linux force reboot or shutdown

Force Reboot :

echo 1 > /proc/sys/kernel/sysrq

echo b > /proc/sysrq-trigger

If you want to force shutdown machine try this.

echo 1 > /proc/sys/kernel/sysrq

echo o > /proc/sysrq-trigger

Thanks to http://linax.wordpress.com/2009/02/16/linux-force-reboot-and-shutdown/