How can I capture STDOUT to a string?

Question

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.

Answer 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 

Answer 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.