When things go wrong, there are ways to fix problems. However, these
methods require that you understand the system well. This chapter will
describe the ways that you can boot into rescue modes, where you can use
your own knowledge to repair the system.
Rescue mode is the ability to boot a small Linux environment entirely
from a diskette or CD, 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 will take a closer look at each of these scenarios.
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
overwrite the Master Boot Record (MBR) that originally contained the
LILO bootloader. If LILO is overwritten in this manner, you will not be
able to boot Red Hat Linux unless you can get into rescue mode.
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. In both of 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.
To boot your system in rescue mode, enter the following command at the
installation boot prompt:
You can get to the installation boot prompt in one of these ways:
By booting your system from an installation boot diskette
[1]
or the Red Hat Linux CD-ROM #1.
By booting from a network or PCMCIA boot diskette. You can only do
this if your network connection is working. You will need to
identify the network host and transfer type. For an explanation of
how to specify this information, see Installing over
the Network in the Official Red Hat Linux x86 Installation Guide.
Once you have your system in rescue mode, a prompt appears on VC
(virtual console) 2 (use the
[Ctrl]-[Alt]-[F2]
key combination to access VC 2):
From this prompt, you can run many useful commands including:
anaconda gzip mkfs.ext2 ps
badblocks head mknod python
bash hwclock mkraid python1.5
cat ifconfig mkswap raidstart
chattr init mlabel raidstop
chmod insmod mmd rcp
chroot less mmount rlogin
clock ln mmove rm
collage loader modprobe rmmod
cp ls mount route
cpio lsattr mpartition rpm
dd lsmod mrd rsh
ddcprobe mattrib mread sed
depmode mbadblocks mren sh
df mcd mshowfat sync
e2fsck mcopy mt tac
fdisk mdel mtools tail
fsck mdeltree mtype tar
fsck.ext2 mdir mv touch
fsck.ext3 mdu mzip traceroute
ftp mformat open umount
gnome-pty-helper minfo pico uncpio
grep mkdir ping uniq
gunzip mke2fs probe zcat |
However, if your root filesystem is undamaged, you can mount it and
then run any standard Linux utility. For example, if your root
filesystem is in /dev/hda5, you can mount
this partition with the following command:
mount -t ext2 /dev/hda5 /foo |
In the above command, /foo is a directory
that you have created.
At this point, you can run chroot,
fsck, man, and other utilities.
You are running Linux in single-user mode.
If you do not know the names of your Linux partitions, you can guess
what they are. Mounting non-existent partitions will do no harm.
You may be able to boot single-user mode directly. If your system
boots, but does not allow you to log in when it has completed booting,
try rebooting and specifying one of these options at the LILO boot
prompt (if you are using the graphical LILO, you must press
[Ctrl]-[x] to exit
the graphical screen and go to the
boot: prompt):
boot: linux single
boot: linux emergency |
In single-user mode, you computer boots to runlevel 1. Your local
filesystems will be mounted, but your network will not be
activated. You will have a usable system maintenance shell.
In emergency mode, you are booted into the most minimal environment
possible. The root filesystem will be mounted read-only and almost
nothing will be set up. The main advantage of emergency mode over
linux single is that your
init files are not loaded. If
init is corrupted or not working, you can
still mount filesystems to recover data that could be lost during a
re-installation.
Have you ever rebuilt a kernel and, eager to try out your new
handiwork, rebooted before running /sbin/lilo? If you did not have an
entry for an older kernel in lilo.conf, you had a
problem. If you would like to know a solution to this problem, read
this section.
In many cases, you can boot your Red Hat Linux system from the Red Hat Linux boot
disk [1] with your root filesystem
mounted and ready to go. Here is how to do it:
Enter the following command at the boot disk'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 command do? First, it starts the boot process 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 disk, which will cause you
to enter single-user mode immediately.
Is there a negative side to using this technique? Unfortunately,
yes. Because the kernel on the Red Hat Linux boot disk only has support for
IDE built-in, if your system is SCSI-based, you will not be able to do
this. In that case, you will have to access rescue mode using the
linux rescue command mentioned above.
