可以这么配置,应该还有更好的写法。
        location /log {
            if ($http_user_agent ~ MSIE ) {
                  access_log /var/log/msie.log;
            }
            if ($http_user_agent ~ Opera ) {
                  access_log /var/log/opera.log;
            }
            if ($http_user_agent ~ Webkit ) {
                  access_log /var/log/webkit.log;
            }
            if ($http_user_agent ~ Mozilla ) {
                  access_log /var/log/mozilla.log;
            }
            add_header "Content-Type" "text/plain;charset=UTF-8";
            return 200  "$http_user_agent";
        }