当我重新启动计算机时，它会在 sda4 上启动到 ubuntu 18.10。我不想这样。我想在 sda2 上启动到 18.04。我可以在启动过程中通过从操作系统列表中进行选择来做到这一点，但如果断电，它会启动到错误的操作系统。该怎么办？
Model: ATA WDC WD1001FALS-0 (scsi)
Disk /dev/sda: 1000GB
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Disk Flags:
Number Start End Size File system Name Flags
1 1049kB 538MB 537MB fat32 EFI System Partition boot, esp
2 538MB 537GB 536GB ext4
3 537GB 537GB 1049kB bios_grub
4 537GB 1000GB 464GB ext4
jcg@jcg-hp:/boot$ sudo dpkg-reconfigure grub-efi-amd64
[sudo] password for jcg:
Sourcing file `/etc/default/grub'
Generating grub configuration file ...
Found linux image: /boot/vmlinuz-4.15.0-041500rc6-generic
Found initrd image: /boot/initrd.img-4.15.0-041500rc6-generic
Found linux image: /boot/vmlinuz-4.15.0-72-generic
Found initrd image: /boot/initrd.img-4.15.0-72-generic
Found linux image: /boot/vmlinuz-4.15.0-70-generic
Found initrd image: /boot/initrd.img-4.15.0-70-generic
Found linux image: /boot/vmlinuz-4.13.0-37-generic
Found initrd image: /boot/initrd.img-4.13.0-37-generic
Found linux image: /boot/vmlinuz-4.13.0-36-generic
Found initrd image: /boot/initrd.img-4.13.0-36-generic
Found linux image: /boot/vmlinuz-4.13.0-31-generic
Found initrd image: /boot/initrd.img-4.13.0-31-generic
Found memtest86+ image: /boot/memtest86+.elf
Found memtest86+ image: /boot/memtest86+.bin
Found Ubuntu 18.10 (18.10) on /dev/sda4
done
Processing triggers for shim-signed (1.37~18.04.3+15+1533136590.3beb971-0ubuntu1) ...
EFI variables are not supported on this system
/sys/firmware/efi/efivars not found, aborting.
jcg@jcg-hp:/boot$