====== reboot into OS ======
  * Get all entries <code bash>grep -oP "^((submenu|menuentry)\s')\K(.+?)(?=\')" /boot/grub/grub.cfg</code>

  * ...and subentries <code bash>grep -oP "^((submenu|menuentry|\tmenuentry)\s')\K(.+?)(?=\')" /boot/grub/grub.cfg</code>
  * now count from 0 and run <code bash>grub reboot '1>2'</code>
  * reboot