$ sudo su -m postgres
postgres@baseubu1210dev:~$ psql
psql (9.1.7)
Type "help" for help.

postgres=#

postgres=# du
 Role name |                 List of roles Attributes       | Member of
-----------+------------------------------------------------
 admin     | Superuser, Create role, Create DB              | {}
 postgres  | Superuser, Create role, Create DB, Replication | {}

$ psql -U postgres
psql: FATAL:  Peer authentication failed for user "postgres"

psql -U admin -h localhost mydb