.::: Enable Crontab on Linux/Unix Platform :::.

Crontab  is  the program used to install, remove or list the tables used to drive the cron(8) daemon.  Each user can have their own crontab, and though these are files in /var/spool/ , they are not intended to be edited directly. For SELinux in mls mode can be even more crontabs - for  each  range.  For  more  see selinux(8).

The  cron jobs could be allow or disallow for different users. For classical crontab there exists cron.allow and cron.deny files.  If cron.allow file exists, then you must be listed therein in order to be allowed to use this command.  If the cron.allow file does not exist but the cron.deny file  does  exist,  then you must not be listed in the cron.deny file in order to use this command.  If neither of these files exists, only the super user will be allowed to use this command.  The second option is using PAM authentication, where you set up users, which could  or  couldn’t  use  crontab  and  also  system  cron  jobs  from /etc/cron.d/.

The temporary directory could be set in enviroment variables. If it’s not set by user than /tmp is used.

Tips for writing crontab expressions:

Tip 1: If the day-of-month or day-of-week part starts with a *, they form an intersection. Otherwise they form a union. * * 3 * 1 runs on the 3rd day of the month and on Wednesday (union), whereas * * */2 * 1 runs on every second day of the month only if it's also a Monday (intersection). The manpage is incorrect about this detail. More info.

Tip 2: Cron jobs can break. Don't rely just on email to find out because the root cause can be outside of cron. Monitor each cronjob with a watchdog timer.

Tip 3: Run your servers including the cron process in UTC timezone. Why?

Tip 4: Some cron implementations allow to specify years and seconds. However, cron is not the best tool if you need to operate at those levels, which is also why crontab.guru doesn't support them.

Tip 5: Don't use @reboot because it has too many issues.

Tip 6: More difficult schedules can be realized by combining multiple cron expressions. For example, if you need to run X every 90 minutes, create one crontab entry that runs X every 3 hours on the hour (0 */3 * * *), and a second crontab entry that runs X every 3 hours with an offset (30 1/3 * * *).

Tip 7: Another alternative for complicated schedules is Mcron.

how to implementation crontab

1. Prepare command to be scheduled monthly by crontab
example

@monthly /opt/ManageEngine/OpManager/bin/backup/BackupDB.sh -mode configdata
@monthly /opt/ManageEngine/OpManager/bin/backup/BackupDB.sh -mode all

2. enable crontab (# crontab -e)
[root@TeguhLab ~]# crontab -e
crontab: installing new crontab
[root@TeguhLab ~]# crontab -l
@monthly /opt/ManageEngine/OpManager/bin/backup/BackupDB.sh -mode configdata
@monthly /opt/ManageEngine/OpManager/bin/backup/BackupDB.sh -mode all
[root@TeguhLab ~]#

4. remove crontab type
if you wan to delete/remove crontab. type
 # crontab -r

5. here example tutorial crontab
https://crontab.guru/examples.html