Add first pass at migration mode.

pre_commit/commands/install.py

@@ -1,24 +1,44 @@
 from __future__ import print_function
 from __future__ import unicode_literals
 
+import io
 import os
+import os.path
 import pkg_resources
 import stat
 
+# This is used to identify the hook file we install
+IDENTIFYING_HASH = 'd8ee923c46731b42cd95cc869add4062'
+
+
+def is_our_pre_commit(filename):
+    return IDENTIFYING_HASH in io.open(filename).read()
+
+
+def make_executable(filename):
+    original_mode = os.stat(filename).st_mode
+    os.chmod(
+        filename,
+        original_mode | stat.S_IXUSR | stat.S_IXGRP | stat.S_IXOTH,
+    )
+
 
 def install(runner):
     """Install the pre-commit hooks."""
     pre_commit_file = pkg_resources.resource_filename(
         'pre_commit', 'resources/pre-commit-hook',
     )
+
+    # If we have an existing hook, move it to pre-commit.legacy
+    if (
+        os.path.exists(runner.pre_commit_path) and
+        not is_our_pre_commit(runner.pre_commit_path)
+    ):
+        os.rename(runner.pre_commit_path, runner.pre_commit_path + '.legacy')
+
     with open(runner.pre_commit_path, 'w') as pre_commit_file_obj:
         pre_commit_file_obj.write(open(pre_commit_file).read())
-
-    original_mode = os.stat(runner.pre_commit_path).st_mode
-    os.chmod(
-        runner.pre_commit_path,
-        original_mode | stat.S_IXUSR | stat.S_IXGRP | stat.S_IXOTH,
-    )
+    make_executable(runner.pre_commit_path)
 
     print('pre-commit installed at {0}'.format(runner.pre_commit_path))
     return 0

pre_commit/resources/pre-commit-hook

@@ -1,4 +1,10 @@
 #!/usr/bin/env bash
+# This is a randomish md5 to identify this script
+# d8ee923c46731b42cd95cc869add4062
+
+HERE=$(dirname $(readlink -f "$0"))
+
+retv=0
 
 which pre-commit > /dev/null
 if [ $? -ne 0 ]; then
@@ -6,4 +12,20 @@ if [ $? -ne 0 ]; then
     exit 1
 fi
 
+
+# Run the legacy pre-commit if it exists
+if [ -x "$HERE"/pre-commit.legacy ]; then
+    "$HERE"/pre-commit.legacy
+    if [ $? -ne 0 ]; then
+        retv=1
+    fi
+fi
+
+
+# Run pre-commit
 pre-commit
+if [ $? -ne 0 ]; then
+    retv=1
+fi
+
+exit $retv