不知道大家最害怕什么linux命令，我最最最最害怕的就是 sudo apt-get update命令。我简直遇到了各种各样形形色色的问题，甚至有一些是网上找不到原因，最后不知道改了什么，莫名其妙又好了的问题。气到肝疼！
今后遇到的apt-get update 问题全都归纳总结到这里，做个记录，看能遇到多少！
会持续更新，一有问题就更新！
1.
E: Failed to fetch http://xxx.xxx.xxx.xxx/files/31210000008FF17B/mirror.neu.edu.cn/ubuntu/dists/trusty-updates/main/binary-amd64/Packages.bz2
E: Failed to fetch http://xxx.xxx.xxx.xxx/files/51210000008A8BF7/mirror.neu.edu.cn/ubuntu/dists/trusty-updates/universe/binary-amd64/Packages.bz2
E: Failed to fetch http://xxx.xxx.xxx.xxx/files/21210000005B2D12/mirror.neu.edu.cn/ubuntu/dists/trusty-updates/universe/binary-i386/Packages.bz2 Hash Sum mismatch
E: Some index files failed to download. They have been ignored, or old ones used instead.
解决方法：
换成阿里源（额。。。其实我之前用的就是阿里源，相当于重新粘了一遍，私以为这步没用）
sudo apt-get clean(好像没有成功，不过懒得管了)
sudo rm /var/lib/apt/lists/* -vf
参考博客：https://blog.csdn.net/tian_ciomp/article/details/51339635
https://blog.csdn.net/theonegis/article/details/59142193
https://blog.csdn.net/qq_35451572/article/details/79516563
https://blog.csdn.net/wang1144/article/details/51604424/
错误瞬间变成了如下：
W: The repository 'http://mirrors.aliyun.com/ubuntu raring Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: The repository 'http://mirrors.aliyun.com/ubuntu raring-security Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: The repository 'http://mirrors.aliyun.com/ubuntu raring-updates Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: The repository 'http://mirrors.aliyun.com/ubuntu raring-proposed Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: The repository 'http://mirrors.aliyun.com/ubuntu raring-backports Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: Failed to fetch http://mirrors.aliyun.com/ubuntu/dists/raring/main/source/Sources 404 Not Found [IP: 111.206.4.253 80]
E: Failed to fetch http://mirrors.aliyun.com/ubuntu/dists/raring-security/main/source/Sources 404 Not Found [IP: 111.206.4.253 80]
E: Failed to fetch http://mirrors.aliyun.com/ubuntu/dists/raring-updates/main/source/Sources 404 Not Found [IP: 111.206.4.253 80]
E: Failed to fetch http://mirrors.aliyun.com/ubuntu/dists/raring-proposed/main/source/Sources 404 Not Found [IP: 111.206.4.253 80]
E: Failed to fetch http://mirrors.aliyun.com/ubuntu/dists/raring-backports/main/source/Sources 404 Not Found [IP: 111.206.4.253 80]
E: Some index files failed to download. They have been ignored, or old ones used instead.
换了个我也不知道是哪里的源(原谅我的孤陋寡闻)，竟然可以了！果然是源的问题吗？
sudo gedit /etc/apt/sources.list
将/etc/apt/sources.list文件替换为下面文件
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
参考博客：https://blog.csdn.net/github_35160620/article/details/52115542
未完待续。。。