问题
当执行sudo apt-get update或sudo apt-get install时，
出现Not Found Package Repository Errors
或E: Some index files failed to download They have been ignored, or old ones used的问题，
可以通过更换包源来解决
解决方法
- 备份
sudo cp /etc/apt/sources.list /etc/apt/sources.list.bak - 修改
sudo vim /etc/apt/sources.list，用下面内容(16.04.3)替换:
deb http://cn.archive.ubuntu.com/ubuntu/ xenial main restricted universe multiverse
deb http://cn.archive.ubuntu.com/ubuntu/ xenial-security main restricted universe multiverse
deb http://cn.archive.ubuntu.com/ubuntu/ xenial-updates main restricted universe multiverse
deb http://cn.archive.ubuntu.com/ubuntu/ xenial-backports main restricted universe multiverse
##测试版源
deb http://cn.archive.ubuntu.com/ubuntu/ xenial-proposed main restricted universe multiverse
# 源码
deb-src http://cn.archive.ubuntu.com/ubuntu/ xenial main restricted universe multiverse
deb-src http://cn.archive.ubuntu.com/ubuntu/ xenial-security main restricted universe multiverse
deb-src http://cn.archive.ubuntu.com/ubuntu/ xenial-updates main restricted universe multiverse
deb-src http://cn.archive.ubuntu.com/ubuntu/ xenial-backports main restricted universe multiverse
##测试版源
deb-src http://cn.archive.ubuntu.com/ubuntu/ xenial-proposed main restricted universe multiverse
更新
sudo apt-get update
