Danger of IO duplicate

io_duplicate_danger.rb

# This illustrates how IO duplicates flush separately.
# The danger is that this can result in disordered
# output.  Run this from the command line:
#
#   % ruby io_duplicate_danger.rb
#   1
#   2
#   3
#
# You get what you expect. Now try with redirection:
#
#   % ruby io_duplicate_danger.rb &> output.txt
#   % cat output.txt
#   2
#   1
#   3
#
a = $stdout
b = a.dup

a.puts 1
b.puts 2
a.puts 3