Cron Useful Documentation

Cron is not too hard to use but if you stay away from it for a while or if  you jump from one Linux flavor to another (not mentioning UNIX and Mac), there is a big change that you are going to run into issues that you cannot really explain. This post highlights the most common
scenarios that you are likely to run into.

Default location of Cron Files
/var/spool/cron/crontabs
use the "> /dev/null 2>&1" (do not use the double quotes) to redirect error messages to void. Use it to disable email on error.
/3 ****  php -f ycsoftware.php (repeat every three minutes)
(the forward slash in a cron job does not work in all Linux flavors)
if your cron job is not working chances are that you do not have permission to run the cron job. Check and make sure that you are not denied access by locating the con.deny on your system. Also if you are not listed on cron.deny make sure that you are added to cron.allow file if that file is not empty.
Make sure that collect the result in a log file for debugging purposes for example
3 ****  php -f ycsoftware.php > /home/ycsoftware/mycrons/ycsoftware.log 2>&1
All asterisk in a cron job :
* * * * *   php -f ycsoftware.php
means every minute of everyday of every week of every month.
for more info just check the manual
man crontab
or man cron

Quick Reference
http://en.wikipedia.org/wiki/Cron

Others
http://adminschoice.com/crontab-quick-reference

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