Difference between revisions of "Cron entry"
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00 16 * * * root /root/cronjobs/external_ip.sh | 00 16 * * * root /root/cronjobs/external_ip.sh | ||
| − | Syntax: (0-59) (0-23) (1-31) (1-12 or Jan-Dec) (0-6 or Sun-Sat) (user) (command) | + | Syntax: Minute(0-59) Hour(0-23) Day(1-31) Month(1-12 or Jan-Dec) Day of Week(0-6 or Sun-Sat) (user) (command) |
| − | + | ||
| + | The asterisk (*) is a wildcard entry meaning all. If you enter day, you have to enter month. To run something once a week, put * for day and month and enter a number for the day. | ||
Save and exit. | Save and exit. | ||
Latest revision as of 21:26, 6 October 2015
Description
How to automatically run a command periodically with Cron daemon.
Cron is a time-based job scheduler in Unix-like computer operating systems. Cron enables users to schedule jobs (commands or shell scripts) to run periodically at certain times or dates. It is commonly used to automate system maintenance or administration, though its general-purpose nature means that it can be used for other purposes, such as connecting to the Internet and downloading email. From Wikipedia
Cron daemon is the Unix and GNU/Linux equivalent to Scheduled Tasks in Windows.
Custom template
This is the preferred method on SME Server, see documentation. Custom templates survive updates and are included in a regular backup.
Check for a custom template directory. If one doesn't exist, make the directory for custom template with the following command
mkdir -p /etc/e-smith/templates-custom/etc/crontab
Create and open a file with the text editor nano:
nano /etc/e-smith/templates-custom/etc/crontab/your_file_name
Add your cron entries, example for running a script daily at 16,00:
# Run custom script with cron 00 16 * * * root /root/cronjobs/external_ip.sh
Syntax: Minute(0-59) Hour(0-23) Day(1-31) Month(1-12 or Jan-Dec) Day of Week(0-6 or Sun-Sat) (user) (command)
The asterisk (*) is a wildcard entry meaning all. If you enter day, you have to enter month. To run something once a week, put * for day and month and enter a number for the day.
Save and exit.
Regenerate the template:
expand-template /etc/crontab
Verify your new entry in the regenerated crontab:
more /etc/crontab
Restart the cron daemon:
service crond restart
Cron directorys
This method is simple, but will not be included in a regular backup and might have problems with updates and upgrades.
There are four directorys in /etc/: cron.hourly, cron.daily, cron.weekly and cron.monthly.
Any executable script dropped in these will be executed automatically each period (indicated by the directory name).
Notes
Cron jobs are executed by the user admin (which is an alias for root).
Cron job results are usually sent by email to the administrator.
A script fired by a cron entry must be executable.
Contribs
Crontab Manager is a contribs which is able to do automatically what is needed above.
Links
Developers Manual on Configuration file templates
