Variables

Variable expansion allows you to retrieve and manipulate the value stored in a Bash variable using the $ syntax. This powerful feature is essential for automation, text processing, and script flexibility.

Introduction

In shell scripting, variables can store filenames, paths, numbers, or strings. By applying parameter expansion, you can:

Access variable contents
Perform string operations
Provide fallback values
Calculate lengths and more

filename="file.txt"
number=100
string="Hello World"
command_output=$(ls)

dir_path="/path/to/directory"
greeting="Hello"
name="User"
echo "$greeting, $name! Welcome to $dir_path"

Basic Variable Expansion

Whenever the shell sees $variable_name, it replaces it with the variable’s value:

name="John Doe"
echo $name
# Output:
# John Doe

Using braces helps delimit variable names clearly:

name="John Doe"
echo "Hello, ${name}"
# Output:
# Hello, John Doe

Variable names are case-sensitive and may contain letters, digits, and underscores:

# Invalid:
# Valid:
name_user="John Doe"

Using Curly Braces

Curly braces {} not only disambiguate names but also unlock advanced parameter expansion features.

Avoiding Ambiguity

Without braces, attached text can be misinterpreted:

#!/usr/bin/env bash
height=170

# Wrong: shell looks for 'heightcm'
echo "Your height is: $heightcm"

# Correct: braces isolate 'height'
echo "Your height is: ${height}cm"
# Output:
# Your height is: 170cm

Default Values

Provide a fallback when a variable is unset or empty:

Use ${var:-default} to safely reference a variable that may not exist.

#!/usr/bin/env bash
echo "Hello, ${name:-Unknown}"
# If 'name' is unset:
# Output:
# Hello, Unknown

Parameter Expansion Operators

Expansion Type	Syntax	Description
Default Value	`${var:-default}`	Use `default` if `var` is unset or null
Substring Extraction	`${var:offset:length}`	Extract a substring starting at `offset`
String Replacement	`${var/pattern/repl}`	Replace the first match of `pattern` with `repl`
Length	`${#var}`	Return the length of `var`’s value

String Manipulation

Substring Extraction

Pull out part of a string using an offset and length:

#!/usr/bin/env bash
name="John Doe"
echo "Hello, ${name:0:4}"
# Output:
# Hello, John

Substring Replacement

Replace the first occurrence of a pattern:

#!/usr/bin/env bash
path="/home/user/file.txt"
echo "${path/file/data}"
# Output:
# /home/user/data.txt

Length of a Variable

Determine the number of characters in a variable’s value:

#!/usr/bin/env bash
name="John Doe"
echo "${#name}"
# Output:
# 8

Quoting and Word Splitting

By default, unquoted expansions undergo word splitting. To preserve spaces, quote your variables. If you want to iterate over words, leave them unquoted:

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

# Iterate over each hostname:
for server in $SERVERS; do
  echo "${server}.kodekloud.com"
done
# Output:
# server1.kodekloud.com
# server2.kodekloud.com
# As a single string when quoted:
for server in "$SERVERS"; do
  echo "${server}.kodekloud.com"
done
# Output:
# server1 server2 server3.kodekloud.com

Always quote expansions containing spaces unless you explicitly need word splitting.

Introduction

Conventions

Refresher

Streams

Expansions - Part One

Globs

Expansions - Part Two

Special Shell Variables

Arrays

Good Practices applied

awk

sed

Introduction

Basic Variable Expansion

Using Curly Braces

Avoiding Ambiguity

Default Values

Parameter Expansion Operators

String Manipulation

Substring Extraction

Substring Replacement

Length of a Variable

Quoting and Word Splitting

Additional Resources

Watch Video

​Introduction

​Basic Variable Expansion

​Using Curly Braces

​Avoiding Ambiguity

​Default Values

​Parameter Expansion Operators

​String Manipulation

​Substring Extraction

​Substring Replacement

​Length of a Variable

​Quoting and Word Splitting

​Additional Resources

Watch Video

Introduction

Basic Variable Expansion

Using Curly Braces

Avoiding Ambiguity

Default Values

Parameter Expansion Operators

String Manipulation

Substring Extraction

Substring Replacement

Length of a Variable

Quoting and Word Splitting

Additional Resources