📂 What is a Directory?
A directory is a special file that contains information about other files and directories. Think of it as a table of contents for the file system.
Functions of a Directory:
- Organize files in a structured way.
- Store file attributes (name, type, size, location, creation date).
- Provide access control and navigation within the file system.
🧱 Types of Directory Structures
🔸 1. Single-Level Directory
- All files are in one directory.
- Easy to implement.
👎 Limitations:
- File name conflicts.
- Poor scalability.
🧪 Example:
/
|-- file1
|-- file2
|-- file3
🔸 2. Two-Level Directory
- A separate directory for each user.
- Solves naming conflict between users.
👎 Limitations:
- Users can’t share files easily.
🧪 Example:
/
|-- user1/
| |-- file1
|-- user2/
| |-- file1
🔸 3. Tree-Structured Directory
- Hierarchical structure with nested directories.
- Each directory can contain files and subdirectories.
✅ Advantages:
- Scalable, organized, supports grouping.
🧪 Example:
/
|-- home/
| |-- user/
| |-- docs/
| |-- report.txt
| |-- music/
|-- etc/
|-- bin/
🔸 4. Acyclic Graph Directory
- Allows shared subdirectories/files via links (soft/hard links).
- Prevents duplication and saves space.
🧪 Example:
/home/user/docs/report.txt
/shared/docs/report.txt → points to same file
👎 Issues:
- Complex traversal.
- Needs care in deletion and backup.
🔸 5. General Graph Directory
- Like acyclic graph, but allows cycles (e.g., symbolic links pointing back).
👎 Risks:
- Infinite loops during traversal.
- Must implement cycle detection.
⚙️ Directory Operations (Management)
The Operating System supports these directory operations:
| Operation | Description |
|---|---|
| Create Directory | mkdir() — makes a new directory. |
| Delete Directory | rmdir() — removes a directory (if empty). |
| Open Directory | Opens a directory to access its contents. |
| Read Directory | Reads entries in the directory (readdir()). |
| Rename Directory | Changes the name of a directory. |
| Traverse | Navigates through the hierarchy. |
| List Contents | Lists all files and subdirectories (ls, dir). |
🔐 Directory Protection
Directories also support access permissions, such as:
- Read (r) – view contents.
- Write (w) – add/delete/rename files.
- Execute (x) – enter the directory.
Unix-like systems use a combination of:
- Owner, Group, Others
- Mode bits (e.g.,
drwxr-xr--)
🧠 Behind the Scenes: Data Structures
The OS uses various internal structures like:
- Directory Tables: Store names and inode numbers.
- Inodes: Contain metadata and pointers to file blocks.
- File Allocation Table (FAT) or NTFS MFT: For locating files in Windows.
📌 Summary
| Structure | Hierarchy | Sharing Allowed | Cycles Possible | Use Case |
|---|---|---|---|---|
| Single-Level | ❌ | ❌ | ❌ | Simple systems, floppy disks |
| Two-Level | ✔️ | ❌ | ❌ | Multi-user systems |
| Tree | ✔️ | ❌ | ❌ | Unix/Linux systems |
| Acyclic Graph | ✔️ | ✔️ | ❌ | Shared files, links |
| General Graph | ✔️ | ✔️ | ✔️ | Advanced linking systems |