Why does -n appear in my bash test [[ condition debug output?

Question

I would like to test if a condition is true or false. Here is a sample bash script:

#!/bin/bash
set -x
var=foo
if [[ $var==bar ]]
then
        echo $var is bar
else
        echo $var is not bar
fi
set +x

I expect that the output will be foo is not bar, but I get the following output instead: foo is bar

While viewing the debug output, I noticed that the condition was being evaluated with -n:

+ var=foo
+ [[ -n foo==bar ]]
+ echo foo is bar
foo is bar
+ set +x

On the Bash Conditional Expressions man page it says

-n string

True if the length of string is non-zero.

But I'm not interested in whether the string length is non-zero, but rather whether the two strings are equal to each other.

Answer 1

[[ is behaving as designed, because $var==bar is not actually a conditional expression comparing two strings, but a single string (that happens to contain two equal sign characters). Since [[ ... ]] only contains a string rather than a conditional expression, its default is to use the -n flag to check whether the string is not zero-length, and thus evaluates with an exit code of 0. This is not what you intended, though, so if you would like to evaluate whether $var is equal to bar, use spaces around the == operator to turn it into a conditional expression:

[[ $var == bar ]]

From the conditional expressions man page:

string1 == string2

string1 = string2

True if the strings are equal. When used with the [[ command, this performs pattern matching as described above (see Conditional Constructs).

‘=’ should be used with the test command for POSIX conformance.