blog/How_is_booting_into_runlevel_1_different_from_single_user_boots_.html

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   6     <title>Petter Reinholdtsen: How is booting into runlevel 1 different from single user boots?</title>
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  12  <h1>
  13      <a href="http://people.skolelinux.org/pere/blog/">Petter Reinholdtsen</a>
  14
  15  </h1>
  16
  17 </div>
  18
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  20     <div class="entry">
  21       <div class="title">How is booting into runlevel 1 different from single user boots?</div>
  22       <div class="date"> 4th August 2011</div>
  23       <div class="body"><p>Wouter Verhelst have some
  24 <a href="http://grep.be/blog/en/retorts/pere_kubuntu_boot">interesting
  25 comments and opinions</a> on my blog post on
  26 <a href="http://people.skolelinux.org/pere/blog/What_should_start_from__etc_rcS_d__in_Debian____almost_nothing.html">the
  27 need to clean up /etc/rcS.d/ in Debian</a> and my blog post about
  28 <a href="http://people.skolelinux.org/pere/blog/What_is_missing_in_the_Debian_desktop__or_why_my_parents_use_Kubuntu.html">the
  29 default KDE desktop in Debian</a>.  I only have time to address one
  30 small piece of his comment now, and though it best to address the
  31 misunderstanding he bring forward:</p>
  32
  33 <p><blockquote>
  34 Currently, a system admin has four options: [...] boot to a
  35 single-user system (by adding 'single' to the kernel command line;
  36 this runs rcS and rc1 scripts)
  37 </blockquote></p>
  38
  39 <p>This make me believe Wouter believe booting into single user mode
  40 and booting into runlevel 1 is the same.  I am not surprised he
  41 believe this, because it would make sense and is a quite sensible
  42 thing to believe.  But because the boot in Debian is slightly broken,
  43 runlevel 1 do not work properly and it isn't the same as single user
  44 mode.  I'll try to explain what is actually happing, but it is a bit
  45 hard to explain.</p>
  46
  47 <p>Single user mode is defined like this in /etc/inittab:
  48 "<tt>~~:S:wait:/sbin/sulogin</tt>".  This means the only thing that is
  49 executed in single user mode is sulogin.  Single user mode is a boot
  50 state "between" the runlevels, and when booting into single user mode,
  51 only the scripts in /etc/rcS.d/ are executed before the init process
  52 enters the single user state.  When switching to runlevel 1, the state
  53 is in fact not ending in runlevel 1, but it passes through runlevel 1
  54 and end up in the single user mode (see /etc/rc1.d/S03single, which
  55 runs "init -t1 S" to switch to single user mode at the end of runlevel
  56 1.  It is confusing that the 'S' (single user) init mode is not the
  57 mode enabled by /etc/rcS.d/ (which is more like the initial boot
  58 mode).</p>
  59
  60 <p>This summary might make it clearer.  When booting for the first
  61 time into single user mode, the following commands are executed:
  62 "<tt>/etc/init.d/rc S; /sbin/sulogin</tt>".  When booting into
  63 runlevel 1, the following commands are executed: "<tt>/etc/init.d/rc
  64 S; /etc/init.d/rc 1; /sbin/sulogin</tt>".  A problem show up when
  65 trying to continue after visiting single user mode.  Not all services
  66 are started again as they should, causing the machine to end up in an
  67 unpredicatble state.  This is why Debian admins recommend rebooting
  68 after visiting single user mode.</p>
  69
  70 <p>A similar problem with runlevel 1 is caused by the amount of
  71 scripts executed from /etc/rcS.d/.  When switching from say runlevel 2
  72 to runlevel 1, the services started from /etc/rcS.d/ are not properly
  73 stopped when passing through the scripts in /etc/rc1.d/, and not
  74 started again when switching away from runlevel 1 to the runlevels
  75 2-5.  I believe the problem is best fixed by moving all the scripts
  76 out of /etc/rcS.d/ that are not <strong>required</strong> to get a
  77 functioning single user mode during boot.</p>
  78
  79 <p>I have spent several years investigating the Debian boot system,
  80 and discovered this problem a few years ago.  I suspect it originates
  81 from when sysvinit was introduced into Debian, a long time ago.</p>
  82 </div>
  83
  84       <div class="tags">Tags: <a href="http://people.skolelinux.org/pere/blog/tags/bootsystem">bootsystem</a>, <a href="http://people.skolelinux.org/pere/blog/tags/debian">debian</a>, <a href="http://people.skolelinux.org/pere/blog/tags/english">english</a>.</div>
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