This is mostly a note for myself, but maybe other people will also find it useful.
There are two types of jobs I'm going to talk about here. They are jobs to be run at regular intervals and jobs to be run periodically. Regular intervals means every x unit of time. Maybe every 4 days. Maybe every half hour. Periodic means once a week or once a day.
Let's say you download your email with the command
do_email and you want to
do this every 30 minutes. Fire up
fcrontab -e and put in this entry:
@ 30 do_email
The @ sign says this is an interval based command and 30 is the time interval in minutes. You can specify more precisely by using multipliers of m, w, d, h, and s for months, weeks, days, hours, and seconds, respectively. For example
@ 2h05 do_email
will download your email every 2 hours and 5 minutes.
Let's say that you want a command to be run in some regular interval. Maybe
you want to download podcasts once a week. If you use the command
get_podcasts to download your podcasts you could use this entry:
%weekly * 9-17 get_podcasts
This will run the command get_podcasts once a week at any minute and sometime between 9am and 5pm. A "week" starts on Monday and goes to Sunday so fcron will try to do this Monday between 9 and 5. If your computer is on, it will try again on Tuesday.
Lot's more good stuff at http://fcron.free.fr/doc/en/fcrontab.5.html