查看GRUB缺省启动顺序
cat /etc/default/grub | grep GRUB_DEFAULT 回显：GRUB_DEFAULT=saved
如果是saved，就是按照指定保存的内核启动，如果为数字（0/1/2等），就是/boot/grub2/grub.cfg中对应位置的内核，建议设置为saved。
查看当前使用的内核：
grub2-editenv list 回显：saved_entry=CentOS Linux (3.10.0-957.21.3.el7.x86_64) 7 (Core) with debugging
查看可以使用的内核：
cat /boot/grub2/grub.cfg | grep ‘menuentry ‘ 回显：
menuentry ‘CentOS Linux (5.18.1-1.el7.elrepo.x86_64) 7 (Core) with debugging’ —class rhel fedora –class gnu-linux –class gnu –class os –unrestricted $menuentry_id_option ‘gnulinux-3.10.0-229.el7.x86_64-advanced-a1003f9f-2f89-4a7d-baea-d7e2d6b5088a’ {
menuentry ‘CentOS Linux (5.18.1-1.el7.elrepo.x86_64) 7 (Core)’ –class rhel fedora –class gnu-linux –class gnu –class os –unrestricted $menuentry_id_option ‘gnulinux-3.10.0-229.el7.x86_64-advanced-a1003f9f-2f89-4a7d-baea-d7e2d6b5088a’ {
menuentry ‘CentOS Linux (3.10.0-957.21.3.el7.x86_64) 7 (Core) with debugging’ –class rhel fedora –class gnu-linux –class gnu –class os –unrestricted $menuentry_id_option ‘gnulinux-3.10.0-229.el7.x86_64-advanced-a1003f9f-2f89-4a7d-baea-d7e2d6b5088a’ {
menuentry ‘CentOS Linux (3.10.0-957.21.3.el7.x86_64) 7 (Core)’ –class rhel fedora –class gnu-linux –class gnu –class os –unrestricted $menuentry_id_option ‘gnulinux-3.10.0-229.el7.x86_64-advanced-a1003f9f-2f89-4a7d-baea-d7e2d6b5088a’ {
手动设置默认启动的内核：
grub2-set-default ‘CentOS Linux (5.18.1-1.el7.elrepo.x86_64) 7 (Core) ‘
查看设置后的结果：
grub2-editenv list 回显： saved_entry=CentOS Linux (5.18.1-1.el7.elrepo.x86_64) 7 (Core)
reboot 重启生效。
