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.