Skip to main content

Understanding Parameter Expansion in Linux

Parameter expansion is a robust and often underutilized feature in Linux shells like Bash, which allows you to manipulate shell variables and their values in different ways. Whether you're an everyday Linux user, a system administrator, or a developer, understanding parameter expansion can significantly increase your productivity and script efficiency. This article offers a deep dive into what parameter expansion is, how it works, and how you can use it effectively.

What Is Parameter Expansion?

Parameter expansion allows you to transform the value of a parameter (a shell variable or a special character), or even use its value to replace part of the command. It's an essential part of the shell's ability to interact with variables and manipulate their content.



The general syntax for parameter expansion is ${parameter}. This is just the simplest form and allows you to reference a variable. However, it becomes powerful when you begin to use various operators and constructs to manipulate these variables. The form often looks like ${parameter:operator} where operator indicates what kind of manipulation you want to perform.

How Does Parameter Expansion Work?

When you issue a command involving parameter expansion, the shell goes through several steps:

  1. Command Parsing: The shell identifies the ${} constructs in the command.
  2. Variable Substitution: The variable name is identified, and its value is fetched.
  3. Transformation or Manipulation: Any specified operators are used to change the variable's value or generate a new string.
  4. Command Reconstruction: The original ${} construct in the command is replaced with the new string.
  5. Execution: Finally, the command, as modified, is executed.

Parameter expansion happens after brace expansion but before word splitting and pathname expansion.

Examples of Parameter Expansion

Basic Example: Variable Substitution

The most basic use case is simply getting the value of a variable:

echo "Hello, ${name}"


Hello, Alice

Nested Expansion: Using Variables within Variables

You can nest parameter expansions to get more complex outcomes:

echo "${!var1}"



Here ${!var1} performs an indirect expansion, using the value of var1 (name) as a variable name, and then expands it (Alice).

Case Modification: Changing Letter Case

Parameter expansion allows you to manipulate the case of alphabetic characters:

echo "${name,,}"



Here ${name,,} converts all characters in $name to lowercase.

Operators in Parameter Expansion

Parameter expansion provides a variety of operators to perform manipulations on variables. These operators enable string manipulations, sub-string extraction, case modification, and more. Understanding these operators will make your work with the shell more effective and less prone to errors.

Available Operators and Their Descriptions

Here's a table summarizing some of the key operators available for parameter expansion:

${var}Basic variable expansionvar="hello"; echo ${var}hello
${var:-val}Use val if var is unset or nullecho ${var:-hello}hello
${var:=val}Set var to val if var is unset or nullecho ${var:=hello}hello
${var:+val}Use val only if var is setvar="world"; echo ${var:+hello}hello
${var:?msg}Display msg if var is unset or null${var:?is null}is null
${#var}Get length of varvar="hello"; echo ${#var}5
${var#pat}Remove shortest match from beginning, pat stands for "pattern"var="file.txt"; echo ${var#*.}txt
${var##pat}Remove longest match from beginningvar="file.txt.old"; echo ${var##*.}old
${var%pat}Remove shortest match from endvar="file.txt"; echo ${var%.*}file
${var%%pat}Remove longest match from endvar="file.txt.old"; echo ${var%%.*}file
${var/pat/rep}Replace first occurrence of pat with repvar="hello"; echo ${var/ll/rr}herro
${var//pat/rep}Replace all occurrences of pat with repvar="hello world"; echo ${var//l/r}herro word
${var:pos:len}Extract substring starting at pos for len lengthvar="hello"; echo ${var:1:3}ell
${var^}Convert first character to uppercasevar="hello"; echo ${var^}Hello
${var^^}Convert all characters to uppercasevar="hello"; echo ${var^^}HELLO
${var,}Convert first character to lowercasevar="Hello"; echo ${var,}hello
${var,,}Convert all characters to lowercasevar="HELLO"; echo ${var,,}hello

Examples of Using Operators

Using Default Values

echo "Hello, $greet"

In this example, if the variable name is not set, the greet variable will be set to "Guest".

String Replacement

echo $newfilename

This will output picture.png, replacing jpg with png.

By using these operators creatively, you can perform complex operations right within the shell, without needing to rely on external programs or utilities. This adds to the power and flexibility of parameter expansion in Linux.


Parameter expansion is a feature rich and powerful tool in Linux shell scripting, offering variable manipulation and string operations without the need for external commands or utilities. By using parameter expansion creatively, you can write more efficient and effective shell scripts or commands. With its various operators and capabilities, parameter expansion provides you with a swiss-army knife for text and variable manipulation directly within the shell.

What Can You Do Next 🙏😊

If you liked the article, consider subscribing to Cloudaffle, my YouTube Channel, where I keep posting in-depth tutorials and all edutainment stuff for software developers.

YouTube @cloudaffle