How can I copy STDOUT to a file without stopping it showing onscreen using Ruby

Question

I'm attempting to copy stdout to a file for logging purposes. I also want it to display in the Ruby console of the IDE I'm using.

I inserted this code into my script and it redirects $stdout to the my.log file:

$stdout.reopen("my.log", "w")

Does anyone know of a gem or technique to copy the contents of $stdout to a file and not redirect it to a file? Also, I am not using Rails just Ruby.

Answer 1

Something like this might help you:

class TeeIO < IO
  
  def initialize orig, file
    @orig = orig
    @file = file
  end
  
  def write string
    @file.write string
    @orig.write string
  end
  
end

Most of the methods in IO that do output ultimately use write, so you only have to override this one method. You can use it like this:

#setup
tee = TeeIO.new $stdout, File.new('out.txt', 'w')
$stdout = tee

# Now lots of example uses:
puts "Hello"
$stdout.puts "Extending IO allows us to expicitly use $stdout"
print "heres", :an, :example, "using", 'print', "\n"
48.upto(57) do |i|
  putc i
end
putc 10 #newline
printf "%s works as well - %d\n", "printf", 42
$stdout.write "Goodbye\n"

After this example, the following is written identically to both the console and to the file:

Hello
Extending IO allows us to expicitly use $stdout
heresanexampleusingprint
0123456789
printf works as well - 42
Goodbye

I won't claim this technique is fail proof, but it should work for simple uses of stdout. Test it for your use.

Note that you don't have to use reopen on $stdout unless you want to redirect output from a child process or an uncooperative extension. Simply assigning a different IO object to it will work for most uses.

---

RSpec

The RSpec command line takes a reference to $stdout before you can get any code to run to reassign it, so this doesn't work. reopen still works in this case as you're changing the actual object pointed to by both $stdout and the reference that RSpec has, but this doesn't give you output to both.

One solution is to monkey-patch $stdout like this:

$out_file = File.new('out.txt', 'w')

def $stdout.write string
  $out_file.write string
  super
end

This works, but as with all monkey patching, be careful. It would be safer to use your OS's tee command.