Occasionally it is useful to be able to reboot a machine into an earlier kernel and watching and waiting for the grub menu isn’t always feasible or convenient.

The command grub-reboot allows you to set a temporary entry to use for the next reboot; however you need to know the correct input in order for it to work.

First, look at /boot/grub/grub.cfg and determine which kernel you want to reboot into. Note the menu and submenus to construct a string of the form menu>submenu. For example if you want to reboot into an older kernel you can do:

sudo grub-reboot "Advanced options for Ubuntu>Ubuntu, with Linux 4.2.0-16-generic"
sudo reboot

After rebooting you’ll be in the 4.2.0-16-generic kernel!