$ grep "linux.*root" /boot/grub/grub.cfg
linux /boot/vmlinuz-4.4.0-36-generic root=UUID=52705d97-cd8f-4ea5-a95e-64ed46ecb232 ro console=tty1 console=ttyS0
linux /boot/vmlinuz-4.4.0-36-generic root=UUID=52705d97-cd8f-4ea5-a95e-64ed46ecb232 ro console=tty1 console=ttyS0
linux /boot/vmlinuz-4.4.0-36-generic root=UUID=52705d97-cd8f-4ea5-a95e-64ed46ecb232 ro recovery nomodeset
linux /boot/vmlinuz-4.2.0-42-generic root=UUID=52705d97-cd8f-4ea5-a95e-64ed46ecb232 ro console=tty1 console=ttyS0
linux /boot/vmlinuz-4.2.0-42-generic root=UUID=52705d97-cd8f-4ea5-a95e-64ed46ecb232 ro recovery nomodeset
linux /boot/vmlinuz-3.16.0-77-generic root=UUID=52705d97-cd8f-4ea5-a95e-64ed46ecb232 ro console=tty1 console=ttyS0
linux /boot/vmlinuz-3.16.0-77-generic root=UUID=52705d97-cd8f-4ea5-a95e-64ed46ecb232 ro recovery nomodeset
$ sudo grub-reboot 3 ; echo $?
0
$ grep -v "###" /boot/grub/grubenv
# GRUB Environment Block
next_entry=3
$ sudo reboot
...
$ uname -r
4.4.0-36-generic