mjhjmtu

In case of test -e for example:

VAR="<this is a file path: /path/to/file/or/dir"
test -e $VAR && do_something || do_anotherthing

Question: Should I use "$VAR" here?, here I don't like verbose if not necessary, because $VAR obviously in this case is a path then if it's empty string it should always fail because there's no path that is empty string, then with my logic, double quote it is not necessary.

But it case of string test, test -n for exmpale:

VAR="<this is a string"
test -n $VAR && do_something || do_anotherthing

then with my logic, $VAR should be put in double quote: "$VAR" because it can be expanded to an empty string that if not in double quote will be expanded to -n argument only and always true.

So the actual question because of that I'm in doubt of that should we only use double quotes in test command only with -n and -z against strings?

A general rule is to double quote any variable, unless you want the shell to perform token splitting and wildcard expansion on its content.

because $VAR obviously in this case is a path then if it's empty string it should always fail [...] then with my logic, double quote it is not necessary.

On contrary. The behavior of test -e with no operands is another reason you should quote the variable in this particular case:

$ unset foo # or foo=""
$ test -e $foo # note this is equivalent to `test -e'
$ echo $?
0
$ test -e "$foo"
$ echo $?
1
$

See this Q&A, Greg's Wiki and the manual for explanations. tl;dr: it is not at all obvious how this works, and you should Use More Quotes™. Or to put it a different way: if your goals are correctness and maintainability, quoting properly is approximately priority 5 (after learning all the other weirdness necessary to write correct and concise shell scripts), while saving individual characters is priority 999,999+.

This would be better, clearer:

if [[ "$VAR" ]]; then
 do_something
else
 do_anotherthing
fi