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Chapter 11
File Systems and Directories
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 System’s logical
view of files it manages. Often organized into
a hierarchy (tree).
Directory: A named group of files.
Also called a folder.
2
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 0’s and 1’s
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.
3
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.
4
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!!
5
File Types
Why are file extensions helpful?
1. OS uses file extension to select display
icon, which helps user identify type of data
in the file.
2. double-click on icon launches associated
application and opens the data file
6
File Types

File names are often
separated, usually by
a period, into two
parts:



Figure 11.1 Some common file types and their
extensions
Main name
File extension
The file extension
indicates the type of
the file.
7
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
8
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.
9
File Access
Figure 11.2 Sequential file access
10
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).
11
File Access
Figure 11.3 Direct file access
12
File Protection



In multiuser systems, file protection is of
primary importance.
We don’t want one user to be able to access
another user’s files unless the access is
specifically allowed.
A file protection mechanism determines who
can use a file and for what general purpose.
13
File Protection

A file’s protection settings in the Unix operating
system is divided into three categories



Owner
Group
World
Page 356
14
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)
15
Directory Trees
Figure 11.4 A Windows directory tree
16
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.
17
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
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