• A file is the most important and basic entity for data storage. In this article we will get some basic knowledge about concept of a file, different attributes of file, types of file and how a file is useful for the operating system.

A file is a collection of related information stored on a peripheral device

  • File is a logical storage unit.
  • Computer store information on different storage area such as magnetic disk, tap drives etc.
  • These storage devices are usually of non-volatile type that means contents are stored despite of power failure.
  • It is a named collection of related information that is recorded on secondary storage. It is the smallest allotment of logical secondary storage as per the user’s point of view.
  • File may represent programs or data.
  • Data files may be numeric, alphabetic, alphanumeric or binary.
  • In general a file is a sequence of bits, bytes, lines or records whose meaning is defined by the file creator.
  • A file can store different information such as source program, object program, executable program, numeric data/text, database records, Graphic Image etc.
  • Each file has certain structure according to its type.
  • A text file is represented as a sequence of characters organized into lines.
  • A source file is a sequence of sub routines and functions.
  • An object file is a sequence of bytes organized into block which is understandable by system linker. Usually it has the .obj extension.
  • Executable file is a series of binary code that can be executed by the CPU. It has the .exe extension.

Types of File

  • To perform an operation on the file, the O/S must recognize its types.
  • Normally file type is implemented by including it as a part of file name.
  • The type of file represents the contents of that file.
  • File name is divided into two parts:
  1. Name
  2. Extension
  • By this way user and O/S can easily identify type of the file.
  • In Dos the name can be of 8 characters long followed by a period and extension up to 3 characters.
  • The system uses the extension to identify the file type and allow operations that can be perform on that file.
  • For example only file with .com, .exe or .bat extension are used for executing a file. .com and .exe are two forms of binary executable file whereas .bat file is a batch files.
  • DOS supports only a few extensions but application program also uses extension to indicate file types.
Files Extension Functions
Executable .exe, .com, bin, none. Need to run m/c lang .prg
Object .o, .obj Compile, m/c Lang; non linked
Source code .c, .cc, .java, .asm, .a, .pas Source code in various Lang
Batch file .bat, .sh Commands used for interpreting by o/s
Text .txt, .rtf, .doc Textual data documents
Word file .wp, .tex, .ry, .doc Various word processor formats
Library .lib, .a, .so, .dll, .mpeg, .mov, .rm Libraries routine for programs
Archive .arc, .zip, rar Related files
Multimedia .mpeg, .mov, .rm, mp4, mkv Binary file with audio, video information
  • UNIX uses a magic number to represent file type.
  • This number is stored at the beginning of a file at the time of creation.
  • Not all files have magic numbers so systems features cannot be based on this type of information.
  • It is roughly allocate to indicate some of the file types such as program file and scripting file
  • UNIX also does not record the name of the creative program.

File Attributes

  • File attributes specifies properties of the file
  • It also defines behavior of any file.
  • A file can be referred by its name.
  • A name is usually a string of characters such as example.c
  • Some system treats upper and lower case character names differently while other systems consider them to be equivalent.
  • A file has certain attributes that vary from the o/s to o/s.
  • Some common file attributes are as follow:
  1. Name: Symbolic file name is the only information which is in human readable form.
  2. Type: It indicates the type of the file such as text, binary, object etc.
  3. Location: It is a pointer to the location of the file where a file is stored.
  4. Size: It determines the current size of the file. (bytes, blocks)
  5. Protection: It provides access control information to a user who can read write and execute it.
  6. Date, Time and User Identifier:  It provides the information about the file creation date/time, last modification and last used date/time. It is helpful for security and usage monitoring.
  • All information about each file is kept in the directory structure which resides on secondary storage.
  • Normally the directory entry consists of files name and identifier.
  • The identifier locates all the attributes of the files.
  • Files and directories are non volatile storage they must be store on the device and brought into memory as needed.

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