Bash Variables, Input and Arithmetic

Variables store values in a script so you can use them again and again.

Creating Variables

Write the name, an equals sign and the value, with no spaces around =. Use $ before the name to read the value.

name="Ayesha"
city="Karachi"
age=20
echo "$name lives in $city and is $age years old."

Ayesha lives in Karachi and is 20 years old.

Curly Braces

Use ${name} when the variable is joined to other text, so Bash knows where the name ends:

file="report"
echo "${file}_2026.txt"
echo "$file_2026.txt"

report_2026.txt
.txt

The second line prints only .txt, because Bash looked for a variable called file_2026, which does not exist.

Single and Double Quotes

name="Ali"
echo "Hello $name"
echo 'Hello $name'
echo "Price: \$50"

Hello Ali
Hello $name
Price: $50

A backslash \ before $ stops it from being treated as a variable. Always put variables in double quotes, especially file names that may contain spaces.

Command Substitution

$(command) runs a command and puts its output in place. You can store the result in a variable:

files=$(ls | wc -l)
today=$(date +%Y-%m-%d)
echo "There are $files files here."
echo "Backup name: backup-$today.zip"

This prints the real number of files and today's date, for example backup-2026-10-15.zip.

Reading User Input

read waits for the user to type something and saves it in a variable. -p shows a prompt first.

#!/bin/bash
read -p "What is your name? " name
read -p "Which course do you want? " course
echo "Welcome $name! You chose $course."

If the user types Hamza and Web Development, the script prints:

What is your name? Hamza
Which course do you want? Web Development
Welcome Hamza! You chose Web Development.

Use read -s for passwords, so the typed characters are not shown.

Arithmetic

Bash does whole-number maths inside $(( )):

price=350
qty=4
total=$((price * qty))
echo "Total: Rs $total"
echo "Half: $((total / 2))"
echo "Remainder: $((17 % 5))"

count=1
((count++))
((count += 10))
echo "Count: $count"

Total: Rs 1400
Half: 700
Remainder: 2
Count: 12

Inside $(( )) you do not need $ before variable names. Bash has no decimals here: $((7 / 2)) is 3. For decimals, use the bc tool: echo "7 / 2" | bc -l.

Environment Variables

Some variables are already set by the system. They are written in capital letters:

Use export NAME=value to make a variable available to programs started from your script.

Exercise