If you want reboot you Linux box and the reboot, init 6 etc. and these commands do not work for you, or you just want to reboot the system in the very, very fast way, type:
echo b > /proc/sysrq-trigger
This is equivalent to the key combination Alt + SysRq + B which reboots the machine. (Immediately reboot the system, without unmounting or syncing filesystems).