bug#15539: Setting user-emacs-directory at command line invocation

["François Févotte" <francois.fevotte@ensta.org>]

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Hello,

attached is a patch which tries to implement the desired feature. It should
apply cleanly atop master (b91eafe31a524b391d5cec079cf8f36c2f9d5f30)

With this patch, emacs accepts a new command line argument:
    --user-emacs-directory=DIR
which has two effects:
1. it sets the `user-emacs-directory' variable to DIR (instead of the
default "~/.emacs.d")
2. it looks for the init file in DIR/init.el (and only there: ~/.emacs & co
are bypassed)

This doesn't impact anything else in emacs' startup sequence.

Implementationwise, I'm not very proud of having to define a new global
variable, but I fail to see how to do otherwise, except maybe wrapping the
whole `command-line' function in a let form to use a local binding.

Please do not hesitate to criticize or ask me for any modification which
would be desirable. This is the first patch I propose to emacs; I don't
expect to have it right on the first try.

Thanks in advance,

    François

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From cb6e52b3cd1bb6ff23a5c89b9838169ea216cf9b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Fran=C3=A7ois=20F=C3=A9votte?= <fevotte@gmail.com>
Date: Fri, 13 Mar 2015 15:01:02 +0100
Subject: [PATCH] Add a `--user-emacs-directory' command-line option

This sets `user-emacs-directory' and looks for `init.el' in it.

Fixes: bug#15539
---
 doc/emacs/cmdargs.texi |   10 +++++++++-
 lisp/startup.el        |   16 +++++++++++++---
 lisp/subr.el           |    4 ++++
 3 files changed, 26 insertions(+), 4 deletions(-)

diff --git a/doc/emacs/cmdargs.texi b/doc/emacs/cmdargs.texi
index 42c8e33..be83aaa 100644
--- a/doc/emacs/cmdargs.texi
+++ b/doc/emacs/cmdargs.texi
@@ -349,7 +349,15 @@ Do not reload any saved desktop.  @xref{Saving Emacs Sessions}.
 @opindex --user
 @cindex load init file of another user
 Load @var{user}'s initialization file instead of your
-own@footnote{This option has no effect on MS-Windows.}.
+own@footnote{This option has no effect on MS-Windows, nor when
+@samp{--user-emacs-directory} is used}.
+
+@item --user-emacs-directory=@var{dir}
+@opindex --user-emacs-directory
+Look for the initialization file in @var{dir}, instead of the default
+@file{~/.emacs.d/}; @pxref{Init File}.  @var{dir} will also be used as
+the default directory where user-specific configuration data will be
+stored; @xref{Standard File Names,,, elisp}.
 
 @item --debug-init
 @opindex --debug-init
diff --git a/lisp/startup.el b/lisp/startup.el
index 999e53e..6f25c65 100644
--- a/lisp/startup.el
+++ b/lisp/startup.el
@@ -877,7 +877,7 @@ please check its value")
     (while (and (not done) args)
       (let* ((longopts '(("--no-init-file") ("--no-site-file") ("--debug-init")
                          ("--user") ("--iconic") ("--icon-type") ("--quick")
-			 ("--no-blinking-cursor") ("--basic-display")))
+			 ("--no-blinking-cursor") ("--basic-display") ("--user-emacs-directory")))
              (argi (pop args))
              (orig-argi argi)
              argval)
@@ -924,6 +924,10 @@ please check its value")
 	  (push '(visibility . icon) initial-frame-alist))
 	 ((member argi '("-nbc" "-no-blinking-cursor"))
 	  (setq no-blinking-cursor t))
+	 ((equal argi "-user-emacs-directory")
+	  (setq user-emacs-directory (file-name-as-directory (or argval (pop args)))
+		user-emacs-directory--set t
+		argval nil))
 	 ;; Push the popped arg back on the list of arguments.
 	 (t
           (push argi args)
@@ -1102,7 +1106,11 @@ please check its value")
 		      ;; This tells `load' to store the file name found
 		      ;; into user-init-file.
 		      (setq user-init-file t)
-		      (load user-init-file-1 t t)
+
+		      ;; Don't try to read `~/.emacs' if the user provided a
+		      ;; custom `user-emacs-directory' in the command-line.
+		      (unless user-emacs-directory--set
+			(load user-init-file-1 t t))
 
 		      (when (eq user-init-file t)
 			;; If we did not find ~/.emacs, try
@@ -1111,7 +1119,9 @@ please check its value")
 			       (expand-file-name
 				"init"
 				(file-name-as-directory
-				 (concat "~" init-file-user "/.emacs.d")))))
+				 (if user-emacs-directory--set
+				     user-emacs-directory
+				   (concat "~" init-file-user "/.emacs.d"))))))
 			  (load otherfile t t)
 
 			  ;; If we did not find the user's init file,
diff --git a/lisp/subr.el b/lisp/subr.el
index deadca6..c5c81b7 100644
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -2545,6 +2545,10 @@ mode.")
 Various programs in Emacs store information in this directory.
 Note that this should end with a directory separator.
 See also `locate-user-emacs-file'.")
+
+(defvar user-emacs-directory--set nil
+  "Non-nil if the user provided a custom value for `user-emacs-directory'.
+This can be done via the `--user-emacs-directory' command-line switch.")
 \f
 ;;;; Misc. useful functions.
 
-- 
1.7.2.5