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

set -x
if [[ $var==bar ]]
        echo $var is bar
        echo $var is not bar
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.


[[ 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.

| improve this answer | |
  • @Kusalananda the problem occurs with [ ... ] also; do you think it is important to differentiate in this case? I get the same behavior with -n whether I use [ or [[ – enharmonic Oct 22 '19 at 21:25
  • [ is the same as test and works the same. [[ is different. For example, variables do not need to be quoted within [[ ... ]], but they would need to be quoted with test or [ ... ] (as they are ordinary utilities, while [[ is a keyword). Also, -n will not occur in the xtrace output when using [ or test in the way done in the question. – Kusalananda Oct 22 '19 at 21:30
  • @Kusalananda no need for quotes ? so if bar is empty, [[ -n $bar ]] is a valid expression ? – Pierre-Antoine Guillaume Oct 24 '19 at 6:27
  • @Pierre-AntoineGuillaume Yes. Also, [[ -z $bar ]] && echo ok would output ok for an empty or unset $bar. – Kusalananda Oct 24 '19 at 6:31

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.