36.1.2. 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:
By booting the system from a boot CD-ROM or DVD.
By booting the system from other installation boot media, such as USB flash devices.
By booting the system from the Red Hat Enterprise Linux installation DVD.
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:
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
/mnt/sysimage/. If it fails to mount a partition, it notifies you. If you select , it attempts to mount your file system under the directory
/mnt/sysimage/, but in read-only mode. If you select , your file system is not mounted. Choose if you think your file system is corrupted.
, it attempts to mount your file system under the directory
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):
If you selected
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:
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
, and typing the following command:
, you can still try to mount a partition or LVM2 logical volume manually inside rescue mode by creating a directory such as
mount -t ext4
In the above command,
is a directory that you have created and
is the LVM2 logical volume you want to mount. If the partition is of type
If you do not know the names of all physical partitions, use the following command to list them:
If you do not know the names of all LVM2 physical volumes, volume groups, or logical volumes, use the following commands to list them:
From the prompt, you can run many useful commands, such as:
ping if the network is started
restore for users with tape drives
fdisk for managing partitions
rpm for installing or upgrading software
joe for editing configuration files
If you try to start other popular editors such as
joe editor is started.
18.104.22.168. 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.
linux rescue at the installation boot prompt to enter the rescue environment.
chroot /mnt/sysimage to mount the root partition.
/sbin/grub-install to reinstall the GRUB boot loader, where
bootpart is the boot partition (typically, /dev/sda).
/boot/grub/grub.conf file, as additional entries may be needed for GRUB to control additional operating systems.
Reboot the system.
36.1.3. 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:
At the GRUB splash screen at boot time, press any key to enter the GRUB interactive menu.
Select Red Hat Enterprise Linux with the version of the kernel that you wish to boot and type
a to append 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.