Filename


A filename is used to uniquely identify a computer file in a file system. Different file systems impose different restrictions on filename lengths.
A filename may include:
The components required to identify a file by utilities and applications varies across operating systems, as does the syntax and format for a valid filename.
The characters allowed in filenames depend on the file system. The letters A–Z and digits 0–9 are allowed by most file systems; many file systems support additional characters, such as the letters a–z, special characters, and other printable characters such as accented letters, symbols in non-Roman alphabets, and symbols in non-alphabetic scripts. Some file systems allow even unprintable characters, including Bell, Null, Return and Linefeed, to be part of a filename, although most utilities do not handle them well.
Filenames may include things like a revision or generation number of the file,
a numerical sequence number,
a date and time,
or a comment such as the name of a subject or a location or any other text to help identify the file.
Some people use the term filename when referring to a complete specification of device, subdirectories and filename such as the Windows C:\Program Files\Microsoft Games\Chess\Chess.exe.
The filename in this case is Chess.exe.
Some utilities have settings to suppress the extension as with MS Windows Explorer.

History

During the 1970s, some mainframe and minicomputers had operating systems where files on the system were identified by a user name, or account number.
For example, on the TOPS-10 and RSTS/E operating systems from Digital Equipment Corporation, files were identified by
  • optional device name followed by an optional unit number, and a colon ":". If not present, it was presumed to be SY:
  • the account number, consisting of a bracket "". If omitted, it was presumed to be yours.
  • mandatory file name, consisting of 1 to 6 characters
  • optional 3-character extension.
On the OS/360 and successor operating systems from IBM, a file name can be up to 44 characters, consisting of upper case letters, digits, and the period; a file name had to start with a letter or number, a period must occur at least once each 8 characters, two consecutive periods can not appear in the name, and the name must end with a letter or digit. By convention, when using TSO, the letters and numbers before the first period is the account number of the owner or the project it belongs to, but there is no requirement to use this convention.
On the McGill University MUSIC/SP system, file names consisted of
  • Optional account number, which was one to four characters followed by a colon.If the account number was missing, it was presumed to be in your account, but if it was not, it was presumed to be in the *COM: pseudo-account, which is where all files marked as public were catalogued.
  • 1–17 character file name, which could be upper case letters or digits, and the period, with the requirement it not begin or end with a period, or have two consecutive periods.
The Univac VS/9 operating system had file names consisting of
  • Account name, consisting of a dollar sign "$", a 1-7 character username, and a period. If not present it was presumed to be in your account, but if it was not, the operating system would look in the system manager's account $TSOS. If you typed in a dollar sign only as the account, this would indicate the file was in the $TSOS account unless the first 1–7 character of the file name before the first period matched an actual account name, then that account was used, e.g. ABLE.BAKER is a file in your account, but if not there the system would search for $TSOS.ABLE.BAKER, but if $ABLE.BAKER was specified, the file $TSOS.ABLE.BAKER would be used unless $ABLE was a valid account, then it would look for a file named BAKER in that account.
  • File name, 1–56 characters separated by periods. File names cannot start or end with a period, nor can two consecutive periods appear.
In 1985, officially defined a pathname to be the character string that must be entered into a file system by a user in order to identify a file.
On early personal computers using the CP/M operating system, filenames were always 11 characters. This was referred to as the 8.3 filename with a maximum of an 8 byte name and a maximum of a 3 byte extension. Utilities and applications allowed users to specify filenames without trailing spaces and include a dot before the extension. The dot was not actually stored in the directory. Using only 7 bit characters allowed several file attributes to be included in the actual filename by using the high-order-bit; these attributes included Readonly, Archive, and System. Eventually this was too restrictive and the number of characters allowed increased. The attribute bits were moved to a special block of the file including additional information.
The original File Allocation Table file system, used by Standalone Disk BASIC-80, had a 6.3 file name, with a maximum of 6 bytes in the name and a maximum of 3 bytes in the extension. The FAT12 and FAT16 file systems in IBM PC DOS/MS-DOS and Microsoft Windows prior to Windows 95 used the same 8.3 convention as the CP/M file system. The FAT file systems supported 8-bit characters, allowing them to support non-ASCII characters in file names, and stored the attributes separately from the file name.
Around 1995, VFAT, an extension to the MS-DOS FAT filesystem, was introduced in Windows 95 and Windows NT. It allowed mixed-case long filenames, using Unicode characters, in addition to classic "8.3" names.

File naming schemes

Programs and devices may automatically assign names to files such as a numerical counter or a time stamp with the current date and time.
The benefit of a time stamped file name is that it facilitates searching files by date, given that file managers usually feature file searching by name. In addition, files from different devices can be merged in one directory without file naming conflicts.
Numbered file names, on the other hand, do not require that the device has a correctly set internal clock. For example, some digital camera users might not bother setting the clock of their camera. Internet-connected devices such as smartphones may synchronize their clock from a NTP server.
Perhaps the most common file naming convention is to limit directory names and file names to the 65 characters in the POSIX portable filename character set.
One common approach is to store the full "title" of a document inside the file itself as arbitrary UTF-8 characters, and then automatically generating a "slug" from that title to use as the filename.

Number of names per file

Unix-like file 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. Windows supports hard links on NTFS file systems, and provides the command fsutil in Windows XP, and mklink in later versions, for creating them. Hard links are different from Windows shortcuts, classic Mac OS/macOS aliases, or symbolic links. The introduction of LFNs with VFAT allowed filename aliases. For example, longfi~1.??? with a maximum of eight plus three characters was a filename alias of "long file name.???" as a way to conform to 8.3 limitations for older programs.
This property was used by the move command algorithm that first creates a second filename and then only removes the first filename.
Other filesystems, by design, provide only one filename per file, which guarantees that alteration of one filename's file does not alter the other filename's file.

Length restrictions

Some filesystems restrict the length of filenames. In some cases, these lengths apply to the entire file name, as in 44 characters in IBM z/OS. In other cases, the length limits may apply to particular portions of the filename, such as the name of a file in a directory, or a directory name. For example, 9, 11, 14, 21, 31, 30, 15, 44, or 255 characters or bytes. Length limits often result from assigning fixed space in a filesystem to storing components of names, so increasing limits often requires an incompatible change, as well as reserving more space.
A particular issue with filesystems that store information in nested directories is that it may be possible to create a file with a complete pathname that exceeds implementation limits, since length checking may apply only to individual parts of the name rather than the entire name. Many Windows applications are limited to a MAX_PATH value of 260, but Windows file names can easily exceed this limit. From Windows 10, version 1607, MAX_PATH limitations have been removed.

Filename extensions

Filenames in some file systems, such as FAT and the ODS-1 and ODS-2 levels of Files-11, are composed of two parts: a base name or stem and an extension or suffix used by some applications to indicate the file type. Some other file systems, such as Unix file systems, VFAT, and NTFS, treat a filename as a single string; a convention often used on those file systems is to treat the characters following the last period in the filename, in a filename containing periods, as the extension part of the filename.
Multiple output files created by an application may use the same basename and various extensions. For example, a Fortran compiler might use the extension FOR for source input file, OBJ for the object output and LST for the listing. Although there are some common extensions, they are arbitrary and a different application might use REL and RPT. Extensions have been restricted, at least historically on some systems, to a length of 3 characters, but in general can have any length, e.g., html.