How do I get the STDOUT of a ruby system() call while it is being run?

Question

Similar to Getting output of system() calls in Ruby , I am running a system command, but in this case I need to output the STDOUT from the command as it runs.

Answer 1

In case someone might want to read stdout and stderr:
It is important to read them in parallel, not first one then the other. Because programs are allowed to output to stdout and stderr by turns and even in parallel. So, you need threads. This fact isn't even Ruby-specific.

Stolen from here.

require 'open3'

cmd = './packer_mock.sh'
data = {:out => [], :err => []}

# see: http://stackoverflow.com/a/1162850/83386
Open3.popen3(cmd) do |stdin, stdout, stderr, thread|
  # read each stream from a new thread
  { :out => stdout, :err => stderr }.each do |key, stream|
    Thread.new do
      until (raw_line = stream.gets).nil? do
        parsed_line = Hash[:timestamp => Time.now, :line => "#{raw_line}"]
        # append new lines
        data[key].push parsed_line

        puts "#{key}: #{parsed_line}"
      end
    end
  end

  thread.join # don't exit until the external process is done
end