Shutdown ubuntu/CentOS/RHEL system using crontab

Run command from user to set cron

For Ubuntu system

$ sudo crontab -e

For CentOS/RHEL system
#crontab -e

00 18 * * * /sbin/shutdown -h now

Check Crontab status

$ sudo crontab -l
or,
#crontab -l

Now cron is set for system shutdonw at 6 PM .