Mac OS X Hacks

Fiddling with Type/Creator Codes and File Extensions

Mac OS X uses a combination of type and creator codes and file extensions to determine the application with which a file is associated.

Every file in OS X and earlier versions of the Mac OS usually have both a type and creator attribute that help determine which application should open them. For example, a .html file may have a type and creator that say it should be opened in Microsoft’s Internet Explorer, while a .mov file would have different attributes that suggest it should be opened by QuickTime. Files also have other attributes, like stationary, locked or unlocked, and timely information like creation and modification dates.

Unlike Windows, however, a Mac file doesn’t need an extension to determine association with an application. A file named webcam could be a JPEG image, a text file to be opened by BBEdit, or even an HTML file associated with Internet Explorer. The type and creator codes rise above petty naming distinctions.

Normally, you’d need special software to set or change these types and creators, making the task more difficult (or expensive) than you’d hope. Longtime users of the Mac OS would often use the venerable ResEdit to perform the dirty deed, perhaps also tweaking other application strings in a fit of mirth and because they’re there.

Thankfully, if you’ve installed Apple’s Developer Tools, you can do this easily with the Terminal [Hack #48].

The easiest way to change a file’s type and creator codes ...