Add a checker for executables without shebangs

tests/check_executables_have_shebangs_test.py

@@ -0,0 +1,35 @@
+# -*- coding: utf-8 -*-
+from __future__ import absolute_import
+from __future__ import unicode_literals
+
+import pytest
+
+from pre_commit_hooks.check_executables_have_shebangs import main
+
+
+@pytest.mark.parametrize('content', (
+    b'#!/bin/bash\nhello world\n',
+    b'#!/usr/bin/env python3.6',
+    b'#!python',
+    '#!☃'.encode('UTF-8'),
+))
+def test_has_shebang(content, tmpdir):
+    path = tmpdir.join('path')
+    path.write(content, 'wb')
+    assert main((path.strpath,)) == 0
+
+
+@pytest.mark.parametrize('content', (
+    b'',
+    b' #!python\n',
+    b'\n#!python\n',
+    b'python\n',
+    '☃'.encode('UTF-8'),
+
+))
+def test_bad_shebang(content, tmpdir, capsys):
+    path = tmpdir.join('path')
+    path.write(content, 'wb')
+    assert main((path.strpath,)) == 1
+    _, stderr = capsys.readouterr()
+    assert stderr.startswith('{}: marked executable but'.format(path.strpath))