String substitution in Nginx config files

Question

How do you mix variables and strings together in a Nginx configuration file?

I have a configuration file that looks like this:

server {
        listen 80;
        server_name example.org;
        root /var/www/comet;
        index index.htm;
        default_type text/plain;
        location /publish {
                push_stream_publisher admin;
                set $push_stream_channel_id             $arg_id;
        }
        location /sub {
                push_stream_subscriber long-polling;
                set $callback "${arg_callback}({\"id\":~id~,\"data\":~text~});";
                push_stream_message_template   $callback;
                set $push_stream_channels_path "/channel1";
        }
}

It's a push server and it's supposed to push a message to the client depending on what they pass to on the callback parameter.

I.e. If the user requests http://example.org/sub?callback=call&id=blah and a message comes to them, it should read call({"id":0, "data":"blah"});

The above code outputs the literal variable name

tangrs@~ $ curl "http://example.org/sub?id=woo&callback=call" -D - && echo
HTTP/1.1 200 OK
Server: nginx/1.0.11
Date: Thu, 12 Jan 2012 04:55:38 GMT
Content-Type: text/plain
Last-Modified: Thu, 12 Jan 2012 04:55:38 GMT
Connection: close
Transfer-Encoding: chunked
Etag: 0

$callback

Does anyone know how to concat strings in Nginx config files?

Answer 1

Store the string in another variable (say, $string) and do this:

set $callback $arg_callback$string;