Bash Variables and Quoting Explained

Variables#

Assign with = and no spaces, and read with $:

name="Asha"
age=24
echo "$name is $age"

Asha is 24

Use ${name} when text follows immediately:

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

report_2026.txt
.txt

The second line looks for a variable called file_2026, which is empty.

Bash variables have no types. Everything is text, unless used in arithmetic. Uppercase names are usually for environment variables, and lowercase for your own.

Single and double quotes#

city="New Delhi"
echo "I live in $city"
echo 'I live in $city'

I live in New Delhi
I live in $city

Why quoting matters#

Without quotes, Bash splits a value on spaces, and it expands wildcards. This is the source of many script bugs:

file="my report.txt"
touch "$file"
ls -l $file 2>&1 | head -n 2
echo ---
ls -l "$file" | awk '{print $NF}'

ls: cannot access 'my': No such file or directory
ls: cannot access 'report.txt': No such file or directory
---
report.txt

Unquoted, ls received two names: my and report.txt.

Command substitution#

$(...) stores a command's output:

count=$(ls -A | wc -l)
today=$(date +%Y)
echo "Year $today, and $count items here"

Year 2026, and 4 items here

Arithmetic#

$(( )) does whole-number arithmetic. Variables do not need $ inside it:

a=17
b=5
echo "$((a + b)) $((a * b)) $((a / b)) $((a % b))"
count=1
((count++))
echo "$count"

22 85 3 2
2

Division drops the remainder. For decimals, use awk or bc:

awk 'BEGIN {printf "%.2f\n", 17 / 5}'

3.40

Default values#

These expansions handle missing values:

echo "Port: ${PORT:-8080}"
PORT=3000
echo "Port: ${PORT:-8080}"
echo "${#PORT} digits"

Port: 8080
Port: 3000
4 digits

This is the normal way to give a script's settings a default.

Special variables#

Read-only values#

readonly VERSION="1.0"
VERSION="2.0"
echo "$VERSION"

bash: VERSION: readonly variable
1.0

Common mistakes#

Practice#

price=250
qty=3
echo "Total: $((price * qty))"
echo "Discount: ${DISCOUNT:-0}"

Total: 750
Discount: 0

Key takeaway#

Next: Conditions with if and case.