filename extension: Extra characters on the end of a filename, usually separated from the filename by a period as a delimiter.

Dos only allows one extension to a filename, Linux allows more than one. In general, filename extensions help to identify the content of the file for a user and for the operating system or application program. In dos / Windows, executable files are recognized by the operating system by their extension -- originally those extensions were .exe, .bat, and .sys, but there are now additional extensions which I can't recite (but help viruses propagate). (I can think of .dll, but I don't know that it fits quite within the normal meaning of an executable file -- it is a (dynamic) library file.) In Linux, executable files are not recognized by their extension, but instead based on their permissions. This is considered more secure because, for example, simply loading a <filename>.exe to a Linux system does not make it executable -- the owner of the file must set the file permissions appropriately to make it executable. (Note that permissions could allow a file to be executable by the owner, the group (owner), or anybody (world).)

Examples:

• <filename>.txt -- a file containing text (probably -- note that nothing prevents you or the system from specifying a misleading extension)
• <filename>.txt,v -- an RCS / CVS history (version) file
• <filename>.exe -- In dos, an executable file. In Linux it could be executable, but that depends on the permissions, not the extension.

Contributors

• RandyKramer - 31 Jan 2002
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