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A filename is a special kind of string used to uniquely identify a file stored on the file system of a computer. Depending on the operating system, such a name may also identify a directory. Different operating systems impose different restrictions regarding length and allowed characters on filenames. A filename includes one or more of these components:

• protocol (or scheme) — access method (e.g., http, ftp, file etc.)
• host (or network-ID) — host name, IP address, domain name, or LAN network name (e.g., wikipedia.org, 207.142.131.206, \\MYCOMPUTER, SYS:, etc.)
• device (or node) — port, socket, drive, root mountpoint, disc, volume (e.g., C:, /, SYSLIB, etc.)
• directory (or path) — directory tree (e.g., /usr/bin, \TEMP, [USR.LIB.SRC], etc.)
• file — base name of the file
• type (format or extension) — indicates the content type of the file (e.g., .txt, .exe, .dir, etc.)
• version — revision number of the file

In order to refer to a file on a remote computer (aka: host, server) the remote computer must be known. The remote computer name or address might part of the file name, or it might be specified at the time a file system is "mounted", in which case it won't necessarily be part of the file name.

In some systems, if a filename does not contain a path part, the file is assumed to be in the current working directory.

Many operating systems, including MS-DOS, Microsoft Windows, and VMS systems, allow a filename extension that consists of one or more characters following the last period in the filename, thus dividing the filename into two parts: the basename (the primary filename) and the extension (usually indicating the file type associated with a certain file format). On these systems the extension is considered part of the filename, and on systems which allow (for example) an eight character basename followed by a three character extension, a filename with an extension of "" or " " (nothing, or three spaces) will still be 11 characters long (since the "." is supplied, but not considered as part of the name, by the OS). On Unix-like systems, files will often have an extension (for example prog.c, denoting the C-language source code of a program called "prog"); but since the extension is not considered a separate part of the filename, a file on a Unix system which allows 14-character filenames, and with a filename which uses "." as an "extension separator" or "delimiter", could possibly have a filename such as a.longxtension

Within a single directory, filenames must be unique. However, two files in different directories may have the same name. On Unix, however, upper-case and lower-case are considered different, so that files MyName and myname would be valid names for different files in the directory; historically, names with upper-case characters in them have come before those with all-lower-case names in them in directory (folder) listings; many Unix software vendors use this scheme to make important files, such as INSTALL or README, appear in listings before relatively less important files or directories (like lib).

Unix-like systems allow a file to have more than one name; in traditional Unix-style file systems, the names are hard links to the file's inode or equivalent. Hard links are different from Windows shortcuts, Mac OS aliases, or symbolic links.

Contents 1 Reserved characters and words 2 Comparison of file name limitations 3 See also 4 References 5 External links

Many operating systems prohibit control characters from appearing in file names. Unix-like systems are an exception, as the only control character forbidden in file names is the null character, as that's the end-of-string indicator in C. Trivially, Unix also excludes the path separator / from appearing in filenames.

Some operating systems prohibit some particular characters from appearing in file names:

Character	Name	Reason
/	slash	used as a path name component separator in Unix-like and Amiga systems. Traditionally used for command line switches in MS-DOS and Windows, but can also be used as a path name component separator in some instances. [1]
\	backslash	used as a path name component separator in MS-DOS and Windows, and as the escape character in Unix systems, see Note 1
?	question mark	used as a wildcard in Unix, Windows and AmigaOS; marks a single character.
%	percent sign	used as a wildcard in RT-11; marks a single character.
*	asterisk	used as a wildcard in Unix, MS-DOS, RT-11, VMS and Windows. Marks any sequence of characters (Unix, Windows, later versions of MS-DOS) or any sequence of characters in either the basename or extension (thus "*.*" in early versions of MS-DOS means "all files". See note 1
:	colon	used to determine the mount point / drive on Windows; used to determine the virtual device or physical device such as a drive on AmigaOS, RT-11 and VMS; used as a pathname separator in classic Mac OS. Doubled after a name on VMS, indicates the DECnet nodename (equivalent to a NetBIOS (Windows networking) hostname preceded by "\\".)
|	vertical bar	designates software pipelining in Windows, see Note 1
"	quotation mark	used to mark beginning and end of filenames containing spaces in Windows, see Note 1
<	less than	used to redirect input, allowed in Unix filenames, see Note 1
>	greater than	used to redirect output, allowed in Unix filenames, see Note 1
.	period	allowed but the last occurrence will be interpreted to be the extension separator in VMS, MS-DOS and Windows. In other OSes, usually considered as part of the filename, and more than one full stop may be allowed.

Note 1: Most Unix shells require certain characters such as spaces, <, >, |, \, and sometimes :, (, ), &, ;, to be quoted or escaped:

five\ and\ six\ 'five and six

In Windows the space and the period are not allowed as the final character of a filename. The period is allowed as the first character, but certain Windows applications, such as Windows Explorer, forbid creating or renaming such files (despite this convention being used in Unix-like systems to describe hidden files and directories). Among workarounds are using different explorer applications or saving a file from an application with the desired name.^[1]

Some file systems on a given operating system (especially file systems originally implemented on other operating systems), and particular applications on that operating system, may apply further restrictions and interpretations. See comparison of file systems for more details on restrictions imposed by particular file systems.

In Unix-like systems, MS-DOS, and Windows, the file names "." and ".." have special meanings (current and parent directory respectively).

In addition, in Windows and DOS, some words might also be reserved and can not be used as filenames.^[1] For example, DOS Device file:

CON, PRN, AUX, CLOCK$, NUL
COM0, COM1, COM2, COM3, COM4, COM5, COM6, COM7, COM8, COM9
LPT0, LPT1, LPT2, LPT3, LPT4, LPT5, LPT6, LPT7, LPT8, and LPT9.

Operating systems that have these restrictions cause incompatibilities with some other filesystems. For example, Windows will fail to handle, or raise error reports for, these legal UNIX filenames: aux.c, q"uote"s.txt, or NUL.txt.

Main article: Comparison of file systems

System	Alphabetic Case Sensitivity	Allowed Character Set	Reserved Characters	Reserved Words	Maximum Length	Comments
MS-DOS FAT	case-insensitive case-destruction	A–Z 0–9 - _	all except allowed		8 + 3
Commodore 64	case-sensitive case-preservation	any	:,= shift-space		16	Actual limit depends on the drive used, but most drives limit the length to 16 characters.
Win95 VFAT	case-insensitive	any	|\?*<":>+[]/ control characters		255
WinXP NTFS	optional	any	|\?*<":>/ control characters	aux, con, prn	260
OS/2 HPFS	case-insensitive case-preservation	any	|\?*<":>/		254
Mac OS HFS	case-insensitive case-preservation	any	:		255	Finder is limited to 31 characters
Mac OS HFS+	case-insensitive case-preservation	any	: on disk, in classic Mac OS, and at the Carbon layer in Mac OS X; / at the Unix layer in Mac OS X		255	Mac OS 8.1 - Mac OS X
most UNIX file systems	case-sensitive case-preservation	any	/ null		255	a leading . indicates that ls and file managers will not by default show the file
early UNIX (AT&T)	case-sensitive case-preservation	any	/		14	a leading . indicates a "hidden" file
POSIX "Fully portable filenames"[2]	case-sensitive case-preservation	A–Za–z0–9._-	/ null	Filenames to avoid include: a.out, core, .profile, .history, .cshrc	14	hyphen must not be first character
AmigaOS	case-insensitive case-preservation	any	:/"		107	dos.library
Amiga OFS	case-insensitive case-preservation	any	:/"		30	Original File System 1985
Amiga FFS	case-insensitive case-preservation	any	:/"		30	Fast File System 1988
Amiga PFS	case-insensitive case-preservation	any	:/"		255	Professional File System 1993
Amiga SFS	case-insensitive case-preservation	any	:/"		32,000	Smart File System 1998
Amiga FFS2	case-insensitive case-preservation	any	:/"		107	Fast File System 2 2002
BeOS BFS	case-sensitive	UTF-8	/		255
DEC PDP-11 RT-11	case-insensitive	RADIX-50			6 + 3	Flat filesystem with no subdirs. A full "file specification" includes device, filename and extension (file type) in the format: dev:filnam.ext.
DEC VAX VMS	case-insensitive	A–Z 0–9 _			32 per component; earlier 9 per component; latterly, 255 for a filename and 32 for an extension.	a full "file specification" includes nodename, diskname, directory/ies, filename, extension and version in the format: OURNODE::MYDISK:[THISDIR.THATDIR]FILENAME.EXTENSION;2 Directories can only go 8 levels deep.
ISO 9660	case-insensitive	A–Z 0–9 _ .			255	8 directory levels max (for Level 1 conformance)

• File system
• Long filename
• Path (computing)
• Uniform Resource Identifier (URI)
• Uniform Resource Locator (URL)

1. ^ ^{a b} Naming a file msdn.microsoft.com (MSDN), filename restrictions on Windows
2. ^ Lewine, Donald. POSIX Programmer's Guide: Writing Portable UNIX Programs 1991 O'Reilly & Associates, Inc. Sebastopol, CA pp63-64

• Very large library: File Extension.com
• List of filename extensions: FILExt
• Large list of filename extensions: File-extensions.org
• Metasearch engine for file extensions: File Extension Seeker
• Filename Strategy for Managing Image Assets: ControlledVocabulary.com
• Recommendations for Limitations on Image Filenaming: ControlledVocabulary.com