s6-envuidgid sets the UID and GID environment variables, then executes into another program.
s6-envuidgid account prog...
s6-envuidgid behaves exactly like envuidgid except for the exit code if account does not exist.
s6-envuidgid is useful when running a program that must start as root but can drop its privileges later. Such a program can read its new uid/gid info from the UID and GID environment variables.