Ubuntu18.04 问题——无法安全地用该源进行更新，所以默认禁用该源。404 NOT FOUND [IP.... ]不含有 'multiverse`' 组件[...]

解决办法：在etc/apt/sources.list.d 目录中删除对应的ppa(稳妥起见可以进行备份成.bak）
$ sudo mv /etc/apt/sources.list.d/ubuntukylin.list /etc/apt/sources.list.d/ubuntukylin.list.bak

然后执行：
sudo apt-get updat

如果任然有下载源错误执行：
$ sudo gedit /etc/apt/sources.list

将内容替换如下：

# deb cdrom:[Ubuntu 18.04 LTS _Bionic Beaver_ - Release amd64 (20180426)]/ bionic main restricted

# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://mirrors.cn99.com/ubuntu/ bionic main restricted
# deb-src http://cn.archive.ubuntu.com/ubuntu/ bionic main restricted

## Major bug fix updates produced after the final release of the
## distribution.
deb http://mirrors.cn99.com/ubuntu/ bionic-updates main restricted
# deb-src http://cn.archive.ubuntu.com/ubuntu/ bionic-updates main restricted

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team. Also, please note that software in universe WILL NOT receive any
## review or updates from the Ubuntu security team.
deb http://mirrors.cn99.com/ubuntu/ bionic universe
# deb-src http://cn.archive.ubuntu.com/ubuntu/ bionic universe
deb http://mirrors.cn99.com/ubuntu/ bionic-updates universe
# deb-src http://cn.archive.ubuntu.com/ubuntu/ bionic-updates universe

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu 
## team, and may not be under a free licence. Please satisfy yourself as to 
## your rights to use the software. Also, please note that software in 
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
deb http://mirrors.cn99.com/ubuntu/ bionic multiverse
# deb-src http://cn.archive.ubuntu.com/ubuntu/ bionic multiverse
deb http://mirrors.cn99.com/ubuntu/ bionic-updates multiverse
# deb-src http://cn.archive.ubuntu.com/ubuntu/ bionic-updates multiverse

## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.
deb http://mirrors.cn99.com/ubuntu/ bionic-backports main restricted universe multiverse
# deb-src http://cn.archive.ubuntu.com/ubuntu/ bionic-backports main restricted universe multiverse

## Uncomment the following two lines to add software from Canonical's
## 'partner' repository.
## This software is not part of Ubuntu, but is offered by Canonical and the
## respective vendors as a service to Ubuntu users.
# deb http://archive.canonical.com/ubuntu bionic partner
# deb-src http://archive.canonical.com/ubuntu bionic partner

deb http://mirrors.cn99.com/ubuntu/ bionic-security main restricted
# deb-src http://security.ubuntu.com/ubuntu bionic-security main restricted
deb http://mirrors.cn99.com/ubuntu/ bionic-security universe
# deb-src http://security.ubuntu.com/ubuntu bionic-security universe
deb http://mirrors.cn99.com/ubuntu/ bionic-security multiverse
# deb-src http://security.ubuntu.com/ubuntu bionic-security multiverse
deb [arch=amd64] https://mirrors.aliyun.com/docker-ce/linux/ubuntu bionic stable
# deb-src [arch=amd64] https://mirrors.aliyun.com/docker-ce/linux/ubuntu bionic stable