Understanding and Fixing "Unary Operator Expected" Errors in Bash Scripts

Introduction

In Bash scripting, encountering an error like "unary operator expected" can be perplexing for beginners. This error typically arises from syntax issues within conditional expressions. Understanding the root causes and how to prevent these errors is crucial for writing robust Bash scripts.

This tutorial will guide you through common pitfalls that lead to this error and provide solutions with examples, focusing on proper syntax and best practices in Bash scripting.

Common Causes of "Unary Operator Expected" Error

1. Unquoted Variables

One frequent cause of the "unary operator expected" error is failing to quote variables within conditional statements. When a variable is not quoted, it may expand to an empty string or include unexpected characters (like spaces), resulting in invalid syntax.

Example:

if [ $aug1 = "add" ]; then
    echo "Man Page for: add"
fi

If $aug1 is undefined or contains spaces, the condition becomes:

if [  = "add" ]; then

This is not valid syntax and leads to the error.

Solution:

Always quote your variables in conditional expressions:

if [ "$aug1" = "add" ]; then
    echo "Man Page for: add"
fi

2. Incorrect Use of Single vs Double Brackets

The choice between single brackets [ ... ] and double brackets [[ ... ]] can also lead to errors if not used correctly. The single bracket syntax is POSIX-compliant but requires careful quoting, whereas double brackets offer more features like pattern matching and are generally easier for string comparisons.

Example with Single Bracket:

if [ "$operation" = "man" ]; then
    # Commands here
fi

Preferred Double Bracket Usage:

if [[ $operation == "man" ]]; then
    # Commands here
fi

Double brackets automatically handle quoting, reducing the risk of syntax errors.

3. Spacing Issues

Incorrect spacing within conditional expressions can also cause this error. Bash requires spaces around operators and after [ but not before ].

Incorrect:

if[$var -eq 1]; then
    echo "Equal"
fi

Correct:

if [ "$var" -eq 1 ]; then
    echo "Equal"
fi

4. Handling Empty or Undefined Variables

When a variable might be empty, it’s essential to check its state before using it in comparisons.

Example:

if [ $operation = "man" ] || [ "$operation" = "" ]; then
    # Handle both cases
fi

This ensures you’re not comparing an empty string inadvertently.

5. Numerical Comparisons

When dealing with numerical inputs, ensure that the variable is sanitized to contain only digits before comparison.

Example:

# Sanitize input
var=$(echo $ans | tr -cd '[:digit:]')

if [ -z "$var" ] || [ "$var" -lt 1 ]; then
    echo "Invalid number"
fi

This approach prevents errors from non-numeric characters.

Best Practices

• Always Quote Variables: This is a simple yet effective way to prevent many common syntax errors.

• Use Double Brackets for String Comparisons: They provide more flexibility and reduce the need for quoting.

• Check Variable State: Always check if variables are set or empty before using them in conditions.

• Maintain Proper Spacing: Ensure correct spacing around operators and brackets to avoid syntax errors.

• Enable Debugging: Use set -x to trace your script’s execution, which can help identify where things go wrong.

Conclusion

By understanding the common causes of "unary operator expected" errors in Bash scripts and adopting best practices, you can write more reliable and error-free scripts. Always quote variables, use double brackets for string comparisons, ensure proper spacing, and handle undefined or empty variables carefully. These steps will help you avoid syntax pitfalls and improve your scripting skills.