Let's say the following is being returned when invoking a bash shell script.

example.sh: line 1: [: ==: unary operator expected

This output is returned when a comparison (typically in an if statement) contains a variable that has not been defined (variable does not exist) or a variable that contains no value.

Here is an example script that will produce this ouput. In this example, the foo variable does not exist.

#!/bin/bash
if [ $foo == "bar" ]; then
  exit
fi

In this example, the foo variable has been defined (exists) but contains no value.

#!/bin/bash
foo = ""
if [ $foo == "bar" ]; then
  exit
fi

This can be resolved by adding a clause to the if statement to only invoke the if statement when the variable is not null.

#!/bin/bash
if [ ! -z $foo ] && [ $foo == "bar" ]; then
  exit
fi