linux day #1

1. What is an Operating System (OS)?

An operating system is system software that:

• Manages computer hardware (CPU, memory, disk, network)
• Provides services for applications
• Allows users and programs to interact with the computer

Examples:

• Windows
• macOS
• Linux

Without an operating system, applications cannot run.

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2. What is Unix?

Unix is a powerful operating system that was created in the late 1960s.

Key characteristics of Unix:

• Multi-user
  Multiple users can use the system at the same time, each with different permissions.

• Multitasking
  Multiple programs can run simultaneously.

• Stable and secure
  This is why Unix became very popular in servers and enterprise systems.

Originally:

• Unix was proprietary (closed source)
• Different companies created their own Unix versions (Solaris, AIX, HP-UX)

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3. What does “multi-user” really mean?

A simple example:

• When Windows asks: “Do you want to allow this app to make changes to your device?”
• That is a multi-user permission model

Unix and Linux were designed around this concept from the beginning, which makes them ideal for servers and security-focused systems.

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4. What is a Kernel?

The kernel is the core part of an operating system.

It is responsible for:

• CPU scheduling
• Memory management
• Disk access
• Network communication
• Talking directly to hardware

Think of it like this:

Applications → Operating System → Kernel → Hardware

Without the kernel, the OS cannot function.

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5. What is Linux?

Linux is a kernel.
Not a full operating system by itself.

Linux kernel responsibilities:

• Manages hardware
• Handles system calls
• Controls memory, CPU, and devices

The Linux kernel is:

• Open source
• Free to use and modify
• Widely used in servers, cloud, security, and embedded systems

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6. What does “Unix-like” mean?

Linux is described as Unix-like because:

• It behaves like Unix
• It follows Unix design principles
• It uses similar commands (ls, cp, chmod, ps, etc.)
• But it is not original Unix

Linux was created as a free replacement for Unix.

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7. What is GNU?

GNU stands for:

GNU is Not Unix

The GNU Project started in 1983 with the goal of creating:

• A free
• Open-source
• Unix-compatible operating system

GNU created:

• Compilers (gcc)
• Shells (bash)
• Core utilities (ls, cat, grep, chmod, etc.)

But GNU did not have a kernel.

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8. How GNU and Linux work together

Here is the key idea:

• Linux → provides the kernel
• GNU → provides the tools and utilities

Together they form a complete operating system:

GNU + Linux = a full Unix-like operating system

Technically, this is called GNU/Linux.

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9. Why do we usually just say “Linux”?

In practice:

• Saying “GNU/Linux” is technically correct
• Saying “Linux” is common and accepted

So when people say:

• “Linux server”
• “Linux OS”
• “Linux system”

They usually mean GNU/Linux.

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10. Linux in real life

Linux is everywhere:

• Servers & Cloud (AWS, Azure, GCP)
• Cybersecurity tools
• DevOps environments
• Microcomputers (Raspberry Pi)
• IoT devices
• Android smartphones (Android runs on the Linux kernel)

Even:

• macOS and iOS are based on Unix principles (BSD/Unix foundation)

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11. Simple summary (interview-friendly)

You can answer like this:

Linux is an open-source kernel used to build Unix-like operating systems.
GNU provides the tools and utilities, and together they form GNU/Linux.
Linux is widely used in servers, cloud computing, cybersecurity, and embedded systems because it is stable, secure, and flexible.

Linux Distributions Explained

1. What is a Linux Distribution?

A Linux distribution (distro) is a complete operating system built on top of:

• Linux kernel (core)
• GNU tools and utilities
• Additional software selected for a specific purpose

In short:

GNU + Linux kernel + extra software = Linux distribution

Examples of extra software:

• Package managers
• Desktop environments
• Web servers
• Security tools
• System utilities

That is why Linux distributions exist — to serve different user needs.

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2. Why are there many Linux distributions?

Different users need different things:

• Beginners → user-friendly systems
• Servers → stability and security
• Enterprises → long-term support
• Developers → modern packages
• Security professionals → hardened systems

Because Linux is open source:

• Anyone can create a distribution
• Many distributions share common tools
• Differences are usually in defaults, configuration, and support model

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3. Free vs Paid Linux Distributions

Most Linux distributions are free.

Some are commercial (paid), but this is allowed by open-source licenses as long as:

• Source code is available
• Users can inspect and modify it

Paid distributions usually offer:

• Enterprise support
• Security guarantees
• Long-term maintenance

1. Ubuntu

• Based on the Debian family
• Designed to be user-friendly

• Very popular for:

  • Beginners
  • Developers
  • Cloud environments

• Excellent documentation and community support

2. CentOS Stream

• Part of the Red Hat family
• Free to use
• Closely related to Red Hat Enterprise Linux (RHEL)

• Ideal for:

  • Enterprise-style environments
  • Learning Red Hat ecosystems
  • Professional and production-like setups

9. Ways to Run Linux on Your Computer

There are multiple options:

Option 1: Install Linux as the main OS

• Replace Windows or macOS
• Linux becomes your primary system

Option 2: Dual Boot

• Install Linux alongside Windows
• Choose OS at boot time

Installing Ubuntu in VirtualBox

you will learn:

• How to install Ubuntu Linux in VirtualBox
• How to install CentOS Stream in VirtualBox
• How to configure VirtualBox so the virtual machine is comfortable to use

• How to enable:

  • Copy & paste
  • Shared folders
  • Better mouse and screen behavior

• How to pause, resume, and shut down virtual machines correctly

PART 1 — Installing Ubuntu in VirtualBox

1. Download Ubuntu Desktop (LTS)

1. Go to ubuntu.com
2. Click Download → Ubuntu Desktop
3. Download the latest LTS version

• LTS = Long Term Support
• Supported for several years
• More stable for learning and courses

Ubuntu releases versions every 6 months, but LTS releases every 2 years.

2. Create a New Ubuntu Virtual Machine

1. Open VirtualBox
2. Click New
3. Name the VM: Ubuntu-LTS
4. Select the downloaded Ubuntu ISO
5. IMPORTANT: ✅ Check “Skip unattended installation” (If you cannot check it, that’s fine — continue anyway)

Why:

• Automatic installs often break:

  • Language settings
  • Terminal
  • sudo permissions

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3. Allocate Resources

Recommended (minimum → better):

• RAM:

  • Minimum: 512 MB
  • Recommended: 2 GB

• CPU:

  • Minimum: 1 core
  • Recommended: 2–4 cores

• Disk:

  • Recommended: 100 GB
  • Disk grows only when used

Click Next → Finish

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4. Install Ubuntu

1. Start the VM
2. Select Try or Install Ubuntu
3. Click Install Ubuntu
4. Keyboard layout

• Choose carefully (very important)

  1. Select:

• Normal installation

• ✅ Install updates

• ✅ Install third-party software

  1. Choose Erase disk and install Ubuntu

• This affects only the virtual disk

  1. Set:

• Name

• Computer name

• Username

• Password (keep it simple)

• Optional: Auto-login

  1. Click Install
  2. Wait ~10 minutes
  3. Click Restart

Ubuntu is now installed.

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5. Basic Ubuntu First Boot

• Skip welcome tour if shown
• If you see a battery icon instead of power icon, this is normal
• Ubuntu is now ready, but not fully optimized yet

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6. Install VirtualBox Guest Additions (Ubuntu)

Guest Additions improve:

• Mouse behavior
• Screen resizing
• Clipboard sharing
• Shared folders

Step 1: Update System

Open Terminal and run:

sudo apt update
sudo apt full-upgrade -y

Restart if kernel updates were installed.

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Step 2: Install Required Tools

sudo apt install -y build-essential linux-headers-generic dkms

Why:

• Guest Additions drivers are compiled from source
• These tools allow compilation and kernel integration

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Step 3: Install Guest Additions

1. In VirtualBox menu:

• Devices → Insert Guest Additions CD Image
  1. Open Terminal
  2. Run (drag the file into terminal):

sudo /media/*/VBoxLinuxAdditions.run

1. Confirm installation
2. Restart the VM

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7. Enable Convenience Features (Ubuntu)

Clipboard

• Devices → Clipboard → Bidirectional

Shared Folders

1. Devices → Shared Folders → Settings
2. Add a folder (e.g., Desktop)
3. Enable:

• Auto-mount
• Make permanent

• Read & Write

  1. Access via:

• Files → Other Locations

Now you can exchange files between Windows/macOS ↔ Ubuntu.

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8. Pausing and Shutting Down Ubuntu

• Close VM window:

  • Save Machine State → pause
  • Power off → shutdown

• Saved state resumes instantly later

Creating and Using Snapshots

Why Snapshots Matter

Snapshots allow you to:

• Go back in time
• Undo mistakes
• Recover broken systems
• Avoid reinstalling Linux

You should always create a snapshot after setup.

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1. Create a Snapshot

1. Power off or keep VM running
2. VirtualBox → Snapshots
3. Click Take Snapshot
4. Name it: After Clean Install
5. Click OK

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2. Restore a Snapshot

If something breaks:

1. Power off VM
2. Open Snapshots
3. Select snapshot
4. Click Restore
5. ❌ Do NOT take another snapshot
6. Start VM

Your system is restored to a working state.

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Important Safety Note

Snapshots do not protect:

• Files deleted from shared folders
• Files deleted from your Windows host

Best practice:

• Use shared folders carefully
• Prefer read-only shared folders if unsure

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Installing Ubuntu on Apple Silicon Macs (M1 / M2 / M3)

This lecture is only for Mac users who:

• Do NOT have an Intel Mac

• Have a Mac with an Apple processor:

  • M1
  • M2
  • M3 (and newer)

If you have an Intel Mac, you should not follow this lecture.
You can use VirtualBox instead.

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Why We Cannot Use VirtualBox on Apple Silicon

Apple Silicon Macs use the ARM CPU architecture, while VirtualBox is mainly optimized for x86 (Intel/AMD) processors.

Because of this:

• VirtualBox support on Apple Silicon is limited
• Performance is poor or features don’t work correctly

So we need a different solution.

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What Is UTM?

UTM is a virtualization tool made specifically for macOS.

It supports two modes:

1. Apple Virtualization (fast – recommended)
2. QEMU Emulation (slow – not recommended)

In this course, we will use:

✅ Apple Virtualization

This requires:

• Guest operating system compiled for ARM
• Host and guest CPU architecture must match

That is why we must download Ubuntu for ARM (arm64).

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CPU Architecture (Simple Explanation)

x86 / x86_64 / AMD64

• Used by Intel and AMD

• Common on:

  • Windows PCs
  • Older Macs

• All names mean the same architecture

ARM (arm64 / aarch64)

• Used by:

  • Apple Silicon Macs
  • Smartphones
  • Many cloud servers

• Very energy efficient

• Different “language” than x86 CPUs

A processor can only execute instructions written for its architecture.

That’s why:

• x86 Linux cannot run directly on ARM
• ARM Linux must be used on Apple Silicon

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Why We Use Apple Virtualization (Not Emulation)

Apple Virtualization

• Uses macOS native virtualization
• Very fast
• Low overhead
• Requires ARM Linux (which we use)

QEMU Emulation

• Emulates a full CPU
• Extremely slow
• Not needed for this course

❌ Do NOT use QEMU unless absolutely necessary

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PART 1 — Download Ubuntu for ARM (LTS)

Important Requirements

• Must be Ubuntu Desktop
• Must be ARM64
• Must be LTS (Long Term Support)

⚠️ Do NOT download:

• Ubuntu Server
• x86 / AMD64 ISO

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How to Find the Correct ISO

The ARM desktop ISO is not always visible on the main Ubuntu page.

Option 1: Direct Search

Search for:

Ubuntu Desktop Live Jammy ARM64

Option 2: Ubuntu Release Page

Look for:

• Desktop
• arm64
• ISO

Example (Jammy = 22.04 LTS):

ubuntu-22.04.x-desktop-arm64.iso

Download the Desktop ISO, not Server.

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PART 2 — Install UTM on macOS

Download UTM

Go to:

https://mac.getutm.app

You have two options:

• Mac App Store (paid – supports developers)
• Direct download (free – open source)

✅ Direct download is perfectly fine

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Install UTM

1. Download the DMG
2. Open it
3. Drag UTM into Applications
4. Open UTM

UTM is now installed.

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PART 3 — Create Ubuntu Virtual Machine in UTM

Step 1: Create New VM

1. Open UTM
2. Click Create
3. Choose Virtualize
4. Choose Linux
5. Choose Apple Virtualization ⚠️ This is critical

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Step 2: Select Ubuntu ARM ISO

1. Click Boot ISO Image
2. Browse to your downloaded:

   ubuntu-*-desktop-arm64.iso

1. Click Continue

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Step 3: Allocate Resources

Recommended settings:

• Memory: 2–4 GB
• CPU cores: 2–4
• Storage: 64–100 GB

Shared directories:

• Leave empty for now (we’ll configure later)

Click Continue

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Step 4: Save and Start VM

• Name the VM (e.g. Ubuntu-Desktop)
• Click Save
• Click Play

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PART 4 — Install Ubuntu Inside UTM

Boot Screen

• Select Try or Install Ubuntu
• Press Enter

⚠️ You may see boot warnings or errors
These are normal on ARM → ignore them

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Ubuntu Installer Steps

1. Click Install Ubuntu
2. Language: English
3. Keyboard layout

• Choose carefully
• If you’re not using US keyboard → change it

• Mac users should select Macintosh layout

  1. Select:

• Normal installation

• Install updates

• Install third-party software

  1. Choose Erase disk and install Ubuntu

• This affects only the virtual disk

  1. Set:

• Name

• Username

• Password (simple is fine)

• Enable automatic login (recommended)

• Set computer name (optional)

Click Install

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Installation Time

• Usually 3–5 minutes
• Progress bar may restart → this is normal
• Wait until completion

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Restart

• Click Restart Now
• Ubuntu boots into your new system

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PART 5 — First Boot on Apple Silicon

After login:

• Skip welcome screens if desired
• Internet should already work
• Copy & paste text usually works
• Keyboard shortcuts may use Ctrl instead of Cmd

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Display Resolution (Important)

UTM does not auto-resize the display.

Fix manually:

1. Open Settings
2. Go to Display
3. Choose higher resolution if needed

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Current Limitations (Normal)

• No automatic screen resize
• Drag & drop may not work
• This is expected behavior in UTM

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Why This Setup Works Well

• Native Apple virtualization
• ARM-optimized Linux
• Fast performance
• Stable for learning
• No CPU emulation overhead

Configuring Ubuntu on macOS Using UTM (Apple Silicon)

This is for Mac users with Apple Silicon (M1 / M2 / M3) who are running:

• Ubuntu in UTM

At this point, your Linux system already works.
We only need to finish configuration, mainly:

• Access files from macOS
• Make shared folders permanent
• Improve usability
• Create a safe backup (clone)

UTM supports shared folders, but they need manual setup inside Linux.

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PART 1 — Adding a Shared Folder in UTM (Ubuntu)

Step 1: Add Shared Directory in UTM

1. Open UTM
2. Select your virtual machine
3. Click Settings (slider icon)
4. Go to Sharing / Directory Sharing
5. Add a folder from macOS 👉 Recommended: create a dedicated folder like:

   ~/utm-shared

1. Confirm

⚠️ The folder may not appear immediately inside Linux.
This is normal.

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Step 2: Create a Mount Directory Inside Linux

Open Terminal in Ubuntu or CentOS and run:

sudo mkdir -p /mnt/shared

• This folder will hold all shared directories
• You can name it anything, but /mnt/shared is a clean convention

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Step 3: Mount the Shared Folder Manually

UTM uses virtiofs for directory sharing.

Run:

sudo mount -t virtiofs share /mnt/shared

Important notes:

• share is always called share (it does NOT depend on your folder name)
• /mnt/shared must exist

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Step 4: Verify Access

1. Open Files
2. Go to:

   Computer → /mnt/shared

1. You should see your macOS shared folder

Test it:

• Create a file in Linux → appears on macOS
• Rename a file → reflected on macOS
• Delete a file → deleted on macOS

✅ File sharing works

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PART 2 — Make Shared Folder Persistent (Auto-Mount)

Why This Is Needed

After reboot:

• Shared folders disappear
• You would need to re-run the mount command

To avoid this, we update /etc/fstab.

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Step 1: Edit /etc/fstab

Open the file with admin privileges:

sudo nano /etc/fstab

(Add nano if prompted — it’s usually installed)

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Step 2: Add This Line at the Bottom

share   /mnt/shared   virtiofs   rw,nofail   0   0

Explanation (high level, no deep theory yet):

• share → virtiofs shared source
• /mnt/shared → mount location
• virtiofs → filesystem type
• rw,nofail → read/write, don’t fail boot
• 0 0 → standard settings

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Step 3: Save and Exit

In nano:

• CTRL + O → save
• Enter
• CTRL + X → exit

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Step 4: Reboot and Test

sudo reboot

After reboot:

• Open Files
• Go to /mnt/shared
• Folder should be available automatically

✅ Permanent shared folder enabled

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Safety Recommendation (Important)

⚠️ Do NOT share your entire home directory

Best practice:

• Share one dedicated folder only
• Assume everything inside can be deleted accidentally

• Never share:

  • Documents
  • Desktop
  • Home directory
  • System folders

This keeps your macOS safe even if you run dangerous commands.

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PART 3 — Clipboard Behavior (macOS ↔ Linux)

Clipboard works automatically in UTM:

• Copy in Linux → paste in macOS
• Copy in macOS → paste in Linux

⚠️ Keyboard shortcuts:

• Linux: Ctrl + C, Ctrl + V
• macOS: Cmd + V

This difference is normal.

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PART 4 — Display Settings (Highly Recommended)

Adjust Resolution & Scaling

Especially useful on MacBooks.

1. Open Settings
2. Go to Displays
3. Choose:

• Higher resolution
• Optional scaling
  1. Click Apply

This improves:

• Sharpness
• Readability
• Comfort during long sessions

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PART 5 — Cloning the Virtual Machine (Your Safety Net)

Why Cloning Is Important

If you:

• Break the system
• Delete critical files
• Misconfigure Linux
• Want to experiment freely

A clone lets you recover instantly.

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How to Clone in UTM

1. Shut down the VM
2. Right-click the VM
3. Select Clone
4. Confirm

Now you have:

• Original (clean)
• Clone (experiment freely)

⚠️ Note:

• Shared folders may need to be re-selected in UTM settings
• /etc/fstab changes remain intact

First Steps in the Terminal (Bash Basics)

1. Opening the Terminal

On Ubuntu

1. Click Show Applications
2. Search for Terminal
3. Click to open it

The terminal may look different on Ubuntu .
This is normal.
The behavior is the same.

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2. What Is the Terminal?

The terminal is a text-based interface to your computer.

Instead of clicking buttons:

• You type commands
• Press Enter
• The system executes them

If you type something invalid (like random text), Linux will respond with an error.

Example:

hello world

Output:

command not found

This simply means:

Linux tried to find a program called hello and could not find it.

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3. Your First Command: echo

The echo command prints text to the terminal.

Basic Example

echo 'bash is amazing!'

Output:

bash is amazing!

Important Notes (for now)

• Use single quotes exactly as shown
• Quotes in Bash work differently than in other programming languages
• We will explain this later in the expansions chapter

For now:

Type it exactly as shown

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4. Understanding a Bash Command

This command:

echo 'bash is amazing!'

Breaks down into:

• Command: echo
• Argument: 'bash is amazing!'

General structure:

command [options] [arguments]

Everything together is called a Bash command.

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5. Line Break Behavior in echo

By default, echo prints a line break at the end.

Example

echo 'hello'

Cursor moves to the next line.

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Disable Line Break: -n

echo -n 'hello'

Now the cursor stays on the same line.

This is normal behavior.

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6. Command History & Editing

You do not need to retype commands.

Useful Keys

• Arrow Up → previous command
• Arrow Down → next command
• Arrow Left / Right → move cursor
• Edit the command
• Press Enter to run again

This is extremely important for productivity.

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7. Clearing the Terminal

To clear the screen:

clear

This does not delete anything.
It only clears the visible output.

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8. Escape Sequences (-e option)

The -e option enables escape sequences like \n.

Without -e

echo 'bash\namazing'

Output:

bash\namazing

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With -e

echo -e 'bash\namazing'

Output:

bash
amazing

\n becomes a real line break.

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9. Combining Options

Options can usually be combined.

These are equivalent:

echo -e -n 'bash\namazing'

echo -en 'bash\namazing'

Result:

• -e → enables \n
• -n → disables final line break

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10. Why This Matters

This may look simple, but these ideas are fundamental:

• Commands
• Options
• Arguments
• Output
• Line breaks

Later you will:

• Write output to files
• Chain commands
• Build scripts
• Automate tasks

Everything starts here.

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11. Confirm You Are Using Bash

Some systems use different shells by default.

Run:

echo "${BASH_VERSION}"

If you see a version number (for example 5.1.x):

✅ You are using Bash

If nothing prints:

❌ You are NOT in Bash

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12. Starting Bash Manually (If Needed)

If Bash is not active:

bash

Press Enter.

Now run again:

echo "${BASH_VERSION}"

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Important Rule

Each terminal window or tab is independent.

If your system does not start Bash automatically:

• You must run bash every time you open a new terminal

Later in the course, we will change the default shell if needed.

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13. Terminal Appearance (Optional)

You can customize the terminal:

1. Right-click → Preferences
2. Change:

• Font size
• Colors
• Theme

This does not affect commands.
Choose what is comfortable.

Navigating the File System in the Shell

1. The Working Directory

Whenever you open a terminal, you are always inside a directory.

This directory is called the working directory.

• Commands run from this directory
• Programs can access files relative to this directory

To find out where you are, we use the pwd command.

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2. pwd — Print Working Directory

pwd stands for print working directory.

Example

pwd

Example output:

/home/yourusername

This means:

• You are currently in the folder /home/yourusername
• This is your home directory

Notes

• On Linux, there are no drive letters like C: or D:
• Everything starts from a single root directory: /

• Other systems may look different:

  • Linux: /home/username
  • macOS: /Users/username

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3. The Home Directory (~)

Often, the terminal shows a ~ symbol.

Example prompt:

~ $

This means:

• ~ is a shortcut for your home directory
• It is equivalent to /home/yourusername

⚠️ This depends on terminal configuration.
If you don’t see it, that’s fine — pwd always works.

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4. Changing Directories with cd

To move between directories, we use:

cd

cd stands for change directory.

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Example 1: Move into a Folder

cd Desktop

Now check:

pwd

Output:

/home/yourusername/Desktop

You are now inside the Desktop folder.

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Tab Autocomplete (Very Important)

You don’t need to type full names.

Example:

cd Des<TAB>

The shell completes:

cd Desktop

This:

• Saves time
• Prevents typing mistakes

Use Tab constantly.

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5. Absolute Paths vs Relative Paths

Absolute Path

• Starts from /
• Always the same, no matter where you are

Example:

cd /home/yourusername/Desktop

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Relative Path

• Depends on your current directory

If you are already in /home/yourusername:

cd Desktop

Both commands lead to the same place.

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6. Moving Up the Directory Tree

One Level Up

cd ..

.. means parent directory.

Example:

• From /home/yourusername/Desktop
• cd .. → /home/yourusername

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Two Levels Up

cd ../..

Example:

• From /home/yourusername/Desktop
• cd ../.. → /home

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7. Going Back to Home

There are two easy ways:

Option 1: Use ~

cd ~

Option 2: Just cd

cd

Both take you to your home directory.

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8. Combining Paths

You can combine shortcuts and paths.

Example:

cd ~/Desktop

This means:

• Go to home directory
• Then into Desktop

Very common and very useful.

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9. Listing Files with ls

To see what’s inside a directory, we use:

ls

This lists the contents of the current working directory.

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Example

ls

Output:

Desktop  Documents  Downloads  Pictures

These are folders inside your home directory.

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10. Showing Hidden Files: -a

Hidden files in Linux:

• Start with a . (dot)
• Example: .bashrc, .config

To show all files, including hidden ones:

ls -a

You will now see:

• .
• ..
• Files starting with .

Meaning

• . → current directory
• .. → parent directory

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11. Sorting Output

Sort by Modification Time: -t

ls -t

Newest files appear first.

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Reverse Order: -r

ls -r

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Combine Options

ls -tr

Options with single letters can be combined.

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12. Long Options (--)

Some options use full words, not single letters.

Example:

ls --color=never

This disables colored output.

Important Rule

• Single-letter options → can be combined (-tr)
• Long options → cannot be combined They must be written separately with --

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13. Listing Another Directory Without Entering It

You can list a directory without changing into it.

Example:

ls /

This lists the contents of the root directory, even if your working directory is still your home directory.

Your location does not change:

pwd

Still shows:

/home/yourusername

This pattern is very important and applies to many commands.

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14. Key Concepts to Remember

• pwd → where you are
• cd → move around
• ls → see contents
• Working directory matters

• Commands use:

  • Command
  • Options
  • Arguments (like paths)

Absolute Paths vs Relative Paths (Very Important)

Because we will work a lot with files and paths, this lecture is dedicated entirely to understanding:

• Absolute paths
• Relative paths
• Why mistakes here cause many Linux problems

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1. Executing Multiple Commands Together

You already know that you can run commands one by one.

You can also run multiple commands in one line using &&.

Example

cd .. && cd Desktop && ls

What happens here:

1. Go one directory up
2. Go into Desktop
3. List the contents of Desktop

If the folder is empty, ls prints nothing — this is normal.

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Another Example

cd / && cd home && ls

This:

1. Moves to the root directory /
2. Moves into home
3. Lists its contents

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Important Warning About Auto-Completion

⚠️ Be careful when combining commands

Tab-completion (TAB) always works based on your current working directory, not future ones.

Example:

cd / && cd Des<TAB>

This fails because:

• While typing, you are still in your original directory
• After execution, your directory changes
• Tab completion cannot predict future directories

Best practice:

• Use single commands when navigating interactively
• Use combined commands only when paths are fully known

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2. What Is an Absolute Path?

An absolute path:

• Always starts with /
• Describes the full path from the root
• Works no matter where you are

Example

/home/username/Desktop

This path:

• Always points to the same folder
• Works from anywhere in the system

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Special Case: ~ (Home Shortcut)

You may see paths like:

~/Desktop

Even though it does not start with /, it is treated as absolute because:

• Bash rewrites ~ internally
• ~ becomes /home/username

So these two are equivalent:

cd ~/Desktop
cd /home/username/Desktop

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3. What Is a Relative Path?

A relative path:

• Does not start with /
• Is resolved relative to your current working directory

Example

cd Desktop

This means:

Go into Desktop from where I am now

---

Using .. (Parent Directory)

cd ..

• Moves one level up

cd ../Documents

• Go one level up
• Then enter Documents

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4. Why Relative Paths Can Break

Imagine you previously ran:

cd ../Documents

This worked because:

• You were in your home directory

Now imagine you moved somewhere else:

cd /

Then you press arrow up and re-run:

cd ../Documents

❌ This fails because:

• You are now in /
• /../Documents does not exist

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Absolute Path Always Works

cd /home/username/Documents

This works from anywhere.

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5. Key Difference (Very Important)

Absolute Path	Relative Path
Starts with /	Starts without /
Independent of location	Depends on current directory
Safer in scripts	Risky if directory changes
Predictable	Context-dependent

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6. Getting Help in Linux

When you don’t know how a command works, Linux gives you multiple help options.

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7. --help Option

Many commands support:

command --help

Example

ls --help

This prints:

• Available options
• Usage syntax
• Short explanations

⚠️ Not all commands support -h or --help, but many do.

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8. Manual Pages (man)

The most important help system in Linux is man.

Example

man ls

This opens the manual page for ls.

Navigation:

• Arrow keys → scroll
• q → quit

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9. Ubuntu: Installing man Pages (If Missing)

On Ubuntu Desktop, manuals may not be installed by default.

If man ls shows an error, do this:

Step 1: Enable manuals

sudo unminimize

Answer Y to all prompts.

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Step 2: Install man pages

sudo apt install man-db

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Step 3: Reboot (recommended)

sudo reboot

After reboot:

man ls

Now works correctly.

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10. If Built-In Help Is Not Enough

You can also:

• Search official documentation online
• Use Stack Overflow
• Use Linux forums or Reddit
• Ask questions in the course Q&A section

When searching online:

• Include your distribution (Ubuntu / CentOS)
• Include the exact error message