Re-open *scratch* buffer in Emacs?

Question

If I accidentally closed the scratch buffer in Emacs, how do I create a new scratch buffer?

Answer 1

I have combined the solutions posted so far into one function:

(defun --scratch-buffer(&optional reset)
  "Get the *scratch* buffer object.
Make new scratch buffer unless it exists. 
If RESET is non-nil arrange it that it can't be killed."
  (let ((R (get-buffer "*scratch*")))
    (unless R
      (message "Creating new *scratch* buffer")
      (setq R (get-buffer-create "*scratch*") reset t))
        (when reset
          (save-excursion
            (set-buffer R)
            (lisp-interaction-mode)
            (make-local-variable 'kill-buffer-query-functions)
            (add-hook 'kill-buffer-query-functions '(lambda()(bury-buffer) nil)
          )))
    R))

To apply this function in your .emacs use:

(--scratch-buffer t)
(run-with-idle-timer 3 t '--scratch-buffer)

This will make the scratch buffer indestructible in the first place, and if saved it will be recreated. Additionally we can use a shortcut function scratch to bring up the buffer quickly:

(defun scratch()
  "Switch to *scratch*.  With prefix-arg delete its contents."
  (interactive)
  (switch-to-buffer (--scratch-buffer))
  (if current-prefix-arg
      (delete-region (point-min) (point-max))
    (goto-char (point-max))))

In the past it has proven useful to know the original startup-directory from which Emacs was started. This is either the value of desktop-dirname or the default-directory local variable of the scratch-buffer:

(defvar --scratch-directory
  (save-excursion (set-buffer "*scratch*") default-directory)
  "The `default-directory' local variable of the *scratch* buffer.")

(defconst --no-desktop (member "--no-desktop" command-line-args)
  "True when no desktop file is loaded (--no-desktop command-line switch set).")

(defun --startup-directory ()
  "Return directory from which Emacs was started: `desktop-dirname' or the `--scratch-directory'.
Note also `default-minibuffer-frame'."
  (if (and (not --no-desktop) desktop-dirname) 
      desktop-dirname
    --scratch-directory))

So --startup-directory will always return the base directory of your makefile, TODO-file etc. In case there is no desktop (--no-desktop commandline-switch or no desktop-file) the --scratch-directory variable will hold directory Emacs was once started under.