bash one-liner: set variable to output of command or to default value if output is empty

Question

I know that in Bash you can use the syntax

editor=${new_editor:-/usr/bin/vi}

which will set editor to new_editor if the variable new_editor is not empty and to /usr/bin/vi otherwise. Is it possible to produce similar one-liner, which will set editor to output of some command and to some default value if the output is empty? Something like (only as example, this does not work)

editor=$( $(which emacs) :-/usr/bin/vi )

I know how to do it in several lines of code, but would like to know if an elegant solution exists.

Answer 1

POSIXLY:

$ : "${editor:=$(command -v emacs)}" "${editor:=/usr/bin/vi}"
$ printf '%s\n' "$editor"
emacs

---

You can't do nested parameter expansion in any Bourne-like shells but zsh:

$ editor=${$(whence -p emacs):-/usr/bin/vi}
$ print -rl -- $editor
/usr/bin/emacs

or:

editor=${commands[emacs]-$commands[vi]}