The s6-envuidgid program

s6-envuidgid sets the UID, GID and GIDLIST environment variables, then executes into another program.

Interface

     s6-envuidgid account prog...

s6-envuidgid looks account up by name in the account database.
If account is unknown, it exits 1.
It sets the UID environment variable to account's uid, and the GID environment variable to account's gid.
It also sets the GIDLIST environment variable to a comma-separated list of supplementary group ids account is a member of according to the group database. (If account doesn't belong to any other group than its primary group, GIDLIST is still set, but empty.)
Then it executes into prog....

Notes

s6-envuidgid behaves like envuidgid, except that:

it also handles supplementary groups
It exits 1 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/groups info from the UID, GID and GIDLIST environment variables. Superservers such as s6-tcpserver4 make use of this.