第一步：查看当前内核
rew $ uname -a
Linux rew 4.15.0-42-generic #45~16.04.1-Ubuntu SMP Mon Nov 19 13:02:27 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
当前使用版本为： 4.15.0-42-generic
第二步：查看所有内核
rew $ dpkg --get-selections | grep linux
console-setup-linux install
libselinux1:amd64 install
libselinux1:i386 install
linux-base install
linux-firmware install
linux-generic-hwe-16.04 install
linux-headers-4.15.0-42 install
linux-headers-4.15.0-39 install
linux-headers-4.15.0-39-generic install
linux-headers-4.15.0-42-generic install
linux-headers-generic-hwe-16.04 install
linux-image-4.15.0-39-generic install
linux-image-4.15.0-42-generic install
linux-image-generic-hwe-16.04 install
linux-libc-dev:amd64 install
linux-libc-dev:i386 install
linux-modules-4.15.0-39-generic install
linux-modules-4.15.0-42-generic install
linux-modules-extra-4.15.0-39-generic install
linux-modules-extra-4.15.0-42-generic install
linux-sound-base install
pptp-linux install
syslinux install
syslinux-common install
syslinux-legacy install
util-linux install
第三步：移除冗余内核
所有 39 版本的对我来说都是多余的，进行删除：
rew $ sudo apt-get remove \
linux-headers-4.15.0-39 \
linux-headers-4.15.0-39-generic \
linux-image-4.15.0-39-generic \
linux-modules-4.15.0-39-generic \
linux-modules-extra-4.15.0-39-generic
卸载后重新检查：
rew $ dpkg --get-selections | grep linux
console-setup-linux install
libselinux1:amd64 install
libselinux1:i386 install
linux-base install
linux-firmware install
linux-generic-hwe-16.04 install
linux-headers-4.15.0-42 install
linux-headers-4.15.0-42-generic install
linux-headers-generic-hwe-16.04 install
linux-image-4.15.0-39-generic deinstall
linux-image-4.15.0-42-generic install
linux-image-generic-hwe-16.04 install
linux-libc-dev:amd64 install
linux-libc-dev:i386 install
linux-modules-4.15.0-39-generic deinstall
linux-modules-4.15.0-42-generic install
linux-modules-extra-4.15.0-39-generic deinstall
linux-modules-extra-4.15.0-42-generic install
linux-sound-base install
pptp-linux install
syslinux install
syslinux-common install
syslinux-legacy install
util-linux install
状态为 deinstall 即已经卸载，如果觉得看着不舒服的话可以使用 purge 连配置文件里一起彻底删除，清理内核列表
rew $ sudo apt-get purge \
linux-headers-4.15.0-39 \
linux-headers-4.15.0-39-generic \
linux-image-4.15.0-39-generic \
linux-modules-4.15.0-39-generic \
linux-modules-extra-4.15.0-39-generic
第四步：更新系统引导
删除内核后需要更新 grub 移除失效的启动项
rew $ sudo update-grub #根据情况选择grub/grub2