What is cron?
Today in this blog we will try to cover some of the basics of cron and cronjobs. Cron is a demon (background process) or is said to be a system executable file that runs tracks we scheduled on our system at a particular time.
If the cron demon service is not running then the scheduled corncobs won’t get triggered on the scheduled timings. You can find the background process state as below:
What is crontab?
Crontab is a client program or utility that is being used for users to schedule/remove automated jobs. Cronjobs can be scheduled per users on a system.
Cron demon reads the crontab file and performs the assigned tasks. We open a crontab file using the following commands.
Crontab -l [ list of our crons ]
If there are no scheduled tasks, then it would simply message out saying that there is no crons.
Crontab -e [ to edit the cron file ]
Use this command to schedule the task. Then we list the cron file.
How to schedule cron tasks:
There are multiple ways of scheduling a cronjob
- Adding a cronjob directly using crontab -e
- Adding the cronjob in /var/spool/cron/crontabs/cronfilename
The schedule on cron jobs using 5 asterisk (*) as below
Cron scheduling format
* * * * * Command_to_execute | | | | | | | | | Day of the Week ( 0 - 6 ) ( Sunday = 0 ) | | | | | | | Month ( 1 - 12 ) | | | | | Day of Month ( 1 - 31 ) | | | Hour ( 0 - 23 ) | Min ( 0 - 59 )
Each user crontab file is stored inside the folder /var/spool/crontabs/user_name
NOTE :System crontabs are stored in /etc/cron.d
Crontab -r [ Clear/Delete all the scheduled crons ]
One of most dangerous command to be run on a production environment as it might clear off all the scheduled cronjob without taking a backup. Always be beware of running this command on an production environment. If a server is going to be decommissioned on going to be repurposed them doing such operations would help to clean up but still do not delete without taking a backup of all cronjobs.
How to take Cronjobs Backup in a file ?
listing out all the cronjob and redirecting that output to a file would allow you to backup all the cronjobs of that particular user.
crontab -l > /home/root/crontab_root_20211006.bkp
Usage of Cronjobs
- Automate the backups
- Daily routine tasks automation
- Clear the log file at any particular time
References and tools around Cronjob Scheduler
https://crontab.guru – Tool to convert cron schedule expressions
https://man7.org/linux/man-pages/man5/crontab.5.html – crontab manual
Please follow our blogs to stay updated with out contents