Auto-change output format if GitHub Actions detected

docs/integration.rst

@@ -17,3 +17,37 @@ Here is an example, to add in your .pre-commit-config.yaml
     hooks:
       - id: yamllint
         args: [-c=/path/to/.yamllint]
+
+Integration with GitHub Actions
+-------------------------------
+
+yamllint auto-detects when it's running inside of `GitHub
+Actions<https://github.com/features/actions>` and automatically uses the suited
+output format to decorate code with linting errors automatically. You can also
+force the GitHub Actions output with ``yamllint --format github``.
+
+An example workflow using GitHub Actions:
+
+.. code:: yaml
+
+   ---
+   name: yamllint test
+
+   on: push
+
+   jobs:
+     test:
+       runs-on: ubuntu-latest
+       steps:
+         - uses: actions/checkout@v2
+
+         - name: Set up Python
+           uses: actions/setup-python@v2
+           with:
+             python-version: 3.8
+
+         - name: Install yamllint
+           run: pip install yamllint
+
+         - name: Lint YAML files
+           run: yamllint .

tests/test_cli.py

@@ -563,6 +563,24 @@ class CommandLineTestCase(unittest.TestCase):
         self.assertEqual(
             (ctx.returncode, ctx.stdout, ctx.stderr), (1, expected_out, ''))
 
+    def test_github_actions_detection(self):
+        path = os.path.join(self.wd, 'a.yaml')
+        self.addCleanup(os.environ.__delitem__, 'GITHUB_ACTIONS')
+        self.addCleanup(os.environ.__delitem__, 'GITHUB_WORKFLOW')
+
+        with RunContext(self) as ctx:
+            os.environ['GITHUB_ACTIONS'] = 'something'
+            os.environ['GITHUB_WORKFLOW'] = 'something'
+            cli.run((path, ))
+        expected_out = (
+            '::error file=%s,line=2,col=4::[trailing-spaces] trailing'
+            ' spaces\n'
+            '::error file=%s,line=3,col=4::[new-line-at-end-of-file] no'
+            ' new line character at the end of file\n'
+            % (path, path))
+        self.assertEqual(
+            (ctx.returncode, ctx.stdout, ctx.stderr), (1, expected_out, ''))
+
     def test_run_read_from_stdin(self):
         # prepares stdin with an invalid yaml string so that we can check
         # for its specific error, and be assured that stdin was read

yamllint/cli.py

@@ -109,7 +109,9 @@ def show_problems(problems, file, args_format, no_warn):
             continue
         if args_format == 'parsable':
             print(Format.parsable(problem, file))
-        elif args_format == 'github':
+        elif args_format == 'github' or (args_format == 'auto' and
+                                         'GITHUB_ACTIONS' in os.environ and
+                                         'GITHUB_WORKFLOW' in os.environ):
             print(Format.github(problem, file))
         elif args_format == 'colored' or \
                 (args_format == 'auto' and supports_color()):