Download Red Hat Enterprise Linux 4 System Administration

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Chapter 5. Basic System Recovery
5.2.1. Reinstalling the Boot Loader
In many cases, the GRUB boot loader can mistakenly be deleted, corrupted, or replaced by other
operating systems.
The following steps detail the process on how GRUB is reinstalled on the master boot record:
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Boot the system from an installation boot medium.
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Type linux rescue at the installation boot prompt to enter the rescue environment.
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Type chroot /mnt/sysimage to mount the root partition.
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Type /sbin/grub-install /dev/hda to reinstall the GRUB boot loader, where /dev/hda is
the boot partition.
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Review the /boot/grub/grub.conf file, as additional entries may be needed for GRUB to control additional operating systems.
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Reboot the system.
5.3. Booting into Single-User Mode
One of the advantages of single-user mode is that you do not need a boot CD-ROM; however, it does
not give you the option to mount the file systems as read-only or not mount them at all.
If your system boots, but does not allow you to log in when it has completed booting, try single-user
mode.
In single-user mode, your computer boots to runlevel 1. Your local file systems are mounted, but your
network is not activated. You have a usable system maintenance shell. Unlike rescue mode, single-user
mode automatically tries to mount your file system. Do not use single-user mode if your file system
cannot be mounted successfully. You cannot use single-user mode if the runlevel 1 configuration on
your system is corrupted.
On an x86 system using GRUB, use the following steps to boot into single-user mode:
1. At the GRUB splash screen at boot time, press any key to enter the GRUB interactive menu.
2. Select Red Hat Enterprise Linux with the version of the kernel that you wish to boot and type
a to append the line.
3. 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.
5.4. Booting into Emergency Mode
In emergency mode, you are booted into the most minimal environment possible. The root file system
is mounted read-only and almost nothing is set up. The main advantage of emergency mode over
single-user mode is that the init files are not loaded. If init is corrupted or not working, you can
still mount file systems to recover data that could be lost during a re-installation.
To boot into emergency mode, use the same method as described for single-user mode in
Section 5.3 Booting into Single-User Mode with one exception, replace the keyword single with
the keyword emergency.