Wouter Verhelst have some
interesting
comments and opinions on my blog post on
the
need to clean up /etc/rcS.d/ in Debian and my blog post about
the
default KDE desktop in Debian. I only have time to address one
small piece of his comment now, and though it best to address the
misunderstanding he bring forward:
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)
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.
Single user mode is defined like this in /etc/inittab:
"~~:S:wait:/sbin/sulogin". This means the only thing that is
executed in single user mode is 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).
This summary might make it clearer. When booting for the first
time into single user mode, the following commands are executed:
"/etc/init.d/rc S; /sbin/sulogin". When booting into
runlevel 1, the following commands are executed: "/etc/init.d/rc
S; /etc/init.d/rc 1; /sbin/sulogin". 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.
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 required to get a
functioning single user mode during boot.
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.