode (see /etc/rc1.d/S03single, which
+runs "init -t1 S" to switch to single user mode at the end of runlevel
+1.  It is confusing that the 'S' (single user) init mode is not the
+mode enabled by /etc/rcS.d/ (which is more like the initial boot
+mode).</p>
+
+<p>This summary might make it clearer.  When booting for the first
+time into single user mode, the following commands are executed:
+"<tt>/etc/init.d/rc S; /sbin/sulogin</tt>".  When booting into
+runlevel 1, the following commands are executed: "<tt>/etc/init.d/rc
+S; /etc/init.d/rc 1; /sbin/sulogin</tt>".  A problem show up when
+trying to continue after visiting single user mode.  Not all services
+are started again as they should, causing the machine to end up in an
+unpredicatble state.  This is why Debian admins recommend rebooting
+after visiting single user mode.</p>
+
+<p>A similar problem with runlevel 1 is caused by the amount of
+scripts executed from /etc/rcS.d/.  When switching from say runlevel 2
+to runlevel 1, the services started from /etc/rcS.d/ are not properly
+stopped when passing through the scripts in /etc/rc1.d/, and not
+started again when switching away from runlevel 1 to the runlevels
+2-5.  I believe the problem is best fixed by moving all the scripts
+out of /etc/rcS.d/ that are not <strong>required</strong> to get a
+functioning single user mode during boot.</p>
+
+<p>I have spent several years investigating the Debian boot system,
+and discovered this problem a few years ago.  I suspect it originates
+from when sysvinit was introduced into Debian, a long time ago.</p>