How can I capture STDOUT to a string?

You can do this by making a call to your R script inside backticks, like this:

result = `./run-your-script`
puts result  # will contain STDOUT from run-your-script

For more information on running subprocesses in Ruby, check out this Stack Overflow question.

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

If activesupport is available in your project you may do the following:

output = capture(:stdout) do
  run_arbitrary_code
end

More info about Kernel.capture can be found here

Redirect Standard Output to a StringIO Object

You can certainly redirect standard output to a variable. For example:

# Set up standard output as a StringIO object.
foo = StringIO.new
$stdout = foo

# Send some text to $stdout.
puts 'hi'
puts 'bye'

# Access the data written to standard output.
$stdout.string
# => "hi\nbye\n"

# Send your captured output to the original output stream.
STDOUT.puts $stdout.string

In practice, this is probably not a great idea, but at least now you know it's possible.