Specify and use inferior shell only once

Question

I know you can customize settings or set environmental variables to get emacs to use other shells in shell-mode (1, 2). Is it possible to simply start a shell specifying which shell you want to use interactively?

Like: M-x other-shell RET /path/to/csh RET

Or set up something in your init file to let you use other shells fairly easily without changing the default?

Answer 1

This appears to do the job:

(defun other-shell (explicit-shell-file-name)
  (interactive "f")
  (if (called-interactively-p)
      (call-interactively #'shell)
    (shell)))

This relies on shell using the variable explicit-shell-file-name as a first choice of the shell to be used.

It also relies on not using lexical scoping.

Answer 2

Building on Harald's answer. If you want to specify the shell by name (interactive "s" option) rather than path and use the default shell-file-name instead of the explicit path:

(defun other-shell (shell-file-name)
  (interactive "s")
  (if (called-interactively-p)
      (call-interactively #'shell)
    (shell)))

I found this to be more useful when using a shell like xonsh where I just want to use the one in the virtualenv that started emacs, rather than specify long explicit paths.