File Management

File management is a core function of the operating system, responsible for managing files and directories, ensuring efficient storage, retrieval, and manipulation of data.

1. File Types

A file is a collection of related information that is stored on a secondary storage device. Files can be of various types:

Text files: Contain readable characters (e.g., .txt, .html).
Binary files: Contain binary data that can only be interpreted by specific programs (e.g., .exe, .bin).
Source files: Contain programming code (e.g., .c, .py).
Object files: Compiled versions of source files (e.g., .o, .obj).

2. Operations on Files

Operating systems provide various operations to manage files:

Create: Establish a new file in the file system.
Read: Retrieve data from a file.
Write: Insert or modify data in a file.
Delete: Remove a file from the file system.
Open: Make a file accessible for operations.
Close: Close an opened file to free up system resources.

3. File Support

The operating system supports file operations through the File System, which defines how files are stored, retrieved, and organized on disk. Some key features provided by the file system include:

File Naming: Provides unique identifiers for files.
File Metadata: Stores information like the file's creation date, size, type, and permissions.
File Organization: Ensures files are stored efficiently for easy access and management.

4. Access Methods

There are two primary methods to access files:

1. Sequential Access

Definition: Data in the file is accessed in a specific order, from the beginning to the end.
Use Case: Suitable for tape storage or logs where data is processed in order.

Example: Reading a log file where entries are processed one after another.

2. Direct Access

Definition: Data can be accessed directly, without reading the preceding information.
Use Case: Suitable for random-access devices like hard disks.

Example: Jumping directly to a specific record in a database file.

5. Indexing

Indexed Access uses a special index file that contains pointers to blocks of data. This allows quick access to specific file sections by referring to the index.

Example: A large database file where an index points to records, allowing fast retrieval of specific data.

6. Allocation Methods

Memory allocation methods ensure efficient file storage in secondary memory. Common allocation methods include:

1. Contiguous Allocation

Definition: Files are stored in contiguous blocks on the disk. This method is simple and provides fast access, but can lead to external fragmentation.

Example: File A is stored in blocks 5-9, File B in blocks 10-14, etc.

2. Linked Allocation

Definition: Files are stored in blocks scattered across the disk, with each block containing a pointer to the next block.

Example: File A is stored in block 5, which points to block 12, then to block 7, and so on.

3. Indexed Allocation

Definition: An index block contains pointers to all the blocks occupied by a file. This method avoids the fragmentation problems of linked allocation.

Example: An index block contains the addresses of all blocks occupied by a file.

7. Free Space Management

Free space management refers to the technique used by the operating system to keep track of available disk space. Common methods include:

Bit Vector: A bitmap where each bit represents a block (0 for free, 1 for occupied).
Linked List: A list of free blocks, each pointing to the next available block.
Grouping: Groups free blocks into clusters for efficient management.
Counting: Tracks contiguous blocks of free space by noting the starting block and the number of blocks available.

8. Directory System

A directory is a special type of file that contains information about other files, like their locations and attributes. Different directory structures include:

1. Single-Level Directory

Definition: All files are stored in one directory. Simple but inefficient for large systems with many users.

2. Two-Level Directory

Definition: Each user has their own directory. Provides better organization and separation of files for multi-user systems.

3. Tree-Structured Directory

Definition: Directories are organized in a hierarchical tree structure, allowing subdirectories to be created under other directories.

Example:

root
 ├── user1
 │   ├── file1
 │   └── file2
 └── user2
     ├── file3
     └── file4

9. File Protection

File protection mechanisms ensure that unauthorized users cannot access or modify files. Key methods include:

Access Control Lists (ACLs): Specify who can read, write, or execute a file.
File Permissions: Set specific permissions (read, write, execute) for different user groups (owner, group, others).
Encryption: Encodes file contents, allowing only authorized users to decrypt and access the file.

Example: In a Unix system, file permissions like rwxr-xr-- specify that the owner can read, write, and execute, the group can read and execute, and others can only read the file.