Filename extension
Encyclopedia
A filename extension is a suffix
(separated from the basefilename by a dot) to the name
of a computer file
applied to indicate the encoding (file format
) of its contents or usage.
Some filesystems
limit the length of the extension (such as the FAT
file system
not allowing more than three characters) while others (such as NTFS
) do not. Unix filesystems accept the separator dot as a legal character.
(example
On file systems of mainframe systems such as MVS
, VMS
, and PC systems such as CP/M
and derivative systems such as MS-DOS
, the extension is a separate namespace
from the filename. Under Microsoft's DOS
and Windows
, extensions such as
. This is different from UNIX-like
filesystems, where a suffix is not a separate namespace.
With the advent of graphical user interface
s, the issue of file management and interface behavior arose. Microsoft Windows allowed multiple applications to be associated with a given extension, and different actions were available for selecting the required application, such as a context menu
offering a choice between viewing, editing or printing the file.
Pre-OS X versions of the Mac OS
disposed of filename extensions entirely, instead using a file type code
to identify the file format. Additionally, a creator code
was specified to determine which application would be launched when the file's icon
was double-clicked. Mac OS X
, however, uses filename suffixes, as well as type and creator codes, as a consequence of being derived from the UNIX-like NEXTSTEP
operating system, which did not have type or creator code support in its file system.
files,
, and
. However, because many different software programs have been made that all handle these data types (and others) in a variety of ways, filename extensions started to become closely associated with certain products—even specific product versions. For example, early WordStar
files used
packages and RealPlayer
Media files; others being
fonts, Quicken
financial ledger
s, and QuickTime
pictures, and
scripts and Game Boy Advance
ROM images.
Some other operating systems such as Multics
that used filename extensions generally had much more liberal sizes for filenames. Many allowed full filename lengths of 14 or more characters, and maximum name lengths up to 255 were not uncommon. The file systems in operating systems such as UNIX
stored the file name as a single string, not split into base name and extension components, with the '.' being just another character allowed in file names. Such systems generally allow for variable-length filenames, permitting more than one dot, and hence multiple suffixes. Some components of Multics and UNIX, and applications running on them, used suffixes, in some cases, to indicate file types, but they did not use them as much — for example, executables and ordinary text files had no suffixes in their names.
The High Performance File System (HPFS), used in Microsoft and IBM
's OS/2
also supported long file names, and did not divide the file name into a name and an extension. The convention of using suffixes continued, even though HPFS supported extended attributes for files, allowing a file's type to be stored with the file as an extended attribute.
Microsoft's Windows NT
's native file system, NTFS
, supported long file names and did not divide the file name into a name and an extension, but again, the convention of using suffixes to simulate extensions continued, for compatibility with existing versions of Windows.
When the Internet
age first arrived, those using Windows systems that were still restricted to 8.3 filename formats had to create web pages with names ending in
, since it requires source code
files to have the four-letter suffix
object code
output files with the five-letter
Eventually, Windows
introduced support for long file names, and removed the 8.3 name/extension split in file names, in an extended version of the commonly used FAT
file system
called VFAT. VFAT first appeared in Windows NT 3.5
and Windows 95
. The internal implementation of long file names in VFAT is largely considered to be a kludge
, but it removed the important length restriction, and allowed files to have a mix of upper case and lower case letters, on machines that would not run Windows NT
well. However, the use of three-character extensions under Microsoft Windows has continued, originally for backward compatibility with older versions of Windows and now by habit, along with the problems it creates.
, Python
, etc.) and the interpreter name being suffixed to the command name, a practice common on systems like Windows
and Mac OS X
, which rely on globally set associations between filename extension and interpreter, but sharply deprecated in UNIX
-derived systems like Linux
and Apple's Mac OS X
, where the interpreter is normally specified as a header in the script.
On association-based systems, the filename extension is generally mapped to a single, system-wide selection of interpreter for that extension (such as ".py" meaning to use Python), and the command itself is runnable from the command line even if the extension is omitted (assuming appropriate setup is done). If the implementation language is changed, the command name extension is changed as well, and the OS provides a consistent API by allowing the same extension-less version of the command to be used in both cases. This method suffers somewhat from the essentially global nature of the association mapping, as well as from developers' incomplete avoidance of extensions when calling programs, and that developers can't force that avoidance. Windows is the only remaining widespread employer of this mechanism.
On systems with interpreter directive
s, command name extensions have no special significance, and are by standard practice not used, since the primary method to set interpreters for scripts is to start them with a single line specifying the interpreter to use (which could be viewed as a degenerate resource fork
).
Developers coming from association-based based culture to the interpreter directive culture often make the very distinctive error of including command name extensions. Embedding the implementation detail of the language used introduces a problem where the command's implementation language cannot be changed (for example, from shell to C++
) without either breaking any tool that refers to the old script name, retaining the now inaccurate old extension, or preferably writing a wrapper in the old language that launches a tool in the new language.
, the file browser provided with Microsoft Windows
, is for filename extensions not to be shown. Malicious users have tried to spread computer virus
es and computer worm
s by using file names formed like
. The hope is that this will appear as
.
Later Windows versions (starting with Windows XP
Service Pack 2 and Windows Server 2003
) included customizable lists of filename extensions that should be considered 'dangerous' in certain 'zones' of operation, such as when downloaded from the web
or received as an e-mail attachment. Modern antivirus software
systems also help to defend users against such attempted attacks where possible.
Some viruses take advantage of the similarity between the ".com
" top-level domain
and the ".COM" file extension
by emailing malicious, executable command-file attachments under names superficially similar to URLs (e.g., "myparty.yahoo.com"), with the effect that some unsophisticated users click on email-embedded links that they think lead to websites but actually download and execute the malicious attachments.
There have been instances of malware
crafted to exploit vulnerabilities in some Windows applications which could cause a stack-based buffer overflow
when opening a file with an overly long, unhandled filename extension.
and do not have filenames or extensions.
In many internet
protocols such as HTTP and MIME email
, the type of a bitstream is stated as the internet media type
of the stream (also called the MIME type or content type). This is given in a line of text preceding the stream, such as:
Content-type: text/plain
Because there is no standard mapping between filename extensions and internet media types, mismatches in file type interpretation between authors, web servers, and client software can be a source of errors when transferring files over the internet. For instance, a content author may specify a .svgz extension on a compressed Scalable Vector Graphics
file, but a web server which does not recognize this extension may not send the proper content type of application/svg+xml and its required compression header, leaving web browsers unable to correctly interpret and display the image.
BeOS
, whose BFS
file system supports extended attributes, would tag a file with its internet media type as an extended attribute. The KDE
and GNOME
desktop environment
s associate an internet media type with a file by examining both the filename suffix and the contents of the file, in the fashion of the file command, as a heuristic
. They choose the application to launch when a file is opened based on that internet media type, reducing the dependency on filename extensions. Mac OS X
uses both filename extensions and media types, as well as file type codes
, to select a Uniform Type Identifier
by which to identify the file type internally.
Substring
A subsequence, substring, prefix or suffix of a string is a subset of the symbols in a string, where the order of the elements is preserved...
(separated from the basefilename by a dot) to the name
Filename
The filename is metadata about a file; a string used to uniquely identify a file stored on the file system. Different file systems impose different restrictions on length and allowed characters on filenames.A filename includes one or more of these components:...
of a computer file
Computer file
A computer file is a block of arbitrary information, or resource for storing information, which is available to a computer program and is usually based on some kind of durable storage. A file is durable in the sense that it remains available for programs to use after the current program has finished...
applied to indicate the encoding (file format
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...
) of its contents or usage.
Some filesystems
File system
A file system is a means to organize data expected to be retained after a program terminates by providing procedures to store, retrieve and update data, as well as manage the available space on the device which contain it. A file system organizes data in an efficient manner and is tuned to the...
limit the length of the extension (such as the FAT
File Allocation Table
File Allocation Table is a computer file system architecture now widely used on many computer systems and most memory cards, such as those used with digital cameras. FAT file systems are commonly found on floppy disks, flash memory cards, digital cameras, and many other portable devices because of...
file system
File system
A file system is a means to organize data expected to be retained after a program terminates by providing procedures to store, retrieve and update data, as well as manage the available space on the device which contain it. A file system organizes data in an efficient manner and is tuned to the...
not allowing more than three characters) while others (such as NTFS
NTFS
NTFS is the standard file system of Windows NT, including its later versions Windows 2000, Windows XP, Windows Server 2003, Windows Server 2008, Windows Vista, and Windows 7....
) do not. Unix filesystems accept the separator dot as a legal character.
Usage
Filename extensions can be considered a type of metadata. They are commonly used to imply information about the way data might be stored in the file. The exact definition, giving the criteria for deciding what part of the file name is its extension, belongs to the rules of the specific filesystem used; usually the extension is the substring which follows the last occurrence, if any, of the dot characterFull stop
A full stop is the punctuation mark commonly placed at the end of sentences. In American English, the term used for this punctuation is period. In the 21st century, it is often also called a dot by young people...
(example
txt is the extension of the filename readme.txt, html the extension of mysite.index.html).On file systems of mainframe systems such as MVS
MVS
Multiple Virtual Storage, more commonly called MVS, was the most commonly used operating system on the System/370 and System/390 IBM mainframe computers...
, VMS
VMS
- Communication and transportation :* Voice Mail System, automated telephone messaging* Video Messaging Service , video messaging for 3G handsets* VMS MobiFone, one of the largest mobile phone operators in Vietnam...
, and PC systems such as CP/M
CP/M
CP/M was a mass-market operating system created for Intel 8080/85 based microcomputers by Gary Kildall of Digital Research, Inc...
and derivative systems such as MS-DOS
MS-DOS
MS-DOS is an operating system for x86-based personal computers. It was the most commonly used member of the DOS family of operating systems, and was the main operating system for IBM PC compatible personal computers during the 1980s to the mid 1990s, until it was gradually superseded by operating...
, the extension is a separate namespace
Namespace
In general, a namespace is a container that provides context for the identifiers it holds, and allows the disambiguation of homonym identifiers residing in different namespaces....
from the filename. Under Microsoft's DOS
DOS
DOS, short for "Disk Operating System", is an acronym for several closely related operating systems that dominated the IBM PC compatible market between 1981 and 1995, or until about 2000 if one includes the partially DOS-based Microsoft Windows versions 95, 98, and Millennium Edition.Related...
and Windows
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
, extensions such as
EXE, COM or BAT indicate that a file is a program executableExecutable
In computing, an executable file causes a computer "to perform indicated tasks according to encoded instructions," as opposed to a data file that must be parsed by a program to be meaningful. These instructions are traditionally machine code instructions for a physical CPU...
. This is different from 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....
filesystems, where a suffix is not a separate namespace.
With the advent of graphical user interface
Graphical user interface
In computing, a graphical user interface is a type of user interface that allows users to interact with electronic devices with images rather than text commands. GUIs can be used in computers, hand-held devices such as MP3 players, portable media players or gaming devices, household appliances and...
s, the issue of file management and interface behavior arose. Microsoft Windows allowed multiple applications to be associated with a given extension, and different actions were available for selecting the required application, such as a context menu
Context menu
A context menu is a menu in a graphical user interface that appears upon user interaction, such as a right mouse click or middle click mouse operation...
offering a choice between viewing, editing or printing the file.
Pre-OS X versions of the 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...
disposed of filename extensions entirely, instead using a file type code
Type code
A type code is the only mechanism used in pre-Mac OS X versions of the Macintosh operating system to denote a file's format, in a manner similar to file extensions in other operating systems. Codes are four-byte OSTypes...
to identify the file format. Additionally, a creator code
Creator code
A creator code is a mechanism introduced in pre-Mac OS X versions of the Macintosh operating system to link a data file to the application program which created it, in a manner similar to file extensions in other operating systems. Codes are four-byte OSTypes. For example, the creator code of the...
was specified to determine which application would be launched when the file's icon
Icon (computing)
A computer icon is a pictogram displayed on a computer screen and used to navigate a computer system or mobile device. The icon itself is a small picture or symbol serving as a quick, intuitive representation of a software tool, function or a data file accessible on the system. It functions as an...
was double-clicked. 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...
, however, uses filename suffixes, as well as type and creator codes, as a consequence of being derived from the UNIX-like NEXTSTEP
NEXTSTEP
NeXTSTEP was the object-oriented, multitasking operating system developed by NeXT Computer to run on its range of proprietary workstation computers, such as the NeXTcube...
operating system, which did not have type or creator code support in its file system.
Improvements
The filename extension was originally used to easily determine the file's generic type. The need to condense a file's type into three characters frequently led to inscrutable extensions. Examples include using.GFX for graphicsGraphics
Graphics are visual presentations on some surface, such as a wall, canvas, computer screen, paper, or stone to brand, inform, illustrate, or entertain. Examples are photographs, drawings, Line Art, graphs, diagrams, typography, numbers, symbols, geometric designs, maps, engineering drawings,or...
files,
.TXT for plain textPlain text
In computing, plain text is the contents of an ordinary sequential file readable as textual material without much processing, usually opposed to formatted text....
, and
.MUS for musicMusic
Music is an art form whose medium is sound and silence. Its common elements are pitch , rhythm , dynamics, and the sonic qualities of timbre and texture...
. However, because many different software programs have been made that all handle these data types (and others) in a variety of ways, filename extensions started to become closely associated with certain products—even specific product versions. For example, early WordStar
WordStar
WordStar is a word processor application, published by MicroPro International, originally written for the CP/M operating system but later ported to DOS, that enjoyed a dominant market share during the early to mid-1980s. Although Seymour I...
files used
.WS or .WSn, where n was the program's version number. Also, filename extensions began to conflict between separate files. One example is .rpm, used for both RPM Package ManagerRPM Package Manager
RPM Package Manager is a package management system. The name RPM variously refers to the .rpm file format, files in this format, software packaged in such files, and the package manager itself...
packages and RealPlayer
RealPlayer
RealPlayer is a cross-platform media player by RealNetworks that plays a number of multimedia formats including MP3, MPEG-4, QuickTime, Windows Media, and multiple versions of proprietary RealAudio and RealVideo formats.-History:...
Media files; others being
.qif, shared by DESQviewDESQview
DESQview was a text mode multitasking program developed by Quarterdeck Office Systems which enjoyed modest popularity in the late 1980s and early 1990s...
fonts, Quicken
Quicken
Quicken is a personal finance management tool developed by Intuit, Inc.Quicken runs on Windows and Macintosh systems. Previous versions ran on DOS. An online version is also available. The last version of Quicken for Macintosh computers was Quicken Essentials for Mac released in February 2010...
financial ledger
Ledger
A ledger is the principal book or computer file for recording and totaling monetary transactions by account, with debits and credits in separate columns and a beginning balance and ending balance for each account. The ledger is a permanent summary of all amounts entered in supporting journals which...
s, and QuickTime
QuickTime
QuickTime is an extensible proprietary multimedia framework developed by Apple Inc., capable of handling various formats of digital video, picture, sound, panoramic images, and interactivity. The classic version of QuickTime is available for Windows XP and later, as well as Mac OS X Leopard and...
pictures, and
.gba, shared between GrabItGrabIt
GrabIt is a Windows-based freeware newsreader programmed and developed by Ilan Shemes.-History:Ilan Shemes has been making GrabIt changes since the program has been introduced...
scripts and Game Boy Advance
Game Boy Advance
The is a 32-bit handheld video game console developed, manufactured, and marketed by Nintendo. It is the successor to the Game Boy Color. It was released in Japan on March 21, 2001; in North America on June 11, 2001; in Australia and Europe on June 22, 2001; and in the People's Republic of China...
ROM images.
Some other operating systems such as Multics
Multics
Multics was an influential early time-sharing operating system. The project was started in 1964 in Cambridge, Massachusetts...
that used filename extensions generally had much more liberal sizes for filenames. Many allowed full filename lengths of 14 or more characters, and maximum name lengths up to 255 were not uncommon. The file systems in operating systems such as 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...
stored the file name as a single string, not split into base name and extension components, with the '.' being just another character allowed in file names. Such systems generally allow for variable-length filenames, permitting more than one dot, and hence multiple suffixes. Some components of Multics and UNIX, and applications running on them, used suffixes, in some cases, to indicate file types, but they did not use them as much — for example, executables and ordinary text files had no suffixes in their names.
The High Performance File System (HPFS), used in Microsoft and IBM
IBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...
's OS/2
OS/2
OS/2 is a computer operating system, initially created by Microsoft and IBM, then later developed by IBM exclusively. The name stands for "Operating System/2," because it was introduced as part of the same generation change release as IBM's "Personal System/2 " line of second-generation personal...
also supported long file names, and did not divide the file name into a name and an extension. The convention of using suffixes continued, even though HPFS supported extended attributes for files, allowing a file's type to be stored with the file as an extended attribute.
Microsoft's Windows NT
Windows NT
Windows NT is a family of operating systems produced by Microsoft, the first version of which was released in July 1993. It was a powerful high-level-language-based, processor-independent, multiprocessing, multiuser operating system with features comparable to Unix. It was intended to complement...
's native file system, NTFS
NTFS
NTFS is the standard file system of Windows NT, including its later versions Windows 2000, Windows XP, Windows Server 2003, Windows Server 2008, Windows Vista, and Windows 7....
, supported long file names and did not divide the file name into a name and an extension, but again, the convention of using suffixes to simulate extensions continued, for compatibility with existing versions of Windows.
When the Internet
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
age first arrived, those using Windows systems that were still restricted to 8.3 filename formats had to create web pages with names ending in
.HTM, while those using Macintosh or UNIX computers could use the recommended .html filename extension. This also became a problem for programmers experimenting with the Java programming languageJava (programming language)
Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
, since it requires source code
Source code
In computer science, source code is text written using the format and syntax of the programming language that it is being written in. Such a language is specially designed to facilitate the work of computer programmers, who specify the actions to be performed by a computer mostly by writing source...
files to have the four-letter suffix
.java and compilesCompiler
A compiler is a computer program that transforms source code written in a programming language into another computer language...
object code
Object code
Object code, or sometimes object module, is what a computer compiler produces. In a general sense object code is a sequence of statements in a computer language, usually a machine code language....
output files with the five-letter
.class suffix.Eventually, Windows
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
introduced support for long file names, and removed the 8.3 name/extension split in file names, in an extended version of the commonly used FAT
File Allocation Table
File Allocation Table is a computer file system architecture now widely used on many computer systems and most memory cards, such as those used with digital cameras. FAT file systems are commonly found on floppy disks, flash memory cards, digital cameras, and many other portable devices because of...
file system
File system
A file system is a means to organize data expected to be retained after a program terminates by providing procedures to store, retrieve and update data, as well as manage the available space on the device which contain it. A file system organizes data in an efficient manner and is tuned to the...
called VFAT. VFAT first appeared in Windows NT 3.5
Windows NT 3.5
Windows NT 3.5 is the second release of the Microsoft Windows NT operating system. It was released on 21 September 1994.One of the primary goals during Windows NT 3.5's development was to increase the speed of the operating system; as a result, the project was given the codename "Daytona" in...
and Windows 95
Windows 95
Windows 95 is a consumer-oriented graphical user interface-based operating system. It was released on August 24, 1995 by Microsoft, and was a significant progression from the company's previous Windows products...
. The internal implementation of long file names in VFAT is largely considered to be a kludge
Kludge
A kludge is a workaround, a quick-and-dirty solution, a clumsy or inelegant, yet effective, solution to a problem, typically using parts that are cobbled together...
, but it removed the important length restriction, and allowed files to have a mix of upper case and lower case letters, on machines that would not run Windows NT
Windows NT
Windows NT is a family of operating systems produced by Microsoft, the first version of which was released in July 1993. It was a powerful high-level-language-based, processor-independent, multiprocessing, multiuser operating system with features comparable to Unix. It was intended to complement...
well. However, the use of three-character extensions under Microsoft Windows has continued, originally for backward compatibility with older versions of Windows and now by habit, along with the problems it creates.
Command name issues
The use of a filename extension in a command name appears occasionally, usually as a side effect of the command having been implemented as a script (in Bourne shellBourne shell
The Bourne shell, or sh, was the default Unix shell of Unix Version 7 and most Unix-like systems continue to have /bin/sh - which will be the Bourne shell, or a symbolic link or hard link to a compatible shell - even when more modern shells are used by most users.Developed by Stephen Bourne at AT&T...
, Python
Python (programming language)
Python is a general-purpose, high-level programming language whose design philosophy emphasizes code readability. Python claims to "[combine] remarkable power with very clear syntax", and its standard library is large and comprehensive...
, etc.) and the interpreter name being suffixed to the command name, a practice common on systems like Windows
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
and 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...
, which rely on globally set associations between filename extension and interpreter, but sharply deprecated in 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...
-derived systems like Linux
Linux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...
and Apple's 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...
, where the interpreter is normally specified as a header in the script.
On association-based systems, the filename extension is generally mapped to a single, system-wide selection of interpreter for that extension (such as ".py" meaning to use Python), and the command itself is runnable from the command line even if the extension is omitted (assuming appropriate setup is done). If the implementation language is changed, the command name extension is changed as well, and the OS provides a consistent API by allowing the same extension-less version of the command to be used in both cases. This method suffers somewhat from the essentially global nature of the association mapping, as well as from developers' incomplete avoidance of extensions when calling programs, and that developers can't force that avoidance. Windows is the only remaining widespread employer of this mechanism.
On systems with interpreter directive
Interpreter directive
An interpreter directive is a computer language construct that is used to control which interpreter parses and interprets the instructions in a computer program.- See also :* Shebang * Bourne-Again Shell* C Shell...
s, command name extensions have no special significance, and are by standard practice not used, since the primary method to set interpreters for scripts is to start them with a single line specifying the interpreter to use (which could be viewed as a degenerate resource fork
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...
).
Developers coming from association-based based culture to the interpreter directive culture often make the very distinctive error of including command name extensions. Embedding the implementation detail of the language used introduces a problem where the command's implementation language cannot be changed (for example, from shell to C++
C++
C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...
) without either breaking any tool that refers to the old script name, retaining the now inaccurate old extension, or preferably writing a wrapper in the old language that launches a tool in the new language.
Security issues
The default behavior of Windows ExplorerWindows Explorer
This article is about the Windows file system browser. For the similarly named web browser, see Internet ExplorerWindows Explorer is a file manager application that is included with releases of the Microsoft Windows operating system from Windows 95 onwards. It provides a graphical user interface...
, the file browser provided with Microsoft Windows
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
, is for filename extensions not to be shown. Malicious users have tried to spread computer virus
Computer virus
A computer virus is a computer program that can replicate itself and spread from one computer to another. The term "virus" is also commonly but erroneously used to refer to other types of malware, including but not limited to adware and spyware programs that do not have the reproductive ability...
es and computer worm
Computer worm
A computer worm is a self-replicating malware computer program, which uses a computer network to send copies of itself to other nodes and it may do so without any user intervention. This is due to security shortcomings on the target computer. Unlike a computer virus, it does not need to attach...
s by using file names formed like
LOVE-LETTER-FOR-YOU.TXT.vbsILOVEYOU
ILOVEYOU, also known as Love Letter, is a computer worm that successfully attacked tens of millions of computers in 2000 when it was sent as an attachment to a user with the text "ILOVEYOU" in the subject line. The worm arrived e-mail on and after May 4, 2000 with the simple subject of "ILOVEYOU"...
. The hope is that this will appear as
LOVE-LETTER-FOR-YOU.TXT, a harmless text file, without alerting the user to the fact that it is a harmful computer program, in this case written in VBScriptVBScript
VBScript is an Active Scripting language developed by Microsoft that is modeled on Visual Basic. It is designed as a “lightweight” language with a fast interpreter for use in a wide variety of Microsoft environments...
.
Later Windows versions (starting with Windows XP
Windows XP
Windows XP is an operating system produced by Microsoft for use on personal computers, including home and business desktops, laptops and media centers. First released to computer manufacturers on August 24, 2001, it is the second most popular version of Windows, based on installed user base...
Service Pack 2 and Windows Server 2003
Windows Server 2003
Windows Server 2003 is a server operating system produced by Microsoft, introduced on 24 April 2003. An updated version, Windows Server 2003 R2, was released to manufacturing on 6 December 2005...
) included customizable lists of filename extensions that should be considered 'dangerous' in certain 'zones' of operation, such as when downloaded from the web
World Wide Web
The World Wide Web is a system of interlinked hypertext documents accessed via the Internet...
or received as an e-mail attachment. Modern antivirus software
Antivirus software
Antivirus or anti-virus software is used to prevent, detect, and remove malware, including but not limited to computer viruses, computer worm, trojan horses, spyware and adware...
systems also help to defend users against such attempted attacks where possible.
Some viruses take advantage of the similarity between the ".com
.com
The domain name com is a generic top-level domain in the Domain Name System of the Internet. Its name is derived from commercial, indicating its original intended purpose for domains registered by commercial organizations...
" top-level domain
Top-level domain
A top-level domain is one of the domains at the highest level in the hierarchical Domain Name System of the Internet. The top-level domain names are installed in the root zone of the name space. For all domains in lower levels, it is the last part of the domain name, that is, the last label of a...
and the ".COM" file extension
COM file
In many computer operating systems, a COM file is a type of executable file; the name is derived from the file name extension .COM. Originally, the term stood for "Command file", a text file containing commands to be issued to the operating system , on many of the Digital Equipment Corporation mini...
by emailing malicious, executable command-file attachments under names superficially similar to URLs (e.g., "myparty.yahoo.com"), with the effect that some unsophisticated users click on email-embedded links that they think lead to websites but actually download and execute the malicious attachments.
There have been instances of malware
Malware
Malware, short for malicious software, consists of programming that is designed to disrupt or deny operation, gather information that leads to loss of privacy or exploitation, or gain unauthorized access to system resources, or that otherwise exhibits abusive behavior...
crafted to exploit vulnerabilities in some Windows applications which could cause a stack-based buffer overflow
Buffer overflow
In computer security and programming, a buffer overflow, or buffer overrun, is an anomaly where a program, while writing data to a buffer, overruns the buffer's boundary and overwrites adjacent memory. This is a special case of violation of memory safety....
when opening a file with an overly long, unhandled filename extension.
Alternatives
In network contexts, files are regarded as streams of bitsBitstream
A bitstream or bit stream is a time series of bits.A bytestream is a series of bytes, typically of 8 bits each, and can be regarded as a special case of a bitstream....
and do not have filenames or extensions.
In many internet
Internet protocol suite
The Internet protocol suite is the set of communications protocols used for the Internet and other similar networks. It is commonly known as TCP/IP from its most important protocols: Transmission Control Protocol and Internet Protocol , which were the first networking protocols defined in this...
protocols such as HTTP and MIME email
MIME
Multipurpose Internet Mail Extensions is an Internet standard that extends the format of email to support:* Text in character sets other than ASCII* Non-text attachments* Message bodies with multiple parts...
, the type of a bitstream is stated as the internet media type
Internet media type
An Internet media type, originally called a MIME type after MIME and sometimes a Content-type after the name of a header in several protocols whose value is such a type, is a two-part identifier for file formats on the Internet.The identifiers were originally defined in RFC 2046 for use in email...
of the stream (also called the MIME type or content type). This is given in a line of text preceding the stream, such as:
Content-type: text/plain
Because there is no standard mapping between filename extensions and internet media types, mismatches in file type interpretation between authors, web servers, and client software can be a source of errors when transferring files over the internet. For instance, a content author may specify a .svgz extension on a compressed Scalable Vector Graphics
Scalable Vector Graphics
Scalable Vector Graphics is a family of specifications of an XML-based file format for describing two-dimensional vector graphics, both static and dynamic . The SVG specification is an open standard that has been under development by the World Wide Web Consortium since 1999.SVG images and their...
file, but a web server which does not recognize this extension may not send the proper content type of application/svg+xml and its required compression header, leaving web browsers unable to correctly interpret and display the image.
BeOS
BeOS
BeOS is an operating system for personal computers which began development by Be Inc. in 1991. It was first written to run on BeBox hardware. BeOS was optimized for digital media work and was written to take advantage of modern hardware facilities such as symmetric multiprocessing by utilizing...
, whose BFS
Be File System
The Be File System is the native file system for the BeOS....
file system supports extended attributes, would tag a file with its internet media type as an extended attribute. The KDE
KDE
KDE is an international free software community producing an integrated set of cross-platform applications designed to run on Linux, FreeBSD, Microsoft Windows, Solaris and Mac OS X systems...
and GNOME
GNOME
GNOME is a desktop environment and graphical user interface that runs on top of a computer operating system. It is composed entirely of free and open source software...
desktop environment
Desktop environment
In graphical computing, a desktop environment commonly refers to a style of graphical user interface derived from the desktop metaphor that is seen on most modern personal computers. These GUIs help the user in easily accessing, configuring, and modifying many important and frequently accessed...
s associate an internet media type with a file by examining both the filename suffix and the contents of the file, in the fashion of the file command, as a heuristic
Heuristic
Heuristic refers to experience-based techniques for problem solving, learning, and discovery. Heuristic methods are used to speed up the process of finding a satisfactory solution, where an exhaustive search is impractical...
. They choose the application to launch when a file is opened based on that internet media type, reducing the dependency on filename extensions. 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...
uses both filename extensions and media types, as well as file type codes
OSType
OSType is the name of a four-byte sequence commonly used as an identifier in Mac OS. While the bytes can have any value, they usually display figures characterized in software programs such as those utilized in ASCII or Mac OS Roman character sets.OSType values are used to identify file data...
, to select a Uniform Type Identifier
Uniform Type Identifier
A Uniform Type Identifier is a text string used on software provided by Apple Inc. to uniquely identify a given class or type of item...
by which to identify the file type internally.
See also
- List of file formats
- List of file formats (alphabetical)
- file (command)
- Windows file types
- MetadataMetadataThe term metadata is an ambiguous term which is used for two fundamentally different concepts . Although the expression "data about data" is often used, it does not apply to both in the same way. Structural metadata, the design and specification of data structures, cannot be about data, because at...
- .properties.properties.properties is a file extension for files mainly used in Java related technologies to store the configurable parameters of an application. They can also be used for storing strings for Internationalization and localization; these are known as Property Resource Bundles.Each parameter is stored as a...