Showing the "database" doesn't (normally) work with libpq / pgsql...

# sudo -u zabbix  psql "host=/var/run/postgresql user=zabbix dbname=" -c 'select 1'
 ?column?
----------
        1

# sudo -u zabbix psql "host=/var/run/postgresql user=zabbix database=" -c 'select 1'
psql: invalid connection option "database"

If the socket file is not there...

Connection failed: failed to connect to `host=/var/run/postgresql user=zabbix database=postgres`: dial error (dial unix /var/run/postg    resql/.s.PGSQL.5432: connect: no such file or directory).