Emacs dotfile for loading an Emacs configuration based on EMACSDIR environment variable

.emacs

;; load configuration from $EMACSDIR/init.el and set $EMACSDIR as
;; `user-emacs-directory'. if $EMACSDIR is not set, use `~/.emacs.d'.
(let* ((emacs-dir-env (getenv "EMACSDIR"))
       (custom-emacs-dir (and emacs-dir-env
                              (file-name-as-directory emacs-dir-env))))
  ;; `user-emacs-directory' default value is emacs.d"
  (when custom-emacs-dir
    (setq user-emacs-directory custom-emacs-dir)))

(setq user-init-file (expand-file-name "init.el" user-emacs-directory))
(load user-init-file t)

start-emacs.bat

@echo off
set EMACSDIR=C:/path/to/emacs.d
set SPACEMACSDIR=C:/path/to/spacemacs.d
C:/path/to/emacs/bin/runemacs.exe %*

start-emacs.sh

#!/bin/bash
EMACSDIR=/path/to/emacs.d SPACEMACSDIR=/path/to/spacemacs.d emacs "$@"