AppleJack is a command-line tool and collection of scripts for file system repair, and is used in Single User Mode when your computer doesn’t boot all the way to the desktop. Very often, we see computers in our shops that will power on but not progress further than a blue screen, or even past the initial gray screen. The root cause in many cases is a failed hard drive, but about half the time these symptoms are related to file system or other corruption that AppleJack may be able to resolve.
This utility can repair your disk’s file system, repair permissions, validate preference files, and purge your cache files. These repairs will, in many cases, get you back up and running.
AppleJack works when your computer won’t boot because it functions in Single User Mode, and is supported under Mac OS X 10.4 and up. There is one key requirement: it must be installed BEFORE trouble arises, because if your computer won’t boot, you won’t be able to install anything!