20191224 本地虛擬機 ubuntu 1804.1 很久沒用，先升級一下 ubuntu ，結果遇到問題，升不上去！
$ cat /etc/issue
Ubuntu 18.04.1 LTS \n \l
照例，sudo apt update & sudo apt upgrade ，但是 update 就出錯了！
sudo apt update
錯誤如下
…
Reading package lists… Done
E: Failed to fetch http://mirrors.aliyun.com/ubuntu/dists/bionic-updates/main/binary-amd64/by-hash/SHA256/1fc26aa9068c94f64aaf27415e896356f170c8f2c2b9d952a7354773f82d389c Hash Sum mismatch
Hashes of expected file:
- Filesize:816660 [weak]
- SHA256:1fc26aa9068c94f64aaf27415e896356f170c8f2c2b9d952a7354773f82d389c
- SHA1:b790e96ef8a8c656bd539b8d7072efeb91d53275 [weak]
- MD5Sum:d59679d8b7bda2355df95e51adce4e09 [weak]
Hashes of received file:
- SHA256:a2b9d89caf25fb7a3ae8267edac104b495790a43b86d98971d123d7f3f11b582
- SHA1:ca06617424e81168a43d285f7509aba8eeb31b46 [weak]
- MD5Sum:86fbe9596b0466ec6919576f0290a6eb [weak]
- Filesize:816660 [weak]
Last modification reported: Sat, 21 Dec 2019 18:59:02 +0000
Release file created at: Tue, 24 Dec 2019 17:14:40 +0000
E: Some index files failed to download. They have been ignored, or old ones used instead.
2、按照建議，嘗試修復
sudo apt clean
sudo apt update --fix-missing
之後，還是一樣的錯誤
3、換個源試試
目前一直使用的是阿里雲的源，換成清華的試試！
找到一個很全的國內源
https://blog.csdn.net/xiefeisd/article/details/86663243
sudo vim sources.list
刪除原來的阿里雲源，複製清華的源
再來
sudo apt update
ok！不報錯誤了！
其中，中途有個提示
The default action is to keep your current version.
*** cloud.cfg (Y/I/N/O/D/Z) [default=N] ?
*** 直接回車使用缺省，也就是保持當前的 cloud.cfg 版本
sudo apt upgrade
完成！
$ cat /etc/issue
Ubuntu 18.04.3 LTS \n \l
** 升級之後，無需重啓（reboot）！
