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A simple mutation testing apparatus
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bin Print total run time at the end of the run Dec 28, 2018
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A Ruby-based, language-agnostic tool for performing mutation testing on a file.

Mutation testing is the act of modifying production code and seeing if the tests still pass. If the tests still pass, you have some effectively useless code (according to the tests). This can be helpful to determine how good a file’s tests are and what code might be useless. This can be much more helpful than traditional code coverage tools, as they will only tell you if a line executes and not if the line is necessary to the program.

This tool is relatively crude (hence the name) in that it will just remove lines one by one to detect which is a dangerous mutation (i.e. a line removed where all tests still pass).

More around the inspiration in this blog post: Better Code through Mutation Testing.


In your preferred shell…

gem install crude-mutant


The script is started by providing the file you wish to modify and then the command used to test the code.

It's assumed that the command will return a non-zero status code on test failure.

Here's a Ruby-centric example:

crude-mutant "bundle exec rspec" "app/models/post.rb"

You can also use the shortened cm:

cm "bundle exec rspec" "app/models/post.rb"

See Also

If you’re looking for more serious mutation testing, please have a look at the mutant gem.


After checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to


Bug reports and pull requests are welcome on GitHub at This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the Contributor Covenant code of conduct.


The gem is available as open source under the terms of the MIT License.

Code of Conduct

Everyone interacting in the Crude::Mutant project’s codebases, issue trackers, chat rooms and mailing lists is expected to follow the code of conduct.

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