DevOps Linux Fundamentals

Linux is the operating system that powers most servers, cloud platforms, and DevOps tools in the world. Whether deploying on AWS, running Docker containers, or configuring a Jenkins server — Linux knowledge is essential. Understanding the Linux command line makes every DevOps task faster and more precise.

Why Linux for DevOps?

• Most cloud servers (AWS EC2, Azure VM, GCP) run Linux by default.
• Docker containers are Linux-based at their core.
• DevOps tools like Jenkins, Ansible, Kubernetes, and Terraform run natively on Linux.
• Linux is open-source, stable, and highly customizable.
• Shell scripting on Linux automates repetitive DevOps tasks with ease.

Linux File System Structure

Linux organizes everything under a single root directory called /. Key directories:

Essential Linux Commands

Navigation Commands

# Show current directory
pwd

# List files and folders
ls
ls -la   # Shows hidden files with details

# Change directory
cd /var/log
cd ..    # Go one level up
cd ~     # Go to home directory

File and Directory Operations

# Create a directory
mkdir myapp

# Create an empty file
touch config.txt

# Copy a file
cp config.txt /tmp/config-backup.txt

# Move or rename a file
mv config.txt settings.txt

# Remove a file
rm settings.txt

# Remove a directory and all contents
rm -rf myapp/

Viewing File Contents

# Print full file content
cat app.log

# View large files page by page
less app.log

# Show first 10 lines
head app.log

# Show last 10 lines (very useful for logs)
tail app.log

# Follow a log file in real time
tail -f /var/log/syslog

Searching and Filtering

# Search for text inside a file
grep "ERROR" app.log

# Search recursively in all files inside a folder
grep -r "database" /etc/myapp/

# Find files by name
find /var/log -name "*.log"

File Permissions in Linux

Every file in Linux has permissions that control who can read, write, or execute it. The permission string looks like this:

-rwxr-xr--

Breaking it down:

• The first character: - means file, d means directory.
• Next three characters (rwx): Owner permissions — read, write, execute.
• Next three (r-x): Group permissions — read, no write, execute.
• Last three (r--): Others — read only.

Changing Permissions

# Give execute permission to the owner
chmod u+x deploy.sh

# Set full permissions using numbers (7=rwx, 5=r-x, 4=r--)
chmod 755 deploy.sh

# Change file owner
chown john:devteam deploy.sh

User and Process Management

User Commands

# Check current user
whoami

# Switch user
su - john

# Run a command as administrator
sudo apt update

Process Commands

# View running processes
ps aux

# Live system resource monitor
top

# Kill a process by ID
kill 1234

# Kill by process name
pkill nginx

Package Management

Installing software on Linux uses a package manager. Two common ones:

APT (Ubuntu/Debian)

# Update package list
sudo apt update

# Install a package
sudo apt install nginx

# Remove a package
sudo apt remove nginx

YUM / DNF (CentOS/RHEL/Amazon Linux)

# Install a package
sudo yum install nginx

# Update all packages
sudo yum update

Networking Commands

# Check IP address
ip addr show

# Test connectivity to a host
ping google.com

# Check open ports
netstat -tuln

# Make an HTTP request
curl https://api.example.com/status

# Download a file
wget https://example.com/setup.sh

Shell Scripting Basics

Shell scripts automate sequences of Linux commands. A script is a plain text file that runs line by line.

Simple Deployment Script Example

#!/bin/bash
# This script deploys a web app

echo "Starting deployment..."

# Pull latest code
cd /var/www/myapp
git pull origin main

# Restart the web server
sudo systemctl restart nginx

echo "Deployment complete!"

Save this as deploy.sh, make it executable with chmod +x deploy.sh, then run it with ./deploy.sh.

systemctl – Managing Services

Linux services (like web servers and databases) are managed with systemctl:

# Start a service
sudo systemctl start nginx

# Stop a service
sudo systemctl stop nginx

# Restart a service
sudo systemctl restart nginx

# Check service status
sudo systemctl status nginx

# Enable a service to start on boot
sudo systemctl enable nginx

Viewing Logs

Logs are critical in DevOps for troubleshooting deployments and monitoring applications.

# View system journal logs
journalctl -u nginx

# Follow live journal logs
journalctl -fu nginx

# View application log
cat /var/log/myapp/error.log

SSH – Remote Access

SSH (Secure Shell) connects to remote Linux servers securely. In DevOps, SSH is used to manage cloud servers.

# Connect to a remote server
ssh john@192.168.1.100

# Connect using a key file (common for AWS EC2)
ssh -i mykey.pem ubuntu@54.10.20.30

# Copy a file to a remote server
scp deploy.sh john@192.168.1.100:/home/john/

Real-World Example

A DevOps engineer needs to deploy a new version of a Node.js app on a Linux server:

1. SSH into the server: ssh -i mykey.pem ubuntu@54.200.100.50
2. Navigate to the app folder: cd /var/www/nodeapp
3. Pull latest code: git pull origin main
4. Restart the app service: sudo systemctl restart nodeapp
5. Check status: sudo systemctl status nodeapp
6. Tail logs to confirm it's working: tail -f /var/log/nodeapp/app.log

Summary

• Linux is the backbone of almost every DevOps environment.
• Key skills include file navigation, permissions, process management, and package installation.
• Shell scripting automates repetitive deployment and maintenance tasks.
• SSH provides secure remote access to servers for hands-on management.
• Log viewing and systemctl are daily tools for monitoring and managing services.