Ruby: Write to stdin and read from stdout?

Try closing stdin before reading the output. Here's an example:

require 'open3'
Open3.popen3("./MyProgram") do |i, o, e, t|
  i.write "Hello World!"
  i.close
  puts o.read
end

Here's a more succint way of writing it using Open3::capture3: (beware, untested!)

o, e, s= Open3.capture3("./MyProgram", stdin_data: "Hello World!")
puts o