I use this trick every time I start a new project, which is just often enough to go look it up but not often enough to commit it to memory, so here it is in blog form:

spec_helper.rb
if ENV['COVERAGE']
    require 'simplecov'
    SimpleCov.start 'rails'
end

Of course if you’re writing a non-rails app omit the ‘rails’ argument. If you’re not using RSpec, put it in test/minitest_helper.rb. (And, of course, if you’re using Test::Unit, stop using Test::Unit and switch to MiniTest!)

Now when you run your specs or tests as normal with ‘rspec spec’ or ‘rake test’ your regular tests will run normally. To run them and get a coverage graph, however, just run

COVERAGE=true rspec spec

or

COVERAGE=true rake test

And there you go. If memory serves me correctly, this trick is credit due to Jake Mallory, from a project we worked on together last year.