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gz indicates that the tar archive file is compressed with gzip). tar indicates that the file is a tar archive of one or more files, and the. More than one extension usually represents nested transformations, such as (the. Sometimes it is said to have more than one extension, although terminology varies in this regard, and most authors define extension in a way that doesn't allow more than one in the same file name. The dot character is just another character in the main filename. In OS/360 and successors, the part of the dataset name following the last period is treated as an extension by some software, e.g., TSO EDIT, but it has no special significance to the operating system itself the same applies to Unix files in MVS.įilesystems for UNIX-like operating systems do not separate the extension metadata from the rest of the file name. Under Microsoft's DOS and Windows, extensions such as EXE, COM or BAT indicate that a file is a program executable. On file systems of some mainframe systems such as CMS in VM, VMS, and of PC systems such as CP/M and derivative systems such as MS-DOS, the extension is a separate namespace from the filename. 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 character ( example: txt is the extension of the filename readme.txt, and html the extension of ). They are commonly used to imply information about the way data might be stored in the file. Some file systems implement filename extensions as a feature of the file system itself and may limit the length and format of the extension, while others treat filename extensions as part of the filename without special distinction.įilename extensions may be considered a type of metadata. Other extension formats include dashes and/or underscores on early versions of Linux and some versions of IBM AIX.

A filename extension is typically delimited from the rest of the filename with a full stop (period), but in some systems it is separated with spaces. The extension indicates a characteristic of the file contents or its intended use.