scheduling tasks

at

at allows you to run commands once at a different chronological location than when you type them. for instance you can run:

$ at 11:50
and it will open a terminal that will allow you to type in a command that will be executed at 11:50 

crontab

crontab allows you to run a repeating command at a different time or date every time that time or date happens. to do this you have to use the command:
$ crontab -e
this allows you to edit the crontab. then you can schedule tasks by typing in the time code then the command. the time codes are formatted like this:

minute hour day week month year

so for instance Wednesday march 16th at 7:30 at night would be:

30 19 16 3 3

for any random unit of time you use a *. so for say every wednesday at 7:00 AM you would use:

0 7 * * 3

now how do you find out what cron tabs are running?  all you have to do is use:

$ crontab -l