How can I capture STDOUT to a string?

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A handy function for capturing stdout into a string…

The following method is a handy general purpose tool to capture stdout and return it as a string. (I use this frequently in unit tests where I want to verify something printed to stdout.) Note especially the use of the ensure clause to restore $stdout (and avoid astonishment):

def with_captured_stdout
  original_stdout = $stdout  # capture previous value of $stdout
  $stdout = StringIO.new     # assign a string buffer to $stdout
  yield                      # perform the body of the user code
  $stdout.string             # return the contents of the string buffer
ensure
  $stdout = original_stdout  # restore $stdout to its previous value
end

So, for example:

>> str = with_captured_stdout { puts "hi"; puts "bye"}
=> "hi\nbye\n"
>> print str
hi
bye
=> nil

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