How to simultaneously print and capture the output of an external command in Ruby

io_popen_example.rb

#!/usr/bin/env ruby

cmd = %q[echo '3...'; sleep 1;
         echo '2...'; sleep 1;
         echo '1...'; sleep 1;
         echo 'Liftoff!']

puts '------ beginning command ------'
output_log = []
IO.popen(cmd).each do |line|
  puts line
  output_log << "[#{Time.now}] #{line}"
end.close # Without close, you won't be able to access $?
puts '------ done with command ------'

puts "The command's exit code was: #{$?.exitstatus}"
puts 'Here is the log:'
puts output_log.join('')