filename extension

filename extension The portion of a filename, following the

final point, which indicates the kind of data stored in the

file.

Many operating systems use filename extensions, e.g. Unix,

VMS, MS-DOS, Microsoft Windows. They are usually from

one to three letters (some sad old OSes support no more than

three). Examples include "c" for C source code, "ps" for

PostScript, "txt" for arbitrary text.

NEXTSTEP and its descendants also use extensions on

directories for a similar purpose.

Apart from informing the user what type of content the file

holds, filename extensions are typically used to decide which

program to launch when a file is "run", e.g. by

double-clicking it in a GUI file browser. They are also

used by Unix's make to determine how to build one kind of

file from another.

Compare: MIME type.

Tony Warr's comprehensive list.

FAQS.org Graphics formats.

(2002-04-19)