How to set a cron job to run every 3 hours [duplicate]
Change Minute to be 0. That's it :)
Note: you can check your "crons" in http://cronchecker.net/
Example
Change Minute parameter to 0.
You can set the cron for every three hours as:
0 */3 * * * your command here ..
The unix setup should be like the following:
0 */3 * * * sh cron/update_old_citations.sh
good reference for how to set various settings in cron at: http://www.thegeekstuff.com/2011/07/cron-every-5-minutes/