Linux Comprehensive Revision Guide (Lectures 1-7)

1. Introduction to Linux & OS Concepts

• Operating System (OS): An OS manages hardware and software, acting as a bridge between the user and the physical computer.
• Layers: User ↔ Applications ↔ OS (Kernel) ↔ Hardware.
• The Kernel: The core of Linux that manages CPU, Memory (RAM), and devices. Hardware drivers in Linux are called Modules.
• Distributions (Distros): A complete OS package including the Kernel, GNU tools (Shell/Terminal), a Graphical Environment (Gnome, KDE), and a Package Manager (YUM, DNF).
• Advantages: Linux is Costless, Open Source, Stable, and Highly Secure.

2. Command Line Interface (CLI) Basics

• CLI vs GUI: CLI is text-based and essential for advanced tasks and server management.
• Terminal: The window where you type.
• Shell: The program that executes your commands (e.g., Bash).
• Essential Commands:
  • whoami: Current logged-in user.
  • pwd: Print Working Directory (where am I?).
  • ls: List files and folders.
  • cd: Change directory (cd .. to go back, cd / for root).
  • mkdir: Create a new folder.
  • rm / rmdir: Delete files or empty folders.
  • cp / mv: Copy or Move/Rename files.
  • man [command]: View manual for a command.

3. Text File Manipulation

• touch: Create an empty file.
• cat: Display file content.
• echo: Write text to a file (> overwrites, >> appends).
• nano: Simple terminal text editor (Ctrl+O to save, Ctrl+X to exit).
• Searching:
  • find: Search for file names (e.g., find . -name "*.txt").
  • grep: Search for text inside files (e.g., grep "error" log.txt).
• Viewing: less (scrollable), head (first lines), tail (last lines), tail -f (real-time monitoring).

4. User and Group Management (CLI)

• Users:
  • sudo useradd -m username: Create user with a home directory.
  • sudo passwd username: Set/change password.
  • sudo userdel -r username: Delete user and their files.
  • /etc/passwd: File containing user information.
• Groups:
  • sudo groupadd groupname: Create a group.
  • sudo usermod -aG groupname username: Add user to a group.
  • groups username: See which groups a user belongs to.
  • /etc/group: File containing group information.

5. File Permissions (chmod)

• Structure: [Type] [User] [Group] [Others] (e.g., - rwx r-x r--).
• Numeric (Absolute) Mode:
  • Read (4), Write (2), Execute (1).
  • 7 (4+2+1) = rwx
  • 6 (4+2) = rw-
  • 5 (4+1) = r-x
  • 4 = r--
• Examples:
  • chmod 755 file: Full for user, read/execute for others.
  • chmod 644 file: Read/write for user, read-only for others.

6. Disk Partitioning (fdisk)

• Partition Types:
  • Primary: Max 4 per disk, bootable.
  • Extended: Holds multiple Logical partitions.
• Process to use a new disk:
  1. Create Partition: fdisk /dev/sdb (Commands: n for new, w to save).
  2. Format: mkfs -t ext4 /dev/sdb1 (Creates the filesystem).
  3. Mount: mount /dev/sdb1 /mnt/data (Attaches it to a directory).
• Verification: df -hT shows mounted disks and their sizes.