1，檢視當前系統有幾個核心

執行： cat /boot/grub2/grub.cfg |grep menuentry

[[email protected] ~]# cat /boot/grub2/grub.cfg |grep menuentry

if [ x"${feature_menuentry_id}" = xy ]; then
  menuentry_id_option="--id"
  menuentry_id_option=""
export menuentry_id_option
menuentry 'CentOS Linux (4.4.110-1.el7.elrepo.x86_64) 7 (Core)' --class centos --class gnu-linux --class gnu --class os --unrestricted $menuentry_id_option 'gnulinux-4.4.110-1.el7.elrepo.x86_64-advanced-c59b9eb7-3a84-43c6-8f1e-29c2ef4ef0e6' {
menuentry 'CentOS Linux (4.4.82-1.el7.elrepo.x86_64) 7 (Core)' --class centos --class gnu-linux --class gnu --class os --unrestricted $menuentry_id_option 'gnulinux-4.4.82-1.el7.elrepo.x86_64-advanced-c59b9eb7-3a84-43c6-8f1e-29c2ef4ef0e6' {
menuentry 'CentOS Linux (3.10.0-693.el7.x86_64) 7 (Core)' --class centos --class gnu-linux --class gnu --class os --unrestricted $menuentry_id_option 'gnulinux-3.10.0-693.el7.x86_64-advanced-c59b9eb7-3a84-43c6-8f1e-29c2ef4ef0e6' {
menuentry 'CentOS Linux (0-rescue-72db9dd831b546f2a155776dc80f6cb4) 7 (Core)' --class centos --class gnu-linux --class gnu --class os --unrestricted $menuentry_id_option 'gnulinux-0-rescue-72db9dd831b546f2a155776dc80f6cb4-advanced-c59b9eb7-3a84-43c6-8f1e-29c2ef4ef0e6' {
 

2，設定預設的啟動核心

grub2-set-default "CentOS Linux (3.10.0-693.el7.x86_64) 7 (Core)"

驗證是否修改成功：

[[email protected] ~]# grub2-editenv list 
saved_entry=CentOS Linux (3.10.0-693.el7.x86_64) 7 (Core)

3，重啟後進行觀察
重啟成功以後檢視下機器的核心是不是該核心：

[[email protected] ~]# uname -r
3.10.0-693.el7.x86_64