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I'm interested in learning more about the purpose of each folder underneath /System and /Library.

Some are more obvious than others. /System/Library/Fonts are a basic set of fonts for the system, and /Library/Fonts are any additional fonts available system-wide. But what's the difference between Frameworks and PrivateFrameworks? Can I do anything with the packages under /Library/Receipts? If I install a new app, where under /Library might I find the different dependencies of that app (Frameworks, Extensions, anything else?)

I suppose I'm really just looking for a list of every standard folder under /System and /Library along with a few sentences about what it is for and what might be found inside.

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up vote 5 down vote accepted

The Mac OS X file system hierarchy is covered in great detail in in the Mac Dev Center article, Introduction to the File System Overview. Pages you'll be most interested in are File System Domains and The Library Directory.

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A-ha, for some reason I didn't even think about using the developer documentation. Thanks! – Stephen Jennings Feb 5 '10 at 0:48

The user and system Library folders, from Apple's Mac Dev Centre:

  • Application Support: Contains application data and support files, under subdirectories named for each app
  • Assistants: Contains utilities that assist in configuration
  • Caches: Contains cached data
  • ColorPickers: Contains color picker palettes, available to all applications
  • ColorSync: Contains ColorSync profiles, which are used for color management
  • Components: Contains system bundles
  • Contextual Menu Items: Contains contextual menu plugins
  • Documentation: Contains the system help files
  • Extensions: Contains drivers and kexts
  • Favorites: Contains file aliases
  • Fonts: Contains font files
  • Frameworks: Contains shared frameworks and libraries
  • Internet Plug-ins: Contains plugins to display content in web browsers
  • Keyboards: Contains keyboard definitions
  • Logs: Contains log files
  • Mail: Contains user's mailbox
  • PreferencePanes: Contains plugins for the System Preferences application
  • Preferences: Contains user preferences
  • Printers: Contains print drivers
  • QuickTime: Contains QuickTime plugins
  • Screen Savers: Contains screen savers
  • Scripting Additions: Contains AppleScript plugins
  • Sounds: Contains system alert sounds
  • StartUpItems: Contains scripts and applications to be run on startup
  • Web Server: Contains the Apache web server root
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Receipts are... well like shopping receipts. You can only check what was installed but the real content is not there. The system uses it, for example, to look file permissions when running maintenance jobs.

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