How to run a cron script every 30 minutes

To run a cron script every 30 minutes, specify minute intervals of both on the hour and 30 minutes past the hour separated by a comma.

0,30 * * * * wget http://dfbills.com > /dev/null

And to suppress email notifications:

0,30 * * * * wget http://dfbills.com >> /dev/null 2>&1

The >> /dev/null 2>&1 part sends the standard output to /dev/null and redirects standard error (2) to the same place as standard output (1).