Mac Software Basics

File System
Macintosh disks are formatted with a Hierarchical File System (HFS), which defines files and subdirectories on the disk. The Mac file system allows file names to be up to 31 characters long, and it may use any character (including accented and other special characters) except for a colon ( : ). This restriction exists because the colon is used to separate subdirectories in the directory structure, like:
Macintosh HD:System Folder:Control Panels:Keyboard

The previous line specifies the complete path of the Keyboard control panel; the colon serves the same purpose as a forward slash in Windows or a backslash in DOS.

Mac OS X is founded around a Unix core. Since Unix uses the forward slash ( / ) the same way the Mac OS uses the colon, the directory structure in Mac OS X is represented with slashes. For example, the same OS 9 directory above would be represented as follows:
macintoshhd/systemfolder/controlpanels/keyboard

You'll also note that most filenames do not have spaces (though Unix is capable of handling spaces, most prefer not to put spaces in file names).

All Macintosh files are composed of two distinct parts, a resource fork and a data fork. This setup is unique to the Macintosh, as PCs and Unix all have only a single fork. Each fork is used for a different purpose. For most applications, the resource fork contains most of the program code, associated icons, document encoding information, file identifiers, sounds and other software necessary for running the program. For a document, such as a Word document or text file, the content is stored in the data fork. Some programs create documents that use both forks, so the data is stored in the data fork, and formatting, configuration, or preferences may be stored in the resource fork. When transporting a file from a Mac to another platform, the only part that is usually kept intact is the data fork; the other platform does not even know the resource fork exists. Macs use "File Creator" and "File Type" resources in the resource fork to keep track of the application to which a file belongs. A common Mac troubleshooting technique is Rebuilding the desktop, a procedure that forces the Mac to check all of the File Types to make sure that the Finder knows to what applications the files belong.

Desktop
The desktop is the workspace you see when you start up the Mac, and it is set up like a desk, where you can look through folders, manage files, throw things into the trash can or put a handy item on top of your desk. The contents of the desktop are actually stored in an "invisible" folder called the Desktop folder, which resides on the hard drive. In OS X, each user has an individual desktop folder in their home directory, meaning every user has an individual desktop and can customize its appearance without affecting other users.

All mounted drives appear as individual icons on the desktop, as well as printers, and, in OS 9 and earlier, the Trash can. In OS X, the Trash can appears on the far right of the Dock.

The menu bar is always constant on the screen, and may be considered part of the "desktop" since it pervades all aspects of Macintosh software. Be aware that when switching applications, it is often only the items on the menu bar that change; there will be no other visual indicator of the switch. In OS 9 and previous versions of the Mac OS, there are three items on the menu bar which never change: the Apple Menu on the far left, the Help Menu and the Application Menu on the far right (though the icon changes). Clicking on the Application Menu allows you to switch between open programs.

In OS X, two menu items remain constant when switching between applications: the apple menu always appears in the top left corner; and the second menu from the left is the Application menu for the currently active application: it will usually be named after the application. Additionally, the OS X desktop includes the Dock, a toolbar along the bottom of the screen, that allows you to find commonly used applications and easily switch between active applications. The Dock will always appear on the screen and indicates active applications with a small, black arrow beneath the icons.

System Folder
The System folder is responsible for most of the Macintosh magic; it holds the basic operating system, configuration software, fonts, sounds and most preferences. You can locate the System folder by double-clicking on the icon for your hard drive on your desktop.

Most items in the System folder have a specific subfolder where they belong. The location of the subfolders determines the order in which they are loaded when booting up as well as if they will work at all, so it is imperative to be sure items are in their proper places. You can select Get Info from the File menu and get a file description for the selected item thatwould help determine where it should be.

If you drag an extension or control panel onto the System folder icon (NOT the open window), the system will prompt you if you would like to place the item in the appropriate folder. This warning only occurs if the system recognizes the file, otherwise it is placed in the System folder with no query.

Mac OS X has an entirely different System folder layout than previous versions of the Mac OS. The System folder contains a Library with information for the operating system's services as well as most preferences and configuration settings. The CoreServices folder contains many of the key OS X services (including the Dock, help tool and the Software Update application as well as the System and Finder files described below).

System File and Finder
The files called System and Finder in the System folder form the basis for the entire operating system. The System file (which has an icon that looks like a suitcase) defines most of the basic interface and system processes; the Finder acts as a "shell" to manage files and launch applications. The term "Finder" is sometimes used interchangeably with "desktop," since the Finder is the "application" which is open when your desktop is active.

The System file and the Finder must always live in the same folder. In theory, this folder may be named anything, and may be anywhere on the hard disk. By convention, though, the System file and Finder are kept in the System folder in the root of the bootup drive. Separating these two files is disastrous, as it makes the disk unbootable!

Trash Can
The Trash is a folder which acts as a holding ground for deleted files. In Mac Os 9 and earlier, the Trash can appears on the dekstop; in OS X it's found on the Dock. To delete a file, move it to the Trash, where it will remain intact until you select "Empty Trash" from the Special menu (or from the Finder menu in OS X).

It is important to realize that the space used by a trashed file is not available until the trash is emptied. At that point, the file is unrecoverable by ordinary means, but may be recoverable by special disk recovery tools.

The Trash can also serve as a recovery location for temporary files that some programs leave behind when they crash.

Extensions
The Extensions folder comprises the root of many of the advanced features of the MacOS. For example, the Extensions folder contains Chooser extensions such as AppleShare (necessary for file sharing), System extensions such as QuickTime (for streaming video) and printer drivers (for example, if you do not see a "LaserWriter 8" icon in the Chooser, it could be because the corresponding driver in the Extensions folder has been disabled). The files in this folder load before the control panels and usually offer no configuration options. It is not uncommon for a Macintosh to have an extension conflict. To troubleshoot such a problem, it may be necessary to disable extensions by removing them from the System folder or using the Extensions Manager control panel. Not all extensions will appear in the Extensions Manager, so be aware of this fact when troubleshooting.

Extensions are a feature of older Macintosh operating systems (OS 9 and earlier); OS X does not use extensions.

Startup Items
After all of the control panels and extensions have loaded, and after the Finder has established itself, the items inside of the Startup Items folder are launched. Like the Extensions and Control Panels folders, the Startup Items folder is in the System folder. The startup items are not loaded like system extensions (so placing any extensions into this folder will disable them and most likely give an error on startup); instead, these files are loaded as if you had double-clicked them.

The Startup Items folder is a handy place to put items you use every time you launch the computer. Usually, instead of putting the original file or document into the Startup Items folder, an alias of the item is used instead. You can disable a startup item by removing it from this folder.

Mac OS X does not use the Startup Items folder the way previous versions of the Mac OS did. Instead of placing files directly into this folder, each individual user can determine what items are loaded on startup by customizing the Login Items tab of the Login System Preferences window.

Apple Menu Items
The Apple menu is located in the top left corner on every Mac. In OS 9 and earlier versions of the Mac OS, this menu is a quick list of commonly used applications, utilities, folders and files, and is a standard way to access control panels. Any item placed inside of the Apple Menu Items folder (located in the System folder), whether it be an alias, application, document or folder, will appear as an icon and name inside of the Apple Menu. As with other folders, removing an item from this folder disables it and deletes it from the menu (which is automatically updated).

In Mac OS X, the Apple menu is intended as an interface to the operating system, rather than a place from which to launch common applications and utilities. You cannot modify the Apple menu in OS X. Other features (such as the Dock) have been created to provide easy access to your applications.

Preferences
Prior to OS X, the Preferences folder, also in the System folder, is a special place used by most Macintosh applications. It is the location where settings and configurations specific to particular applications and system settings are stored. You will usually find things such as disk cache, preference files and even some data files in this folder. If you are troubleshooting and having problems with a specific application, however, this folder would be a good place to start. You can remove the preferences for an application. You will then need to recreate the preferences, but it may resolve a problem caused by a corrupted preferences file.

OS X does not use preferences in the same way, so troubleshooting a problem this way is not possible.