Introduction

In UNIX, the file system is organized as an inverted tree structure (also called a hierarchical structure). Every file and directory has a parent, and directories can contain children (files or subdirectories). This parent-child relationship is fundamental to how UNIX organizes and accesses data.

The Root Directory

• The topmost directory is called the root directory, denoted by /.
• Every file and directory in the system is a descendant of root.
• Root is its own parent (i.e., / is the parent of /).

Directory Hierarchy

/                       (Root)
├── bin/                (Essential user binaries)
├── etc/                (Configuration files)
├── home/               (User home directories)
│   ├── user1/          (Home of user1)
│   │   ├── file1.txt
│   │   └── docs/
│   │       └── report.pdf
│   └── user2/          (Home of user2)
├── tmp/                (Temporary files)
├── usr/                (User programs)
│   ├── bin/
│   ├── lib/
│   └── local/
├── var/                (Variable data - logs, mail)
└── dev/                (Device files)

Key Concepts

1. Parent Directory:

• Every directory (except root) has a parent directory.
• Referred to as .. (dot-dot).
• Example: The parent of /home/user1/docs is /home/user1.

2. Current Directory:

• The directory you are currently working in.
• Referred to as . (single dot).

3. Absolute Path:

• Full path from the root directory.
• Example: /home/user1/docs/report.pdf
• Always starts with /.

4. Relative Path:

• Path relative to the current working directory.
• Example: If you are in /home/user1, then docs/report.pdf is a relative path.
• Uses .. to go up: ../../etc/passwd

Relationship Between Files and Directories

• A directory is a special file that contains a list of (filename, inode number) pairs.
• When you create a file inside a directory, a new entry is added to the parent directory mapping the filename to the file's inode.
• Multiple filenames can point to the same inode — this is called a hard link.
• Deleting a file removes the directory entry, not the inode itself (the inode is freed only when its link count reaches 0).

Important Standard Directories

Summary

• UNIX uses an inverted tree structure with / (root) at the top.
• Each directory entry maps a filename to an inode number.
• Absolute paths start from root; relative paths start from the current directory.
• The .. entry refers to the parent directory, and . refers to the current directory.