how to get query parameter in lua or nginx?
You should be able to use ngx.var.arg_name where name is the name of the query parameter you want to access. See Variables with Infinite Names section in this tutorial for details on query parameter handling; you may also check my blog post for Lua nginx/openresty examples.
As an alternative, you can use ngx.req.get_uri_args() to retrieve all query parameters as one table. See this section in the same tutorial for the brief comparison between these methods.
You can also use ngx.var.QUERY_STRING to access the query string and unescape and parse it.
You can obtain the query parameter with just nginx by using $arg_test, test is the name of the query parameter in this example.
This is documented in http://nginx.org/en/docs/http/ngx_http_core_module.html#var_arg_.