Mac OS X to the Max: Exploring Mac OS X

If you have read most of this book to this point, you have already explored quite a bit of Mac OS X and have seen how its structure and organization differ from previous versions of the Mac OS. The purpose of this section is to give you an overview of the Mac OS X system so you are familiar with its most important parts from a holistic perspective.

Starting from the top level or root of the machine is the Computer folder. As you learned earlier, this directory contains each of the volumes mounted on your machine as well as the Network folder.

In this section, I refer to the Mac OS X startup volume as the Mac OS X volume (a clever feat on my part, eh?). If you have named your Mac OS X startup volume something different from this, you need to swap your name for mine to understand my references to system items.

If you open the Mac OS X volume, you see the following four system directories: Applications, Library, System, and Users.

The Applications Directory

As you know from Chapter 6, "Installing and Using Mac OS X Applications," and other chapters, the Applications directory is the default installation directory for all Mac OS X applications. This directory contains application package files as well as application folders and folders that contain other applications (such as the Utilities directory). Mac OS X includes a large number of applications in this directory by default, and you generally install all other Carbon or Cocoa applications in this directory as well.

The Library Directory

The Library directory contains system-level Library resources that are modifiable when you are using an administrator account. As you saw earlier in this chapter, one of the directories in this directory is the Fonts folder in which you store the fonts available to all user accounts. However, this directory contains many more directories than just this one, such as the Sounds directory that you also learned to change earlier in the chapter. Basically, any system-level resources that can be changed (without logging as root) are stored in this Library directory. Some examples are the following:

  • Application Support? This directory contains files that provide various types of support to specific applications. For example, the StuffIt Engine that provides services to several StuffIt applications (such as StuffIt Expander) resides here.

  • Desktop Pictures? These graphics files are available for you to use as desktop pictures, which you configure using the Desktop pane of the System Preferences application.

  • Documentation? This is an interesting directory; it contains documentation and support files for various services that are part of Mac OS X. For example, you can find files for the Mac OS X help system here as well as information about Unix services such as Apache. Applications can also add documentation here. You should explore this folder to see what documentation is available.

  • Internet Plug-ins? This directory contains plug-ins accessible to Internet applications you have installed on your system.

  • Preferences? System-level preferences (as opposed to user-level preferences) are stored here. Examples are Software Update preferences ( and login window preferences (

  • Printers? This directory contains the printer drivers. Mac OS X includes native support for many printers, and the drivers for any printers you add to the system are also stored here. Within the directory, the drivers are organized into folders, one for each brand of printer installed.

  • Receipts? This is another interesting directory you should explore. It contains various packages for applications and software updates you have installed on your machine, such as OS updates you install using the Software Update feature of the OS. You can view information about these updates or applications by opening one of the packages you find here.


When you download a system update using the Software Update, you can run its package to read about what was installed. Launch the update's installer and move to the readme page to read about that update.

The System Directory

The System directory contains the basic software that makes the system work. It contains one directory, which is the Library directory (not to be confused with the Library directory on the root level of the Mac OS X volume).


The System directory is the most analogous to the System Folder in previous versions of the Mac OS. However, the System Folder was really more a combination of the System and Library directories under Mac OS X.

This Library directory contains the fundamental operating system files that provide the services needed to make Mac OS X work. It contains many directories, which you can't modify without being logged in as root. And there aren't really many times when you will need to access the files and directories stored here.


The Mac OS X directory has many other files and subdirectories that are invisible to you when you are logged in under the Mac OS X interface, even if you are logged in under the root account. You can see all the files installed on your Mac using the Terminal application and Unix commands.

Examples of the directories contained in the System's Library are the following:

  • ColorPickers? These files provide the Color Picker services you learned about earlier in this chapter.

  • Components? Component files (filename extension .component) provide various system services such as AppleScript support and the Sound Manager.

  • CoreServices? As its name implies, this directory contains files that provide core services to the operating system, such as the Dock, Finder, Help Viewer, Login window, and so on. Several of the items in this directory are applications, including the Finder and Dock.

  • Extensions? Although extensions in the traditional Mac OS sense are not part of Mac OS X, there are Mac OS X extensions. These files provide support to various hardware devices and hardware-related services. Mac OS X extension files have .kext as their filename extension (which stands for Kernel extension).

  • Fonts? This Fonts directory provides the fonts that are fundamental to the system, such as those used in the menu bar.

  • Frameworks? As you learned in Chapter 1, "Mac OS X: Foundations," Mac OS X frameworks are the subsystems within the OS that provide various services. The files related to these frameworks are stored in the Frameworks directory. Examples of frameworks installed in this directory are AppleShare, Cocoa, Java, QuickTime, and Security.

  • OpenSSL? Secure Sockets Layer (SSL) is the most common encoding scheme used to securely transmit data over the Internet. This directory contains information related to SSL on your machine, such as the various SSL certificates you have installed.

  • PreferencePanes? This directory contains the panes in the System Preferences application such as for the date and time (DateAndTime.prefPane), Dock (Dock.prefPane), keyboard (Keyboard.prefPane), and QuickTime (QuickTime.prefPane). Interestingly, third-party preference panes aren't added to this directory when they are installed on the System Preferences application. This folder contains all possible Apple System Preferences application panes, even if they don't appear to you (such as the Ink pane that doesn't appear unless you have a tablet installed).

  • Sounds? As you learned earlier in this chapter, this directory contains the alert sounds available to all the users on your machine.

  • StartupItems? This directory contains additional system services that become active when the system starts up. Examples include AppleShare, Network services, and so on. Many startup items are listed in this directory, such as the Apache Web server, AppleTalk, Network services, and so on.

The Users Directory

The Users directory contains the Home directory for each user account configured on your machine. Within this directory are a directory for each active user account, a directory for the user accounts you have deleted (if you chose to save the user's Home folder), and the Shared directory for items that can be shared.


The one user directory you won't see is the root directory. That is hidden except when you are logged in as root.

To learn more about the contents of a user's Home directory, see "Understanding the Home Folder," p. 21.

    Part I: Mac OS X: Exploring the Core
    Part III: Mac OS X: Living the Digital Life