🚀 Day 1: Linux Essentials – Basic Commands, File System & Permissions

Series: 30 Days DevOps Interview Preparation
Author: Tathagat Gaikwad

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👋 Introduction

Welcome to Day 1 of the 30 Days of DevOps Interview Preparation Challenge! Whether you're transitioning from support, just starting in IT, or preparing for a DevOps role, mastering Linux is non-negotiable.

Linux is the backbone of most cloud environments and containerized systems. Knowing how to navigate and manipulate it efficiently will give you an edge in any DevOps interview.

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🧠 What You’ll Learn Today

1️⃣ Linux File System Basics

Understanding the file system hierarchy is crucial. Here's a quick rundown of important directories:

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2️⃣ Commonly Used Linux Commands

Master these commands for everyday DevOps work:

📂 File/Directory Handling

ls -l           # List with details
cd /path        # Change directory
pwd             # Print current directory
mkdir demo      # Make new directory
touch file.txt  # Create a file
rm -rf dir/     # Delete folder forcefully

🔍 System Information

uname -a        # OS details
df -h           # Disk usage
top             # Live system resource usage
whoami          # Current user
uptime          # System running time

🔎 Searching Files

find /var/log -type f -name "*.log"
grep "error" /var/log/syslog

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3️⃣ Understanding Permissions in Linux

Permissions determine who can do what on a file/folder.

🔑 Types of Permissions

• r – read

• w – write

• x – execute

🔐 Command Examples

chmod 755 script.sh     # rwxr-xr-x
chown user:group file   # Change owner and group
ls -l file.txt          # View permissions

🧮 Numeric Mode

Example: chmod 644 file.txt gives read/write to owner and read-only to group and others.

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🎯 Interview Questions to Practice

Here are some commonly asked Linux questions in DevOps interviews:

• What does chmod 777 do? Is it safe?

• Explain the Linux file permission model.

• What is the difference between a symbolic link and a hard link?

• How do you find large files on the system?

• What is the sticky bit in Linux?

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🧪 Hands-on Tasks

Try these on a local VM or WSL (Windows Subsystem for Linux):

• Create a directory /demo, add a file, and set custom permissions.

• Use find to locate .log files larger than 10MB.

• View and explain /etc/passwd and /etc/shadow.

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📌 Wrap-Up

Today was all about building a strong Linux foundation. As we move ahead in this 30-day journey, this knowledge will be the base for mastering automation, scripting, containerization, and cloud infrastructure.