Sometimes, things go wrong, and the system you've carefully installed is no longer bootable. Perhaps the boot loader configuration broke while trying out a change, or perhaps a new kernel you installed won't boot, or perhaps cosmic rays hit your disk and flipped a bit in /sbin/init. Regardless of the cause, you'll need to have a system to work from while you fix it, and rescue mode can be useful for this.

To access rescue mode, select rescue from the boot menu, type rescue at the boot: prompt, or boot with the rescue/enable=true boot parameter. You'll be shown the first few screens of the installer, with a note in the corner of the display to indicate that this is rescue mode, not a full installation. Don't worry, your system is not about to be overwritten! Rescue mode simply takes advantage of the hardware detection facilities available in the installer to ensure that your disks, network devices, and so on are available to you while repairing your system.

Instead of the partitioning tool, you should now be presented with a list of the partitions on your system, and asked to select one of them. Normally, you should select the partition containing the root file system that you need to repair. You may select partitions on RAID and LVM devices as well as those created directly on disks.

If possible, the installer will now present you with a shell prompt in the file system you selected, which you can use to perform any necessary repairs. For example, if you need to reinstall the GRUB boot loader into the master boot record of the first hard disk, you could enter the command grub-install '(hd0)' to do so.

If the installer cannot run a usable shell in the root file system you selected, perhaps because the file system is corrupt, then it will issue a warning and offer to give you a shell in the installer environment instead. You may not have as many tools available in this environment, but they will often be enough to repair your system anyway. The root file system you selected will be mounted on the /target directory.

In either case, after you exit the shell, the system will reboot.

Finally, note that repairing broken systems can be difficult, and this manual does not attempt to go into all the things that might have gone wrong or how to fix them. If you have problems, consult an expert.