Programmatically integrate Rubocop formatter with stdin/stdout

Question

I'm trying to integrate Rubocop's formatter in an editor: given some Ruby code as an input, return a string representing the equivalent formatted code. I was considering the -s option as suggested by Rubocop's help:

-s FILE - Pipe source from STDIN, using FILE in offense reports. This is useful for editor integration.

In a terminal, the following command successfully reads the input contents and prints the formatted output to stdout:

cat some_file.rb | rubocop -a -f fi -s some_file.rb

However, I'm not able to programmatically reproduce something similar using the Rubocop API. I was hoping to somehow feed the input to a RuboCop::Runner, however, simply parsing the options hangs before I get to that point:

require 'rubocop'

input = "some_var    = 'hello'"
options, paths = RuboCop::Options.new.parse(['-a', '-f', 'fi', '-s', 'some_file.rb'])
runner = RuboCop::Runner.new(options, RuboCop::ConfigStore.new)
...

Of course, one option would be to drop -s and operate on temporary files, but that adds complexity and is less efficient.

How do I conveniently produce a formatted output using the API?

Answer 1

Just call RuboCop as an external process, e.g. using Open3.capture2:

stdout_str, status = Open3.capture2("rubocop -a -f fi -s some_file.rb", input)