Chapter 11 File Systems and Directories

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Chapter 11File Systems and Directories

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File Systems (Chapter 11.1)

• File 1. A named collection of related data.
  2. smallest amount of information that
  can be written to secondary storage.
  3. sequence of bits, bytes, lines or records
• File system Operating Systems logical view of
  files it manages. Often organized into a
  hierarchy (tree).
• Directory A named group of files. Also
  called a folder.

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Text and Binary Files

• file - contains a program or data
  - creator of a file decides how it is
  organized - all computer files are patterns of
  0s and 1s
• Text file A file in which the bytes of data
  are organized as characters from the ASCII or
  Unicode character sets.
• Binary file A file that contains data in a
  specific format, requiring interpretation.

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Text and Binary Files (qualifier)

• The terms text file and binary file are somewhat
  misleading
• They seem to imply that the information in a text
  file is not stored as binary data.
• Ultimately, all information on a computer is
  stored as binary digits.
• These terms refer to how those bits are
  formatted as chunks of 8 or 16 bits, interpreted
  as characters, or in some other special format.

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File Types

• file type identifies the kind of information in
  a file, and, therefore, the organization of the
  file
• For example, a file may contain a program, an
  image, or an audio clip.
• file extension indicates the file type
• Example MyProg.exe
• Warning file extension is only helper
  information, it does not guarantee the file
  contents are actually organized that way. You
  can lie!!

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File Types

• Why are file extensions helpful?
• OS uses file extension to select display icon,
  which helps user identify type of data in the
  file.
• double-click on icon launches associated
  application and opens the data file

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File Types

• File names are often separated, usually by a
  period, into two parts
• Main name
• File extension
• The file extension indicates the type of the file.

Figure 11.1 Some common file types and their
extensions
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File Operations

• Must distinguish between operations on files as
  atomic units and processing data inside
• Atomic operations delete, open, close, rename,
  copy
• Processing data read data, write data,
  reposition file pointer, append data, truncate

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File Access

• Sequential access Information in the file is
  processed in order, and read and write operations
  move the current file pointer as far as needed to
  read or write the data.
• The most common file access technique, and the
  simplest to implement.
• Note this largely describes file access and data
  processing, pre 1965. Files were stored on
  magnetic tape and batch processed.

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File Access
Figure 11.2 Sequential file access
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File Access

• Direct access Files are conceptually divided
  into numbered logical records and each logical
  record can be accessed directly by number.
• Note Files are now stored on magnetic (hard
  disc) or optical discs (CD/DVD-ROM), or other
  direct access devices (Flash drive).

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File Access
Figure 11.3 Direct file access
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File Protection

• In multiuser systems, file protection is of
  primary importance.
• We dont want one user to be able to access
  another users files unless the access is
  specifically allowed.
• A file protection mechanism determines who can
  use a file and for what general purpose.

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File Protection

• A files protection settings in the Unix
  operating system is divided into three categories
• Owner
• Group
• World

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Directory Trees (Chapter 11.2)

• A directory of files can be contained within
  another directory.
• The directory containing another is usually
  called the parent directory, and the one inside
  is called a subdirectory.
• Directory tree file system is commonly
  organized into a hierarchy called a directory
  tree. Looks like an upside-down tree. (UNIX OS,
  1974)
• Root directory where the file system starts
  (indicated by \ (Windows), or / (UNIX)

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Directory Trees
Figure 11.4 A Windows directory tree
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Directory Trees

• At any point in time, you can be thought of as
  working in a particular location (that is, in a
  particular subdirectory).
• Working directory The subdirectory in which you
  are working.

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Path Names

• Path location of a file or subdirectory in a
  file system, consisting of the series of
  directories through which you must go to find the
  file or subdirectory.
• Absolute path begins at the root directory
• Relative path begins at the current or working
  directory.

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Path Names

• Examples of absolute paths
• C\Program Files\MS Office\WinWord.exe
• C\My Documents\letters\applications\vaTech.doc
• C\Windows\System\QuickTime

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Path Names

• Examples of relative paths
• Suppose the current working directory is
• C\My Documents\letters
• Then the following relative path names could be
  used
• cancelMag.doc
• applications\calState.doc
• ..\csc101\proj2.java
• ..\..\WINDOWS\Drivers\E55IC.ICM