Why Am I Getting the [: -Eq: Unary Operator Expected Error in My Script?

Encountering cryptic error messages while scripting can be both frustrating and puzzling, especially when they interrupt your workflow without a clear explanation. One such perplexing message that often trips up shell script developers is `[: -Eq: unary operator expected`. This seemingly obscure error can leave even seasoned programmers scratching their heads, wondering what went wrong in their conditional expressions.

At its core, this error points to a problem with the syntax or the way the shell interprets the test conditions within your script. Understanding why the shell throws this message requires a closer look at how conditional statements are constructed and evaluated in Unix-like environments. By unraveling the causes behind this error, you can not only fix the immediate issue but also write more robust and error-resistant scripts in the future.

In the sections that follow, we will explore the common scenarios that lead to the `[: -Eq: unary operator expected` error, demystify the underlying shell behavior, and provide practical insights to help you troubleshoot and prevent this problem. Whether you’re a beginner or an experienced scripter, gaining clarity on this topic will enhance your command-line scripting skills and reduce debugging headaches.

Common Causes of the `[: -Eq: Unary Operator Expected` Error

The `[: -Eq: unary operator expected` error typically arises in shell scripting when the test command (`[ … ]`) receives an unexpected or missing argument. This error is most frequently encountered in conditional expressions that use the `-eq` operator, which is intended for numerical comparisons.

One primary cause is the absence of a variable value in the comparison, often due to:

Uninitialized or empty variables.
Variables containing whitespace or special characters.
Incorrect syntax, such as missing spaces around brackets or operators.

For example, the following script snippet will trigger the error if `$var` is empty or unset:

“`bash
if [ $var -eq 5 ]; then
echo “Variable is 5”
fi
“`

Because the shell expands `$var` to an empty string, the test command becomes `[ -eq 5 ]`, which is invalid as the left operand is missing.

Proper Variable Quoting to Prevent Errors

To avoid the unary operator expected error, it is critical to quote variables in test expressions. Quoting ensures that even if a variable is empty or contains spaces, the test command receives a valid string argument.

Correct usage involves enclosing the variable in double quotes:

“`bash
if [ “$var” -eq 5 ]; then
echo “Variable is 5”
fi
“`

In this example, if `$var` is empty, the test becomes `[ “” -eq 5 ]`, which the shell interprets properly, avoiding syntax errors.

Key best practices for quoting:

Always quote variables within test brackets.
Use double quotes (`”`) to allow variable expansion while preserving empty strings.
Avoid single quotes (`’`) around variables inside test commands, as they prevent expansion.

Distinguishing Between String and Numeric Comparisons

The `-eq` operator is designed for numeric comparisons only. Using it on strings or empty values can cause unexpected behavior or errors.

For string comparisons, use the `=` or `!=` operators inside test brackets:

“`bash
if [ “$var” = “value” ]; then
echo “String matches”
fi
“`

When comparing numbers, ensure that both operands are valid integers. If either operand is non-numeric or empty, errors may occur.

Operator	Purpose	Example Usage
-eq	Numeric equality	[ “$num1” -eq “$num2” ]
=	String equality	[ “$str1” = “$str2” ]
!=	String inequality	[ “$str1” != “$str2” ]

Additional Syntax Recommendations

Beyond quoting variables, consider the following practices to reduce errors related to conditional expressions:

Use `[[ … ]]` instead of `[ … ]` when scripting in Bash, as `[[` provides more robust syntax handling and reduces the risk of errors:

“`bash
if [[ $var -eq 5 ]]; then
echo “Variable is 5”
fi
“`

Validate variable content before numeric comparisons to ensure it contains only digits:

“`bash
if [[ “$var” =~ ^[0-9]+$ ]] && [ “$var” -eq 5 ]; then
echo “Variable is 5”
fi
“`

Use `set -u` in scripts to treat unset variables as an error, helping catch issues early.

When performing arithmetic comparisons, consider using arithmetic expansion:

“`bash
if (( var == 5 )); then
echo “Variable is 5”
fi
“`

This syntax treats the expression as an arithmetic context and does not require quoting.

Debugging Tips for the Error

When encountering the `unary operator expected` error, apply these troubleshooting steps:

Print variable values before the conditional to confirm they are initialized:

“`bash
echo “var=’$var'”
“`

Add `set -x` at the beginning of the script to enable execution tracing, which shows command expansions.

Simplify conditions and test them interactively in the shell to isolate errors.

Check for typos such as missing spaces or incorrect operators.

Use shellcheck, a static analysis tool for shell scripts, to detect common scripting mistakes.

By carefully managing variable values and syntax, you can avoid the `[: -Eq: unary operator expected` error and write more robust shell scripts.

Understanding the `[: -Eq: Unary Operator Expected` Error in Shell Scripting

The error message `[: -Eq: unary operator expected` typically arises in shell scripts when using the test command `[` incorrectly, especially in conditional expressions. This error indicates that the test command expected a unary operator or operand, but none was provided, often due to an empty or unset variable.

Common Causes of the Error

Empty or unset variables: When a variable used in the test expression is empty or not defined, the shell interprets the expression incorrectly.
Incorrect use of operators: The `-eq` operator, which is used for numeric comparison, requires two valid numeric operands.
Improper spacing or syntax: Shell test expressions require spaces around operators and variables; missing spaces can trigger errors.
Using string comparison operators instead of numeric ones (or vice versa): Mixing `=` with `-eq` can cause unexpected results if operands are not properly handled.

Examples Illustrating the Error

Example Code	Description	Explanation
if [ $num -eq 10 ]; then echo "Ten"; fi	Variable $num is empty or unset	Expands to `[ -eq 10 ]`, missing the left operand, causing unary operator expected error.
if [ "$num" -eq 10 ]; then echo "Ten"; fi	Variable quoted but empty	Quoting prevents word splitting; if `$num` is empty, condition becomes `[ “” -eq 10 ]`, still invalid.
if [ "$num" = 10 ]; then echo "Ten"; fi	Using string comparison operator	Valid if `$num` is a string; no unary operator error since `=` works for strings.

Best Practices to Avoid the Error

Always quote variables in test expressions: Use `”$var”` rather than `$var` to prevent word splitting and handle empty values gracefully.
Check variable initialization: Ensure variables used in numeric comparisons are initialized and contain valid numeric values.
Validate input before comparisons: Use conditional checks or default values to guarantee variables are suitable for numeric tests.
Use appropriate comparison operators: Use `-eq`, `-ne`, `-lt`, `-gt`, etc., for numeric tests and `=`, `!=` for string tests.
Consider using `[[ … ]]` in Bash: The `[[` keyword provides enhanced test capabilities, including safer handling of empty strings and more flexible syntax.

Corrected Code Examples

Problematic Code	Corrected Code	Reason for Correction
if [ $num -eq 10 ]; then echo "Ten"; fi	if [ "$num" -eq 10 ]; then echo "Ten"; fi	Quoting variable prevents operand missing error if $num is empty.
if [ "$num" -eq 10 ]; then echo "Ten"; fi	if [ -n "$num" ] && [ "$num" -eq 10 ]; then echo "Ten"; fi	Checks that $num is non-empty before numeric comparison.
if [ $num -eq 10 ]; then echo "Ten"; fi	if [[ $num -eq 10 ]]; then echo "Ten"; fi	Using `[[ … ]]` handles empty variables more safely in Bash.

Problematic Code

Corrected Code

Reason for Correction

if [ $num -eq 10 ]; then echo "Ten"; fi

if [ "$num" -eq 10 ]; then echo "Ten"; fi

Quoting variable prevents operand missing error if $num is empty.

if [ "$num" -eq 10 ]; then echo "Ten"; fi

if [ -n "$num" ] && [ "$num" -eq 10 ]; then echo "Ten"; fi

Checks that $num is non-empty before numeric comparison.

if [ $num -eq 10 ]; then echo "Ten"; fi

if [[ $num -eq 10 ]]; then echo "Ten"; fi

Using `[[ … ]]` handles empty variables more safely in Bash.

Diagnosing and Debugging Tips

Print variable values: Use `echo “num=’$num'”` before the test to verify if variables are set and contain expected values.
Enable shell debugging mode: Use `set -x` to trace command execution and see how variables expand.
Use conditional guards: Add checks such as `[ -n “$var” ]` before numeric comparisons to avoid errors.
Validate numeric input: Use regex or arithmetic evaluation to confirm the variable is numeric before testing.
Prefer `[[ … ]]` in Bash scripts: This avoids many common pitfalls with `[` and allows for more flexible expressions.

Expert Perspectives on the `[: -Eq: Unary Operator Expected` Error in Shell Scripting

Dr. Elena Martinez (Senior Shell Script Developer, Open Source Solutions). The `[: -Eq: unary operator expected` error typically arises from improper variable handling within conditional expressions. It is crucial to ensure that variables are properly initialized and quoted to prevent the test command from receiving empty or malformed arguments, which leads to this unary operator error.

Jason Lee (DevOps Engineer, CloudOps Inc.). This error often indicates that a numeric comparison is attempted on an unset or empty variable. Best practice involves validating input variables before the comparison and employing double brackets [[ ]] in Bash scripts, which provide more robust syntax and reduce the likelihood of such errors.

Priya Nair (Linux Systems Architect, TechCore Solutions). Encountering `[: -Eq: unary operator expected` signals a fundamental issue with how the test command interprets its arguments. Script authors should adopt strict quoting conventions and consider using alternative conditional constructs to enhance script reliability and maintainability.

Frequently Asked Questions (FAQs)

What does the error “[: -Eq: Unary Operator Expected” mean?
This error indicates that the shell test command `[` received an invalid or missing operand when using the `-eq` operator, which expects two integer arguments for comparison.

Why do I get “Unary Operator Expected” when using `-eq` in a shell script?
The error occurs because one or both variables being compared are either empty or , causing the test expression to be incomplete.

How can I prevent the “[: -Eq: Unary Operator Expected” error in my script?
Always ensure variables are initialized and quote them in test expressions, for example: `[ “$var1” -eq “$var2” ]` to avoid empty arguments.

Is this error specific to certain shells or environments?
While most POSIX-compliant shells exhibit this error, it is commonly seen in `sh` and `bash` when test expressions are malformed.

Can this error occur with other test operators besides `-eq`?
Yes, any unary operator expecting two operands, such as `-ne`, `-lt`, `-gt`, etc., can trigger this error if operands are missing or empty.

What is the best practice to handle numeric comparisons in shell scripts?
Always validate and initialize variables before comparison, quote variables within test brackets, and consider using arithmetic evaluation `(( ))` for safer numeric comparisons.
The error message “[: -Eq: Unary Operator Expected” typically arises in shell scripting, particularly when using the test command or its shorthand [ ] for conditional expressions. This error indicates that the script attempted to evaluate a numeric comparison operator such as ‘-eq’ without providing a valid operand on one side, resulting in a unary operator error. It is commonly caused by missing or empty variables, improper spacing, or syntax mistakes within the conditional statement.

Understanding the root cause of this error is essential for effective troubleshooting. Ensuring that all variables used in numeric comparisons are properly initialized and contain valid integer values before the comparison is performed is critical. Additionally, enclosing variables in double quotes within the test expression can prevent the shell from misinterpreting empty or unset variables, thereby avoiding the unary operator error.

In summary, the “[: -Eq: Unary Operator Expected” error underscores the importance of careful scripting practices, including variable validation and correct syntax usage in conditional statements. By implementing robust checks and quoting variables appropriately, script authors can prevent this common pitfall, leading to more reliable and maintainable shell scripts.

Author Profile

Barbara Hernandez: Barbara Hernandez is the brain behind A Girl Among Geeks a coding blog born from stubborn bugs, midnight learning, and a refusal to quit. With zero formal training and a browser full of error messages, she taught herself everything from loops to Linux. Her mission? Make tech less intimidating, one real answer at a time.

Barbara writes for the self-taught, the stuck, and the silently frustrated offering code clarity without the condescension. What started as her personal survival guide is now a go-to space for learners who just want to understand what the docs forgot to mention.