In brief, you can boot even an unbootable system using the linux rescue environment. Using the first RHEL installation CD, type linux rescue at the boot: prompt. The first couple of steps are the same as those for installing RHEL 5. If the linux rescue environment detects your system, it may mount the standard directories in /mnt/ sysimage subdirectories in read-write or in read-only mode. If your filesystems are not mountable, you can open a command prompt and continue with your troubleshooting.
When you type linux rescue at the installation boot prompt and go through the steps, the installation discs install a compact version of a root filesystem. To boot into linux rescue mode, first boot your system using the first installation CD in a bootable CD-ROM drive,
Now take the following steps:
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Boot your system from the first RHEL 5 installation CD.
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Type linux rescue at the boot: prompt . Your system boots a basic Linux system from the first installation CD.
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Select an appropriate language when prompted.
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Select an appropriate keyboard type when prompted.
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You'll see the following message, briefly:
Running anaconda, the Red Hat Enterprise Linux rescue mode - please wait...
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You'll be asked whether you want to set up the network interfaces on the local system. Select Yes if you need to connect to a network installation source to install other packages; otherwise, select No and skip to step 8.
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You'll see a network configuration window for the local network card, similar to what's . If directed by your instructor or exam proctor to set up a static IP address, follow the instructions carefully; otherwise, try to configure this interface using a local network DHCP server. If you set up a static IP address, you'll see another screen where you're prompted to enter a gateway, a primary DNS, and a secondary DNS IP address.
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Select one of the three options for the rescue environment, . Generally, you should try the Continue option first, followed by Read-Only. Continue mounts your RHEL filesystems in read-write mode. Read-Only mounts RHEL file systems in read-only mode. Skip does not mount any of your RHEL filesystems. I address each of these three options in detail in the following sections.
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When successful, you'll see a message to the effect that your system has been mounted under /mnt/sysimage. When you select OK (the only option), you'll see the following prompt, where you have root privileges.
When you select Continue from the screen
, you're taken through the standard linux rescue environment. The rescue files search for your root directory (/) filesystem. If found, your standard root directory (/) is mounted on /mnt/sysimage. All of your other regular filesystems are subdirectories of root; for example, your /boot directory will be found on /mnt/sysimage/boot.
Not all of your filesystems may mount properly. You may see error messages such as:
An error occurred trying to mount some or all of your filesystem
This suggests that at least one of the filesystems listed in /etc/fstab isn't mounting properly for some reason. If the linux rescue environment has no problems, you'll see a message noting that your system has been mounted,
Select OK. You should see the following prompt messages:
Your system is mounted under the /mnt/sysimage directory.
When finished please exit from the shell and your system will reboot.
You'll use the chroot /mnt/sysimage command shortly. Now you
can work on repairing any files or filesystems that might be damaged. First,
check for unmounted filesystems. Run a df command.
Compare the result to the /mnt/sysimage/etc/fstab configuration file. If some filesystem is not mounted, it may be configured incorrectly in the fstab file. Alternatively, the label associated with a partition may not match the filesystem shown in your fstab file. For example, to find the label associated with /dev/sda1, run the following command:
# e2label /dev/sda1
This should return the name of a filesystem to be mounted on that partition such as /boot.
Sometimes an unmounted filesystem just needs a little cleaning; remember, a command such as the following cleans the /dev/sdb1 partition:
# fsck /dev/sdb1
The fsck command works only on an unmounted filesystem. For example, if you get a message such as:
WARNING!!! Running e2fsck on a mounted filesystem may cause
SEVERE filesystem damage.
unmount the subject filesystem with a command such as umount /mnt/sysimage/boot. If that doesn't work, restart the rescue process. When you get to the screen select Skip and read the "No Mount linux rescue Environment" section later in this topic
Alternatively, you may see a message like:
fsck.ext2: Device or resource busy while trying to open /dev/hda2
filesystem mounted or opened exclusively by another program?
This is presented when the partition, in this case /dev/hda2, is part of a Logical Volume Manager (LVM) array as described in topic 8. In that case, you'll need to review your /mnt/sysimage/etc/fstab file for the appropriate logical volume (and unmount it), before trying to apply the fsck
Read-Only linux rescue Environment
When you select the Read-Only option , you'll get the same basic prompt. There is little difference between regular and read-only rescue mode. The rescue system attempts to do everything that it would under regular mode, except all partitions associated with your standard system are mounted read-only. (Some of the rescue system filesystems are still mounted as read-write.)
This is appropriate if you have a large number of mounted filesystems; it can help you cull through what is and isn't working with less risk of overwriting key configuration files.
No Mount linux rescue Environment
When you select the Skip option shown in Figure 16-6, the rescue environment doesn't even search for a Linux installation. A minimal root image is loaded into a RAM disk created by the kernel, and you're taken to a root shell prompt (#), as shown:
When finished, please exit from the shell and your system will reboot.
At this point, you have access to a basic set of commands. You can mount filesystems, create directories, move files, and use editors such as vi. As nothing from your physical drives is mounted, you can apply the fdisc and fsck commands to various hard disks and partitions. A few other basic commands are also available.
The great difficulty in operating from the rescue environment is that you are working with a minimal version of the Linux operating system. Many of the commands you are accustomed to having at your disposal are not available at this level. If your root partition has not been completely destroyed, you may be able to mount this partition to your temporary root directory in memory and access commands from there.
But you may need a little help identifying the partitions on your system. As I'll show you shortly, the fdisk -l /dev/hda command lists the configured partitions on the first IDE hard drive. You can create a new directory such as /mnt/sysimage, mount a partition such as /dev/hda2 on that directory, and check the result with the following commands:
# mkdir /mnt/sysimage
# mount /dev/hda2 /mnt/sysimage
# ls /mnt/sysimage
If you can verify that you've mounted the standard root directory (/) filesystem on the /mnt/sysimage directory, you can run the chroot /mnt/sysimage command. You can then have full access to the commands and configuration files available under that mounted partition.