Before using the Bitcoin Core daemon, bitcoind, you need to create its configuration file with a user name and password. First create the .bitcoin directory, create (touch) the file, and set the file’s permissions so that only your user account can read it. From your terminal or console, type:

mkdir ~/.bitcoin
touch ~/.bitcoin/bitcoin.conf
chmod 600 ~/.bitcoin/bitcoin.conf

Then you can run the command bitcoind. It will print output similar to this:

bitcoind
Error: To use the "-server" option, you must set a rpcpassword in the configuration file:
/home/bitcoinorg/.bitcoin/bitcoin.conf
It is recommended you use the following random password:
rpcuser=bitcoinrpc
rpcpassword=XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
(you do not need to remember this password)
The username and password MUST NOT be the same.
If the file does not exist, create it with owner-readable-only file permissions.
It is also recommended to set alertnotify so you are notified of problems;
for example: alertnotify=echo %s | mail -s "Bitcoin Alert" admin@foo.com