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11.6 Rescue Modes

Rescue Modes

When things go wrong, there are several ways to work on fixing them. However, they require that you understand the system well. This manual can't teach you what to do, but we will present the ways that you can use our products to get into rescue modes where you can use your own knowlege to rescue the system.

11.6.1 Through LILO

Through LILO

If your system boots, but does not allow you to log in when it has completed booting, you can use the single or emergency boot option. At the LILO boot: prompt, type linux single in order to boot in single-user mode. In single-user mode, your local filesystems will be mounted, but your network will not be activated. In emergency mode, almost nothing will be set up. Only the root filesystem will be mounted, and it will be mounted read-only.

11.6.2 Emergency Boot Diskettes

Emergency Boot Diskettes

The installation diskette set can also be used as a rescue disk set. When you boot the boot diskette, type rescue at the boot prompt (on non-Intel platforms, add rescue as an option to the kernel you are booting, for example, linux rescue), and the install process will ask a few questions, request that you insert the second disk, and set up shells on VC 1 and VC 2. The shell that is running is ash, a minimal bourne shell with no history or command-line editing.

The PATH environment variable is set so that once you have mounted your root partition on /mnt, binaries from your root partition will be run if they do not exist on the diskette. Also, the libraries in /mnt/lib and /mnt/usr/lib will be used if needed. You may wish to change your PATH to not include /bin and /usr/bin, leaving /mnt/bin, /mnt/usr/bin, etc., while you are working on repairing your system. If you do that, you will need to use /bin/umount to unmount /mnt.

Limited versions of important utilities are available before you mount /mnt:

mount
The mount command does not require that the device node being mounted already exist, and doesn't require the full name of the device. It assumes ext2 filesystems unless told otherwise.
mount /dev/sda1 /mnt -t ext2
is functionally equivalent to
mount sda1 /mnt
ash
The full version
cat
Does not accept any options
chmod
The chmod command does not accept symbolic mode names, only octal ones.
cpio
The full version
e2fsck
The full version
fdisk
The full version
gzip
The full version
gunzip
The full version
insmod
The full version
ls
The full version
lsmod
Does not accept any options
mkdir
No GNU long options
mke2fs
The full version
mknod
The mknod command does not accept symbolic mode names, only octal ones.
open
The full version
rm
No GNU long options
rmmod
The full version
sh
Symlink to ash

11.6.2.1 A Handy Trick

A Handy Trick

Have you ever rebuilt a kernel and, eager to try out your new handiwork, rebooted before running LILO? And you didn't have an entry for an older kernel in lilo.conf? Read on...

Here's a handy trick. In many cases, it's possible to boot your Red Hat Linux/Intel from the Red Hat Linux boot diskette with your root filesystem mounted and ready to go. Here's how:

Enter the following command at the boot diskette's boot: prompt:

linux single root=/dev/hdXX initrd=

(Replace the XX in /dev/hdXX with the appropriate letter and number for your root partition.)

What does this do? First, it starts the boot in single-user mode, with the root partition set to your root partition. The empty initrd specification bypasses the installation-related image on the boot diskette, which will cause you to enter single-user mode immediately.

Is there a downside to this trick? Unfortunately, yes. Because the kernel on the Red Hat Linux boot diskette only has support for IDE built-in, those of you with SCSI-based systems won't be able to use this trick. In that case, you'll have to use the boot diskette's normal rescue mode (which also requires the supplemental diskette).


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