centos7 选定默认启动内核，及删除无用内核

#使用cat /boot/grub2/grub.cfg |grep menuentry  查看系统可用内核
[root@bigapp-slave27 ~]# 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 (3.10.0-514.16.1.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-6f5840d0-55ac-4d3b-899b-61c0297c5347' {
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-6f5840d0-55ac-4d3b-899b-61c0297c5347' {
menuentry 'CentOS Linux (0-rescue-d57307c454c0437d91c309347178cdf5) 7 (Core)' --class centos --class gnu-linux --class gnu --class os --unrestricted $menuentry_id_option 'gnulinux-0-rescue-d57307c454c0437d91c309347178cdf5-advanced-6f5840d0-55ac-4d3b-899b-61c0297c5347' {

#查看当前内核
[root@bigapp-slave27 ~]# uname -r
3.10.0-514.16.1.el7.x86_64

#修改开机时默认使用的内核
grub2-set-default 'CentOS Linux (3.10.0-327.el7.x86_64) 7 (Core)'

#查看内核修改结果
[root@bigapp-slave27 ~]# grub2-editenv list 
saved_entry=CentOS Linux (3.10.0-327.el7.x86_64) 7 (Core)

#查看系统安装了哪些内核包
[root@bigapp-slave27 ~]# rpm -qa |grep kernel
kernel-3.10.0-327.el7.x86_64
kernel-headers-3.10.0-514.6.1.el7.x86_64
kernel-tools-libs-3.10.0-327.el7.x86_64
kernel-3.10.0-514.16.1.el7.x86_64
kernel-tools-3.10.0-327.el7.x86_64

#使用yum remove 或rpm -e 删除无用内核
yum remove kernel-3.10.0-327.el7.x86_64