🔵 1. What is an Operating System (OS)?

An Operating System is a software layer that acts as a bridge between hardware and software.

✔ OS Role

• User runs an application (software)

• Software sends request → Operating System

• OS interacts with Hardware (CPU, RAM, Storage, IO)

• Response travels back the same way

✔ Simple Definition

OS = Medium that enables communication between Hardware and Software

Without an OS, applications cannot use CPU/RAM or perform any tasks.

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🔵 2. Why Linux? Why not Windows?

Linux is the most used OS in servers & DevOps.

✔ Key Reasons

1. Free / Open-source

   • No licensing cost

2. Secure

   • No need for antivirus in most cases

3. Fast & Lightweight

   • Ideal for production workloads

4. Stable

   • Rarely crashes

5. Flexible (Many Distributions)
   Ubuntu, CentOS, RedHat, Debian, Alpine, Fedora…

✔ Where Linux is used?

• Servers

• Cloud VMs

• Containers

• Kubernetes

• DevOps Tools (Jenkins, Docker, Ansible, GitLab CI…)

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🔵 3. Linux Architecture (Simple Version)

+-------------------------+
| Applications / Softwares|
+-------------------------+
| Compilers | User Process|
| System Softwares         |
+-------------------------+
| System Libraries (glibc) |
+-------------------------+
| Kernel (Heart of OS)     |
+-------------------------+
| Hardware (CPU | RAM | IO)|
+-------------------------+

✔ Kernel Responsibilities

1. Device Management

2. Memory Management

3. Process Management

4. System Calls Handling

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🔵 4. What is Shell?

Shell = Interface to talk to the Linux OS using commands.

• Windows → GUI

• Linux Servers → CLI (Command Line Interface)

• Popular shell → Bash

Shell allows:

• Creating files

• Navigating directories

• Managing processes

• Checking CPU/RAM

• Writing scripts (.sh)

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🔵 5. Basic Linux Commands

📌 Navigation

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📌 File & Folder Operations

✔ Create

touch file.txt      # create file
mkdir folder        # create directory

✔ Edit File (VI Editor)

vi file.txt

Inside VI:

• Press i → insert mode

• Type content

• Press Esc

• Type :wq → save & exit

✔ View File

cat file.txt

✔ Delete

rm file.txt         # delete file
rm -r folder        # delete folder

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🔵 6. System Monitoring Commands

✔ Memory

free -m

✔ CPU Cores

nproc

✔ Disk Usage

df -h

✔ Combined Monitoring

top

Shows:

• CPU usage

• Memory usage

• Running processes

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🔵 7. Important Commands (Quick List)

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🔵 8. Why Shell Scripting in DevOps?

Shell scripts are used for:

• Automating deployments

• Creating scheduled jobs (cron)

• Server maintenance

• Log monitoring

• Starting/stopping services

• CI/CD pipelines