Cron> is a time-based scheduling service in a Linux or Unix-like computer operating systems. Cron job are used to schedule commands to be executed periodically. You can setup commands or scripts, which will repeatedly run at a set time. Cron is one of the most useful tool in Linux or UNIX like operating systems. The cron service (daemon) runs in the background and constantly checks the /etc/crontab file, /etc/cron.*/ directories. It also checks the /var/spool/cron/ directory.
First, Login to UNIX/Linux system.
Type the following command to enter cronjob:
$ crontab -e
Each cronjob has the following syntax:
# +---------------- minute (0 - 59) # | +------------- hour (0 - 23) # | | +---------- day of month (1 - 31) # | | | +------- month (1 - 12) # | | | | +---- day of week (0 - 6) (Sunday=0 or 7) # | | | | | * * * * * command to be executed
To get crontab to run a task every 10 minutes you could type as follow:
*/10 * * * * /path/to/command
*/10 * * * * /path/to/script
Save and close the file.
- The asterisk (*) operator specifies all possible values for a field. For example, an asterisk in the hour time field would be equivalent to every hour or an asterisk in the month field would be equivalent to every month.
- The */10 is used in conjunction with ranges. For example, 0-23/2 can be used in the hours field to specify command execution every other hour. Steps are also permitted after an asterisk, so if you want to say every two hours just use */2. In this example, */10 in the minutes field to specify command execution every 10 minute.
- HowTo: Add Jobs To cron Under Linux or UNIX
- Man pages: cron(8)