hmmm, let's see. You haven't specified PostgreSQL version or O/S as is common sense and courtesy, so I will choose one for you.
You are using PostgreSQL version 8.4 on Ubuntu 14.04
Since pg_dump requires an output file, and the database you are dumping must be known, just just the db name in the path.
eg: pg_dump unknown_db > /dumpdir/unknown_db/whatever_filename_you_want.dmp
Then you can get the db name by lookiing at the directory you dumped to! At least that is what most dba's would do.