Mac OS X Hacks

Inspecting the Contents of an .app Package

If you were an OS 9 fiddler, tweaker, or deviant, there was one piece of software you simply had to have: ResEdit, Apple’s venerable, unsupported, use-at-your-own risk utility. ResEdit is no longer applicable under OS X, but package editing is.

In earlier versions of the Mac OS, files could have data forks and resource forks. The data fork was the gooey inside, and the resource fork was the fluffy outside — whether it be image thumbnails, saved editing data from applications like BBEdit, or application widgets, like window layouts, user interface images, and so forth. With Apple’s ResEdit, you could easily access this resource fork and change the fluff — it wasn’t easily possible to change the coding of an application, but it was certainly mindless to change interface elements.

In OS X, with its grounding in the BSD operating system, resource forks are rarely used for applications, effectively making ResEdit useless. Instead, we’ve got packages or, less jargony, files that end in .app. You’ve got .app files spread all over your OS X system already — you just may not know it. Take, for instance, Apple’s popular Mail program. It sits innocently in your Applications folder, acting as if it were a single file. Instead, it’s really called Mail.app; the .app is hidden from view (you can confirm its existence by examining the Get Info properties).

The magic of these .app files is that they’re really a special kind of folder called a package; ...