Bash nonzero test case (-n), [ and [[ are doing different things

Question

Usually i only use [[ for all kinds of test cases, because it\'s the most advanced way and it\'s more safe to use (Regex, ...). I know that [[ executes different code than [

Answer 1

I think that your problem is related to quotes.

When you use [ -n $VAR ] the command that is executed won't contain any argument where $VAR should be:

$ set -x
$ [ -n $VAR ]
+ '[' -n ']'

This means that you are essentially testing whether the string -n is non-empty, because the following two tests are equivalent:

[ string ]    # is a shorthand for
[ -n string ] # which is always true!

If you use quotes, then you get different behaviour:

$ [ -n "$VAR" ]
+ '[' -n '' ']'

Now you are testing whether the variable is non-empty, so you get the expected behaviour.

Answer 2

Quoting. You have to quote variables that you use in [:

$ VAR=    
$ [ -n $VAR ]
$ echo $?
0
$ [ -n "$VAR" ]
$ echo $?
1