diff --git a/infra/helper.py b/infra/helper.py
index 6048d9771..6c2950ab4 100755
--- a/infra/helper.py
+++ b/infra/helper.py
@@ -64,12 +64,56 @@ LANGUAGES_WITH_COVERAGE_SUPPORT = ['c', 'c++', 'go', 'rust']
 # pylint: disable=too-many-lines
 
 
-def main():  # pylint: disable=too-many-branches,too-many-return-statements,too-many-statements
+def main():  # pylint: disable=too-many-branches,too-many-return-statements
   """Get subcommand from program arguments and do it."""
   os.chdir(OSS_FUZZ_DIR)
   if not os.path.exists(BUILD_DIR):
     os.mkdir(BUILD_DIR)
 
+  args = parse_args()
+
+  # We have different default values for `sanitizer` depending on the `engine`.
+  # Some commands do not have `sanitizer` argument, so `hasattr` is necessary.
+  if hasattr(args, 'sanitizer') and not args.sanitizer:
+    if args.engine == 'dataflow':
+      args.sanitizer = 'dataflow'
+    else:
+      args.sanitizer = 'address'
+
+  if args.command == 'generate':
+    return generate(args)
+  if args.command == 'build_image':
+    return build_image(args)
+  if args.command == 'build_fuzzers':
+    return build_fuzzers(args)
+  if args.command == 'check_build':
+    return check_build(args)
+  if args.command == 'download_corpora':
+    return download_corpora(args)
+  if args.command == 'run_fuzzer':
+    return run_fuzzer(args)
+  if args.command == 'coverage':
+    return coverage(args)
+  if args.command == 'reproduce':
+    return reproduce(args)
+  if args.command == 'shell':
+    return shell(args)
+  if args.command == 'pull_images':
+    return pull_images(args)
+
+  return 0
+
+
+def parse_args(args=None):
+  """Parses args using argparser and returns parsed args."""
+  # Use default argument None for args so that in production, argparse does its
+  # normal behavior, but unittesting is easier.
+  parser = get_parser()
+  return parser.parse_args(args)
+
+
+def get_parser():  # pylint: disable=too-many-statements
+  """Returns an argparse parser."""
   parser = argparse.ArgumentParser('helper.py', description='oss-fuzz helpers')
   subparsers = parser.add_subparsers(dest='command')
 
@@ -192,39 +236,7 @@ def main():  # pylint: disable=too-many-branches,too-many-return-statements,too-
   _add_environment_args(shell_parser)
 
   subparsers.add_parser('pull_images', help='Pull base images.')
-
-  args = parser.parse_args()
-
-  # We have different default values for `sanitizer` depending on the `engine`.
-  # Some commands do not have `sanitizer` argument, so `hasattr` is necessary.
-  if hasattr(args, 'sanitizer') and not args.sanitizer:
-    if args.engine == 'dataflow':
-      args.sanitizer = 'dataflow'
-    else:
-      args.sanitizer = 'address'
-
-  if args.command == 'generate':
-    return generate(args)
-  if args.command == 'build_image':
-    return build_image(args)
-  if args.command == 'build_fuzzers':
-    return build_fuzzers(args)
-  if args.command == 'check_build':
-    return check_build(args)
-  if args.command == 'download_corpora':
-    return download_corpora(args)
-  if args.command == 'run_fuzzer':
-    return run_fuzzer(args)
-  if args.command == 'coverage':
-    return coverage(args)
-  if args.command == 'reproduce':
-    return reproduce(args)
-  if args.command == 'shell':
-    return shell(args)
-  if args.command == 'pull_images':
-    return pull_images(args)
-
-  return 0
+  return parser
 
 
 def is_base_image(image_name):
diff --git a/infra/helper_test.py b/infra/helper_test.py
new file mode 100644
index 000000000..d899a835b
--- /dev/null
+++ b/infra/helper_test.py
@@ -0,0 +1,35 @@
+# Copyright 2021 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+"""Tests for helper.py"""
+
+import unittest
+from unittest import mock
+
+import helper
+
+
+class TestShell(unittest.TestCase):
+  """Tests 'shell' command."""
+
+  @mock.patch('helper.docker_run')
+  @mock.patch('helper.build_image_impl')
+  def test_base_runner_debug(self, mocked_build_image_impl, _):
+    """Tests that shell base-runner-debug works as intended."""
+    image_name = 'base-runner-debug'
+    unparsed_args = ['shell', image_name]
+    args = helper.parse_args(unparsed_args)
+    args.sanitizer = 'address'
+    result = helper.shell(args)
+    mocked_build_image_impl.assert_called_with(image_name)
+    self.assertEqual(result, 0)