Hello Julian,
crontab 101 very brief :
The crontab daemon loads an ascii text file with commands based on user and runs these commands.
crontab mycrontab.txt <enter> loads mycrontab.txt into crontab
Edit mycrontab and reload as above is the easy way to make changes, each load overwrites the previous cron job.
Assuming you are at a linux command prompt type: crontab -l <enter> [ that's a small L not an i ]
This will list your crontab jobs, if any. And you will find out if you are in the /etc/cron.allow file or the /etc/cron.deny.
man crontab -> will explain the mighty Cron , god of repetative tasks.
crontab -h -> Help
Search google for crontab should give you many guides to creating the ascii text file.
One thing to note , crontab creates mail for the user it runs under, filling up a mail box until read/deleted.
You can turn off mail with MAILTO='' [ 2 single quotes] or MAILTO=/dev/null at the top of the text file.
Always use full paths in a crontab text file or $HOME things :
# Send mail to no one
MAILTO=''
# run five minutes after midnight, every day
5 0 * * * /usr/bin/perl /home/bob/test.pl
See the many examples you will find on the web for details on the syntax for timing.
Hope this helps
kode