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