As someone who's written books and many articles on Mac troubleshooting, one of my favorite utilities has long been The Apotek's AppleJack, a clever utility that lets you perform a number of troubleshooting procedures at startup--without requiring a Mac OS X or third-party CD or DVD. Unfortunately, Leopard rendered the previous version of AppleJack inoperable, so I've done without for nearly a year.

With the recent release of AppleJack 1.5, you can now install and use the utility on Macs running Leopard (OS X 10.5), so today's as good a time as any to revisit this essential troubleshooting tool.

Installing AppleJack requires an admin-level user account, as it modifies OS X's startup process and makes a minor (and safe) tweak to the root account. Once installed, AppleJack is available in OS X's single-user mode, accessed by holding down Command+S at startup. Just below the standard "If you want to make modifications to files" message, you see a new message instructing you to type applejack for troubleshooting assistance. Doing so brings up a textual menu of options; you simply type the desired number or letter and press return to perform that task.

Repair Disks uses OS X's built in fsck program--the same one used by Disk Utility--to examine and, if necessary, repair minor damage to your startup drive's directory. Repair Permissions is identical to using Disk Utility's Repair Disk Permissions function (except it takes a bit longer in single-user mode). Cleanup Cache Files removes temporary cache files in case a corrupt cache is the cause of your problems. Validate Preference Files checks preference files for possible corruption; those that don't pass muster are moved to a new "Preferences (Corrupt)" folder in the same parent Library folder as the originals. Finally, Clean Up Virtual Memory manually deletes any virtual-memory swap files that weren't cleaned up automatically by the OS.

You can choose to run all these tasks in one fell swoop, in order, by typing auto (all lowercase) and pressing return. If you instead type AUTO (all uppercase), AppleJack goes into "deep clean" mode, which means it deletes all system cache files, including the LaunchServices database. The developer recommends this mode only if the standard auto procedure doesn't fix enough problems to let your Mac boot properly.

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