NGINX browser redirect

I have one web-app, on server-side all is the same, database and procedure on client-side I need to have different interface: one for desktop and one for iPad.

First of all, we map nginx variable and from a github example and add on my server block:

# before check if UA - user agent - come from desktop
map $http_user_agent $is_desktop {
    default 0;
    ~*linux.*android|windows\s+(?:ce|phone) 0; # exceptions to the rule
    ~*spider|crawl|slurp|bot 1; # bots
    ~*windows|linux|os\s+x\s*[\d\._]+|solaris|bsd 1; # OSes
}

## Revert the logic.
map $is_desktop $is_mobile {
    1 0;
    0 1;
}