如何使用旧版本的rspec-core运行rspec

Question

When I run rspec, I get this error message

/Users/umezo/.rvm/gems/ruby-2.1.1@itsonme/gems/bundler-1.6.1/lib/bundler/runtime.rb:34:in `block in setup': You have already activated rspec-core 2.14.8, but your Gemfile requires rspec-core 2.13.1. Prepending `bundle exec` to your command may solve this. (Gem::LoadError)

I do in fact specify rspec-core 2.13.1 in my gemfile...

gem "rspec-rails", "~> 2.13.0"

and I do have multiple versions of rspec and rspec core, as gem list rspec returns...

*** LOCAL GEMS ***

guard-rspec (4.2.8, 3.1.0, 1.2.1)
rspec (2.14.1, 2.13.0, 2.5.0)
rspec-core (2.14.8, 2.13.1)
rspec-expectations (2.14.5, 2.13.0, 2.5.0)
rspec-mocks (2.14.6, 2.13.0, 2.5.0)
rspec-rails (2.14.2, 2.13.0, 2.5.0)

I understand that three ways of solving this problem are: 1) prepending bundle exec to the command; 2) upgrading the rspec in the Gemfile to 2.14.8; or 3) removing rspec/rspec-core 2.14.1/8.

However, I would like to know how I can run rspec 2.13.0 by default, without removing rspec 2.14.1 . I know this is possible, as I am able to do this on my other OSX machine, which also has the same two versions of rspec, and the same version of rspec specified in the gemfile. Is there a specific configuration file that bundler/rails/rspec looks at to determine which version of rspec to run?

EDIT: Here's more of my config.

System OSX Mavericks
Ruby 2.1.1p76
rvm 1.25.31    
Bundler 1.6.1
Rails 4.1.0

Answer 1

Found this when looking at the rspec executable generated by RubyGems:

if ARGV.first
  str = ARGV.first
  str = str.dup.force_encoding("BINARY") if str.respond_to? :force_encoding
  if str =~ /\A_(.*)_\z/
    version = $1
    ARGV.shift
  end
end

gem 'rspec-core', version
load Gem.bin_path('rspec-core', 'rspec', version)

Which means we can do something like this:

rspec _2.99.2_ ./spec/workers/base_worker_spec.rb