Using Ruby on Rails app to control console application

If you only need to run a "one-off" command, just use backticks. If you need to maintain a long-running background process, which accepts commands and returns responses, you can do something like this (some of the details have been edited out, since this code is from a proprietary application):

class Backend
  def initialize
    @running = false
    @server  = nil
    # if we forget to call "stop", make sure to close down background process on exit
    ObjectSpace.define_finalizer(self,lambda { stop if @running })
  end

  def start
    stop if @running
    @server = IO.popen("backend","w+")
    @running = true
  end

  def stop
    return if not @running
    @server << "exit\n"
    @server.flush
    @running = false
 end

 def query(*args)
   raise "backend not running" if not @running
   @server << "details edited out\n"
   @server.flush
   loop do
     response = parse_response
     # handle response
     # break loop when backend is finished sending data
   end
 end

 private
   def parse_response
     # details edited out, uses c = @server.getc to read data from backend
     # getc will block if there is nothing to read,
     # so there needs to be an unambiguous terminator telling you where
     # to stop reading
   end
 end

end

You can adapt this to your own needs. Beware of situations where the background process dies and leaves the main process hanging.

Although it doesn't apply to your situation, if you are designing the background program yourself: Build the background process so that if ANYTHING makes it crash, it will send an unambiguous message like "PANIC" or something which tells the main process to either exit with an error message, or try starting another background process. Also, make sure it is completely unambiguous where each "message" begins and ends, and test the coordination between main/background process well -- if there are bugs on either end, it is very easy to get a situation where both processes get stuck waiting for each other. One more thing: design the "protocol" which the 2 processes speak to each other in a way which makes it easy to maintain synchronization between the 2.