(cl-defun fish-prompt (&optional (filename default-directory))
  "A simple prompt with a fish-like abbreviation of `$PWD'."
  (let ((cwd (or (file-remote-p filename 'localname) filename)))
    (concat
     "\f\n"
     (file-remote-p cwd)
     (replace-regexp-in-string
      "[^./][^/]*/"
      (lambda (match) (string (aref match 0) ?/))
      (substring (abbreviate-file-name cwd) 0 -1))
     ;; Add slash if it was the only character in the filename.
     (when (file-equal-p "/" cwd) "/")
     (if (zerop (user-uid)) " # " " $ "))))