Linux Basics, Architecture, AWS, Users, Permissions & SSH (Beginner-Friendly Guide)

Today was a deep dive into the world of Linux, a skill every DevOps/DevSecOps engineer must master.
I explored Linux history, architecture, authentication, AWS security groups, file permissions, users, groups, and even connecting VM-to-VM using SSH keys.

Here’s everything I learned — explained simply, visually, and with emojis 👇

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🏁 1️⃣ Introduction to Linux — How It All Started

• Unix was originally a closed-source operating system.

• Linux was created as an open-source alternative, built on Unix concepts.

• Over time, Linux grew into many distributions (flavors) such as:

  • 🟦 Ubuntu

  • 🟩 Debian

  • 🔴 RedHat

  • 🟢 CentOS

  • 🟧 Amazon EC2 Linux

• Best practice: Always use LTS (Long-Term Support) versions for stability & security.

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🧱 2️⃣ Linux Architecture — Visual & Easy Explanation

Linux is built in layers, just like an organization:

🧑‍💻 User Applications

Apps we interact with: browsers, Docker, Jenkins, etc.

⚙️ System Utilities

Basic commands & tools:
ls, cp, mv, mkdir, etc.

📚 System Libraries

Pre-written code that applications use to perform tasks.

🧠 Kernel

The “heart” of Linux — manages:

• Memory

• Processes

• File systems

• Communication between software & hardware

💽 Hardware Layer

CPU, RAM, Storage, Network Cards, etc.

Simple visual:

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| User Applications    |
------------------------
| System Utilities     |
------------------------
| System Libraries     |
------------------------
| Kernel               |
------------------------
| Hardware             |
------------------------

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💿 3️⃣ Setting Up Ubuntu & Authentication Methods

Linux supports two main authentication types:

🔐 1. Username + Password

Simple but less secure.

🗝️ 2. SSH Key Pair (Recommended)

• More secure

• No password required

• Safer for production environments

Also learned how certain ports (like 8080, 3200 etc.) need to be open for remote access.

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🔥 4️⃣ Understanding Security Groups & Networking

Security Groups (SGs) work like firewalls:

📥 Inbound Rules

Define what traffic can enter the machine (e.g., SSH, HTTP).

📤 Outbound Rules

Define what traffic can leave the machine.

🔐 Why VPN is needed sometimes?

Some internal sites are only accessible through a VPN.
Your laptop → VPN → Secure Network → VM
Without VPN, access is denied because the VM trusts only specific IP ranges or ports.

🟧 AWS Hands-On

I learned how to:

• Select an AMI

• Configure an instance

• Open ports

• Launch an EC2 Linux server

  

  

  

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📁 5️⃣ Important Linux Folders Explained

I explored key directories:

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👥 6️⃣ User & Group Management in Linux

⭐ Root User

The superuser → full permissions.

👤 Regular Users

Restricted access → must use sudo for admin tasks.

🔑 Key Learnings:

• Create users

• Create groups

• Add/remove users from groups

• Switch users (su)

• Change passwords

• Lock/unlock accounts

• Delete users safely

• Use multiple SSH keys for multiple user access

Linux commands I practiced:

• ls, cd, cp, mv, mkdir, touch, cat

• useradd, groupadd, usermod

• passwd, deluser, gpasswd

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🔏 7️⃣ File & Folder Permissions

Linux permissions follow this pattern:

r = read  
w = write  
x = execute

Used chmod to:

• Add/remove permissions

• Manage file execution

• Secure sensitive files

Example:

chmod 755 file.sh
chmod u+x script.sh

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🔑 8️⃣ SSH Key Pair Generation & VM-to-VM Communication

Generated SSH keys using:

ssh-keygen -t rsa -b 4096 -C "practise-keys"

Then learned how to:

• Copy public key to another VM

• Connect two VMs using SSH

• Login between servers without passwords

This was a great hands-on practice for real infrastructure setups.

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🎯 Day 3 Takeaway

Linux is at the core of DevOps & DevSecOps, and today, it helped me build a strong foundation in:

✔️ Architecture
✔️ Security
✔️ Networking
✔️ AWS concepts
✔️ Users & Permissions
✔️ SSH & Authentication

I’ll be sharing my Day 4 learnings tomorrow — Stay Tuned 🫡