Ubuntu 指定默认启动内核

版本：Ubuntu 18.04

可以通过开机启动Ubuntu时按 ESC 键来进入GRUB选项，

选择 Advanced options for Ubuntu，然后可以看到所有内核启动菜单，Index从0开始。

或者输入以下命令：

wayde@wayde-desktop:~$ cat /boot/grub/grub.cfg |grep menuentry
if [ x"${feature_menuentry_id}" = xy ]; then
  menuentry_id_option="--id"
  menuentry_id_option=""
export menuentry_id_option
menuentry 'Ubuntu' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-simple-a0b77529-08f4-4fbc-b3a8-85bb77db44e8' {
submenu 'Advanced options for Ubuntu' $menuentry_id_option 'gnulinux-advanced-a0b77529-08f4-4fbc-b3a8-85bb77db44e8' {
    menuentry 'Ubuntu, with Linux 5.3.0-59-generic' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.3.0-59-generic-advanced-a0b77529-08f4-4fbc-b3a8-85bb77db44e8' {
    menuentry 'Ubuntu, with Linux 5.3.0-59-generic (recovery mode)' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.3.0-59-generic-recovery-a0b77529-08f4-4fbc-b3a8-85bb77db44e8' {
    menuentry 'Ubuntu, with Linux 5.3.0-53-generic' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.3.0-53-generic-advanced-a0b77529-08f4-4fbc-b3a8-85bb77db44e8' {
    menuentry 'Ubuntu, with Linux 5.3.0-53-generic (recovery mode)' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.3.0-53-generic-recovery-a0b77529-08f4-4fbc-b3a8-85bb77db44e8' {
    menuentry 'Ubuntu, with Linux 5.3.0' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.3.0-advanced-a0b77529-08f4-4fbc-b3a8-85bb77db44e8' {
    menuentry 'Ubuntu, with Linux 5.3.0 (recovery mode)' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.3.0-recovery-a0b77529-08f4-4fbc-b3a8-85bb77db44e8' {
    menuentry 'Ubuntu, with Linux 4.19.65-d1' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-4.19.65-d1-advanced-a0b77529-08f4-4fbc-b3a8-85bb77db44e8' {
    menuentry 'Ubuntu, with Linux 4.19.65-d1 (recovery mode)' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-4.19.65-d1-recovery-a0b77529-08f4-4fbc-b3a8-85bb77db44e8' {
menuentry 'System setup' $menuentry_id_option 'uefi-firmware' {

Index同样从0开始，记住你需要指定内核的Index

修改grub文件，修改后如下：

wayde@wayde-desktop:~$ cat /etc/default/grub
# If you change this file, run 'update-grub' afterwards to update
# /boot/grub/grub.cfg.
# For full documentation of the options in this file, see:
#   info -f grub -n 'Simple configuration'

GRUB_DEFAULT="1>6"
GRUB_TIMEOUT_STYLE=hidden
GRUB_TIMEOUT=0
GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian`
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash ignore_loglevel log_buf_len=32M console=tty i915.enable_gvt=1 i915.enable_fbc=0 kvm.ignore_msrs=1 intel_iommu=on,igfx_off drm.debug=0"
GRUB_CMDLINE_LINUX=""

# Uncomment to enable BadRAM filtering, modify to suit your needs
# This works with Linux (no patch required) and with any kernel that obtains
# the memory map information from GRUB (GNU Mach, kernel of FreeBSD ...)
#GRUB_BADRAM="0x01234567,0xfefefefe,0x89abcdef,0xefefefef"

# Uncomment to disable graphical terminal (grub-pc only)
#GRUB_TERMINAL=console

# The resolution used on graphical terminal
# note that you can use only modes which your graphic card supports via VBE
# you can see them in real GRUB with the command `vbeinfo'
#GRUB_GFXMODE=640x480

# Uncomment if you don't want GRUB to pass "root=UUID=xxx" parameter to Linux
#GRUB_DISABLE_LINUX_UUID=true

# Uncomment to disable generation of recovery mode menu entries
#GRUB_DISABLE_RECOVERY="true"

# Uncomment to get a beep at grub start
#GRUB_INIT_TUNE="480 440 1"

以上只修改了 GRUB_DEFAULT="1>6" 部分，1代表启动时第一层菜单里的 Advanced options for Ubuntu，6即为指定内核的Index。

注意引号是必须的。

另一种方法是直接填入内核的名称：

GRUB_DEFAULT="Ubuntu, with Linux 4.19.65-d1"

update grub后会有警告提示使用新的名称格式：

root@wayde-desktop:/home/wayde# update-grub
Sourcing file `/etc/default/grub'
Generating grub configuration file ...
Found linux image: /boot/vmlinuz-5.3.0-59-generic
Found initrd image: /boot/initrd.img-5.3.0-59-generic
Found linux image: /boot/vmlinuz-5.3.0-53-generic
Found initrd image: /boot/initrd.img-5.3.0-53-generic
Found linux image: /boot/vmlinuz-5.3.0
Found initrd image: /boot/initrd.img-5.3.0
Found linux image: /boot/vmlinuz-4.19.65-d1
Found initrd image: /boot/initrd.img-4.19.65-d1
Warning: Please don't use old title `Ubuntu, with Linux 4.19.65-d1' for GRUB_DEFAULT, use `Advanced options for Ubuntu>Ubuntu, with Linux 4.19.65-d1' (for versions before 2.00) or `gnulinux-advanced-a0b77529-08f4-4fbc-b3a8-85bb77db44e8>gnulinux-4.19.65-d1-advanced-a0b77529-08f4-4fbc-b3a8-85bb77db44e8' (for 2.00 or later)
Adding boot menu entry for EFI firmware configuration
done

按照其提示的名字再次进行修改即可。

修改完毕之后：

sudo update-grub

重启即可自动从指定内核启动。