Appendix B: Rescue Mode

When things go wrong, there are ways to fix problems. However, these methods require that you understand the system well. This appendix describes how to boot into rescue mode and single-user mode, where you can use your own knowledge to repair the system.

What Is Rescue Mode?

Rescue mode provides the ability to boot a small Linux environment entirely from a diskette or CD-ROM, or via some other method. 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 Linux running completely enough to access its 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 Linux from that hard drive. Normally, you will need to get into rescue mode for one of two reasons:

• You are unable to boot Linux.

• You are having hardware or software problems, and you want to get a few important files off your system’s hard drive.

Next, we take a closer look at each of these scenarios.

Unable to Boot 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, and 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.

Another common problem occurs when using a partitioning tool to resize a partition or create a new partition from free space after installation and the tool changes the order of your partitions. If the number of your / partition changes, the boot loader will 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.

Hardware/Software Problems

This category includes a wide variety of different situations. Two examples include failing hard drives and failing to run LILO after building a new kernel (if you are using LILO as your boot loader). If you are using GRUB, you do not have to execute a command to reread the GRUB configuration file.

However, if you specify an invalid root device or kernel in the GRUB configuration file, you might not know it until you reboot your computer. In these situations, you may be unable to boot Red Hat Linux. If you can get into rescue mode, you might be able to resolve the problem or at least get copies of your most important files.