To use Captain with ExUnit, you need to configure your test suite to output test results to a file and then tell Captain where to find those test results.

Getting started

ExUnit does not natively produce a test results artifact that Captain can parse. To integrate, you'll need to install and configure the junit_formatter package. Once installed, you can configure it by making these changes to your test/test_helper.exs and config/test.exs:

# test/test_helper.exs

ExUnit.configure formatters: [JUnitFormatter, ExUnit.CLIFormatter]
# config/test.exs

config :junit_formatter,
  report_dir: Path.expand("../tmp/junit", __DIR__),
  project_dir: Path.expand("..", __DIR__),
  automatic_create_dir?: true,
  print_report_file: true,
  prepend_project_name?: true,
  include_filename?: true,
  include_file_line?: true

Once you have junit_formatter installed and configured, configure Captain by creating a .captain/config.yaml file in the root directory of your repository:

    command: mix test
      language: Elixir
      framework: ExUnit
      path: tmp/junit/*.xml

You can change your-project-exunit to any name you like, but we typically recommend using the name of your project followed by a dash followed by exunit. The command is the command you already use to run your test suite. Captain will invoke this command to run your tests. The example above shows what you might use if you use mix test and want to store test results in tmp/junit.

Once Captain is configured, you can run captain run your-project-exunit --print-summary. If you see your typical test output following by a captain block like this:

----------------------------------- Captain ------------------------------------

then you've configured everything correctly! You can now supercharge your test framework's capabilities. See below for configuring each of Captain's features.

Quarantining Tests

Traditionally, you might mark a test as pending or skipped to triage flaky or failing tests. With Captain, you can quarantine them instead. When only quarantined tests fail, Captain will still report your build as successful and exit with a 0 exit code. Unlike skipped tests, quarantined tests will continue to run, so you can still view their failure messages and see how frequently they are failing.

You can quarantine tests in OSS mode with captain add quarantine like so:

captain add quarantine your-project-exunit \
  --file test/exunitexample_web/controllers/page_controller_test.exs  \
  --description "Elixir.ExunitexampleWeb.PageControllerTest test GET /"

See the identifying tests section of this page for more information on finding the file and description, and see the OSS quarantining guide for more information on managing quarantined tests in OSS mode.

Retrying Tests

You can configure Captain to automatically retry failed tests to help you determine if failing tests are flaky or are genuinely failing. To configure retries, update your .captain/config.yaml file like so:

    command: mix test
      language: Elixir
      framework: ExUnit
      path: tmp/junit/*.xml
      print-summary: true
      attempts: 2
      command: mix test {{ tests }}

Once configured, Captain will invoke your original test command, check for any failures, and retry your tests however many times you've specified (in this example, two additional times) by templating the failures into the command specified by retries.command. The output.print-summary option is not required, but we've added it for convenience in understanding the overall results after the retries have been factored in.


Captain can optimally partition your test suite's files into multiple groups for execution on multiple CI nodes. Captain tracks your test file runtime so that it can balance each partition.

You can configure Captain to partition your tests by updating your .captain/config.yaml file like so:

    command: mix test
      language: Elixir
      framework: ExUnit
      path: tmp/junit/*.xml
      print-summary: true
      command: mix test {{ testFiles }}
        - tests/**/*_test.exs

Captain will fill in the testFiles placeholder of your partition.command with the files resulting from expanding your configured partition.globs.

Running with Partitioning

Partitioning the files requires specification of the total number of partitions (--partition-total) and the specific, 0-based partition (--partition-index) being run. These values are used alongside the configured partition command and glob patterns.

captain run your-project-exunit --partition-index 0 --partition-total 8

This command partitions your suite into 8 groups and fills in the testFiles at the specified index -- in this case, partition 0.

When initially configuring partitioning, we recommend comparing the test count with and without partitioning to ensure all of your tests are being run (i.e. all of your test files are covered by the configured globs).

GitHub Actions Example

Partitioning works on any CI provider with parallelized or matrix jobs. Partition your tests by passing the appropriate values through to captain run.

Here is a full example using a GitHub Actions workflow:

  runs-on: ubuntu-latest
    fail-fast: false
      partition_index: [0, 1, 2, 3, 4, 5, 6, 7]
      partition_total: [8]
    - uses: actions/checkout@v3
    - uses: erlef/setup-beam@v1
        otp-version: '25'
        elixir-version: '1.14'
    - run: mix deps.get
    - uses: rwx-research/setup-captain@v1
    - name: run tests
      run: |
        captain run your-project-exunit \
        --partition-index ${{ matrix.partition_index }} \
        --partition-total ${{ matrix.partition_total }}

Identifying Tests

Captain uses framework specific "identity recipes" to identify the tests in your suite. These recipes are order dependent components extracted from native test framework output.

We use this identity to track the executions of a test over the course of their lifetime in your suite. This enables us to do things like flake detection, quarantining, and retries.

For exUnit, Captain constructs the identity by parsing out the file and description attributes.