Expanding

In shell scripting, variable expansion uses the dollar sign ($) to tell the shell to replace the variable name with its stored value.

#!/bin/bash
var="value of var"
echo ${var}

Running this script:

$ ./var-sample.sh
value of var

Braces vs No Braces

You can reference variables with or without braces. Braces become essential when you append characters immediately after the variable name.

With Braces

#!/bin/bash
var="value of var"
echo ${var}

Without Braces

#!/bin/bash
var="value of var"
echo $var

Both scripts output:

$ ./var-sample.sh
value of var

Delimiting Variable Names

Without braces, the shell cannot determine where the variable name ends:

#!/bin/bash
height=170

# Incorrect: $heightcm is undefined
echo "Your height is = $heightcm"

# Correct: ${height}cm expands properly
echo "Your height is = ${height}cm"

$ ./height.sh
Your height is = 
Your height is = 170cm

Quoting and Word Splitting

By default, unquoted expansions are split on whitespace defined by IFS (space, tab, newline). Use quotes to preserve the exact value.

The image illustrates how different whitespace characters (tabs, spaces, linebreaks) affect the formatting of a string variable containing "Hello World".

Unquoted Expansion

#!/bin/bash
string="One Two Three"

# Splits into words
for element in ${string}; do
  echo "${element}"
done

Quoted Expansion

#!/bin/bash
string="One Two Three"

# Preserves the entire string as one element
for element in "${string}"; do
  echo "${element}"
done

$ ./string.sh
One
Two
Three

$ ./string2.sh
One Two Three

Always quote expansions when dealing with filenames, paths, or URLs to prevent unintended splitting.

Intentional Splitting

Sometimes you want to iterate over each word in a list:

#!/bin/bash
readonly SERVERS="server1 server2 server3"

for server in ${SERVERS}; do
  echo "${server}.example.com"
done

$ ./expanding.sh
server1.example.com
server2.example.com
server3.example.com

Quoting the variable in this case treats the entire list as one element:

#!/bin/bash
readonly SERVERS="server1 server2 server3"

for server in "${SERVERS}"; do
  echo "${server}.example.com"
done

$ ./expanding2.sh
server1 server2 server3.example.com

Best Practices

Use this quick reference to decide when to quote or brace variables:

Scenario	Quoting	Braces	Example
Simple expansion	Optional	Optional	`echo $var`
Appending text to a variable	Optional	Required	`echo "${var}suffix"`
File paths and filenames	Recommended	Optional	`ls "${directory}/file.txt"`
URLs and complex strings	Recommended	Optional	`curl "${URL}?id=123&name=abc"`
Iterating over words intentionally	Optional	Optional	`for x in ${list}; do ...; done`
Preventing word splitting	Required	Optional	`read -r line <<< "${input}"`

The image is a slide titled "Expanding Variables" with checkmarks next to "Directory paths, filenames" and "Assigning URLs to variables."

Never rely on unquoted variable expansions for user input or file names—they can introduce security risks or unexpected behavior.

Introduction

Conventions

Refresher

Streams

Expansions - Part One

Globs

Expansions - Part Two

Special Shell Variables

Arrays

Good Practices applied

awk

sed

Braces vs No Braces

With Braces

Without Braces

Delimiting Variable Names

Quoting and Word Splitting

Unquoted Expansion

Quoted Expansion

Intentional Splitting

Best Practices

Further Reading

Watch Video

​Braces vs No Braces

​With Braces

​Without Braces

​Delimiting Variable Names

​Quoting and Word Splitting

​Unquoted Expansion

​Quoted Expansion

​Intentional Splitting

​Best Practices

​Further Reading

Watch Video

Braces vs No Braces

With Braces

Without Braces

Delimiting Variable Names

Quoting and Word Splitting

Unquoted Expansion

Quoted Expansion

Intentional Splitting

Best Practices

Further Reading