Linux Shell Scripting for Beginners – Part 1: What is a Variable?

Welcome to this Shell Scripting series! In this first part, we will learn about variables in a very simple and easy-to-understand way.

📑 Table of Contents

• What is a Variable?
• How to Create a Variable
• How to Use a Variable
• Variables That Cannot Change (readonly)
• Global and Local Variables
• Store Command Output in a Variable
• Simple Practical Example: System Info
• Simple Use Case: App Deployment Path
• Multi-Word Values (Use Quotes)
• Use Variables as Counters
• Check if a Variable is Empty
• Use Time in Variable (Backup File Name)
• Small Real-Life Script
• Quick Summary
• Practice for You

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🌟 What is a Variable?

A variable is like a small box where you store some information. Later, you can open the box and use that information again in your script.

✏️ How to Create a Variable

name="Pradeep"
course="Shell Scripting"
count=10

Important: There should be no space before or after the = sign.

❌ Wrong

name = "Pradeep"

✅ Correct

name="Pradeep"

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📢 How to Use a Variable

echo "Hello $name, welcome to $course training!"

Hello Pradeep, welcome to Shell Scripting training!

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🧊 Variables That Cannot Change (readonly)

readonly company="MiddlewareBox"

If you try to change it again, the script will not allow it.

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🌍 Global and Local Variables

name="Pradeep"   # Global variable

greet() {
  local name="DevOps Engineer"
  echo "Inside function: $name"
}

greet
echo "Outside function: $name"

Inside function: DevOps Engineer
Outside function: Pradeep

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🧠 Store Command Output in a Variable

today=$(date)
echo "Today is: $today"

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💻 Simple Practical Example: System Info

host=$(hostname)
os=$(uname -s)
kernel=$(uname -r)

echo "Hostname: $host"
echo "OS: $os"
echo "Kernel: $kernel"

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🚀 Simple Use Case: App Deployment Path

app="orderservice"
version="v2.3"
path="/opt/apps/$app"

echo "Deploying $app version $version"
mkdir -p $path
cp $app-$version.jar $path/

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🧵 Multi-Word Values (Use Quotes)

msg="Hello Shell Scripting Learners"
echo "$msg"

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🔁 Use Variables as Counters

count=1

while [ $count -le 3 ]; do
  echo "Count: $count"
  count=$((count+1))
done

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❓ Check if a Variable is Empty

if [ -z "$name" ]; then
  echo "Name is empty"
else
  echo "Name is $name"
fi

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⏱️ Use Time in Variable (Backup File Name)

time=$(date +%Y-%m-%d_%H-%M-%S)
file="backup_$time.tar.gz"
echo "Backup file name: $file"

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🧰 Small Real-Life Script: Create User and Log

username="john"
log="/var/log/user_creation.log"

echo "Creating user: $username" | tee -a $log
useradd $username
echo "User $username created at $(date)" | tee -a $log

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👍 Quick Summary

Topic	Meaning
Variable	A box to store data
$variable	Use the value
readonly	Lock the value
local	Use only inside a function
$(command)	Save output of a command

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Next Part: Reading User Input in Shell Scripting