When things go wrong, there are ways to fix problems. However, these
methods require that you understand the system well. This chapter
describes how to boot into rescue mode and single user mode,
where you can use your own knowledge to repair the system.
What is Rescue Mode?
Rescue mode provides the ability to boot a small Linux environment entirely
from a diskette, CD-ROM, or using some other method.
As the name implies, rescue mode is provided to rescue you from
something. During normal operation, your Red Hat Linux system uses files
located on your system's hard drive to do everything — run
programs, store your files, and more.
However, there may be times when you are unable to get Linux running
completely enough to access its files on your system's hard drive.
Using rescue mode, you can access the files stored on your system's hard
drive, even if you cannot actually run Linux from that hard drive.
Normally, you will need to get into rescue mode for one of two reasons:
You are unable to boot Linux.
You are having hardware or software problems, and you want to get
a few important files off your system's hard drive.
Next, we take a closer look at each of these scenarios.
Unable to Boot Linux
This problem is often caused by the installation of another operating
system after you have installed Red Hat Linux. Some other operating systems
assume that you have no other operating systems on your computer, and
they overwrite the Master Boot Record (MBR) that originally contained
the GRUB or LILO boot loader. If the boot loader is overwritten in this
manner, you will not be able to boot Red Hat Linux unless you can get into
rescue mode.
Another common problem occurs when using a partitioning tool to resize a
partition or create a new partition from free space after installation
and it changes the order of your partitions. If the partition number
of your / partition changes, the boot loader will
not be able to find it to mount the partition. To fix this problem,
boot in rescue mode and modify
/boot/grub/grub.conf if you are using GRUB or
/etc/lilo.conf if you are using LILO.
Hardware/Software Problems
This category includes a wide variety of different situations. Two
examples include failing hard drives and forgetting to run LILO after
building a new kernel (if you are using LILO as your boot loader). If
you are using GRUB, you do not have to execute a command to reread the
GRUB configuration file. However, if you specify an invalid root
device or kernel in the GRUB configuration file, you might not know
until you reboot your computer.
In these situations, you may be unable to boot Red Hat Linux. If you can get
into rescue mode, you might be able to resolve the problem or at least
get copies of your most important files.
