reboot into OS

Get all entries

grep -oP "^((submenu|menuentry)\s')\K(.+?)(?=\')" /boot/grub/grub.cfg

…and subentries

grep -oP "^((submenu|menuentry|\tmenuentry)\s')\K(.+?)(?=\')" /boot/grub/grub.cfg

now count from 0 and run

grub reboot '1>2'

reboot