Filename

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(Redirected from File name)

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:

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

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Reserved Words

On most operating systems a filename may not contain any of the following characters because they have special meanings in the filesystem:

any control character (0-31)
/ SLASH (used as a path separator; symbol for the root on UNIX)
| PIPE
\ BACKSLASH (used as a path separator)
? QUESTIONMARK (used as a wildcard in Windows)
" DOUBLE-QUOTATIONMARK (used to mark beginning and end of filenames containing spaces)
* STAR (used as a wildcard in Windows)
: COLON (used to determine the mount point / drive on windows)
< LESS THAN
> GREATER THAN
. (allowed but the last occurrence will be interpreted to be the extension separator)

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

In addition, some words may 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

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The comparison of the filename in different operating systems

Although possible on most newer operating systems, for portability reasons it is not recommended to use non-western characters or spaces when naming files. Also some operating systems (UNIX) are case sensitive meaning - myFile is not the same as myfile - while others are not (Windows).

The maximum length of a filename including the extension on different systems is:

System	CaseSensitive	AllowedCharset	ReservedChars	ReservedWords	MaxLength	Comments
MS DOS	no	A-Z,0-9,-,_	all except allowed		12	ISO-9660, CD Filesystem
Win95	no	A-Z, a-z,0-9,-,_	\?*<":>+[] control characters		255
WinXP NTFS	optional	Any	\?*<":> control characters	aux, con, prn	2048	maximum length of entire url in IE
WinXP HPFS	no	Any	\?*<":>		254
MAC OS9 (Classic)	case-preserving	Any	:		255, though Finder limited to 31
MAC OSX	case-preserving	Any	:		255
UNIX	yes	any	/		256	a leading . makes it a system file

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.

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