How To Resolving “unary operator expected” in “[ ]” Brackets

Are you a Bash script enthusiast? Do you find yourself grappling with the frustrating ‘unary operator expected’ error in your scripts? You’re not alone. This common error can stump even the most experienced scriptwriters. In this comprehensive guide, we’ll unravel the mystery behind this error and equip you with the knowledge to resolve it.

Understanding the Problem

Let’s start by dissecting the error message:

calc_1.2: line 75: [: =: unary operator expected

This error typically occurs in a Bash script when you’re using conditional statements enclosed in single brackets [] for comparisons. In this case, it’s likely triggered by an issue in the script snippet you provided:

elif [ $operation = "man" ]; then

The error is telling us that it’s expecting an operand (the value to compare) before encountering the equals sign.

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Why Does This Error Occur?

The ‘unary operator expected’ error usually occurs due to one of the following reasons:

1. Undefined Variable: If the variable you’re trying to compare is undefined or empty, it can lead to this error. In your script, $aug1 might be the culprit.
2. Lack of Quoting: Failing to enclose your variables in double quotes can lead to issues when they contain special characters or are empty.
3. Syntax Errors: Sometimes, simple syntax errors like missing spaces or brackets can trigger this error.

Resolving the Error

Let’s delve into some solutions to get your Bash script back on track:

1. Use Double Brackets [[ ... ]]

One effective way to avoid this error is to use double brackets for your conditional statements. Unlike single brackets, double brackets don’t subject variables to word-splitting and pathname expansion, which can help prevent this error. Here’s how you can rewrite your conditional statement:

if [[ $operation == "man" ]]; then

The == operator is used for string comparison in double brackets.

2. Always Quote Variables

It’s good practice to enclose your variables in double quotes, especially when using single brackets. This prevents issues with empty or undefined variables. For example:

if [ "$aug1" = "add" ]; then

3. Check for Undefined Variables

Before using a variable in a conditional statement, it’s a good idea to check if it’s defined. You can do this with an if statement. Here’s an example:

if [ -z "$aug1" ]; then
    # Handle the case where $aug1 is undefined or empty
else
    # Your regular script logic
fi

This conditional structure ensures that you won’t encounter the ‘unary operator expected’ error due to undefined variables.

4. Debugging with set -x

If you’re still having trouble identifying the source of the error, you can add set -x to your script. This command will enable debugging and provide a detailed trace of the script’s execution. It can be instrumental in pinpointing the exact location of the error.

Conclusion

The ‘unary operator expected’ error in Bash scripting can be frustrating, but with the right understanding and best practices, you can overcome it. By using double brackets for comparisons, quoting variables, and handling undefined variables, you’ll be well on your way to error-free scripting.

Remember that debugging is your ally, and don’t hesitate to utilize tools like set -x to trace the issue.

Happy scripting!