>  (defcustom org-src-lang-modes
> -  '(("C" . c)
> +  `(("C" . c)
>      ("C++" . c++)
>      ("asymptote" . asy)
> -    ("bash" . sh)
>      ("beamer" . latex)
>      ("calc" . fundamental)
>      ("cpp" . c++)
> @@ -208,9 +215,10 @@ but which mess up the display of a snippet in Org exported files.")
>      ("elisp" . emacs-lisp)
>      ("ocaml" . tuareg)
>      ("screen" . shell-script)
> -    ("shell" . sh)
>      ("sqlite" . sql)
> -    ("toml" . conf-toml))
> +    ("toml" . conf-toml)
> +    ("shell" . sh)
> +    ,@(org-src--get-known-shells))
>    "Alist mapping languages to their major mode.

Side note: while it really doesn't matter here for such trivial
top-level code, I prefer to put such ,@ at the beginning rather than the
end of lists, when it's an option.  Basically because it's more
efficient to add to the beginning rather than to the end of a list:

    ELISP> (macroexpand '`(,@(list 1 2) a b c d))
    (append (list 1 2) '(a b c d))
    ELISP> (macroexpand '`(a b c d ,@(list 1 2)))
    (cons 'a (cons 'b (cons 'c (cons 'd (list 1 2)))))
    ELISP>