Using Ruby to control another process?

Question

I would like to control a separate process with a classic 1), 2), 3), etc. menu system. Similar to piping an input file in to control the process, I would like to use Ruby to control the process' $stdin and $stdout. I've experimented with IO.popen and Open3.popen3, but cannot seem to get it to work. The documentation examples are not clear (but I'm also quite new to this sort of programming).

The basic idea is:

Open3.popen3("./server") do |stdin,stdout,stderr|
  stdout.gets
  stdin.puts "1"
  stdout.gets
  stdin.puts "2"
  stdout.gets
end

Currently, the first stdout.gets gets the correct header, but then the program seems to hang. Can anybody offer any advice? I've been googling for a while now but haven't found anything.

Thanks!

Linuxios · Accepted Answer · 2012-01-29 01:46:07Z

1

IPC (Inter Process Communication) is very complicated. When working with IPC you need to flush the buffer after every write operation. It would look like this:

Open3.popen3("./server") do |stdin,stdout,stderr|
  stdout.gets
  stdin.puts "1"
  stdin.flush
  stdout.gets
  stdin.puts "2"
  stdin.flush
  stdout.gets
end

It's ugly, but you can redefine puts as a singleton method on stdin (you can do the same with print and write, if needed):

class <<stdin
  old_puts=method(puts)
  def puts(str)
    old_puts[str]
    flush
  end
end

This will save allot of trouble if you are doing a lot of IPC.

answered Jan 29, 2012 at 1:46

Linuxios

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2 Comments

icz Over a year ago

Thanks! Now, suppose instead of a single line on stdout, an unknown amount of lines were put to stdout. When I try to iterate over them with something like until stdout.eof? or while stdout.ready?, but it hangs at the end of the first menu in the loop. Any ideas?

Linuxios Over a year ago

Generally, this is better done with read or each_line. The other process must flush its output too, so that might be a problem.

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