https://stackoverflow.com/questions/47743015/fish-shell-how-to-check-if-a-variable-is-set-empty
unset 就是 empty. 但是 string length empty 不会输出，string length "" 输出 0
test -n “$var”: var set and its length >= 1
test -z “$var”: var set and its length == 0
但是 test -z 可以不接收参数，test -n 不接收居然会返回 true.