A shell pipeline in Ruby

pipeline.rb

# Read initial standard input from the data segment at the end of the file
# You can also see what is ultimately output down there
pos = DATA.pos          # => 1617
$stdin.reopen __FILE__  # => #<IO:/var/folders/7g/mbft22555w3_2nqs_h1kbglw0000gn/T/seeing_is_believing_temp_dir20161210-36802-czciw7/program.rb>
$stdin.seek pos, :SET   # => 0

# The commands that will be piped together
commands = [
  %w[cat],
  %w[tr a A],
  %w[tr c C],
]

# Connnect each command's to our stderr, first has our stdin, last has our stdout
pipeline = commands.map do |command|
  { command:      command,
    stdin:        $stdin,
    stdout:       $stdout,
    stderr:       $stderr,
    close_stdin:  false,
    close_stdout: false,
  }
end

# Pipe each command's stdout to the next command's stdin
pipeline.each_cons(2) do |(left, right)|
  right[:stdin], left[:stdout] = IO.pipe
  right[:close_stdin] = true
  left[:close_stdout] = true
end

# Execute each command
pipeline.each do |attrs|
  # if we are the parent process
  if pid = fork
    # Document the pid, close the piped file descriptors, on to the next
    attrs[:pid] = pid
    attrs[:stdin].close  if attrs[:close_stdin]
    attrs[:stdout].close if attrs[:close_stdout]
    next
  end

  # Subprocess sets the file descriptors and execs the command
  STDIN.reopen  attrs[:stdin]
  STDOUT.reopen attrs[:stdout]
  STDERR.reopen attrs[:stderr]
  exec *attrs[:command]
end

# Wait for the children to finish, record their exit statuses
pipeline.each do |attrs|
  Process.wait attrs[:pid]
  attrs[:status] = $?
end

pipeline
# => [{:command=>["cat"],
#      :stdin=>
#       #<IO:/var/folders/7g/mbft22555w3_2nqs_h1kbglw0000gn/T/seeing_is_believing_temp_dir20161210-36802-czciw7/program.rb>,
#      :stdout=>#<IO:(closed)>,
#      :stderr=>#<IO:<STDERR>>,
#      :close_stdin=>false,
#      :close_stdout=>true,
#      :pid=>36804,
#      :status=>#<Process::Status: pid 36804 exit 0>},
#     {:command=>["tr", "a", "A"],
#      :stdin=>#<IO:(closed)>,
#      :stdout=>#<IO:(closed)>,
#      :stderr=>#<IO:<STDERR>>,
#      :close_stdin=>true,
#      :close_stdout=>true,
#      :pid=>36805,
#      :status=>#<Process::Status: pid 36805 exit 0>},
#     {:command=>["tr", "c", "C"],
#      :stdin=>#<IO:(closed)>,
#      :stdout=>#<IO:<STDOUT>>,
#      :stderr=>#<IO:<STDERR>>,
#      :close_stdin=>true,
#      :close_stdout=>false,
#      :pid=>36806,
#      :status=>#<Process::Status: pid 36806 exit 0>}]

# >> AAA
# >> bbb
# >> CCC

__END__
aaa
bbb
ccc