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
dpkg --get-selections | grep linux

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

多余的内核可以通过命令删除 需要删除header和image

sudo apt-get remove linux-image-*.*.*-**（*号用你想删除的实际情况改写）
sudo apt-get remove linux-headers-*.*.*-**（*号用你想删除的实际情况改写）
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：
dpkg --get-selections | grep linux

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