org-mode and babel export - setting tangled (exported) files as readonly

Question

I want to maintain a single org file which contained a plethora of different init files (.bashrc, .profile, .tmux etc that type of thing). Here's a portion:-

#+PROPERTY: header-args :tangle yes :comments no

* bashrc
  :PROPERTIES:
  :header-args+: :tangle "./DotFiles/.bashrc"
  :END:
** bashrc 1
   #+begin_src shell
     # -*- mode: read-only -*-
     # ~/.bashrc: executed by bash(1) for non-login shells.
     # see /usr/share/doc/bash/examples/startup-files (in the package bash-doc)
     # for examples

Now when I tangle (C-c C-v t), the bashrc is correctly tangled and written to ./DotFiles/.bashrc. Notice the first line inside the #+begin_src :-

 # -*- mode: read-only -*-

Now, this works in that when I open that .bashrc in emacs then it's readonly, a subtle reminder that I need to edit the master org-file.

Currently I need to add that to each and every dotfile section I have in my master org file. Is there a way to specify a global "first line" in each exported file? I would have thought a

header-args: :comments "# -*- mode: read-only -*-"

would do that but alas not. Is there cleaner way to do this than the way I'm currently doing it?

Answer 1

#+PROPERTY: header-args:conf :tangle-mode (identity #o444)

Sets the files as read-only (not just for emacs, but for any editor). Then, each block can be:

#+BEGIN_SRC conf :tangle ~/.file_rc
#+END_SRC

Repeat for bash and other specific languages if you want them in BEGIN_SRC bash blocks rather than conf blocks.

Tested on Org mode version 9.3.7 (9.3.7-18-g093b47-elpaplus @ ~/.emacs.d/elpa/org-plus-contrib-20200810/)

Answer 2

Define a function to add the string and add it to org-babel-tangle-body-hook:

#+PROPERTY: header-args :tangle yes :comments no

* bashrc
  :PROPERTIES:
  :header-args+: :tangle "./DotFiles/.bashrc"
  :END:
** bashrc 1
   #+begin_src shell
     # ~/.bashrc: executed by bash(1) for non-login shells.
     # see /usr/share/doc/bash/examples/startup-files (in the package bash-doc)
     # for examples
   #+end_src


* Code

#+begin_src emacs-lisp
  (defun ndk/add-read-only ()
    (goto-char (point-min))
    (insert "# -*- mode: read-only -*-\n\n"))

 (add-hook 'org-babel-tangle-body-hook #'ndk/add-read-only)
#+end_src

#+begin_src emacs-lisp
  (remove-hook 'org-babel-tangle-body-hook #'ndk/add-read-only)
#+end_src

You will need to remove the function from the hook afterwards, hence the second code block.

Answer 3

Use :noweb weaving to include common snippets of code between blocks

#+PROPERTY: header-args :tangle yes :comments no :noweb yes

#+NAME: common-mode-line
#+BEGIN_SRC shell :eval never :tangle no
  # -*- mode: read-only -*-
#+END_SRC

* bashrc
  :PROPERTIES:
  :header-args+: :tangle "./DotFiles/.bashrc"
  :END:
** bashrc 1
   #+begin_src shell
     <<common-mode-line>>
     # ~/.bashrc: executed by bash(1) for non-login shells.
     # see /usr/share/doc/bash/examples/startup-files (in the package bash-doc)
     # for examples
   #+end_src

If you want to get really fancy with metaprogramming, you can use :noweb features to include the results of code block instead.

e.g.

#+NAME: common-mode-line-meta
#+BEGIN_SRC shell :tangle no :var MODE_LINE="mode: read-only"
  echo "# -*- ${MODE_LINE}  -*-"
#+END_SRC


* bashrc
  :PROPERTIES:
  :header-args+: :tangle "./DotFiles/.bashrc-meta"
  :END:
** bashrc 1
   #+begin_src shell
     <<common-mode-line-meta()>>
     # ~/.bashrc: executed by bash(1) for non-login shells.
     # see /usr/share/doc/bash/examples/startup-files (in the package bash-doc)
     # for examples
   #+end_src

This would allow you to customize the mode by passing parameter via the noweb statement <<common-mode-line-meta(MODE_LINE="mode: Shell-script")>> which would output # -*- mode: Shell-script -*- at top of file instead.

I use results from custom code blocks like this to create data structures for yaml, json, ruby, python and perl files from org-tables for Literate DevOps.

Note: If you do not want to add noweb syntax to each code block by hand then just add common-mode-line-meta code block and then add :prologue (org-sbe common-mode-line-meta) to your header-args statement.

Thanks for asking your question!

---

This answer was tested using:
emacs version: GNU Emacs 25.2.1
org-mode version: 9.1.2

Answer 4

Not an answer per se, but the problem seems to be with tangling through a symlink. In my example above DotFiles was a symlink.

When I tangled directly to the "hard" file then the identity worked.

I've been unable to find out why this is but am happy I can work around it.

Update: 15/8/2020

The plot thickens. It's now apparent that the reason the identify "didn't work" is... Dropbox. It does indeed work but Dropbox switches it back to o644 almost immediately. I've been unable to find out why other than assuming Dropbox runs as user and must have user write permissions.