[mongodb] how to set mongod --dbpath

You could also configure mongod to run on start up so that it is automatically running on start up and the dbpath is set upon configuration. To do this try:

mongod --smallfiles --config /etc/mongod.conf

The --smallfiles tag is there in case you get an error with size. It is, of course, optional. Doing this should solve your problem while also automating your mongodb setup.