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'failsafe' is a standard boot option in all Mandrake
Linux systems.
Under normal circumstances, the system switches right
into the preferred runlevel
during boot ('3' for console, '5' for X). 'failsafe' on the other hand first
boots into runlevel 1 (Single User Mode, see below), then tries to switch
to runlevel 3 (console) and then, if 5 is the default runlevel, into runlevel
5.
If the 'Linuxconf' administration suite is installed, it will be started in
console mode upon reaching runlevel 1. You will be presented with a runlevel
menu or the possibility to use 'Linuxconf' to do system maintenance tasks.
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Linux also provides two built-in rescue systems, one
of them is the 'single user mode', aka runlevel 1. This 'single user' is 'root'.
There will only be a minimum of processes running.
There are several ways to get into this runlevel:
-
From within a running system (as 'root'): init
1. Notice that this command will shutdown almost everything on your
machine. It's also a popular way to simulate a reboot.
-
From the prompt of a boot loader: linux single
or linux init 1. You might also be dropped off here when using
the 'failsafe' boot option if the system can't go to runlevel 3.
There's no login required.
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The single user mode still relies on a working 'init'.
But what if 'init' is corrupted or even missing? If you boot your system with
this boot loader option:
linux init=/bin/sh
only the kernel will be loaded into system memory and
you will be dropped almost immediately to a shell.
On a Mandrake Linux 8.1 system, you should add another
option to turn off devfsd, because otherwise you will run into trouble with
hardware related utilities:
linux init=/bin/sh devfs=nomount
Things you do not have initially in this shell:
- A national key map and font. The default ASCII font
and an American key map is used.
- A PATH containing '/sbin'. Execute all commands from
that directory with their full path, e.g. /sbin/fdisk -l /dev/hda
instead of fdisk -l /dev/hda.
- Mounts apart from the partition containing '/'. Only
'/' is mounted, nothing else (thus the name).
- A writable mount. Not only that just '/' is mounted,
it is also mounted read-only.
- init. Obviously ;-). There's no 'init', no 'shutdown'
or 'halt'. You have to sync and unmount all media you might have mounted during
your root shell session by yourself.
The first thing you should try is getting write access
to your '/' partition:
mount -o remount,rw /dev/device
Run mount to find out the name of device.
Another file system you want to mount is the virtual 'proc' file system, which
provides you and the system utilities with information about what's going
on in your system:
mount /proc /proc -t proc
From here on you should be able to do your repair tasks.
Your main objective should be getting init to work again, so that you can
do further repairs in single user mode.
Before leaving this shell, flush all buffers with
sync
unmount all mounts with
umount -a
and remount the '/' mount read-only again with
mount -o remount,ro /
Press <ALT> <CTRL> <DEL>
simultaneously to leave the shell and reboot the machine.
Notice:
- The flags of the 'mount' command are wrong and do
not reflect changes, e.g. a remount from read-only to read-write. Don't trust
them.
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There's quite a number of Linux distributions which run
off a removable medium (floppy, CD, ZIP) or a Windows partition. CD based
distributions often offer the added advantage of providing a graphical interface.
You'll find a fairly updated list of those Linux distributions
on The LWN.net Linux Distribution
List.
Things to keep in mind when using a third party rescue
distribution:
-
Make sure it works. If you've downloaded a CD image,
run
md5sum name.iso
and compare the resulting number with the one provided
on the server you downloaded the image from.
Floppy images are less prone to transmission CRC errors but media failures
are much more likely. Having put the image onto a floppy, run
cmp /dev/fd0 name.img
to make sure the image on the floppy and the image
you've downloaded are identical.
Boot your new rescue system to check if everything's OK.
-
Check if the distribution is actively maintained.
Linux is a fast moving target. If you can't mount your hard disk partition
because of an out-of-date file system driver on your rescue system, you're
back to square one.
-
Have a look at the included software. If your partitions
are formatted with a less common file system like XFS or JFS, it might happen
that the distribution does not contain the necessary utilities. Use the content
list of the Mandrake Linux rescue system as a template.
The next two pages of this article will list some common
(and some less common) emergency scenarios and describe how to handle them.
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 Emergency scenarios
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