It's not much a guide, but it's here!
command using a Bourne-compatible shell, returns (values output error-output exit-status).
command can be a full path to a shell executable binary or just its name. In the later case, the variable
*shell-search-paths* will be used to find the executable.
Depending on the implementation, the variable
*bourne-compatible-shell* may be used to find a shell to use in executing