现象：
CentOS7开机启动界面显示多个内核选项原因：
正常情况下，有两个启动项，一个是“正常启动”，另一个是“救援模式启动”（rescue）。 如果启动项多于2个，说明当前系统有旧内核未删除。原因是CentOS更新后不会自动删除旧内核。 默认以新内核启动，可以在启动选项中临时选择，也可以修改配置永久指定。示例：修改默认启动的内核
[root@CentOS-7 ~]# uname -r # 查看当前内核版本3.10.0-693.5.2.el7.x86_64[root@CentOS-7 ~]# [root@CentOS-7 ~]# cat /boot/grub2/grub.cfg |grep "menuentry " # 查看所有可用内核menuentry 'CentOS Linux (3.10.0-693.5.2.el7.x86_64) 7 (Core)' --class centos --class gnu-linux --class gnu --class os --unrestricted $menuentry_id_option 'gnulinux-3.10.0-327.el7.x86_64-advanced-88ae7d3d-ffaf-4402-9d44-56b6845789e4' {menuentry 'CentOS Linux (3.10.0-327.el7.x86_64) 7 (Core)' --class centos --class gnu-linux --class gnu --class os --unrestricted $menuentry_id_option 'gnulinux-3.10.0-327.el7.x86_64-advanced-88ae7d3d-ffaf-4402-9d44-56b6845789e4' {menuentry 'CentOS Linux (0-rescue-8405cda22c0b421db40478edcf9c1fb2) 7 (Core)' --class centos --class gnu-linux --class gnu --class os --unrestricted $menuentry_id_option 'gnulinux-0-rescue-8405cda22c0b421db40478edcf9c1fb2-advanced-88ae7d3d-ffaf-4402-9d44-56b6845789e4' {[root@CentOS-7 ~]# [root@CentOS-7 ~]# grub2-set-default 'CentOS Linux (3.10.0-327.el7.x86_64) 7 (Core)'[root@CentOS-7 ~]# [root@CentOS-7 ~]# grub2-editenv list # 查看内核修改结果saved_entry=CentOS Linux (3.10.0-327.el7.x86_64) 7 (Core)[root@CentOS-7 ~]#
示例：删除无用内核
[root@CentOS-7 ~]# uname -r # 查看当前内核版本3.10.0-693.5.2.el7.x86_64[root@CentOS-7 ~]# [root@CentOS-7 ~]# rpm -qa |grep kernel-[0-9] # 查看全部内核包kernel-3.10.0-693.5.2.el7.x86_64kernel-3.10.0-327.el7.x86_64[root@CentOS-7 ~]# [root@CentOS-7 ~]# yum remove kernel-3.10.0-327.el7.x86_64 # 删除指定的无用内核Loaded plugins: fastestmirror, langpacksResolving Dependencies--> Running transaction check---> Package kernel.x86_64 0:3.10.0-327.el7 will be erased--> Finished Dependency ResolutionDependencies Resolved============================================================================================================================================================= Package Arch Version Repository Size=============================================================================================================================================================Removing: kernel x86_64 3.10.0-327.el7 @anaconda 136 MTransaction Summary=============================================================================================================================================================Remove 1 PackageInstalled size: 136 MIs this ok [y/N]: yDownloading packages:Running transaction checkRunning transaction testTransaction test succeededRunning transaction Erasing : kernel-3.10.0-327.el7.x86_64 1/1 Verifying : kernel-3.10.0-327.el7.x86_64 1/1 Removed: kernel.x86_64 0:3.10.0-327.el7 Complete![root@CentOS-7 ~]# [root@CentOS-7 ~]# rpm -qa |grep kernel-[0-9] # 查看全部内核包kernel-3.10.0-693.5.2.el7.x86_64[root@CentOS-7 ~]# [root@CentOS-7 ~]# cat /boot/grub2/grub.cfg |grep "menuentry "menuentry 'CentOS Linux (3.10.0-693.5.2.el7.x86_64) 7 (Core)' --class centos --class gnu-linux --class gnu --class os --unrestricted $menuentry_id_option 'gnulinux-3.10.0-327.el7.x86_64-advanced-88ae7d3d-ffaf-4402-9d44-56b6845789e4' {menuentry 'CentOS Linux (0-rescue-8405cda22c0b421db40478edcf9c1fb2) 7 (Core)' --class centos --class gnu-linux --class gnu --class os --unrestricted $menuentry_id_option 'gnulinux-0-rescue-8405cda22c0b421db40478edcf9c1fb2-advanced-88ae7d3d-ffaf-4402-9d44-56b6845789e4' {[root@CentOS-7 ~]#