How can I capture STDOUT to a string?

puts "hi"
puts "bye"

I want to store the STDOUT of the code so far (in this case hi \nbye into a variable say 'result' and print it )

puts result

The reason I am doing this is I have integrate an R code into my Ruby code, output of which is given to the STDOUT as the R code runs , but the ouput cannot be accessed inside the code to do some evaluations. Sorry if this is confusing. So the "puts result" line should give me hi and bye.

Solution 1:

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

Solution 2:

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.