Run command from user to set cron
For Ubuntu system
$ sudo crontab -e
For CentOS/RHEL system
#crontab -e
For CentOS/RHEL system
#crontab -e
00 18 * * * /sbin/shutdown -h now
Check Crontab status
$ sudo crontab -l
or,
#crontab -l
or,
#crontab -l
Now cron is set for system shutdonw at 6 PM .