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AppleJack - A utility for use in single user mode Pick of the Week
AppleJack is a different kind of utility -- it's one that you'll probably only use when you're in a real bind, say with a Mac that crashes every time you login. In such cases, the usual advice is to boot off the installer disk, and use the various tools available there to try to fix the problem.

But what if you're traveling with your laptop, and you don't happen to have your install disk with you? Enter AppleJack, which runs only in single user mode (Command-S at startup). In its basic mode, AppleJack does five things (either as a group, or one by one): repair disks, repair permissions, clean up cache files, validate preference files, and remove swap files. You access these tasks through a text-based menu (as you're in single user mode, there's no GUI).

Beyond the basics, an expert menu (press 'x' on the main menu) offers some additional (though unproven and potentially dangerous) options, including checking hard drive integrity, blessing the system folder, disabling auto login, and more. One of the less-dangerous and potentially very useful features in the advanced section is a memory tester -- if your Mac is having a series of odd, ever-changing crashes, it's possible you've got some bad RAM, and a memory test is one way to find such problems. Apple's got a RAM tester on their Hardware Test disc, but it's great having one built into single user mode via AppleJack. AppleJack bundles Memtest, an excellent memory tester. (Note that to get access to memtest, you'll need to install it when you install AppleJack; read the Read Me for all the details.)

I've been waiting on this Pick of the Week for nearly a year -- until very recently, AppleJack wasn't compatible with Leopard. Now that it is, though, it's a great addition to your troubleshooting toolkit. You may not ever need to use it, but having it with you at all times (especially if you travel a lot) is a nice security blanket. (Dan Frakes recently covered AppleJack for Macworld; read his writeup if you'd like more info on the program.)
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AppleJack - A utility for use in single user mode
Authored by: thermophile on Sep 09, '08 07:09:38AM

I used Applejack extensively when I was on 10.4, but with switch to 10.5, I stopped using it. I recall some site saying that it was not compatible with Leopard. I went to the site and the most recent version is 1.4.3, released Nov, 2006 - before Leopard. So, has anyone verified that this excellent tool does work as designed with 10.5?

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AppleJack - A utility for use in single user mode
Authored by: Thomas_U on Sep 10, '08 11:42:30AM

I have been betatesting AJ 1.5beta and have used the release version 1.5 about 10 times so far and it works without problems on my system.

OSX 10.5.4 (BUILD 9E17), G4 DUAL 867 MDD, 2 GB RAM


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AppleJack - A utility for use in single user mode
Authored by: davidanders on Sep 09, '08 07:27:30AM

The 1.5 version was released on Sept 2, 2008. It can be downloaded at or or sourceforge.
There is a possible problem with improperly uninstalled apps, Little Snitch is one, that leave incompatible kexts that can cause problems. The bug report on AppleJack has the details.
I have run it on three Intel machines, so far.

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