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 mode and single user mode,
where you can use your own knowledge to repair the system.
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 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
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 is if you use 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.
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). 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, boot off of a Red Hat Linux boot disk or
Red Hat Linux CD-ROM #1, and 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 made
from the boot.img image. This method requires
that the Red Hat Linux CD-ROM #1 be inserted as the rescue image or that
the rescue image be on the hard drive as an ISO image.
[1]
By booting your system from the Red Hat Linux CD-ROM #1.
By booting from a network disk made from the
bootnet.img or PCMCIA boot disk made from
pcmcia.img. 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 Installation Guide.
After booting off a boot disk or Red Hat Linux CD-ROM #1 and providing a valid
rescue image, you will see the following message:
The rescue environment will now attempt to find your Red Hat
Linux installation and mount it under the directory
/mnt/sysimage. You can then make any changes required to your
system. If you want to proceed with this step choose
'Continue'.
If for some reason this process fails you can choose 'Skip'
and this step will be skipped and you will go directly to a
command shell. |
If you select Continue, it will attempt to
mount your filesystem under the directory
/mnt/sysimage. If it fails to mount a partition,
it will notify you. If you select Skip, your
filesystem will not be mounted. Choose Skip if
you think your filesystem is corrupted.
Once you have your system in rescue mode, a prompt appears on VC
(virtual console) 1 and VC 2 (use the
[Ctrl]-[Alt]-[F1] key
combination to access VC 1 and
[Ctrl]-[Alt]-[F2] key
combination to access VC 2):
If you selected Continue to mount your
partitions automatically and they were mounted successfully, you are
in single-user mode.
To mount a Linux partition manually inside rescue mode, create a
directory such as /foo,
and type the following command:
mount -t ext3 /dev/hda5 /foo |
In the above command, /foo is a directory
that you have created and
/dev/hda5 is the
partition you want to mount. If the partition is of type
ext2, replace ext3 with
ext2.
If you do not know the names of your partitions, use the following
command to list them:
If your filesystem is mounted and you want to make your system the
root partition, use the command chroot
/mnt/sysimage. This is useful if you need to run commands
such as rpm that require your root partition to be
mounted as /. To exit the chroot environment,
type exit, and you will return to the
bash# prompt.
From the bash# 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 |
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 single-user mode.
If you are using GRUB, use the following steps to boot into
single-user mode:
If you have a GRUB password configured, type p
and enter the password.
Select Red Hat Linux with the version of the kernel
that you wish to boot and type e for edit. You
will be presented with a list of items in the configuration file
for the title you just selected.
Select the line that starts with
kernel and type
e to edit the line.
Go to the end of the line and type single
as a separate word (press the [Spacebar] and then
type single). Press [Enter]
to exit edit mode.
Back at the GRUB screen, type b to boot into
single user mode.
If you are using LILO, specify 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.
