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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.

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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"

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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"

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Using Curly Braces

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

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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

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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

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Parameter Expansion Operators

Expansion TypeSyntaxDescription
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

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String Manipulation

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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

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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

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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

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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.

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Additional Resources

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