Rescue Mode
Basic System Recovery
When things go wrong, there are ways to fix problems. However, these methods require that you understand the system well. This chapter describes how to boot into rescue mode, single-user mode, and emergency mode, where you can use your own knowledge to repair the system.
You might need to boot into one of these recovery modes for any of the following reasons:
- You are unable to boot normally into Red Hat Linux runlevel(3 or 5).
- You are having hardware or software problems, and you want to get a few important files off of your system's hard drive.
- You forgot the root password.
Unable to Boot into Red Hat Linux
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. 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 and reconfigure the boot loader.
Another common problem occurs when using 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 might 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. You must also run the /sbin/lilo command anytime you modify the LILO configuration file.
Hardware/Software Problems
This category includes a wide variety of different situations. Two examples include failing hard drives and specifying an invalid root device or kernel in the boot loader configuration file. If either of these occur, you might not be able to reboot into Red Hat Linux. However, if you boot into one of the system recovery modes, you might be able to resolve the problem or at least get copies of your most important files.
Root Password
What can you do if you forget your root password? To reset it to a different password, boot into rescue mode or single-user mode and use the passwd command to reset the root password.
Booting into Rescue Mode
Rescue mode provides the ability to boot a small Red Hat Linux environment entirely from a diskette, 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 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 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 Linux from that hard drive.
To boot into rescue mode, be able to boot the system using one of the following methods:
- By booting the system from an installation boot diskette made from the bootdisk.img image.
- By booting the system from an installation boot CD-ROM.
- By booting the system from the Red Hat Linux CD-ROM #1.
Once you have booted using one of the described methods, enter the following command at the installation boot prompt:
"linux rescue"
You are prompted 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 Linux as the Red Hat Linux CD-ROM #1 from which you booted. If you used a boot CD-ROM or diskette to start rescue mode, the installation tree must be from the same tree from which the media was created.
If you select a rescue image that does not require a network connect, 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.
You will also 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'. 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 will attempt to mount your file system under the directory /mnt/sysimage. If it fails to mount a partition, it will notify you. If you select "Read-Only", it will attempt to mount your file system under the directory /mnt/sysimage, but in read-only mode. If you select "Skip", your file system will not be 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
-/bin/sh-2.05b#
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, and you will return to the prompt.
If you selected "Skip", you can still try to mount a partition manually inside rescue mode by creating a directory such as /foo, and typing 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:
fdisk -l
From the prompt, you can run many useful commands such as
- list-harddrives to list the hard drives in the system
- ssh,scp, and ping if the network is started
- dump and restore for users with tape drives
- parted and 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 emacs, pico, or vi, the joe editor will be started.)
To create an installation boot diskette, insert a blank diskette and use the images/bootdisk.img file on the Red Hat Linux CD-ROM #1 with the command
dd if=bootdisk.img of=/dev/fd0
Booting into Emergency Mode
In emergency mode, you are booted into the most minimal environment possible. The root file system will be mounted read-only and almost nothing will be 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 9.3 Booting into Single-User Mode with one exception, replace the keyword "single" with the keyword "emergency".
Booting into Single-User Mode
One of the advantages of single-user mode is that you do not need a boot diskette or CD-ROM; however, it does not give you the option to mount the file systems as read-only or not mount them at all.
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 can not be mounted successfully. You can not use single-user mode if the runlevel 1 configuration on your system is corrupted.
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 have 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, at the LILO boot prompt (if you are using the
graphical LILO, press
"linux single"
