How to Check if a Variable is Empty in Bash

In Bash, you can check if a variable is empty by using if [ -z "$variable" ]. This condition returns true if the variable has no value or is unset.

Syntax

The basic syntax to check if a variable is empty in Bash uses the -z test inside an if statement.

• -z: Tests if the string length is zero (empty).
• "$variable": The variable to check, quoted to avoid errors if empty or unset.
• if [ condition ]; then ... fi: Standard Bash conditional syntax.

bash

if [ -z "$variable" ]; then
  echo "Variable is empty"
else
  echo "Variable is not empty"
fi

Example

This example shows how to check if a variable is empty and prints a message accordingly.

bash

#!/bin/bash

variable=""

if [ -z "$variable" ]; then
  echo "Variable is empty"
else
  echo "Variable is not empty"
fi

variable="Hello"

if [ -z "$variable" ]; then
  echo "Variable is empty"
else
  echo "Variable is not empty"
fi

Output

Variable is empty
Variable is not empty

Common Pitfalls

Common mistakes when checking if a variable is empty include:

• Not quoting the variable, which can cause errors if the variable is unset or contains spaces.
• Using -n incorrectly; -n checks if a variable is not empty.
• Confusing empty with unset variables; -z treats both as empty.

bash

# Wrong: Missing quotes can cause errors
if [ -z $variable ]; then
  echo "Empty"
fi

# Correct: Always quote variables
if [ -z "$variable" ]; then
  echo "Empty"
fi

Quick Reference

Test	Meaning	Example
-z "$var"	True if variable is empty or unset	if [ -z "$var" ]; then ... fi
-n "$var"	True if variable is not empty	if [ -n "$var" ]; then ... fi
"$var" = ""	True if variable equals empty string	if [ "$var" = "" ]; then ... fi

Key Takeaways

Use if [ -z "$variable" ] to check if a variable is empty or unset in Bash.

Always quote variables in tests to avoid errors with empty or unset values.

The -z test returns true when the variable is empty; -n returns true when not empty.

Empty and unset variables are treated the same by the -z test.

Avoid missing quotes and confusing -z with -n to prevent bugs.