Add check for executability of scripts with shebangs

Closes https://github.com/pre-commit/pre-commit-hooks/issues/543

pre_commit_hooks/check_shebang_scripts_are_executable.py

@@ -0,0 +1,53 @@
+"""Check that text files with a shebang are executable."""
+import argparse
+import shlex
+import sys
+from typing import List
+from typing import Optional
+from typing import Sequence
+from typing import Set
+
+from pre_commit_hooks.check_executables_have_shebangs import EXECUTABLE_VALUES
+from pre_commit_hooks.check_executables_have_shebangs import git_ls_files
+from pre_commit_hooks.check_executables_have_shebangs import has_shebang
+
+
+def check_shebangs(paths: List[str]) -> int:
+    # Cannot optimize on non-executability here if we intend this check to
+    # work on win32 -- and that's where problems caused by non-executability
+    # (elsewhere) are most likely to arise from.
+    return _check_git_filemode(paths)
+
+
+def _check_git_filemode(paths: Sequence[str]) -> int:
+    seen: Set[str] = set()
+    for ls_file in git_ls_files(paths):
+        is_executable = any(b in EXECUTABLE_VALUES for b in ls_file.mode[-3:])
+        if not is_executable and has_shebang(ls_file.filename):
+            _message(ls_file.filename)
+            seen.add(ls_file.filename)
+
+    return int(bool(seen))
+
+
+def _message(path: str) -> None:
+    print(
+        f'{path}: has a shebang but is not marked executable!\n'
+        f'  If it is supposed to be executable, try: '
+        f'`chmod +x {shlex.quote(path)}`\n'
+        f'  If it not supposed to be executable, double-check its shebang '
+        f'is wanted.\n',
+        file=sys.stderr,
+    )
+
+
+def main(argv: Optional[Sequence[str]] = None) -> int:
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument('filenames', nargs='*')
+    args = parser.parse_args(argv)
+
+    return check_shebangs(args.filenames)
+
+
+if __name__ == '__main__':
+    exit(main())