Alias (Mac OS)
||This article needs additional citations for verification. (May 2010)|
||This article includes a list of references, but its sources remain unclear because it has insufficient inline citations. (April 2009)|
In Mac OS System 7 and later, 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 OS X). In Windows, the same function is performed with a "shortcut", a file with a .lnk extension.
It is similar to the Unix symbolic link, but with the added benefit of working even if the target file moves to another location on the same disk (in this case it acts like hard link, but the source and target of the link may be on different filesystems). As a descendant of BSD, OS X 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 OS X it can be fairly large, more than 5000 KB for the alias to a folder.
Preventing Alias Failure 
An alias is a dynamic reference to an object. The original may be moved to another place within the same filesystem, 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:
- file ID
- directory ID
- file size
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. For example, an application can be moved from one directory to another within the same filesystem, but an existing alias would still launch the same application when double-clicked.
The question can arise of how an alias should work if a file is moved, and then a file is created with the same name as the original moved file, since the alias can be used to locate both the original name and the new location of the original file. With symbolic links the reference is unambiguous and refers to the new file. Before Mac OS 10.2, however, such an ambiguous alias would consistently find the original moved file, rather than the recreated file. From Mac OS 10.2, the new file is found, matching the behaviour of symbolic links . OS X applications can programmatically use the old behavior if required.
Distinguishing marks 
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 OS X, the filenames of aliases are not italicized, but the arrow badge remains.
File structure 
The alias files in Mac OS start by the magic number
62 6F 6F 6B 00 00 00 00 6D 61 72 6B 00 00 00 00 which is in ASCII
book␀␀␀␀mark␀␀␀␀ (␀ representing the Null character); and has a set of records inside it, each record is 150 bytes long and consists of:
- 4 bytes user type name/app creator code = long ASCII text string (none = 0)
- 2 bytes record size = short unsigned total length
- 2 bytes record version = short integer version (current version = 2)
- 2 bytes alias kind = short integer value (file = 0; directory = 1)
- 1 byte volume name string length = byte unsigned length
- 27 bytes volume name string (if volume name string < 27 chars then pad with zeros)
- 4 bytes volume created mac date = long unsigned value in seconds since beginning 1904 to 2040
- 2 bytes volume signature = short unsigned HFS value
- 2 bytes volume type = short integer mac os value (types are Fixed HD = 0; Network Disk = 1; 400kB FD = 2;800kB FD = 3; 1.4MB FD = 4; Other Ejectable Media = 5 )
- 4 bytes parent directory id = short unsigned HFS value
- 1 bytes file name string length = byte unsigned length
- 63 bytes file name string (if file name string < 63 chars then pad with zeros)
- 4 bytes file number = long unsigned HFS value
- 4 bytes file created mac date = long unsigned value in seconds since beginning 1904 to 2040
- 4 bytes file type name = long ASCII text string
- 4 bytes file creator name = long ASCII text string
- 2 bytes nlvl From (directories from alias thru to root) = short integer range
- 2 bytes nlvl To (directories from root thru to source) = short integer range (if alias on different volume then set above to -1)
- 4 bytes volume attributes = long hex flags
- 2 bytes volume file system id = short integer HFS value
- 10 bytes reserved = 80-bit value set to zero
- 4+ bytes optional extra data strings = short integer type + short unsigned string length (types are Extended Info End = -1; Directory Name = 0; Directory IDs = 1; Absolute Path = 2; AppleShare Zone Name = 3; AppleShare Server Name = 4; AppleShare User Name = 5; Driver Name = 6; Revised AppleShare info = 9; AppleRemoteAccess dialup info = 10)
- string data = hex dump
- odd lengths have a 1 byte odd string length pad = byte value set to zero
Alias record structure outside of size length 
The following is for use with the Apple's Alias Resource Manager.
- 4 bytes resource type name = long ASCII text string
- 2 bytes resource ID = short integer value
- 2 bytes resource end pad = short value set to zero
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 (when the simple search algorithms failed to find a reliable replacement).
- Alias Manager Reference — Inside Macintosh developer documentation
- System 7 aliases — Article about System 7 aliases, from 1992