How to detect ssl in shared 80 / 443 ssl nginx server config?

http://nginx.org/en/docs/http/ngx_http_core_module.html#variables

• $https — “on” if connection operates in SSL mode, or an empty string otherwise
• $scheme — request scheme, “http” or “https”

For example, if you want to limit a location to https and only:

location /admin {
    if ($https = "") {
        return 404;
    }
    # ... and so on
}