Unit 1: Linux Ecosystem & Infrastructure

1. Linux Distributions & Families

The Linux operating system is composed of the Linux Kernel (the core interface between hardware and software) and a collection of software packages/tools (GNU tools). Because the kernel is open-source, various organizations bundle the kernel with different software managers, desktop environments, and configurations. These bundles are called Distributions (Distros).

Core Architecture

• The Kernel: Manages CPU, memory, and devices.
• The Shell: The command-line interface (CLI) for user interaction.
• The Desktop Environment (DE): The GUI (GNOME, KDE Plasma, XFCE).
• Package Manager: Software handling installation and updates.

Major Distribution Families

Linux distributions are generally categorized by their lineage and the package management system they use.

1. Debian Family:

   • Base: Debian (Stability focused).
   • Package Format: .deb
   • Package Manager: apt, dpkg
   • Notable Distros:
     • Ubuntu: Most popular, user-friendly, vast community support.
     • Linux Mint: Windows-like interface, great for beginners.
     • Kali Linux: Specialized for cybersecurity and penetration testing.

2. Red Hat (RPM) Family:

   • Base: Red Hat Enterprise Linux (RHEL) (Enterprise/Server focused).
   • Package Format: .rpm
   • Package Manager: dnf (formerly yum), rpm.
   • Notable Distros:
     • Fedora: Upstream for RHEL, features the latest technologies.
     • CentOS Stream / Rocky Linux: Free enterprise-grade alternatives to RHEL.

3. Arch Linux Family:

   • Philosophy: KISS (Keep It Simple, Stupid), Rolling Release model.
   • Package Manager: pacman
   • Notable Distros:
     • Arch Linux: Do-it-yourself installation.
     • Manjaro: User-friendly Arch derivative.

2. Command Line Fundamentals

The Command Line Interface (CLI) allows direct interaction with the OS via the Shell (usually Bash or Zsh).

Directory Structure (FHS - Filesystem Hierarchy Standard)

• /: Root directory.
• /home: User personal data.
• /etc: Configuration files.
• /bin & /usr/bin: User binaries (commands).
• /var: Variable data (logs, websites).

Essential Commands

Navigation

• pwd: Print Working Directory. Shows where you are.
• ls: List directory contents.
  • ls -l: Long listing (shows permissions).
  • ls -a: Show hidden files.
• cd [path]: Change Directory.
  • cd ..: Move up one level.
  • cd ~: Move to home directory.

File Manipulation

• mkdir [name]: Make a new directory.
• touch [file]: Create an empty file or update timestamps.
• cp [source] [dest]: Copy files.
  • cp -r: Copy recursively (for directories).
• mv [source] [dest]: Move or Rename files.
• rm [file]: Remove/Delete files.
  • rm -rf: Force remove directory recursively (Use with caution).

File Viewing

• cat [file]: Concatenate and display file content.
• less [file]: View file one page at a time.
• head [file]: View first 10 lines.
• tail [file]: View last 10 lines.

I/O Redirection and Piping

Linux treats input and output as streams.

1. stdin (0): Standard Input (Keyboard).
2. stdout (1): Standard Output (Screen).
3. stderr (2): Standard Error (Screen, error messages).

• Redirection (>, >>): Sends output to a file instead of the screen.
  
  BASH

      ls > filelist.txt   # Overwrites file with output
      ls >> filelist.txt  # Appends output to file
      

• Piping (|): Takes the output of Command A and uses it as input for Command B.
  
  BASH

      cat largefile.txt | grep "search_term" | less
      

3. User & Group Management

Linux is a multi-user system. Access is controlled via User IDs (UID) and Group IDs (GID).

User Configuration Files

• /etc/passwd: Stores user account information (public).
• /etc/shadow: Stores encrypted passwords (secure).
• /etc/group: Stores group information.

User Management Commands (Root privileges required)

• Creating Users:
  
  BASH

      sudo useradd -m -s /bin/bash username
      # -m: Create home directory
      # -s: Set default shell
      

• Setting Password:
  
  BASH

      sudo passwd username
      

• Modifying Users:
  
  BASH

      sudo usermod -aG groupname username
      # -aG: Append to secondary group (e.g., sudo group)
      

• Deleting Users:
  
  BASH

      sudo userdel -r username
      # -r: Remove home directory
      

File Permissions & Ownership

Every file has three sets of permissions: User (Owner), Group, and Others.

Permission Types

• Read (r): View file contents / List directory contents. (Octal: 4)
• Write (w): Modify file / Create or delete files in directory. (Octal: 2)
• Execute (x): Run file as script / Enter directory. (Octal: 1)

Changing Permissions (chmod)

Can be done symbolically or numerically (Octal).

• Symbolic: chmod u+x file (Add execute to User).
• Numeric: chmod 755 file (rwx for User, r-x for Group, r-x for Others).

Changing Ownership (chown)

• chown user:group file

4. Package Management

Package managers automate the process of installing, upgrading, configuring, and removing computer programs. They resolve dependencies (libraries required for software to run).

Debian/Ubuntu (apt)

Advanced Package Tool. Uses .deb files.

• Update Repositories: sudo apt update (Refreshes list of available packages).
• Upgrade System: sudo apt upgrade (Installs newer versions).
• Install Package: sudo apt install [package_name]
• Remove Package: sudo apt remove [package_name]
• Search: apt search [keyword]

Red Hat/CentOS (dnf / yum)

Dandified YUM. Uses .rpm files.

• Update/Upgrade: sudo dnf update
• Install: sudo dnf install [package_name]
• Remove: sudo dnf remove [package_name]

Low-Level Tools

These tools install specific package files but do not resolve dependencies automatically.

• Debian: dpkg -i package.deb
• Red Hat: rpm -ivh package.rpm

Universal Package Managers

These work across different distributions by bundling dependencies within the application.

1. Snap: Canonical's system (Ubuntu).
   • sudo snap install vlc
2. Flatpak: Community-driven, focused on sandboxing.
   • flatpak install flathub org.gimp.GIMP