grub-reboot [,OPTION/] ,MENU_ENTRY/
Set the default boot menu entry for GRUB, for the next boot only.
MENU_ENTRY is a number, a menu item title or a menu item identifier. Please note that menu items in submenus or sub-submenus require specifying the submenu components and then the menu item component. The titles should be separated using the greater-than character (>) with no extra spaces. Depending on your shell some characters including > may need escaping. More information about this is available in the GRUB Manual in the section about the 'default' command.
Report bugs to <email@example.com>.
The full documentation for grub-reboot is maintained as a Texinfo manual. If the info and grub-reboot programs are properly installed at your site, the command
should give you access to the complete manual.