How to create Cron jobs UNIX/LINUX

$crontab -l
//shows/prints existing cronjob

$crontab -e
//create a cronjob

$crontab -r 
//removes all cronjobs



below code writes test********** to testCronLog file every 1 min

• */1 * * * * echo "test***********" >>/home/app/testCronLog

--below runs every 3 mins and prints output to appLog file

• */3 * * * * /home/app/startProcess.sh >>/home/app/appLog

Actually, it's not recommended to handle those files by hand. Per crontab man page:

Each user can have their own crontab, and though
these are files in /var/spool/cron/crontabs, they are not
intended to be edited directly.

Files under /var/spool are considered temporary/working, that's why they probably get deleted during an upgrade, though a closer look at the cron package's upgrade scripts may shed some light on this.

Anyway, it's always a good practice to back up your cron entries or keep them in a file in your home directory.

I assume you're using crontab -e to create crontab files on the fly. If so, you can get a "copy" of your crontab file by doing crontab -l. Pipe that to a file to get a "backup":

crontab -l > my-crontab

Then you can edit that my-crontab file to add or modify entries, and then "install" it by giving it to crontab:

crontab my-crontab

This does the same syntax checking as crontab -e.

Reference:

https://docs.oracle.com/cd/E23824_01/html/821-1451/sysrescron-24589.html

https://www.taniarascia.com/setting-up-a-basic-cron-job-in-linux/

https://askubuntu.com/questions/216692/where-is-the-user-crontab-stored
https://kb.iu.edu/d/afiz