Test Ruby executable with code injection to have stubs/mocks

foo.rb

#!/usr/bin/env ruby
puts "from original executable, args: #{ARGV.join(" ")}"

inject_and_exec.rb

require "open4"
exec = "foo.rb"
injection = "puts \\\"injected!\\\""    # useful for injecting stub/mock code
args = "passed args"
cmd = "ruby -e \"#{injection}; `cat #{exec}`\" -- #{args}"
Open4::popen4(cmd) { |p, i, o| puts o.readlines }