Whenever a file is saved on a hard disk, the operating system associates several attributes with it:

1. Name

Every file has a name used to identify and perform operations on it.
Example: a.txt, report.docx

2. Identifier

When a file is created, the operating system assigns a unique number to it internally.
This number is used by the OS to perform operations on the file.

3. Type

This specifies the kind of file.
Different file types include:

• Text file (.txt)
• PowerPoint file (.ppt)
• C++ file (.cpp)
• Word file (.docx)

File extension is used to indicate the file type.

4. Location

This shows the path where the file is stored on the hard disk.

Example:
C:\Program Files\Microsoft Word\a.docx

5. Size

This represents the size of the file in bytes, kilobytes (KB), megabytes (MB), etc.

Example:
a.txt → 10 KB

6. Protection

This defines access permissions for the file. There are three main types:

• Read (R) – allows reading the file
• Write (W) – allows modifying the file
• Execute (X) – allows running the file

These permissions are given to users based on user ID, time, and access rules.

7. Date

This records timestamps of the file:

• Creation time
• Last modified time
• Last accessed time

Example Summary

File operations are actions performed by the operating system on files using system calls.

1. Create (create())

To create a new file, the OS allocates space on the disk and makes an entry in the directory.

Example:
You create a file named notes.txt.

• OS allocates space on disk
• Adds entry in directory: notes.txt

2. Open

Before using a file, it must be opened. The OS searches the directory and loads the file entry into the open-file table.

Example:
Opening notes.txt in Notepad:

• OS finds file in directory
• Loads it for editing

3. Close

After finishing work, the file is closed and removed from the open-file table.

Example:
After editing notes.txt, you click Save & Close:

• File entry removed from memory table
• File is safely saved

4. Write

Used to add or modify data in a file.

Example:
Writing in notes.txt:

Hello World

• OS writes data at current write pointer position

5. Read

Used to read data from a file.

Example:
Opening notes.txt to view content:

• OS reads data from file
• Displays “Hello World”

6. Reposition (Seek)

Moves the file pointer to a specific location in the file.

Example:
In a 100-line file, you jump directly to line 50:

• OS moves pointer to line 50
• No reading of previous lines needed

7. Delete

Removes the file completely from disk.

Example:
Deleting notes.txt:

• File entry removed from directory
• Storage space becomes free

8. Truncate

Deletes all content but keeps the file.

Example:
notes.txt contains 500 words.
After truncate:

• File still exists
• Content becomes empty (0 bytes)

9. Append

Adds new data at the end of the file.

Example:
notes.txt has:

Hello

You append:

World

Final file:

Hello
World

10. Copy

Creates a duplicate file.

Example:
Copy notes.txt → copy_notes.txt

• Both files now exist with same content

11. Rename

Changes the file name without changing content.

Example:
notes.txt → my_notes.txt

• Content stays same, only name changes

Different types of files are identified by their extensions. Each file type has a specific function.

1. Executable Files

• Extensions: .exe, .bin, .com
• Function: Ready-to-run machine language programs.

Example:
Double-clicking chrome.exe starts Google Chrome.

2. Object Files

• Extensions: .obj, .o
• Function: Compiled machine code, but not yet linked into an executable program.

Example:
After compiling program.c, the compiler creates program.o.

3. Source Code Files

• Extensions: .c, .cpp, .java
• Function: Files containing program source code written by programmers.

Example:
hello.cpp contains C++ code written by the user.

4. Text Files

• Extensions: .txt, .doc
• Function: Simple text documents or notes.

Example:
notes.txt contains written notes in Notepad.

5. Library Files

• Extensions: .lib, .dll
• Function: Contain reusable functions used by programs.

Example:
A .dll file may provide printing or graphics functions to software.

6. Print/View Files

• Extensions: .pdf, .zip, .rar
• Function: Files used for viewing, printing, or sharing compressed information.

Example:
report.pdf is used for reading and printing documents.

7. Archive Files

• Extensions: .arc, .zip, .tar
• Function: Group multiple files into one file, often compressed for storage or transfer.

Example:
project.zip contains multiple project files in one package.

8. Multimedia Files

• Extensions: .mpeg, .mp3, .mp4, .mov
• Function: Store audio, video, or both (audio-visual) data.

Example:
song.mp3 contains audio music
movie.mp4 contains video and audio

Quick Revision Table

A directory is used to organize and manage files in a computer system.

Key Points:

• A directory is a collection of files.
• It may also contain subdirectories.
• It stores file information such as:
  • Name
  • Size
  • Location
  • Access permissions
  • Date and time

Types of Directory Structures

There are four main types:

1. Single-Level Directory
2. Two-Level Directory
3. Tree-Structured Directory
4. Acyclic-Graph Directory

1. Single-Level Directory

Concept:

Only one directory (root directory) exists for all users. All files are stored in the same place.

Structure Diagram:

Root Directory
 ├── file1.txt
 ├── file2.cpp
 ├── file3.doc
 ├── file4.mp3

Advantages:

• Very simple to implement
• Easy to access files

Disadvantages:

• File naming conflict (same name cannot be used twice)
• No grouping of files (all files mixed together)
• Difficult to manage when number of files increases

2. Two-Level Directory

Concept:

Each user has their own separate directory under a root directory.

Structure Diagram:

Root Directory
 ├── User1
 │     ├── fileA.txt
 │     ├── fileB.cpp
 │
 ├── User2
 │     ├── fileA.txt
 │     ├── fileC.doc
 │
 ├── User3
       ├── fileX.mp3

Advantages:

• Same file names allowed for different users
• Better file organization
• Faster search within user directory

Disadvantages:

• No proper grouping within a user directory
• Limited sharing of files between users

3. Tree-Structured Directory

Concept:

A hierarchical structure where directories can contain subdirectories and files.

Structure Diagram:

Root
 ├── UserA
 │     ├── C_Files
 │     │      ├── file1.c
 │     │      ├── file2.c
 │     ├── Java_Files
 │            ├── file1.java
 │
 ├── UserB
       ├── Docs
              ├── fileA.doc

Advantages:

• Very well organized structure
• Easy file grouping
• Efficient searching using path

Example Path:

To access a file:

Root/UserA/C_Files/file1.c

Disadvantages:

• File sharing between different branches is difficult

4. Acyclic Graph Directory

Concept:

An extension of tree structure where files or directories can be shared between multiple directories, but cycles are not allowed.

Structure Diagram:

        Root
       /    \
    UserA   UserB
       \      /
        Shared_File

Explanation:

• Same file can be shared between multiple users
• No loops (acyclic structure)

Advantages:

• Efficient file sharing
• Saves storage space (no duplicate files)
• Flexible structure

Disadvantages:

• More complex to manage
• Requires link/reference handling
• Deletion becomes difficult (shared files issue)