su - postgres

su - postgres && psql
psql (10.12 (Ubuntu 10.12-0ubuntu0.18.04.1))
Type "help" for help.
postgres=#

See also

• PostgreSQL: psql, .psqlrc, .pgpass, $HOME/.pg service.conf, -U, -W, -d, meta-commands: \du, \dg, \ddp, \dp, \list, \dn, \dt, \c, \l, \di, \q, show users, show, su - postgres, psql -c, \?, \connect, \conninfo, --version, --help, PL/pgSQL