From: Brian Candler <lists@...>
Date: 2013-01-03T22:37:45+09:00
Subject: Re: temporarily override a methode

However, what you have written doesn't handle your desired case of 
redirecting system("ls"). This is because commands run by system() 
inherit the ruby process's stdout/stderr file descriptors, and write 
directly to them (not touching any Ruby objects)

Here's a possible solution, setting stdout and stderr to point to new 
pipes which the ruby process consumes in the background.

class Tee
  def self.tee(logfile, append=true)
    file = File.open(logfile, append ? 'a' : 'w')
    stdout_old = $stdout.dup
    stderr_old = $stderr.dup
    #stdout_old.close_on_exec = true  # only in ruby 1.9
    #stderr_old.close_on_exec = true  # only in ruby 1.9
    stdout_r, stdout_w = IO.pipe
    stderr_r, stderr_w = IO.pipe
    t1 = Thread.new do
      while data = stdout_r.readpartial(1024) rescue nil
        stdout_old.write(data)
        file.write(data)
      end
      stdout_r.close
    end
    t2 = Thread.new do
      while data = stderr_r.readpartial(1024) rescue nil
        stderr_old.write(data)
        file.write(data)
      end
      stderr_r.close
    end
    begin
      $stdout.reopen(stdout_w)
      $stderr.reopen(stderr_w)
      yield
    ensure
      $stdout.reopen(stdout_old)
      $stderr.reopen(stderr_old)
      [stdout_w,stderr_w].each { |f| f.close }
      t1.join
      t2.join
      [stdout_old,stderr_old,file].each { |f| f.close }
    end
  end
end

puts "start"
Tee::tee("logfile.log", true) {
  puts "Hello"
  printf "Hello again"
  system("ls -l")
}
puts "end"

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