Outdated. Please go to
http://yong321.freeshell.org/computer/CronJobs.html



* Once in a while save crontab entries to a file: crontab -l > somefile. Or create a cron job to 
save crontab to a file.

* Make sure crontab command is not accidentally followed by a valid filename, including . 
(representing current directory) and .. (parent directory), or the crontab would be replaced with a 
blank table.

* Since e and r are close on the keyboard, every time after you type crontab -e, pause and check 
it's not -r before you hit Enter. If crontab is wiped out, restore from saved backup. Check against 
the latest actual cron job running times stored in /var/log/cron (text file readable by root).

* Even if your script does logging, always redirect stdout and stderr to some file instead of not 
redirecting (and you end up having lots of mails you rarely bother to read and clean) or 
redirecting to /dev/null (and you lose valuable debugging info whenever there's any): 
/full_path_to_script [optional arguments] > /tmp/scriptname.out 2>&1

* Group similar cron jobs in crontab, with each group preceded by a comment line, optionally with 
each job preceded by a job-specific comment line