If you are coming from a MySQL background, SHOW DATABASES; will not work. Here are two different ways you can show PostgreSQL databases, using the command line:

The \l or \list command#

The psql command line interface, provides \l or \list commands, which presents a list of all available databases:

postgres=# \l

Good 'ol select#

Another way, is to just run a select against the pg_database system table:

postgres=# select * from pg_database;