Linux has a powerful task scheduler called Cron. Cron will allow you to run commands automatically at times specified by you. Cron is similar to
task scheduler you find in Windows. To keep track of schedules and tasks it has to run, Cron requires a file called Crontab (CRON TABle). All Cron schedules and tasks should be stored in this table. The Crontab files cannot be directly edited. You can add or delete entries in crontab file using crontab command.What's Cron and Crontab ?
You must be wondering what difference between cron and crontab or wether they are same. Cron is a process or program which wakes up every minute and looks for jobs it has to execute at that moment. Crontab is list of jobs and times at which they have to execute.
Crontab Format:
Each entry in Crontab has at least 6 fields separated by a single space. Field 1 Minute Range of Values : 0-59 Field 2 Hour Range of Values : 0-23 Field 3 Day Range of Values : 1-31 Field 4 Month Range of Values : 1-12 Field 5 Day of week Range of Values : 0-6 (Sunday being 0) Field 6 Command to Execute
Now let's see how to make a crontab entry. Let's say you want to run a script backup.sh every day at 6:00pm.The entry would look like this:
0 18 * * * /home/user/backup.sh
The asterisk (*) is used to indicate that every instance of particular time period will be used (i.e. every hour, every weekday, etc.). I've used to full path to script /home/user/backup.sh instead of just using backup.sh. This is because cron runs as root, you should fully qualify your path names to any scripts that will be run. Let's see some more examples :
* Let's run script printinvoices.sh every sunday at 12:45pm.
45 12 * * 0 /home/account/printinvoices.sh
* How about clearaccount.sh every month beginning at 1:32am ?
