If you meant to start automatically each time the node / raspberry starts then take a look to the StartForging add-on.
From the changelog:
Quote
The StartForging add-on reads the passphrases stored in a plain text file, as
defined in the property nxt.startForgingFile, one passphrase per line, and
automatically starts forging with each of these accounts when the server is
started. Since passphrases are stored unencrypted, this add-on should only
be used on well secured servers, or with accounts of insignificant value, or
in test environments.