Cron jobs

Cron job is a time based scheduling service in Unix-like operating systems. The schedules modified in crontab and executed by a daemon, crond, which runs constantly in the background and checks once a minute to see if any of the scheduled jobs need to be executed.

Example:

The following will run jar file at one minute past midnight each day.

01 00 * * * echo "" > java -jar MyApplication.jar param1

Fields:

# .---------------- minute (0 - 59)
# |  .------------- hour (0 - 23)
# |  |  .---------- day of month (1 - 31)
# |  |  |  .------- month (1 - 12) OR jan,feb,mar,apr ...
# |  |  |  |  .---- day of week (0 - 6) (Sunday=0 or 7)  OR sun,mon,tue,wed,thu,fri,sat
# |  |  |  |  |
*  *  *  *  *  command to be executed

Examples:

01 * * * * root echo "This command is run at one min past every hour"
17 8 * * * root echo "This command is run daily at 8:17 am"
17 20 * * * root echo "This command is run daily at 8:17 pm"
00 4 * * 0 root echo "This command is run at 4 am every Sunday"
* 4 * * Sun root echo "So is this"
42 4 1 * * root echo "This command is run 4:42 am every 1st of the month"
01 * 19 07 * root echo "This command is run hourly on the 19th of July"


References:
http://en.wikipedia.org/wiki/Cron
http://www.unixgeeks.org/security/newbie/unix/cron-1.html