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A filename is a special kind of string used to uniquely identify a file stored on a computer. Different operating systems impose different restrictions regarding length and allowed characters on filenames. The address (unique resource identifier - URI) of a file includes at least 4 different parts:

1. computer id (IP address, domain name or LAN computer name eg: wikipedia.org, 207.142.131.206 or \\MYCOMPUTER )
2. device(drive, root mountpoint, disc, volume, eg: C:, /)
3. path (position in directory tree : everything between the first and last path separator)
4. filename

In order to refer to a file on a remote computer (aka: host, server) its network id must be provided. If an URI does not contain a path part, the file is assumed to be in the current working directory.

Many systems, including DOS and UNIX, allow a filename extension that consists of one or more characters following the last period thus dividing the filename into two parts

• the basename (aka: proper filename, primary filename)
• and the extension (usually indicating the file type associated with a certain file format or mime type)

Within a single directory, filenames must be unique. However, two files in different directories may have the same name. Some operating systems, such as UNIX and the Macintosh operating system, allow a file to have more than one name, called an alias or hard link (see also: inode).

Note: These are different from windows shortcuts, symbolic links, soft links or hyperlinks.

Reserved characters and words

On all operating systems a filename might be prohibited from containing some or all of the following characters:

Character	Name	Reason
any control character (0-31)
/	slash	used as a path separator in Unix-like systems
\	backslash	used as a path separator in MS-DOS and Windows
?	question mark	used as a wildcard in Windows and AmigaOS
*	asterisk	used as a wildcard in MS-DOS and Windows
:	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; used as a pathname separator in classic Mac OS
|	vertical bar	designates software pipelining
"	quotation mark	used to mark beginning and end of filenames containing spaces
<	less than	used to redirect input
>	greater than	used to redirect output
.	period	allowed but the last occurrence will be interpreted to be the extension separator

In Windows the space and the period are not allowed as the final character of a filename.

In addition, some words might also be reserved and can not be used as filenames. 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.

You must avoid using those reserved names as a filename suffix or file name body, eg: aux.c, file.aux or NUL.txt

The comparison of the filename in different operating systems

Although possible on most newer operating systems, for maximum portability it is not recommended to use non-ASCII characters or spaces when naming files. Also some file systems, such as most file systems on Unix and Unix-like systems, are case sensitive, meaning that, for example, a file named myFile cannot be referred to as myfile, and that files with both of those names can exist in the same directory, while other file systems, such as file systems on Windows and the HFS and HFS+ file systems on Mac OS, are case-insensitive, meaning that, for example, a filenamed myFile can be referred to as myfile (or MYFILE or MyFile), and only one file with any of those names can exist in a directory.

The maximum length of a filename including the extension on different file system/operating system combinations is:

System	CaseSensitive	Allowed Charset	ReservedChars	Reserved Words	MaxLength	Comments
MS-DOS FAT	case-insensitive case-destruction	A-Z,0-9,-,_	all except allowed		12
Commodore 64	case-sensitive case-preservation	any	"		16	Flat filesystem with no subdirs. Space and shift-space are different chars.
Win95 VFAT	case-insensitive	Any	\?*<":>+[] control characters		255
WinXP NTFS	optional	Any	\?*<":> control characters	aux, con, prn	2048	maximum length of entire url in IE
WinXP HPFS	case-insensitive case-preservation	Any	\?*<":>		254
Mac OS 9 (Classic) [[HFS+]]	case-insensitive case-preservation	Any	:		255, though Finder limited to 31
Mac OS X [[HFS+]]	case-insensitive case-preservation	Any	/ at the Unix layer, : on disk and at the Carbon layer		255
most UNIX file systems	case-sensitive case-preservation	any	/		255	a leading . means ls and file managers will not by default show the file
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 1998
Amiga SFS	case-insensitive case-preservation	any	:/"		32000	Smart File System 1998

Also: consider that ISO 9660 defines a maximum directory depth / level of 8, assuming a maximum length of 255, this leads to an overall maximum url length of 2040.

External links

• Naming a file (from MSDN) – filename restrictions on Windows