How do I temporarily redirect stderr in Ruby?

Question

I'd like to temporarily redirect stderr in a Ruby script for the duration of a block, ensuring that I reset it to its original value at the end of the block.

I had trouble finding how to do this in the ruby docs.

Answer 1

In Ruby, $stderr refers to the output stream that is currently used as stderr, whereas STDERR is the default stderr stream. It is easy to temporarily assign a different output stream to $stderr.

require "stringio"  def capture_stderr   # The output stream must be an IO-like object. In this case we capture it in   # an in-memory IO object so we can return the string value. You can assign any   # IO object here.   previous_stderr, $stderr = $stderr, StringIO.new   yield   $stderr.string ensure   # Restore the previous value of stderr (typically equal to STDERR).   $stderr = previous_stderr end 

Now you can do the following:

captured_output = capture_stderr do   # Does not output anything directly.   $stderr.puts "test" end  captured_output #=> "test\n" 

The same principle also works for $stdout and STDOUT.