I use ert-runner for running unit tests in my Emacs packages. It’s a handy way to run all unit tests in a single go, and it works nicely with Cask to make repeatable runs easier.

One thing I missed was automatic testing when files are saved. I use Guard when developing Rails projects and wanted to hook it into my Emacs setup.

The following Guardfile uses guard-shell to run ert-runner whenever an .el file is modified. I added a workaround for ignoring flycheck files, which Emacs saves when a file is modified and can trigger a huge amount of builds.

guard :shell do
  watch (/(.*)\.el$/) do |file|
    # Ugly fix but I couldn't spend time messing around with regex.
    unless file[0].to_s.include? 'flycheck'
      if system('cask exec ert-runner')
        n 'Tests succeeded', 'ERT', :success
        n 'Tests failed', 'ERT', :failed


Using :success and :failed with the process result also means Guard’s notifier works correctly, so it’s possible to use the Emacs notifier to get realtime test pass & fail colours whenever a file is saved. Pretty neat!