Set a global default directory

Question

I want a "default directory" which is constant across all contexts (buffers, packages, etc). That is, when working on multiple files, opening a new terminal, doing anything which requires a starting directory, I want that starting directory to always be the same.

How can I set a global default directory without having to write an arbitrary number of wrapper functions?

---

To illustrate the problem, suppose my project has the following structure:

.
└───my-project
    │   bar.py
    │   foo.py
    │
    └───tests
            test_bar.py
            test_foo.py

If I'm working on test_foo.py and I do find-file (C-x C-f) to open foo.py, the prompt will start at my-project/tests. I have to go up a level. Now suppose I'm editing foo.py and I want to look at test_bar.py. When I do find-file within the foo.py buffer, the prompt will be at my-project. I need to navigate to tests. Similar things happen when using eshell and other applications which have a default directory.

According to this, this, and this, the starting directory is handled by default-directory. They all indicate that to change the starting directory, you need to do something like,

(setq default-directory "/my/default/path")

or

(cd "/my/default/path")

The trouble is, default-directory is buffer local. You need to constantly re-assign default-directory.

In the case of find-file, I overcome this by creating a wrapper which changes the directory to the default before calling find-file. I then reassign the keybinding:

(defun my-set-global-default-directory (new-default-directory)
  "Set my-global-default-directory to NEW-DEFAULT-DIRECTORY."
  (interactive "DNew global default directory: ")
  (setq my-global-default-directory new-default-directory))

(defun my-find-file ()
  (interactive)
  (cd my-global-default-directory)
  (call-interactively 'find-file))

(global-set-key (kbd "C-x C-f") 'my-find-file)

This works for find-file, but for this to work universally, I would need to manually wrap any function that uses default-directory!

Answer 1

The code below uses a new feature in 26.1 called variable-watcher. It allows a function to observe changes in some variable's value. But more or less the same could be achieved by using hooks and/or advices covering all standard functions which are changing the directory (e.g. find-file-hook).

There may be unforeseen consequences to this.

;; -*- lexical-binding: t -*-

(defun run-with-hook (hook fn &optional append local)
  "Like `add-hook', but runs only once."
  (declare (indent 1))
  (let* ((fname (make-symbol "run-with-hook"))
         (fn `(lambda (&rest args)
                (unwind-protect
                    (apply (function ,fn) args)
                  (remove-hook ',hook ',fname ,local)))))

    (fset fname fn)
    (add-hook hook fname append local)))

(defvar global-default-directory (expand-file-name "~"))

(defun global-default-directory-watcher (_symbol _newval op where)
  (when (and (eq op 'set)
             (buffer-live-p where))
    (run-with-hook 'post-command-hook
      (lambda ()
        (when (buffer-live-p where)
          (with-current-buffer where
            (cd global-default-directory)))))))

(define-minor-mode global-default-directory-mode
  "Use a global `default-directory' value in every buffer."
  :global t
  (cond
   (global-default-directory-mode
    (setq-default default-directory global-default-directory)
    (dolist (buffer (buffer-list))
      (with-current-buffer buffer
        (when (local-variable-p 'default-directory)
          (cd global-default-directory))))
    (add-variable-watcher 'default-directory
                          #'global-default-directory-watcher))
   (t
    (remove-variable-watcher 'default-directory
                             #'global-default-directory-watcher))))

Answer 2

I am using this setting in my ~/.emacs.d/init.el file to set $FOO as my default directory:

(add-hook 'find-file-hook
          (lambda() (setq default-directory (substitute-in-file-name "$FOO/"))))