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Small file that represents another file

In classic Mac OS System 7 and later, and in macOS, an alias is a small file that represents another object in a local, remote, or removable file system and provides a dynamic link to it; the target object may be moved or renamed, and the alias will still link to it (unless the original file is recreated; such an alias is ambiguous and how it is resolved depends on the version of macOS). In Windows, a "shortcut", a file with a lnk extension, performs a similar function. It is similar to the Unix symbolic link, but with the distinction of working even if the target file moves to another location on the same disk (in this case it acts like a hard link, but the source and target of the link may be on different filesystems, and the target of the link may be a directory). As a descendant of BSD, macOS supports Unix symbolic (and hard) links as well.

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. 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. The purpose of an alias is to assist the user in managing large numbers of files by providing alternative ways to access them without having to copy the files themselves. While a typical alias under the classic Mac OS was small, between 1 and 5 KB, under macOS it can be fairly large, more than 5 MB (5000 KB) for the alias to a folder.

In System 7 through Mac OS 9, aliases distinguished themselves visually to the user by the fact that their file names were in italics. To accommodate languages that don't have italics (such as Japanese), in Mac OS 8.5 another distinguishing mark was added, badging with an "alias arrow"—a black arrow with a small white border—similar to that used for shortcuts in Microsoft Windows. In macOS, the filenames of aliases are not italicized, but the arrow badge remains.

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 (when the simple search algorithms failed to find a reliable replacement). This was done by selecting a new target through the standard Open File dialog. In Mac OS 8.5 options were added for command option dragging an object in the Finder to create an alias at that location. This is where the alias cursor was added to the system. The cursor mirrors the appearance of the "create shortcut" cursor on Windows systems.

The Alias Manager API is part of Carbon that allows developers to create aliases and access the aliased file. Mac OS X 10.6 introduced the Bookmarks API to Cocoa as a set of methods on and functions for 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. In Mac OS X 10.8 the Alias Manager API was officially deprecated in favor of the Bookmarks API. Given that, commands such as cd can be set up to check for aliases and treat them just like symbolic or hard links.