备份：cp /etc/apt/sources.list /etc/apt/sources.list.bak

清空source.list:echo > /etc/apt/sources.list

再将以下源文件复制到/etc/apt/sources.list

• deb http://mirrors.aliyun.com/ubuntu/ bionic main restricted universe multiverse
  deb-src http://mirrors.aliyun.com/ubuntu/ bionic main restricted universe multiverse
  deb http://mirrors.aliyun.com/ubuntu/ bionic-security main restricted universe multiverse
  deb-src http://mirrors.aliyun.com/ubuntu/ bionic-security main restricted universe multiverse
  deb http://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted universe multiverse
  deb-src http://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted universe multiverse
  deb http://mirrors.aliyun.com/ubuntu/ bionic-backports main restricted universe multiverse
  deb-src http://mirrors.aliyun.com/ubuntu/ bionic-backports main restricted universe multiverse
  deb http://mirrors.aliyun.com/ubuntu/ bionic-proposed main restricted universe multiverse
  deb-src http://mirrors.aliyun.com/ubuntu/ bionic-proposed main restricted universe multiverse