How to list users in a postgreSQL database

Use the psql meta-command "\du" or "\du+" to list the users (or roles as they are called in PostgreSQL):

psql
psql (11.2)
Type "help" for help.

postgres=# \du
                                            List of roles
     Role name     |                         Attributes                         |      Member of
-------------------+------------------------------------------------------------+---------------------
 music             | Create DB                                                 +| {}
                   | Password valid until 2019-02-10 00:00:00+01                |
 postgres          | Superuser, Create role, Create DB, Replication, Bypass RLS+| {}
                   | Password valid until 2019-01-16 00:00:00+01                |
 resource          | Cannot login                                               | {}
 superuser         | Superuser                                                 +| {}
                   | Password valid until 2019-03-02 00:00:00+01                |
 vegard            | Password valid until 2019-03-02 00:00:00+01                | {}

Sources: List schemas and List users
from dataedo.com