Alias (Mac OS)

From Academic Kids

In System 7 and later, an alias is a small file that represents another object in the file system.

A typical alias is small, between 1 and 5 KB. It acts as a stand-in for any object in the file system, such as a document, an application, a folder, a hard disk, a network share or removable medium or a printer. When double-clicked, the computer will act the same way as if the original file had been double-clicked. Likewise, choosing an alias file from within a 'File Open' dialog box would open the original file.

An alias is a dynamic reference to an object. The original may be moved to another place in the file system, without breaking the link. The operating system stores several pieces of information about the original in the resource fork of the alias file. Examples of the information used to locate the original are:

  • path
  • file ID
  • directory ID
  • name
  • file size

Since any of these properties can change behind the OS' back as a result of user activity, various search algorithms are used to find the most plausible target. This fault-tolerance sets the alias apart from similar functions in other operating systems, such as the Unix symbolic link or the Microsoft Windows shortcut, at the expense of increased complexity. For example, an application can be moved from one directory to another, but an existing alias would still launch the same application when double-clicked.

Distinguishing marks

In System 7 through Mac OS 8.1, aliases distinguished themselves visually to the user by the fact that their file names were in italics. In Mac OS 8.5, another distinguishing mark was added, badging with an "alias arrow" - a black arrow on a small white square - similar to that used for shortcuts in Microsoft Windows.

In Mac OS X, the filenames of aliases is not italicized, but the arrow badge remains.

Managing aliases

In System 7, the only way to create an alias was to select the original and choose "Make Alias" from the "File" menu. An alias, with the same name and "(alias)" appended would then be created in the same folder. In later versions, it became possible to create aliases by drag-and-drop, while holding down the command and option modifier keys.

Mac OS 8.5 added a feature for re-connecting aliases that had been broken for one reason or another (and the simple search algorithms failed to find a reliable replacement).

External links

  • Alias Manager (http://developer.apple.com/documentation/mac/Files/Files-340.html)Inside Macintosh developer documentation
  • System 7 aliases (http://www.upenn.edu/computing/printout/archive/v08/5/aliases.html) — Article about System 7 aliases, from 1992
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