diff --git a/git_updater.py b/git_updater.py
new file mode 100644
index 0000000..bde48a7
--- /dev/null
+++ b/git_updater.py
@@ -0,0 +1,118 @@
+# Copyright (C) 2018 The Android Open Source Project
+#
+# 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.
+"""Module to check updates from Git upstream."""
+
+
+import datetime
+
+import fileutils
+import git_utils
+import metadata_pb2    # pylint: disable=import-error
+
+
+class GitUpdater():
+    """Updater for Git upstream."""
+
+    def __init__(self, url, proj_path, metadata):
+        if url.type != metadata_pb2.URL.GIT:
+            raise ValueError('Only support GIT upstream.')
+        self.proj_path = proj_path
+        self.metadata = metadata
+        self.upstream_url = url
+        self.upstream_remote_name = None
+        self.android_remote_name = None
+        self.latest_commit = None
+
+    def _setup_remote(self):
+        remotes = git_utils.list_remotes(self.proj_path)
+        for name, url in remotes.items():
+            if url == self.upstream_url.value:
+                self.upstream_remote_name = name
+
+            # Guess android remote name.
+            if '/platform/external/' in url:
+                self.android_remote_name = name
+
+        if self.upstream_remote_name is None:
+            self.upstream_remote_name = "update_origin"
+            git_utils.add_remote(self.proj_path, self.upstream_remote_name,
+                                 self.upstream_url.value)
+
+        git_utils.fetch(self.proj_path,
+                        [self.upstream_remote_name, self.android_remote_name])
+
+    def check(self):
+        """Checks upstream and returns whether a new version is available."""
+
+        self._setup_remote()
+        commits = git_utils.get_commits_ahead(
+            self.proj_path, self.upstream_remote_name + '/master',
+            self.android_remote_name + '/master')
+
+        if not commits:
+            return False
+
+        self.latest_commit = commits[0]
+        commit_time = git_utils.get_commit_time(self.proj_path, commits[-1])
+        time_behind = datetime.datetime.now() - commit_time
+        print('{} commits ({} days) behind.'.format(
+            len(commits), time_behind.days), end='')
+        return True
+
+    def _write_metadata(self, path):
+        updated_metadata = metadata_pb2.MetaData()
+        updated_metadata.CopyFrom(self.metadata)
+        updated_metadata.third_party.version = self.latest_commit
+        fileutils.write_metadata(path, updated_metadata)
+
+    def update(self):
+        """Updates the package.
+
+        Has to call check() before this function.
+        """
+        # See whether we have a local upstream.
+        branches = git_utils.list_remote_branches(
+            self.proj_path, self.android_remote_name)
+        upstreams = [
+            branch for branch in branches if branch.startswith('upstream-')]
+        if len(upstreams) == 1:
+            merge_branch = '{}/{}'.format(
+                self.android_remote_name, upstreams[0])
+        elif not upstreams:
+            merge_branch = 'update_origin/master'
+        else:
+            raise ValueError('Ambiguous upstream branch. ' + upstreams)
+
+        upstream_branch = self.upstream_remote_name + '/master'
+
+        commits = git_utils.get_commits_ahead(
+            self.proj_path, merge_branch, upstream_branch)
+        if commits:
+            print('Warning! {} is {} commits ahead of {}. {}'.format(
+                merge_branch, len(commits), upstream_branch, commits))
+
+        commits = git_utils.get_commits_ahead(
+            self.proj_path, upstream_branch, merge_branch)
+        if commits:
+            print('Warning! {} is {} commits behind of {}.'.format(
+                merge_branch, len(commits), upstream_branch))
+
+        self._write_metadata(self.proj_path)
+        print("""
+This tool only updates METADATA. Run the following command to update:
+    git merge {merge_branch}
+
+To check all local changes:
+    git diff {merge_branch} HEAD
+""".format(merge_branch=merge_branch))