Unit 2. Overview

Unit 2. Overview

Unit 2. Overview

  • 2.1. Logging in & out

  • 2.2 I-node and File System Structure

  • 2.3 Booting Sequence & ‘init’ process

  • 2.4 File Access Permissions

Logging In and Logging Out in Linux.

Logging In to a Linux System

  • When you start or access a Linux system, you must first log in with a valid username and password.
  • This process authenticates your identity and starts a session.

✅ Login Methods

There are different ways users can log into a Linux system depending on the interface and environment:

1. Graphical Login (GUI-based Login)

  • Used in desktop environments like GNOME, KDE, or XFCE.
  • A login screen appears with fields for username and password.
  • After successful login, the user is directed to their desktop environment.

2. Terminal Login (Command-Line Interface)

🔹 When using a local terminal:
  • Press Ctrl + Alt + F1 to F6 to switch to a virtual terminal.
  • You’ll see a login prompt like:
    makefile
     
    login: your_username Password: ********
  • Upon success, you enter the command-line shell (e.g., Bash).
🔹 When connecting remotely via SSH:
  • SSH (Secure Shell) allows login to a Linux machine over a network.
  • Example command (from another system):
     username@hostname_or_ip
  • You’ll be prompted to enter your password.

🛠 What Happens After You Log In?

  1. The system verifies your username and password.
  2. It loads your user environment (like .bash_profile or .bashrc).
  3. A shell session is started (usually Bash).
  4. You can now run commands, launch applications, or manage files.

🔒 Security Tips During Login

  • Always use a strong password.
  • Avoid logging in as root unless necessary.
  • Use SSH keys for secure remote login.
  • Monitor login attempts using:
  • last who

Logging Out of a Linux System

Logging out ends your user session and releases system resources.

✅ Logout Methods

1. In GUI (Graphical Interface)

  • Click the logout, shutdown, or exit option from the system menu.
  • This closes the graphical session and returns to the login screen or powers off.

2. In Terminal (Command Line)

You can log out of the shell session using one of the following:

Command Description
exit Ends the current shell session
logout Logs out from a login shell
Ctrl + D Sends EOF (End of File) signal to log out

🔁 Ending SSH Session

If you logged in via SSH, just type:

exit

Or press Ctrl + D. This will close the connection to the remote machine.

📋 Commands to View Login Information.

Command Purpose
whoami Displays your current username
who Shows who is logged into the system
w Lists users and what they are doing
last Shows the history of user logins
id Displays user ID and group ID information

Understanding I-node and File System Structure in Linux/UNIX.

📁 What is a File System?

  • A file system in Linux/UNIX is a method for storing and organizing files and directories on a storage device like a hard disk or SSD.
  • It allows the operating system to keep track of where files are stored and how to access them.

📂 Structure of a Linux/UNIX File System

The typical structure of a file system includes the following components:

Boot Block → Contains bootloader code
Super Block → Holds metadata about the file system
I-node Table → Stores information (metadata) about files
Data Blocks → Stores actual file contents

🔧 1. Boot Block

  • Located at the beginning of the file system.
  • Contains the boot loader, responsible for loading the operating system during startup.

📑 2. Super Block

  • Contains vital information about the file system:
    • File system type
    • Size
    • Number of inodes and blocks
    • Block size
    • Mount status
  • It is essential for mounting and managing the file system.

🧱 3. I-node Table

  • The inode (index node) is a critical data structure used to store information about files and directories (except their name).

  • Every file or directory has an associated inode.

📌 I-node (Index Node) Explained

An inode stores metadata about a file, such as:
Metadata Description
File type Regular file, directory, link, etc.
Permissions Read, write, execute permissions
Owner and group UID and GID
Size File size in bytes
Time stamps Created, modified, accessed
Link count Number of hard links
Pointers to data blocks Locations of the actual file content
🔸 Important: File names are not stored in inodes — they are stored in directory entries, which map filenames to inodes.

🧾 4. Data Blocks

  • These hold the actual contents of files and directories.
  • Inodes point to these blocks so the system knows where the file data is stored.

🔗 How It All Connects

  1. When a file is accessed, the directory entry provides the inode number.
  2. The inode contains metadata and points to the data blocks.
  3. The data blocks contain the file’s actual content.
🔍 Example Flow: Accessing a File
User accesses: /home/hardik/report.txt
→ OS finds 'report.txt' in the directory structure
→ Retrieves its inode number from directory entry
→ Reads metadata and block pointers from the inode
→ Loads actual data from the data blocks

🧠 Benefits of the I-node-Based File System

  • Efficient file metadata handling
  • Supports multiple links to the same file (via inode sharing)
  • Flexible and scalable for large numbers of files
  • Separates metadata from filenames for better management

Booting Sequence and the ‘init’ Process in Linux/UNIX.

  • Booting is the process that starts a computer system and loads the operating system so that it becomes usable.
  • In Linux and UNIX-based systems, this process is well-structured and involves multiple steps before the user gets access to the system.

🧱 1. BIOS/UEFI Stage.

  • BIOS (Basic Input/Output System) or UEFI (Unified Extensible Firmware Interface) is the first code that runs when a system is powered on.
  • It performs a POST (Power-On Self Test) to check hardware functionality.
  • Then it searches for a bootable device like a hard drive or USB.
  • Once found, it hands control over to the bootloader stored on that device.

💻 2. Bootloader Stage (GRUB or LILO)

  • The bootloader (e.g., GRUB – GRand Unified Bootloader) is a small program responsible for:
    • Loading and executing the Linux kernel.
    • Providing a menu to select different OS or kernel versions.
  • It also passes parameters to the kernel if needed.

🧠 3. Kernel Initialization

  • Once loaded into memory, the kernel:
    • Initializes system hardware (CPU, memory, devices).
    • Sets up memory management and process scheduling.
    • Mounts the root file system.
    • Starts the first user-space process, usually /sbin/init.

⚙️ 4. The ‘init’ Process

  • init is the first user-level process started by the Linux kernel.
  • It always has PID 1 (Process ID 1).
  • It remains active throughout the session and becomes the parent of all other processes.

🔍 What ‘init’ Does:

  1. Reads Configuration Files:

    • Traditional init refers to /etc/inittab to determine which processes to start.
    • Defines default runlevel (system state).

  2. Starts System Services:

    • Based on the runlevel, it launches services like:
      • Network
      • Login services
      • Scheduled jobs
      • Graphical interface (if applicable)

  3. Maintains Process Tree:

    • Monitors background services and can restart them if needed.
    • Handles orphaned processes.

  4. Controls System Shutdown/Reboot:

    • Initiates proper shutdown procedures by killing processes and unmounting file systems.

🔢 Common Runlevels (in SysV init systems):

Runlevel Description
0 Halt the system
1 Single-user mode (maintenance)
3 Multi-user mode without GUI
5 Multi-user mode with GUI
6 Reboot the system

🆕 Modern Alternative: systemd.

  • Many Linux distributions now use systemd, a more advanced and faster init system.
  • Replaces traditional init but still keeps compatibility.
  • Uses units instead of runlevels and supports parallel startup of services.
  • Logs events using journalctl.

🔁 Summary of the Boot Process:

Booting Step Description
Power ON Initial hardware power-up
BIOS/UEFI → POST Performs Power-On Self Test to check hardware
Bootloader (GRUB/LILO) Loads and executes the Linux kernel
Kernel Loaded Initializes system hardware and mounts root file system
init or systemd starts Begins user-space initialization and service loading
Services started → Login/Desktop Displays login screen or graphical desktop environment

🔐 File Access Permissions in Linux (Explained in Detail).

  • In Linux, every file and directory has a set of permissions that controls who can access and manipulate it.
  • These permissions help ensure security and multi-user control on the system.

📁 Basic Concepts

  • Each file or directory in Linux has three types of users associated with it:
User Type Description
Owner The user who created the file or directory.
Group A set of users who share access permissions.
Others Everyone else on the system.

🧪 Types of Permissions.

There are three types of permissions in Linux:
Symbol Permission Applies To
r Read View file content or list directory contents
w Write Modify file content or create/delete files in a directory
x Execute Run the file as a program or access a directory

🔍 Permission Structure (Using ls -l).

When you list files using ls -l, you get output like:
ex :- -rwxr-xr– 1 user group 1234 May 20 12:00 file.sh
Part Meaning
- File type (- for file, d for directory, l for link)
rwx Owner has read, write, and execute
r-x Group has read and execute, but no write
r-- Others have read-only access

⚙️ Changing Permissions with chmod

You can change permissions using the chmod command.

1. Symbolic Method.

example :-
  • chmod u+x file.sh # Add execute permission to the user
  • chmod g-w file.txt # Remove write permission from the group
  • chmod o=r file.txt # Set read-only for others 
Symbol Refers To
u User (Owner)
g Group
o Others
a All (user + group + others)

2. Numeric (Octal) Method.

Permissions are represented using numbers:
Permission Binary Octal
r 100 4
w 010 2
x 001 1
Example:
  • chmod 755 file.sh means:
    • 7 (4+2+1) → user: read, write, execute
    • 5 (4+0+1) → group: read, execute
    • 5 (4+0+1) → others: read, execute

🛡️ Default Permissions: umask.

  • umask defines default permission settings when new files/directories are created.
  • For example, a umask of 022 means:
    • New files: 644 (rw-r--r--)
    • New directories: 755 (rwxr-xr-x)

📌 Summary Table.

User Type Read (r) Write (w) Execute (x)
Owner Yes Yes Yes
Group Yes No Yes
Others Yes No No

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