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Re: How to choose safe configuration at boot time?

> I am unable to use my Debian Linux system after the major update I did
> on Tuesday.
> WHAT HAPPENED: I used dselect in an xterm window under XFree to update the
> system after an idle period of 1 week. I update from stable, unstable,
> contrib, and nonfree. I allowed one of the installers to shut down xdm,
> which led to a reboot. I finished the installation by starting dselect again
> from an Xless console. The next reboot led to a login prompt on a non-X
> console screen, flashing and ignoring all input, except for CTL-ALT-DEL
> which led to another reboot ending in the same state.

Something similar happened to me.  I figured out what was wrong in my
case, which was similar.

In my case, because I knew that xbase required restarting X, I
intentionally stopped X before doing the install.

I also ran the new XF86Setup program, and configured it for my card, a
Cirrus 5426-based VLB video card.  XF86Setup decided that probing my
dot clocks and adding a "clocks" line to the XF86Config file would be
a good thing.  My video card doesn't need a clocks line, and in fact,
the X server won't run with it.

So at reboot, xdm starts up.  It runs the X server, which finds the
problem with the configuration, and exits.  So xdm starts it again.
It exits, and repeat.

I basically dealt with it by careful typing... (flash) r (blank)
(flash) o (blank) (flash) o (blank) (flash) t (blank) (flash) enter
(blank) (flash) and so on.  It took me several attempts before I was
able to time -all- the needed keypresses in my password correctly
(that'll teach me to use a 11-character root password).  But then I
was able to kill xdm, start X manually (and thus discover the
problem), and fix.

What happens is that while the VC is on the login prompt, it is
listening to the keyboard, but when X takes over, it switches the VC.
WHen I was able to finally kill it, the other VC contained all of my
mistimed keystrokes.

> WHAT I THINK IS WRONG: I think that my configuration of xdm is broken. The
> last two outputs from the boot process, before the login prompt and the
> flashing, are an indication that xdm is starting, and an indication that the
> nas, which started earlier, has failed.

It is probably true that it is a misconfigured X server.  That is
exactly what I saw causing my similar problems.

> WHAT I THINK I NEED: I think that I need to boot the system in single-user
> mode, or otherwise avoid starting up xdm, so that I can seek out the bad
> configuration files and fix them. 

This will work, but I was also very surprised that when upgrading X on
my system, the install scripts decided that starting XDM for me was
good.  Even though I was in single-user mode.

> I have *only* Debian Linux on the system,
> and I start it with LILO. I have been able to run several versions of the
> kernel from LILO, including a very primitive one which I keep on a diskette
> for emergencies. They all lead to the same behavior. Probably, there is an
> appropriate option to give LILO, and it's probably mentioned in the online
> documentation which I am now unable to read :^{ (I hope that this emoticon
> portrays embarassment). In the past, I have used CTL-ALT-F1 to bring up a
> single-user state, but I have been getting no useful response to this
> signal, although it resets the phase of the flashing, so something is
> evidently reading and ignoring it.

When starting with LILO, hit the shift key to get a boot: prompt, then
type linux emergency, which will start it in single user mode.  

Or try timing your keystrokes, like I did.  (My system has a long
uptime I didn't want to ruin even when upgrading).

> I'll be grateful for all suggestions. Useful ones will help me, and useless
> ones will at least distract me from despair for a while :^)
> Mike O'Donnell
> odonnell@cs.uchicago.edu
> http://www.cs.uiowa.edu/~odonnell
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