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This project was previously used by Codewars and to execute small sets of code within various languages, using various testing frameworks. Each time code is run, it is executed within a Docker container in order to secure unsafe code execution.

All execution is done within Docker, with a Node CLI app contained within each container that manages the code execution for that specific language environment and returns the result via stdout.


This project has been open-sourced so that the Codewars and Qualified communities can contribute support for new languages and frameworks. See the Language Support Status section for more information regarding which languages are currently supported and where their Codewars/Qualified support has been added.

Basic Usage

Within each Docker image, there is a copy of the Node executable and a run script. That script accepts multiple options for executing code. For example to run a simple javascript script which would output 2:

# this is how you run the Node CLI. For this to work you would have to bash into the correct Docker image
node run -l javascript -c "console.log(1+1)"

Because everything runs inside of Docker, you would normally not run Node directly from your host but instead via a Docker run command. To do this, you would either bash into the correct Docker image like so:

# direct Docker call:
docker run --rm -it --entrypoint bash codewars/node-runner

# alternatively you can use the provided Docker Compose configuration:
docker-compose run node-runner

Or you could choose to execute the code outside of Docker by creating a container that will remove itself after it executes:

# direct Docker call:
docker run --rm codewars/node-runner run -l javascript -c "console.log('I ran inside of Docker using NodeJS')"
docker run --rm codewars/ruby-runner run -l ruby -c "puts 'I ran inside of Docker using Ruby'"

# alternatively you can use the provided Docker Compose configuration:
docker-compose run javascript -c "console.log('I ran inside of Docker using NodeJS')"
docker-compose run ruby -c "puts 'I ran inside of Docker using Ruby'"

Integrated Test Suites

The most significant aspect of this project is that integrated test support is built in to many languages. This is how Codewars and Qualified work, instead of testing STDOUT of a program, the executed code is tested using traditional code testing methods, with unit-tests and language-specific testing frameworks.

Here is a very simple example of running tests using the simplified Codewars testing framework.

# manually running docker
docker run --rm codewars/node-runner run -l javascript -c "var a = 1;" -t cw -f "Test.assertEquals(a, 1)"

# using docker compose
docker-compose run javascript -c "var a = 1;" -t cw -f "Test.assertEquals(a, 1)"

Which would output <PASSED::>Test Passed: Value == 1 to STDOUT.

Language Support Status

Many languages are currently supported in various states of completeness. This list tries to keep track of each.

Legend: !!! = Failing Specs, ??? = Status is unknown, * = Any

Language Version Basic Run Project Mode Test Integration Codewars Qualified Docker Image Examples Notes
Assembly (GAS) !!! systems-runner Travis is failing, tests pass locally
Bash rspec rspec rspec ruby-runner
Brainf**k 20041219 cw-2 cw-2 esolangs-runner
C Clang 3.6/C11 criterion criterion systems-runner
Clojure 1.8.0 clojure.test clojure.test clojure.test jvm-runner clojure.test
CoffeeScript 1.10.0 cw-2 cw-2 cw-2 node-runner cw-2
C++ 14 igloo igloo igloo systems-runner
C# Mono 4.8 nunit nunit nunit dotnet-runner nunit
Chapel 1.15.0 cw-2 cw-2 chapel-runner cw-2
Crystal 0.21.1 spec spec spec crystal-runner spec
Dart 1.23.0 test test dart-runner test
Elixir 1.2.4 exunit exunit elixir-runner
Erlang 20.0 eunit erlang-runner
F# 4.1 fuchu fuchu dotnet-runner Fuchu Tests should be placed in a module called "Tests", in a Fuchu testList
Go 1.8 ginkgo ginkgo ginkgo go-runner ginkgo
Groovy 2.4.12 junit4,spock gradle-runner
Haskell 7.10.3 hspec!!! hspec hspec haskell-runner hspec An older version is running on CW & Qualified that is fully functional
Java 1.8.0_91 junit junit junit java-runner junit
JavaScript 0.10.33 cw-2 cw-2 cw-2, node-runner cw-2
JavaScript 0.10.33/Babel cw-2 cw-2 cw-2, node-runner cw-2
JavaScript 6.11.0 cw-2, mocha cw-2 cw-2, mocha node-runner cw-2
JavaScript 6.11.0/Babel cw-2, mocha cw-2 cw-2, mocha node-runner cw-2
JavaScript 8.1.3 cw-2, mocha cw-2 cw-2, mocha node-runner cw-2
JavaScript 8.1.3/Babel cw-2, mocha cw-2 cw-2, mocha node-runner cw-2
Julia 0.4.6 factcheck factcheck
Kotlin 1.1.4 junit4,kotlintest gradle-runner
Lisp Kumite Only func-runner
Lua 5.2 busted busted lua-runner
Objective-C 2.0 UnitKit UnitKit objc-runner
OCaml 4.02.3 ounit ounit ocaml-runner Tests should be placed in a module called "Tests", in an array of OUnit labels named "suite"
Perl Kumite Only *
PHP 7.0 cw-2, phpunit phpunit phpunit, cw-2 alt-runner
PowerShell 6.0.0-beta.4 Pester powershell-runner Pester
Python 2.7.6 cw-2, unittest cw-2 cw-2, unittest python-runner cw-2
Python 3.4.3 cw-2, unittest cw-2, unittest python-runner cw-2
Python 3.6 cw-2, unittest cw-2, unittest python-runner cw-2
R 3.4.1 testthat r-runner
Racket Kumite Only func-runner
Ruby 2.3.0 cw-2, rspec cw-2 cw-2, rspec ruby-runner cw-2
Rust 1.15.1 rust rust
Scala 2.12.3 junit4,scalatest gradle-runner
Scss/Sass ??? ???
SQL SQLite3 rspec
SQL Postgres 9.6 rspec
Swift 3.1.1 xctest xctest
TypeScript 2.4 mocha mocha node-runner TypeScript utilizes require instead of concatenating files


You should have Docker installed, if not do that first. Before you can run any of the code environments you will need to build the proper Docker image. To get started lets work with the node image.

Run make node to build the base and node images. This will take a few minutes. You can speed up the process by first downloading the existing images that you intend to work on, which will allow you to only need to build when you make a change.

For example, if you intend to work on the jvm image, you would do this:

# download existing images first to greatly speed up time
docker pull codewars/base-runner
docker pull codewars/jvm-runner

# now build the jvm image, so that any recent changes that have may not have been pushed yet get added into the image
make jvm

Once you image is downloaded/built, you can create a container to work within it. Doing this means you do not have to worry about having any of the project dependencies loaded directly on your machine.

Run the following command:

docker run \
    -it \
    --rm \
    --entrypoint bash \
    -v $(pwd)/lib:/runner/lib \
    -v $(pwd)/examples:/runner/examples \
    -v $(pwd)/frameworks:/runner/frameworks \
    -v $(pwd)/test:/runner/test \

This will create a new container and send you into the instance with your project's lib and test directories mounted as volumes. Mounting as a volume allows you to change files on your local machine and have those changes available to you from within the container.

Notice: We did not mount the entire directory because that would overwrite things such as your node_modules directory. If you need to update these you should make {image_you_want_to_update} the image to ensure you are always testing against the correct packages.

Docker Compose

We mentioned before that you also have the option of using Docker Compose to run the CLI tool. We have setup the docker-compose.yml file to provide very useful pre-configured services for making development easier. Instead of having to issue the long command mentioned above, you can simply run docker-compose run node-runner to bash into a fresh container with your local volumes already mounted.

All of the docker compose services are setup to mount volumes to make development easier, so that is the recommended way of interacting with the codebase. You should note though that the compose file is unable to build images due to how the directory structure is laid out, so you have to first make {runner_name} the image before you can run it. Otherwise it will pull down the latest copy from Docker Hub.

Running Tests

Once you are in the Docker image, you can run tests as a part of your development flow. For example:

# inside of container
mocha test/runners/typescript_spec.js

# or from outside the container
docker-compose run typescript_test

Test Suite Output Format

A custom and very basic format is used for sending data out of the CLI tool. All formatted data is returned via STDOUT. If you do nothing but write normal strings to STDOUT, then will display each line as you would expect, unformatted.

A small subset of commands is supported that can be used to format output. They are:

  • <DESCRIBE::>
  • <IT::>
  • <PASSED::>
  • <FAILED::>
  • <ERROR::>

Prefixing a new line with these commands will cause that line to be formatted. Since each new STDOUT line is considered a new piece of data, if you wish to format multiple lines as one item (such as a multi line "passed" message), then you must replace all \n line feed characters with the <:LF:> token.

For example, in Ruby, if you wanted to write a multi-line passed message:

def passed(msg)
  puts "<PASSED::>#{msg.gsub("\n", "<:LF:>")}"

Nested Describes

Some test frameworks support nested levels of describes. In order for our output to support multiple levels, you must also use the <COMPLETEDIN::> token, which acts as a terminator for the current item. This should be used to terminate both <DESCRIBE::> and <IT::> statements.

The following is a full example of what the output might look like, that supports nested describes:

<IT::>It should return a string
<PASSED::>Test Passed
<IT::>It should return "foo"
This is some direct output (i.e. console.log("..."))
<FAILED::>Expected "foo" but instead got ""
<DESCRIBE::>This is a nested describe
<IT::>Should not be null
<PASSED::>Test Passed

Notice how there are 3 <COMPLETEDIN::> statements at the end. The first one completes the last IT statement, the 2nd completes the nested DESCRIBE and the 3rd completes the top level "Foo" DESCRIBE.


The value of COMPLETEDIN should be time spent executing the related statement, in milliseconds. It is not required to support time. <COMPLETEDIN::> is valid on its own, and in that case it is only used to terminate the current statement.

Why the custom format?

Getting different test suites in different languages to all play together with the same format can be tricky. In many cases, customizing the test suite output is very limited (sometimes requiring hacking). Because of this, using formats such as XML and JSON are complicated because its not always possibly to correctly close out the data format when a program raises an exception.

The format chosen was originally done so that at any point in time the program could exit while still having readable data. Other formats, such as TAP (Test Anything Protocol) could also be an option. However another requirement that we had when designing the format was to have it be incredibly simple yet flexible, so that could support more than simply outputting test results. With the current format there is nothing stopping you from outputting HTML, JS canvas code, etc in order to create a rich and even interactive test result output.

How to add a new language

Note: These steps all assume you are adding a completely new language to the project. Many languages are currently in an incomplete state so not all steps may be needed in your case

  1. Install the language and its related packages on one of the Docker images. We have grouped many of the Docker images together so if that grouping makes sense then add it there, otherwise you will need to create a new docker image within the docker folder
  2. Add the newly created docker file to the Make and docker-compose files (if applicable)
  3. Add a new runner script within lib/runners. More details about this script later on.
  4. Add a new runner spec within test/runners. You will also need to add the runner spec to the Docker image so that it is tested as a part of the build process.
  5. Add a new examples yml file within the examples folder. These are the code examples that are used on when a user clicks the "Insert Example" button within the kata editor. There is also a helper available for running your examples as a part of the test suite.

Runner Script

The runner script is responsible for outputting a run method. This method utilizes the shovel helper which will handle all of the inter-process communication for you. The shovel config accepts strategies to determine how to handle running the code based on the options passed in. There are currently two types of strategies.

  • solutionOnly: Code is simply executed and its STDOUT returned. There is no test integration.
  • testIntegration: Code is executed within a configurable test suite and the test output is returned via STDOUT.

Each of these strategies is passed in a run method which is used to ultimately execute the final command.