I got an error about dbpath (/data/db/) does not exist, but /etc/mongodb.conf named it dbpath = /var/lib/mongodb.
So, which is the de
The default dbpath for mongodb is /data/db.
There is no default config file, so you will either need to specify this when starting mongod with:
mongod --config /etc/mongodb.conf
.. or use a packaged install of MongoDB (such as for Redhat or Debian/Ubuntu) which will include a config file path in the service definition.
Note: to check the dbpath and command-line options for a running mongod, connect via the mongo shell and run:
db.serverCmdLineOpts()
In particular, if a custom dbpath is set it will be the value of:
db.serverCmdLineOpts().parsed.dbpath // MongoDB 2.4 and older
db.serverCmdLineOpts().parsed.storage.dbPath // MongoDB 2.6+