Change nginx root directory dynamically

Question

How to change nginx root directory to another directory dynamically without a redirect, depending on a certain condition? For instance, if:

$http_user_agent ~* "googlebot"

Answer 1

something like this

server {
    ...

    if ($allow = no) {
        set $variable "folder_1";
    }
    if ($allow = yes) {
        set $variable "folder_2";
    }
    root  /var/www/$variable;

    ...

    location / {
      ...
    }
}