Add a checker for executables without shebangs

pre_commit_hooks/check_executables_have_shebangs.py

@@ -0,0 +1,40 @@
+"""Check that executable text files have a shebang."""
+from __future__ import absolute_import
+from __future__ import print_function
+from __future__ import unicode_literals
+
+import argparse
+import pipes
+import sys
+
+
+def check_has_shebang(path):
+    with open(path, 'rb') as f:
+        first_bytes = f.read(2)
+
+    if first_bytes != b'#!':
+        print(
+            '{path}: marked executable but has no (or invalid) shebang!\n'
+            "  If it isn't supposed to be executable, try: chmod -x {quoted}\n"
+            '  If it is supposed to be executable, double-check its shebang.'.format(
+                path=path,
+                quoted=pipes.quote(path),
+            ),
+            file=sys.stderr,
+        )
+        return 1
+    else:
+        return 0
+
+
+def main(argv=None):
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument('filenames', nargs='*')
+    args = parser.parse_args(argv)
+
+    retv = 0
+
+    for filename in args.filenames:
+        retv |= check_has_shebang(filename)
+
+    return retv