The most straightforward method uses the -z operator to test for empty strings:

if [ -z "$variable" ]; then
    echo "Variable is empty"
else
    echo "Variable contains: $variable"
fi

Always quote your variables to handle cases where they might contain spaces or special characters:

# Correct way
if [ "$variable" = "" ]; then

# Risky without quotes
if [ $variable = "" ]; then  # May fail for unset variables

There's an important distinction between unset variables and empty strings:

# Check if variable is unset
if [ -z "${variable+x}" ]; then
    echo "Variable is unset"

# Check if variable is empty (could be set to "")
elif [ -z "$variable" ]; then
    echo "Variable is set but empty"
fi

For bash-specific scripts, double brackets provide more robust string handling:

if [[ -z $variable ]]; then  # Quotes optional inside [[ ]]
    echo "Empty or unset"
fi

# More advanced pattern matching
if [[ ! $variable ]]; then  # Also catches empty strings
    echo "Variable has no meaningful value"
fi

Here's how you might check for empty arguments in a script:

#!/bin/bash

if [ -z "$1" ]; then
    echo "Error: No input file specified" >&2
    exit 1
fi

input_file="$1"
echo "Processing $input_file"

While all these methods are fast, here's a quick benchmark for 10,000 iterations:

time for i in {1..10000}; do [ -z "$var" ]; done
# real 0m0.045s

time for i in {1..10000}; do [[ -z $var ]]; done
# real 0m0.032s

When writing bash scripts, checking whether a variable is empty or undefined is a common task. There are several ways to accomplish this, each with its own advantages and potential pitfalls.

The simplest way to check for an empty variable is:

if [ -z "$variable" ]; then
    echo "Variable is empty"
fi

This works for both empty strings ("") and undefined variables.

Be aware of these common mistakes:

# Wrong: missing quotes can cause syntax errors
if [ -z $variable ]; then

# Wrong: using -z on unquoted undefined variable
if [ -z $undefined_var ]; then

Here are some other valid approaches:

# Method 1: Using parameter expansion
if [ "${variable:-}" = "" ]; then

# Method 2: The 'x' pattern (old but reliable)
if [ "x$variable" = "x" ]; then

# Method 3: Using double brackets
if [[ -z $variable ]]; then

To distinguish between undefined and empty variables:

# Check if variable is undefined
if [ -z "${variable+set}" ]; then
    echo "Variable is undefined"
fi

# Check if variable is empty (but defined)
if [ -z "$variable" ] && [ "${variable+set}" = "set" ]; then
    echo "Variable is empty but defined"
fi

Here's how you might use this in a real script:

#!/bin/bash

read -p "Enter your name: " username

if [ -z "$username" ]; then
    echo "Error: No name entered" >&2
    exit 1
fi

echo "Hello, $username!"

For scripts where performance matters:

• The -z test is generally fastest
• Parameter expansion methods are slightly slower but more flexible
• The 'x' pattern is reliable but considered somewhat archaic

While there are multiple ways to check for empty variables in bash, the -z test with proper quoting is generally the most straightforward and reliable method for most use cases.