首先启动进入到linux系统中，修改grub设置

Centos7下：

1.查看内核版本

登录Linux终端，执行cat /boot/grub2/grub.cfg |grep menuentry命令查看当前操作系统有几个内核版本。

2.修改需要启动的内核版本（红色段为修改的首选启动项）

使用命令 grub2-set-default  'Oracle Linux Server (3.10.0-693.el7.x86_64 with Linux) 7.4'设置默认启动内核为3.10.0。

centos8下：

1.

查看当前内核版本

[root@aikaiyuan ~]# grubby --default-kernel

/boot/vmlinuz-4.18.0-80.7.1.el8_0.x86_64

2.

查看所有内核

讯享网

[root@aikaiyuan ~]# grubby --info=ALL
index=0
kernel="/boot/vmlinuz-4.18.0-80.11.2.el8_0.x86_64"
args="ro crashkernel=auto resume=UUID=b9b14ba1-0b5c-49ac-842f-f11fa2081cfd console=ttyS0 tuned_params"
root="UUID=3db5c437-b4f7-46cf-a20e-0f3"
initrd="/boot/initramfs-4.18.0-80.11.2.el8_0.x86_64.imgtuned_initrd"
title="CentOS Linux (4.18.0-80.11.2.el8_0.x86_64) 8 (Core)"
id="f56bd0e5ce8b45e594c3fb6d1753d61d-4.18.0-80.11.2.el8_0.x86_64"
index=1
kernel="/boot/vmlinuz-4.18.0-80.7.1.el8_0.x86_64"
args="ro crashkernel=auto resume=UUID=b9b14ba1-0b5c-49ac-842f-f11fa2081cfd console=ttyS0 tuned_params"
root="UUID=3db5c437-b4f7-46cf-a20e-0f3"
initrd="/boot/initramfs-4.18.0-80.7.1.el8_0.x86_64.imgtuned_initrd"
title="CentOS Linux (4.18.0-80.7.1.el8_0.x86_64) 8 (Core)"
id="f56bd0e5ce8b45e594c3fb6d1753d61d-4.18.0-80.7.1.el8_0.x86_64"
index=2
kernel="/boot/vmlinuz-4.18.0-80.el8.x86_64"
args="ro crashkernel=auto resume=UUID=b9b14ba1-0b5c-49ac-842f-f11fa2081cfd console=ttyS0 tuned_params"
root="UUID=3db5c437-b4f7-46cf-a20e-0f3"
initrd="/boot/initramfs-4.18.0-80.el8.x86_64.imgtuned_initrd"
title="CentOS Linux (4.18.0-80.el8.x86_64) 8 (Core)"
id="f56bd0e5ce8b45e594c3fb6d1753d61d-4.18.0-80.el8.x86_64"
index=3
kernel="/boot/vmlinuz-0-rescue-f56bd0e5ce8b45e594c3fb6d1753d61d"
args="ro crashkernel=auto resume=UUID=b9b14ba1-0b5c-49ac-842f-f11fa2081cfd console=ttyS0"
root="UUID=3db5c437-b4f7-46cf-a20e-0f3"
initrd="/boot/initramfs-0-rescue-f56bd0e5ce8b45e594c3fb6d1753d61d.img"
title="CentOS Linux (0-rescue-f56bd0e5ce8b45e594c3fb6d1753d61d) 8 (Core)"
 

3.

修改当前默认配置

[root@aikaiyuan ~]# grubby --set-default /boot/vmlinuz-4.18.0-80.11.2.el8_0.x86_64
The default is /boot/loader/entries/f56bd0e5ce8b45e594c3fb6d1753d61d-4.18.0-80.11.2.el8_0.x86_64.conf with index 0 and kernel /boot/vmlinuz-4.18.0-80.11.2.el8_0.x86_64
[root@aikaiyuan ~]# grubby --default-kernel
/boot/vmlinuz-4.18.0-80.11.2.el8_0.x86_64

4.

修改默认内核

# 使用路径来指定内核，可以使用--set-default=kernel-path
[root@tiny-server ~]# grubby --set-default=/boot/vmlinuz-5.9.1-1.el8.elrepo.x86_64
The default is /boot/loader/entries/12ab47b22fef4c02bcdc88b340d5f706-5.9.1-1.el8.elrepo.x86_64.conf with index 0 and kernel /boot/vmlinuz-5.9.1-1.el8.elrepo.x86_64
[root@tiny-server ~]# grubby --default-kernel
/boot/vmlinuz-5.9.1-1.el8.elrepo.x86_64

# 使用index来指定内核，则使用--set-default-index=entry-index
[root@tiny-server ~]# grubby --set-default-index=1
The default is /boot/loader/entries/12ab47b22fef4c02bcdc88b340d5f706-4.18.0-193.28.1.el8_2.x86_64.conf with index 1 and kernel /boot/vmlinuz-4.18.0-193.28.1.el8_2.x86_64
[root@tiny-server ~]# grubby --default-kernel
/boot/vmlinuz-4.18.0-193.28.1.el8_2.x86_64

5.一些小注释