select.rb

rp, wp = IO.pipe
mesg = "ping "
20.times { |i|
  print "##{i} "
  rs, ws, = IO.select([rp], [wp])
  require'pry-byebug';binding.pry
  if r = rs[0]
    ret = r.read(5)
    print ret
    case ret
    when /ping/
      mesg = "pong\n"
    when /pong/
      mesg = "ping "
    end
  end
  if w = ws[0]
    w.write(mesg)
  end
}

# 0 1 ping 2 pong
# 3 ping 4 pong
# 5 ping 6 pong
# 7 ping 8 pong
# 9 ping 10 pong
# 11 ping 12 pong
# 13 ping 14 pong
# 15 ping 16 pong
# 17 ping 18 pong
# 19 ping

rp, wp = IO.pipe
rs, ws = IO.select([rp], [wp])
# [
#   [],
#   [#<IO:fd 10>],
#   []
# ]