How to solve "Unary operator expected" errors?

The "unary operator expected" error is a common syntax error in Bash scripting that occurs when a conditional expression lacks the required operands. This typically happens when variables are empty or undefined, causing the shell to receive fewer arguments than expected for comparison operations.

What Causes the Error?

The error occurs when Bash encounters a comparison operator with missing or empty operands. Let's examine a script that demonstrates this issue

#!/bin/bash
read -p "Enter the input: " num1
if [ $num1 -eq 1 ]
then
   echo "Number entered is 1"
else
   echo "Not equal to One !!"
fi

When we run this script and press Enter without input, we get

$ ./unity_check.sh
Enter the input: 
./unity_check.sh: line 3: [: -eq: unary operator expected
Not equal to One !!

Using debug mode reveals the problem

+ '[' -eq 1 ']'
./unity_check.sh: line 3: [: -eq: unary operator expected

Due to the empty variable, the comparison becomes [ -eq 1 ] instead of [ value -eq 1 ], leaving the -eq operator without a left operand.

Solution 1 Quote Variables

The first solution is to quote variables to prevent word splitting

#!/bin/bash
read -p "Enter the input: " num1
if [ "$num1" -eq 1 ]
then
   echo "Number entered is 1"
else
   echo "Not equal to One !!"
fi

However, this still produces an error for empty input because -eq expects numeric values

$ ./unity_check.sh
Enter the input: 
./unity_check.sh: line 3: [: : integer expression expected

For string comparisons, use the = operator instead

#!/bin/bash
read -p "Enter the input: " num1
if [ "$num1" = 1 ]
then
   echo "Number entered is 1"
else
   echo "Not equal to One !!"
fi

Solution 2 Double Bracket Syntax

Bash's double bracket syntax [[ ]] is more robust and handles empty variables gracefully

#!/bin/bash
read -p "Enter the input: " num1
if [[ $num1 -eq 1 ]]
then
   echo "Number entered is 1"
else
   echo "Not equal to One !!"
fi

The double bracket construct doesn't perform word splitting, making it more tolerant of unset or empty variables.

Solution 3 Default Values

You can assign default values using parameter expansion

#!/bin/bash
read -p "Enter the input: " num1
if [ "${num1:-0}" -eq 1 ]
then
   echo "Number entered is 1"
else
   echo "Not equal to One !!"
fi

The syntax ${num1:-0} uses 0 as the default value if num1 is empty or unset.

Solution 4 Check for Empty Variables

Use the -z operator to explicitly test for empty variables

#!/bin/bash
read -p "Enter the input: " num1
if [ -z "$num1" ]
then
   echo "The number is empty"
   exit 0
fi
if [ "$num1" -eq 1 ]
then
   echo "Number entered is 1"
else
   echo "Not equal to One !!"
fi

Common Solutions Summary

Conclusion

The "unary operator expected" error occurs when Bash comparison operators lack the required operands due to empty or unset variables. The most reliable solutions are using double bracket syntax for numeric comparisons and properly quoting variables for string comparisons.