I know that this is probably an edge case, but it might make sense to have a ghost update global (or all) command in order to update multiple ghost instances in the same directory/folder, similar with how ghost ls works (maybe).
Of course this can be easily done with a simple script, like:
for d in ./*/ ; do (cd "$d/public/" && ghost update); done
(this is the case for a series of Ghost sites all hosted on a Laravel Forge server).
For a generic installation this could be:
for d in /var/www/*/ ; do (cd "$d" && ghost update); done
Cheers! 