master.cr

reader, writer = IO.pipe  # writes in parent -> reads in child
reader2, writer2 = IO.pipe  # writes in child -> reads in parent

Process.run(command: "./child", input: reader, output: writer2) do
  puts "inside master block"
  writer.puts "message from master 2"
end 

reader2.each_line { |x| puts "rcvd: #{x}"}
puts "done" # doesn't get to this line
# Outputs:
# inside master block
# rcvd: incoming:message from master 2
# rcvd: writing to standard output from child
# rcvd: child finished


#Child code:
puts "child process"
incoming = STDIN.gets
puts "incoming:#{incoming}"

STDOUT << "writing to standard output from child"
puts "\nchild finished"
STDOUT.close