Day 5 — Bash Scripting for Automation

Modern infrastructure runs on automation.

Whether you're a:

DevOps Engineer
Linux Administrator
Cloud Engineer
Cybersecurity Professional
Backend Developer

You will eventually write Bash scripts.

From automating backups to deploying servers, monitoring systems, managing logs, and running CI/CD pipelines — Bash is everywhere.

If Linux is the operating system of the internet, then Bash is its automation language.

🔗 Resources

GitHub Repo:
https://github.com/17J/30-Days-Cloud-DevSecOps-Journey
Command Sheet:
https://bash-command-sheets-k51c.vercel.app/
Linux Cron Job Scheduler:
https://crontab.guru/

many beginners get confused between:

Bash
SH
ZSH
Fish
Dash
Yum
Apt
DNF

So before learning Bash scripting, let’s first understand the Linux shell ecosystem.

🧠 Understanding Linux Shells & Package Managers

Linux has multiple shells.

A shell is simply a command interpreter that lets users communicate with the operating system.

Think of it like:

User → Shell → Linux Kernel → Hardware

🐚 Popular Linux Shells

📦 Linux Package Managers

Package managers install software.

Different Linux distributions use different package managers.

Distribution	Package Manager	Example
Ubuntu	APT	`apt install nginx`
Debian	APT	`apt update`
CentOS 7	YUM	`yum install docker`
RHEL	YUM/DNF	`dnf install git`
Fedora	DNF	`dnf update`
Arch Linux	Pacman	`pacman -S nginx`
Alpine Linux	APK	`apk add curl`

🚀 What is Bash?

Bash stands for:

Bourne Again SHell

It is the default shell for most Linux distributions.

A shell is simply a program that allows users to interact with the operating system.

Example:

pwd
ls
mkdir project

These commands are executed through the shell.

🧠 What is Bash Scripting?

A Bash script is a file containing Linux commands executed sequentially.

Instead of manually typing commands repeatedly, you automate them inside a script.

Example:

#!/bin/bash

echo "Hello Rahul"
date
uptime

Save as:

hello.sh

Make executable:

chmod +x hello.sh

Run:

./hello.sh

⚡ Why Bash Automation Matters

Without automation:

❌ Manual server setup
❌ Repetitive deployments
❌ Manual backups
❌ Human errors
❌ Slow operations

With Bash automation:

✅ Faster workflows
✅ Repeatable processes
✅ Infrastructure consistency
✅ Reduced human mistakes
✅ Better productivity

📂 Bash Script Structure

Basic structure:

#!/bin/bash

# Comments start with #

echo "Starting Script"

# Commands here

🔥 Variables in Bash

Variables store data.

✅ Creating Variables

name="Rahul"
age=22
city="Delhi"

Access variables using $.

echo $name
echo $age

⚠️ Important Rule

No spaces around =.

❌ Wrong:

name = "Rahul"

✅ Correct:

name="Rahul"

🧾 User Input

Take input dynamically.

#!/bin/bash

echo "Enter your name:"
read name

echo "Welcome $name"

📌 Command Line Arguments

Arguments passed while running scripts.

#!/bin/bash

echo "First argument: $1"
echo "Second argument: $2"

Run:

./script.sh Rahul Linux

Output:

First argument: Rahul
Second argument: Linux

🔍 Conditional Statements

Conditions allow decision-making.

✅ If Statement

#!/bin/bash

age=20

if [ $age -ge 18 ]
then
    echo "Adult"
fi

✅ If-Else

#!/bin/bash

num=5

if [ $num -gt 10 ]
then
    echo "Greater than 10"
else
    echo "Less than or equal to 10"
fi

✅ If-Elif-Else

#!/bin/bash

marks=75

if [ $marks -ge 90 ]
then
    echo "Grade A"
elif [ $marks -ge 70 ]
then
    echo "Grade B"
else
    echo "Grade C"
fi

🧠 Comparison Operators

Operator	Meaning
`-eq`	Equal
`-ne`	Not equal
`-gt`	Greater than
`-lt`	Less than
`-ge`	Greater or equal
`-le`	Less or equal

🔁 Loops in Bash

Loops repeat tasks automatically.

✅ For Loop

#!/bin/bash

for i in 1 2 3 4 5
do
    echo "Number: $i"
done

✅ Range Loop

for i in {1..10}
do
    echo $i
done

✅ While Loop

#!/bin/bash

count=1

while [ $count -le 5 ]
do
    echo $count
    ((count++))
done

✅ Infinite Loop

while true
do
    echo "Running..."
    sleep 2
done

🔨 Functions in Bash

Functions help organize reusable code.

#!/bin/bash

greet() {
    echo "Hello $1"
}

greet Rahul

📦 Arrays in Bash

#!/bin/bash

fruits=("apple" "banana" "mango")

echo ${fruits[0]}
echo ${fruits[1]}

Loop through array:

for fruit in "${fruits[@]}"
do
    echo $fruit
done

📁 File Operations

✅ Check if File Exists

#!/bin/bash

if [ -f test.txt ]
then
    echo "File exists"
else
    echo "File not found"
fi

✅ Create File

touch file.txt

✅ Append to File

echo "New Log Entry" >> logs.txt

⚙️ Process Automation

✅ Kill Process

pkill nginx

✅ Restart Service

systemctl restart nginx

✅ Check Service Status

systemctl status docker

🕒 Cron Jobs for Scheduling

Cron automates scripts at scheduled times.

Open cron:

crontab -e

✅ Run Every Day at Midnight

0 0 * * * /home/ubuntu/backup.sh

✅ Run Every 5 Minutes

*/5 * * * * /home/ubuntu/monitor.sh

🚀 Real-World Automation Scripts

Now the fun begins.

🔥 1️⃣ Automated Backup Script

#!/bin/bash

SOURCE="/home/ubuntu/data"
DEST="/backup"

DATE=$(date +%Y-%m-%d)

tar -czf $DEST/backup-$DATE.tar.gz $SOURCE

echo "Backup completed"

What it does:

✅ Compresses files
✅ Creates timestamp backup
✅ Automates backup process

🔥 2️⃣ Disk Usage Monitoring Script

#!/bin/bash

THRESHOLD=80

USAGE=$(df / | grep / | awk '{ print $5 }' | sed 's/%//g')

if [ $USAGE -gt $THRESHOLD ]
then
    echo "Disk usage exceeded threshold!"
else
    echo "Disk usage normal"
fi

Useful for:

Servers
Cloud VMs
Production systems

🔥 3️⃣ Website Monitoring Script

#!/bin/bash

URL="https://example.com"

STATUS=$(curl -o /dev/null -s -w "%{http_code}" $URL)

if [ $STATUS -eq 200 ]
then
    echo "Website is UP"
else
    echo "Website is DOWN"
fi

🔥 4️⃣ Auto Deployment Script

#!/bin/bash

echo "Pulling latest code..."

git pull origin main

echo "Installing dependencies..."

npm install

echo "Restarting application..."

pm2 restart app

Used heavily in:

DevOps
CI/CD
Production deployments

🔥 5️⃣ Log Cleanup Script

#!/bin/bash

find /var/log -type f -name "*.log" -mtime +7 -delete

echo "Old logs deleted"

Deletes logs older than 7 days.

🛡️ Error Handling in Bash

Always validate failures.

#!/bin/bash

mkdir test

if [ $? -eq 0 ]
then
    echo "Directory created"
else
    echo "Failed"
fi

$? stores previous command status.

0 = success
Non-zero = failure

📌 Exit Codes

exit 0

Success.

exit 1

Failure.

🧪 Debugging Bash Scripts

✅ Run in Debug Mode

bash -x script.sh

Shows command execution step-by-step.

✅ Strict Mode (Highly Recommended)

#!/bin/bash

set -euo pipefail

This helps catch:

Undefined variables
Failed commands
Pipeline failures

⚡ Bash Automation in DevOps

Bash is heavily used in:

Area	Usage
CI/CD	Build automation
Kubernetes	Cluster scripts
Docker	Container automation
AWS	EC2 setup scripts
Monitoring	Health checks
Security	Log scanning
Linux	System administration

🔐 Security Best Practices

Never write insecure scripts.

❌ Avoid Hardcoding Passwords

Bad:

password="admin123"

Better:

read -s password

✅ Quote Variables