Subject: bug#49229: 27.2; `M-x shell' fails over TRAMP from local MS Windows

From: Michael Albinus <[email-masked]>
Date: Sun, 27 Jun 2021 10:41:52 +0200

Jim Porter <[email-masked]> writes:

Hi Jim,

>> I've pushed a fix to the master branches of Emacs and Tramp. Could you,
>> pls, check?
>
> I tested on MS Windows and it works correctly for me. (Note that I
> just copied and eval'ed the new version of `tramp-file-name-handler',
> since I don't have a proper build environment on MS Windows.)

Thanks for confirmation.

> If I erase the default text and instead enter
> "C:/Windows/System32/cmd.exe" into the `M-x shell' prompt, it treats
> *that* as a remote path too. Now, the prompt does say to enter a
> *remote* shell path, so if I enter a local path, I made a mistake.
> However, the default shell path for `M-x shell' from a remote
> directory is a TRAMP path ("/sshx:server:/path/to/some//bin/sh"), so
> it's surprising that when I delete the TRAMP host prefix, I still end
> up running a shell on the remote server.

`read-file-name' as used in `shell' just reads a file name, no matter
whether a local or remote one.

> Perhaps it would be nicer if, when `M-x shell' prompted for the remote
> shell path, it didn't include the TRAMP prefix by default (e.g. the
> default value would just be "/bin/sh"). That might not interact well
> with `read-file-name' completion though; is it possible to use
> file-name completion on a remote path without the TRAMP prefix?

No, that doesn't work. File name completion and alike wouldn't work any
more.

But we could teach `read-file-name' to accept only remote file
names. What about this patch:

diff --git a/lisp/shell.el b/lisp/shell.el
index 62de5be817..4339e8c0a3 100644
--- a/lisp/shell.el
+++ b/lisp/shell.el
@@ -759,7 +759,8 @@ shell
                  (file-local-name
                   (expand-file-name
                    (read-file-name "Remote shell path: " default-directory
-                                   shell-file-name t shell-file-name)))))
+                                   shell-file-name t shell-file-name
+                                   #'file-remote-p)))))

    ;; Rain or shine, BUFFER must be current by now.
    (unless (comint-check-proc buffer)

It is not perfect, one can still enter "/sudo::/bin/sh" when
`default-directory' is "/ssh::". But I wouldn't count this as mistake,
it would be rather a "user error by intention" :-)

Best regards, Michael.