Hi Milan,
For automatic backups, you can use:
* 0 * * * cd [PATH TO ERPNEXT INSTALLATION] && python lib/wnf.py --backup >> /var/log/erpnext-backup.log 2>&1
For now, you will need to manually clear old backups or write a script to do that, so that space doesn't run out.
Using the above crontab entry, the backups will be taken at midnight every day and will be available in public/backups folder.
Thanks,
Anand.
PS.
For future questions related to development / setup, please post your questions at
erpnext-dev...@googlegroups.com -- we don't want to burden people in this group who are here for functional queries.