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 that 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 directory. As you learned earlier, this directory contains each of the volumes that are 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 in order to understand my references to system items.

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


The sections that follow assume that you log in under the Administrator or other user account. If you log in under the root account, you will see a fifth directory under the Mac OS X volume. This is the Private directory and contains various files and folders you will probably never have to work with. If you are very comfortable with using the root account, you can explore this directory; however, be careful not to change anything unless you really know what you are doing.

The Applications Directory

As you know from Chapter 6 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 will 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 that are 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 kinds 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 utility.


    As you might expect, you can add your own images to this file to make them selectable from the Desktop pane of the System Preferences utility. Just place the image file in this folder and you will see it among the available choices in that pane. This has the effect of "permanently" installing an image that you like to use on your desktop on the Desktop pane.

  • 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 SendMail and Apache. Applications can also add documentation here as well. You should explore this folder to see what documentation is available to you here.

  • Internet Plug-ins This directory contains plug-ins that are 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 (com.apple.SoftwareUpdate.plist) and login window preferences (loginwindow.plist).

  • 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 will also be stored here. Within the directory, the drivers are organized into folders, one for each brand of printer that is 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 that you install using the Software Update feature of the OS. You can reinstall these updates or applications by running 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.

  • StartupItems This directory contains startup services that are available on the system-level. For example, when you configure a firewall, the files it needs to start up when the system does are installed in this directory. If you haven't installed any startup services, you won't see this directory.

The System Directory

The System directory contains the basic software that makes the system work. Within it, you will see 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; you can't modify these directories without being logged in as root. And there aren't really many times when you will need to access to the files and directories that are stored here.

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 the filename extension (which stands for Kernel extension).

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

  • Frameworks As you learned back in Chapter 1, 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 utility such as for the date and time (DateAndTime.prefPane), dock (Dock.prefPane), keyboard Keyboard.prefPane), and QuickTime (QuickTime.prefPane). Theoretically, third-party preference panes could be added to this directory to be added to the System Preferences utility.

  • Sounds As you learned earlier in this chapter, this directory contains the alert sounds that are 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. Unlike the StartupItems directory in the top-level Library, there are many startup items isted 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, you will see a directory for each active user account, a directory for the user accounts you have deleted, 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 Directory," p. 19.

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