AppleSingle
Encyclopedia
AppleSingle Format and AppleDouble Format are file format
s developed by Apple Computer
to store Mac OS
"dual-forked" files
on the Unix
filesystem being used in A/UX
, Apple's first Unix-like
operating system
. AppleSingle combined both file forks and the related Finder
meta-file information into a single file, whereas AppleDouble stored them as two separate files. Support for the formats was later added to Unix software such as NFS and MAE
, but they saw little use outside this small market.
AppleSingle is similar in concept to the more popular MacBinary
format, in that the resource and data forks are combined together with a header containing the Finder information. In fact, the format is so similar there is no obvious reason why Apple did not simply use MacBinary instead, which by that point was widely known and used. Some not-so-obvious reasons are explained in http://tools.ietf.org/html/draft-newman-macbin-binhex-harmful-00. The format was later assigned the MIME type application/applefile.
AppleDouble leaves the data fork in its original format, allowing it to be edited by normal Unix utilities. The resource fork and Finder information, both proprietary and lacking editors under Unix, were combined into a second file. A MIME type was also assigned to AppleDouble, multipart/appledouble. For sending to an AppleDouble un-aware system, the file was generally encoded using Base64
, as opposed to being converted to AppleSingle.
Before Mac OS X
, AppleSingle and Double had little presence in the Mac market, due largely to the small market share of A/UX. Nevertheless they did force various file compression vendors to add support for the formats, and confuse future MacBinary versions.
Mac OS X revived the use of AppleDouble; on file systems such as NFS and WebDAV
that don't natively support resource forks, Finder information, or extended attributes, that information is stored in AppleDouble format, with the second file having a name generated by prepending "._" to the name of the first file (thus, this information acts as a hidden file when viewed from a non-Apple Unix-based operating system).
File format
A file format is a particular way that information is encoded for storage in a computer file.Since a disk drive, or indeed any computer storage, can store only bits, the computer must have some way of converting information to 0s and 1s and vice-versa. There are different kinds of formats for...
s developed by Apple Computer
Apple Computer
Apple Inc. is an American multinational corporation that designs and markets consumer electronics, computer software, and personal computers. The company's best-known hardware products include the Macintosh line of computers, the iPod, the iPhone and the iPad...
to store Mac OS
Mac OS
Mac OS is a series of graphical user interface-based operating systems developed by Apple Inc. for their Macintosh line of computer systems. The Macintosh user experience is credited with popularizing the graphical user interface...
"dual-forked" files
Resource fork
The resource fork is a construct of the Mac OS operating system used to store structured data in a file, alongside unstructured data stored within the data fork. A resource fork stores information in a specific form, such as icons, the shapes of windows, definitions of menus and their contents, and...
on the Unix
Unix
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
filesystem being used in A/UX
A/UX
A/UX was Apple Computer’s implementation of the Unix operating system for some of their Macintosh computers. The later versions of A/UX ran on the Macintosh II, Quadra and Centris series of machines as well as the SE/30. A/UX was first released in 1988, with the final version released in 1995...
, Apple's first Unix-like
Unix-like
A Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification....
operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
. AppleSingle combined both file forks and the related Finder
Macintosh Finder
The Finder is the default file manager used on Mac OS and Mac OS X operating systems; it is responsible for the overall user-management of files, disks, network volumes and the launching of other applications...
meta-file information into a single file, whereas AppleDouble stored them as two separate files. Support for the formats was later added to Unix software such as NFS and MAE
Macintosh Application Environment
The Macintosh Application Environment was a software package introduced by Apple Computer in 1994 which allowed users of certain Unix-based computer workstations to run Apple Macintosh application software....
, but they saw little use outside this small market.
AppleSingle is similar in concept to the more popular MacBinary
MacBinary
Due to the metadata-rich nature of the Macintosh Hierarchical File System, transferring Mac OS files to platforms that do not support HFS can be problematic. MacBinary was developed as a means of preserving this structure without sacrificing portability. It combines the data and resource forks...
format, in that the resource and data forks are combined together with a header containing the Finder information. In fact, the format is so similar there is no obvious reason why Apple did not simply use MacBinary instead, which by that point was widely known and used. Some not-so-obvious reasons are explained in http://tools.ietf.org/html/draft-newman-macbin-binhex-harmful-00. The format was later assigned the MIME type application/applefile.
AppleDouble leaves the data fork in its original format, allowing it to be edited by normal Unix utilities. The resource fork and Finder information, both proprietary and lacking editors under Unix, were combined into a second file. A MIME type was also assigned to AppleDouble, multipart/appledouble. For sending to an AppleDouble un-aware system, the file was generally encoded using Base64
Base64
Base64 is a group of similar encoding schemes that represent binary data in an ASCII string format by translating it into a radix-64 representation...
, as opposed to being converted to AppleSingle.
Before Mac OS X
Mac OS X
Mac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...
, AppleSingle and Double had little presence in the Mac market, due largely to the small market share of A/UX. Nevertheless they did force various file compression vendors to add support for the formats, and confuse future MacBinary versions.
Mac OS X revived the use of AppleDouble; on file systems such as NFS and WebDAV
WebDAV
Web-based Distributed Authoring and Versioning is a set of methods based on the Hypertext Transfer Protocol that facilitates collaboration between users in editing and managing documents and files stored on World Wide Web servers...
that don't natively support resource forks, Finder information, or extended attributes, that information is stored in AppleDouble format, with the second file having a name generated by prepending "._" to the name of the first file (thus, this information acts as a hidden file when viewed from a non-Apple Unix-based operating system).
External links
- Applesingle and AppleDouble format internals - from the original A/UX documentation
- RFC 1740 - MIME Encapsulation of Macintosh files - contains information on MIME types and a little on AppleSingle/Double
- Mac Binary Converter, an open source tool for converting between different Macintosh file encodings.
- Mac::AppleSingleDouble, a Perl module for reading AppleSingle and AppleDouble files
- Mac OS X: Apple Double Format Creates File Name With the Prefix '._', Apple Inc. knowledgebase note about the AppleDouble format