An alias 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.
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: Since any of these properties can change without the computer's knowledge, 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 some other operating systems, such as the Unix symbolic link or the Microsoft Windows shortcut, at the expense of increased complexity and unpredictability.
In System 7 through Mac OS 9, aliases distinguished themselves visually to the user by the fact that their file names were in italics.
Mac OS 8.5 added a feature for re-connecting aliases that had been broken for one reason or another (when the simple search algorithms failed to find a reliable replacement).
In Mac OS 8.5 options were added for command-option dragging an object in the Finder to create an alias at that location.
[5] Mac OS X 10.6 introduced the Bookmarks API to Cocoa as a set of methods on NSURL and functions for CFURL.
[6][7] In Mac OS X 10.7.3, the API was enhanced for the App Sandbox with security-scoped bookmarks, which add security permissions to aliases on a per-application or per-document basis.