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 Enterprise 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.
26.1.1. Unable to Boot into Red Hat Enterprise Linux
This problem is often caused by the installation of another operating system after you have installed Red Hat Enterprise Linux. Some other operating systems assume that you have no other operating system(s) on your computer. They overwrite the Master Boot Record (MBR) that originally contained the GRUB boot loader. If the boot loader is overwritten in this manner, you cannot boot Red Hat Enterprise 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 the /boot/grub/grub.conf file.
For instructions on how to reinstall the GRUB boot loader from a rescue environment, refer to Section 26.2.1, “Reinstalling the Boot Loader”.
26.1.2. 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 Enterprise 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.
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.