Desktop Items

The most common files are application programs and associated documents. However, you’ll also come across special files that are used by the system, the Finder or by other applications.

Persistent Trash

Sometimes, you’ll encounter an icon in the Trash that simply won’t go away when you select Empty Trash whilst in the Finder. If this fails, you should check that the item isn’t in use. If it’s a document this can usually be solved by closing the file from inside the appropriate application. If this doesn’t work, or the item in question is a folder, you should quit the application that last used it and try again. Failing this, try a special utility, such as Trash It!

File Kinds

Selecting View ➡ as List in the Finder presents a view that includes a Kind column, which provides a short description of each item. As well as applications, documents and special files you’ll encounter the following:-


An alias is a special kind of file that points to another file or folder, allowing you to open the original item from one or more alternative locations. Unlike the item itself, which can occupy any amount of disk space, an alias only occupies a few kilobytes.


A clipping is created when you drag and drop selected material out of the window of a suitable application and onto the desktop. Its Kind is given as a text clipping, picture clipping, sound clipping or, if the dragged item is a URL, an email address or web page location.

Invisible Files

Some files, although they exist, can’t be seen in the Finder. Such documents, when created by the system or applications, are made invisible by using a . (period or full-stop) at the start of the filename.

The files called .DS_Store,, which exist in many folders, are particularly important, since they contain Finder information about the folder’s content, location and icon position.


Desktop items often have a characteristic icon, although this can be replaced by a custom icon. To do this, select the file, choose File ➡ Get Info or press -I, click on the icon and then paste an alternative image in its place.


If you select your startup drive on the desktop and choose File ➡ Get Info or press -I you’ll see an information window, giving the Kind as disk, the Format, usually Mac OS Extended, its Capacity and the free space Available.

Other drives or removable media also appear on the desktop when mounted. You can also mount a disk image file, either through the Disk Copy application or by using a self-mounting image file. This kind of disk doesn’t actually exist, although the illusion can be useful.


A folder is really an illusion created by the computer. It doesn’t actually contain anything but allows you to see a chosen group of files at a single location. This means that when a file is moved, the disk’s directory is modified, although the file actually stays in the same place on the drive.


A package, formerly known as a bundle in Mac OS 9.x, is a folder disguised as a single file. Unlike a normal folder, however, a package usually has a filename extension. Applications in Mac OS X normally have a filename extension of .app and can consist of either a single file or several files inside a special package known as an application program package (APP). Other types of packages are assigned alternative extensions, such as .bndl (bundle), .menu (used for adding extra items to the menubar) and .pkg (as used with Apple’s Installer application and other software).

When you double-click on a package the Carbon or Cocoa application that it contains is launched.

Inside an APP

A package used for a Cocoa application incorporates numerous ‘flat’ files (without resources), usually without a specific file type or creator code, as well as subfolders. Fortunately, the package itself normally prevents the user from seeing or modifying all of these horribly complicated items.

Most .app packages contain a .DS_Store file and a Contents folder. A Carbonized application package may also contain an alias to the application itself, the original application being contained within the hierarchy of the package. The Contents folder usually contains a Resources folder and Mac OS folder, the latter used for a Cocoa application, as well as other files. The Contents folder can also contain a Mac OS Classic or similar folder that holds a Carbonised version of the program.

The data used by a Cocoa application is stored as Unix-style text files, with filename extensions such as .dat, .icns, .lproj, .plist and .strings, whilst graphical elements are kept as binary files, usually in .gif, .jpg, .png, .tiff or other standard formats. Any sounds used by an application are also stored in binary files, typically in .mp3, .snd or .wav formats.

Special resources can be included as .rsrc text files. Despite the latter often being recognised by the old ResEdit application, the resources in these documents are actually kept in the data fork (although it’s possible to transfer these to the resource fork to see them in ResEdit). Other resources are in.nib files, as created by InterfaceBuilder, the special developer’s application supplied with Mac OS X.

Help information for Cocoa programs can come in .rtf files. These avoid the styl resources used for styled text in the Classic Mac OS, which aren’t recognised by the Unix mechanisms of Mac OS X.

©Ray White 2004.