AppleJack checks boot volume integrity, clear accumulated caches, and repair permission issues. It is one of the important preventative tools, and it is a great practice to run it once a week as you do with DiskWarrior.

How to use AppleJack

After downloading AppleJack from above link and ran installer package, below is the steps to run AppleJack. Please note that it is highly recommended to reinstall AppleJack when you update OSX. i.e., OSX10.4.10 > OSX10.4.11, etc, and confirm the current AppleJack version is compatible when upgrading OSX, i.e., OSX10.4.x > OSX10.5.x, etc.

1. Restart your Macintosh computer.
2. 
   As soon as you heard the startup chime, hold Command (Apple) + S keys. This will start Single User Mode without Graphical User Interface. If you see Graphical Interface, the key stroke was not engaged before OSX started up. Simply restart and try again. You will see the Command Line Interface starts to show texts and stops.
3. 
   Type "applejack" (without quotes) followed by [Enter/Return] key. AppleJack starts up an stops for you to enter the option. Type "a" (without quotes) to run all the tasks.
4. 
   When AppleJack finishes, type the letter as instructed to restart or shut down. Note that following boot will take time since all the system cache has been cleared.