The shutdown command is used to bring the system down in a safe and controlled way. It can take various options and arguments, such as the time of shutdown, the message to broadcast to users, the halt or reboot mode, etc. The option -r instructs the shutdown command to reboot the system after shutting down. The argument now means to shut down immediately. Therefore, shutdown -r now will reboot the system without delay. The telinit command is used to change the run level of the system. It takes a single argument that specifies the new run level. The run level 6 is reserved for rebooting the system. Therefore, telinit 6 will also reboot the system. The other options are either incorrect or irrelevant. shutdown -r ''rebooting'' will also reboot the system, but with a delay of one minute and a message to the users. telinit 0 will halt the system, not reboot it. shutdown -k now ''rebooting'' will only send a warning message to the users, but not actually shut down or reboot the system.Reference:LPI Linux Essentials - 1.101.2, LPI Linux Administrator - 101.3