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diff --git a/tools/migration/ctoolchain_comparator.py b/tools/migration/ctoolchain_comparator.py
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+++ b/tools/migration/ctoolchain_comparator.py
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+# Copyright 2018 The Bazel Authors. All rights reserved.
+#
+# 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.
+r"""A script that compares 2 CToolchains from proto format.
+
+This script accepts two files in either a CROSSTOOL proto text format or a
+CToolchain proto text format. It then locates the CToolchains with the given
+toolchain_identifier and checks if the resulting CToolchain objects in Java
+are the same.
+
+Example usage:
+
+bazel run \
+@rules_cc//tools/migration:ctoolchain_comparator -- \
+--before=/path/to/CROSSTOOL1 \
+--after=/path/to/CROSSTOOL2 \
+--toolchain_identifier=id
+"""
+
+import os
+from absl import app
+from absl import flags
+from google.protobuf import text_format
+from third_party.com.github.bazelbuild.bazel.src.main.protobuf import crosstool_config_pb2
+from tools.migration.ctoolchain_comparator_lib import compare_ctoolchains
+
+flags.DEFINE_string(
+    "before", None,
+    ("A text proto file containing the relevant CTooclchain before the change, "
+     "either a CROSSTOOL file or a single CToolchain proto text"))
+flags.DEFINE_string(
+    "after", None,
+    ("A text proto file containing the relevant CToolchain after the change, "
+     "either a CROSSTOOL file or a single CToolchain proto text"))
+flags.DEFINE_string("toolchain_identifier", None,
+                    "The identifier of the CToolchain that is being compared.")
+flags.mark_flag_as_required("before")
+flags.mark_flag_as_required("after")
+
+
+def _to_absolute_path(path):
+  path = os.path.expanduser(path)
+  if os.path.isabs(path):
+    return path
+  else:
+    if "BUILD_WORKING_DIRECTORY" in os.environ:
+      return os.path.join(os.environ["BUILD_WORKING_DIRECTORY"], path)
+    else:
+      return path
+
+
+def _find_toolchain(crosstool, toolchain_identifier):
+  for toolchain in crosstool.toolchain:
+    if toolchain.toolchain_identifier == toolchain_identifier:
+      return toolchain
+  return None
+
+
+def _read_crosstool_or_ctoolchain_proto(input_file, toolchain_identifier=None):
+  """Reads a proto file and finds the CToolchain with the given identifier."""
+  with open(input_file, "r") as f:
+    text = f.read()
+  crosstool_release = crosstool_config_pb2.CrosstoolRelease()
+  c_toolchain = crosstool_config_pb2.CToolchain()
+  try:
+    text_format.Merge(text, crosstool_release)
+    if toolchain_identifier is None:
+      print("CROSSTOOL proto needs a 'toolchain_identifier' specified in "
+            "order to be able to select the right toolchain for comparison.")
+      return None
+    toolchain = _find_toolchain(crosstool_release, toolchain_identifier)
+    if toolchain is None:
+      print(("Cannot find a CToolchain with an identifier '%s' in CROSSTOOL "
+             "file") % toolchain_identifier)
+      return None
+    return toolchain
+  except text_format.ParseError as crosstool_error:
+    try:
+      text_format.Merge(text, c_toolchain)
+      if (toolchain_identifier is not None and
+          c_toolchain.toolchain_identifier != toolchain_identifier):
+        print(("Expected CToolchain with identifier '%s', got CToolchain with "
+               "identifier '%s'" % (toolchain_identifier,
+                                    c_toolchain.toolchain_identifier)))
+        return None
+      return c_toolchain
+    except text_format.ParseError as toolchain_error:
+      print(("Error parsing file '%s':" % input_file))  # pylint: disable=superfluous-parens
+      print("Attempt to parse it as a CROSSTOOL proto:")  # pylint: disable=superfluous-parens
+      print(crosstool_error)  # pylint: disable=superfluous-parens
+      print("Attempt to parse it as a CToolchain proto:")  # pylint: disable=superfluous-parens
+      print(toolchain_error)  # pylint: disable=superfluous-parens
+      return None
+
+
+def main(unused_argv):
+
+  before_file = _to_absolute_path(flags.FLAGS.before)
+  after_file = _to_absolute_path(flags.FLAGS.after)
+  toolchain_identifier = flags.FLAGS.toolchain_identifier
+
+  toolchain_before = _read_crosstool_or_ctoolchain_proto(
+      before_file, toolchain_identifier)
+  toolchain_after = _read_crosstool_or_ctoolchain_proto(after_file,
+                                                        toolchain_identifier)
+
+  if not toolchain_before or not toolchain_after:
+    print("There was an error getting the required toolchains.")
+    exit(1)
+
+  found_difference = compare_ctoolchains(toolchain_before, toolchain_after)
+  if found_difference:
+    exit(1)
+
+
+if __name__ == "__main__":
+  app.run(main)