26.2. Booting into Rescue Mode

26.2. Booting into Rescue Mode

Rescue mode provides the ability to boot a small Red Hat Enterprise Linux environment entirely from CD-ROM, or some other boot method, instead of the system's hard drive.

As the name implies, rescue mode is provided to rescue you from something. During normal operation, your Red Hat Enterprise 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 Red Hat Enterprise Linux running completely enough to access 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 Red Hat Enterprise Linux from that hard drive.

To boot into rescue mode, you must be able to boot the system using one of the following methods[12]:

Once you have booted using one of the described methods, add the keyword rescue as a kernel parameter. For example, for an x86 system, type the following command at the installation boot prompt:

        
          linux rescue
        
      

You are prompted to answer a few basic questions, including which language to use. It also prompts you to select where a valid rescue image is located. Select from Local CD-ROM, Hard Drive, NFS image, FTP, or HTTP. The location selected must contain a valid installation tree, and the installation tree must be for the same version of Red Hat Enterprise Linux as the Red Hat Enterprise Linux disk from which you booted. If you used a boot CD-ROM or other media to start rescue mode, the installation tree must be from the same tree from which the media was created. For more information about how to setup an installation tree on a hard drive, NFS server, FTP server, or HTTP server, refer to the earlier section of this guide.

If you select a rescue image that does not require a network connection, you are asked whether or not you want to establish a network connection. A network connection is useful if you need to backup files to a different computer or install some RPM packages from a shared network location, for example.

The following message is displayed:

        The rescue environment will now attempt to find your 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'. You can also choose to mount your file systems read-only instead of read-write by choosing 'Read-only'. 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 attempts to mount your file system under the directory /mnt/sysimage/. If it fails to mount a partition, it notifies you. If you select Read-Only, it attempts to mount your file system under the directory /mnt/sysimage/, but in read-only mode. If you select Skip, your file system is not mounted. Choose Skip if you think your file system 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 to access VC 2):

        sh-3.00b#
      

If you selected Continue to mount your partitions automatically and they were mounted successfully, you are in single-user mode.

Even if your file system is mounted, the default root partition while in rescue mode is a temporary root partition, not the root partition of the file system used during normal user mode (runlevel 3 or 5). If you selected to mount your file system and it mounted successfully, you can change the root partition of the rescue mode environment to the root partition of your file system by executing the following 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 to return to the prompt.

If you selected Skip, you can still try to mount a partition or LVM2 logical volume manually inside rescue mode by creating a directory such as /foo, and typing the following command:

        mount -t ext3 /dev/mapper/VolGroup00-LogVol02/foo
      

In the above command, /foo is a directory that you have created and /dev/mapper/VolGroup00-LogVol02 is the LVM2 logical volume you want to mount. If the partition is of type ext2, replace ext3 with ext2.

If you do not know the names of all physical partitions, use the following command to list them:

        fdisk -l
      

If you do not know the names of all LVM2 physical volumes, volume groups, or logical volumes, use the following commands to list them:

        pvdisplay
      
        vgdisplay
      
        lvdisplay
      

From the prompt, you can run many useful commands, such as:

26.2.1. 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:

  • Boot the system from an installation boot medium.

  • Type linux rescue at the installation boot prompt to enter the rescue environment.

  • Type chroot /mnt/sysimage to mount the root partition.

  • Type /sbin/grub-install /dev/hda to reinstall the GRUB boot loader, where /dev/hda is the boot partition.

  • Review the /boot/grub/grub.conf file, as additional entries may be needed for GRUB to control additional operating systems.

  • Reboot the system.



[12] Refer to the earlier sections of this guide for more details.


Note: This documentation is provided {and copyrighted} by Red Hat®, Inc. and is released via the Open Publication License. The copyright holder has added the further requirement that Distribution of substantively modified versions of this document is prohibited without the explicit permission of the copyright holder. The CentOS project redistributes these original works (in their unmodified form) as a reference for CentOS-5 because CentOS-5 is built from publicly available, open source SRPMS. The documentation is unmodified to be compliant with upstream distribution policy. Neither CentOS-5 nor the CentOS Project are in any way affiliated with or sponsored by Red Hat®, Inc.