Science Fair Project Encyclopedia
While a file is usually presented as a single stream, it most often is stored as multiple fragments of data at different places on a disk (or even multiple disks). One of the services operating systems usually perform for applications is that organization of files in a file system.
Files are created by software and usually conform to a particular file format. They are almost always assigned file names by the file system on which they are stored, so that they can be referred to at a later time.
Some operating systems allow the contents of a file to be segmented into fixed and variable length records. For example OpenVMS allows any arbitrary set of characters to be defined as the terminators to variable length record within a file. Others like Microsoft Windows, have only one specialised subclass of file, called a text file, where a sequence of characters separate the data into lines of text (a specialised variable length record). Some operating systems like UNIX do not have ability to handle file records at the operating system level, instead it is done at the application level.
A special file is a file system object which is accessed as though it was a file, but the sequence of bits is supplied or consumed by another process (or by the operating system itself) such as a device driver or network interface. Indeed, the philosophy that "everything is a file" is one of the best known design decisions in Unix and Unix-like operating systems (such as Linux).
- A collection of bytes in RAM isn't usually known as a file, unless it's stored in a RAM disk.
- Historically it was common for files to be defined as sequences of records. However this is now uncommon except on certain mainframe operating systems. On most systems, the application or a library creates the "record" abstraction from the byte stream according to the file format.
- Some operating systems use a three letter extension to differentiate files . For example executable files in MS DOS have an extension ".EXE " , for text files it is " .TXT " . 'Unix like' operating systems do not have any fixed extension for files, although .tar .gz .sh and others are standard (see also Magic number (programming)) .
The contents of this article is licensed from www.wikipedia.org under the GNU Free Documentation License. Click here to see the transparent copy and copyright details