前情提要：
今天突然ubuntu apt update 无法用了，通过调查才知道ubuntu产品售后是有期限的。过了期限的版本，会被切换到old release的源。
尝试切换到中科大源：
##中科大源
deb https://mirrors.ustc.edu.cn/ubuntu/ cosmic main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ cosmic main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ cosmic-updates main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ cosmic-updates main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ cosmic-backports main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ cosmic-backports main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ cosmic-security main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ cosmic-security main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ cosmic-proposed main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ cosmic-proposed main restricted universe multiverse
仍然报错：
Err:6 https://mirrors.ustc.edu.cn/ubuntu cosmic Release
404 Not Found [IP: 202.141.176.110 443]
通过中科大官方了解，原来也有过期一说，于是根据上面的说法，切换到old release:
##中科大源
deb https://mirrors.ustc.edu.cn/ubuntu-old-releases/ubuntu/ cosmic main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu-old-releases/ubuntu/ cosmic main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu-old-releases/ubuntu/ cosmic-updates main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu-old-releases/ubuntu/ cosmic-updates main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu-old-releases/ubuntu/ cosmic-backports main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu-old-releases/ubuntu/ cosmic-backports main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu-old-releases/ubuntu/ cosmic-security main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu-old-releases/ubuntu/ cosmic-security main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu-old-releases/ubuntu/ cosmic-proposed main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu-old-releases/ubuntu/ cosmic-proposed main restricted universe multiverse
sudo apt update成功。
参考文献：
ubuntu当前支持的版本（未过期）：
https://wiki.ubuntu.com/Releases
中科大的过期帮助文档：
https://mirrors.ustc.edu.cn/help/ubuntu.html
中科大的原帮助文档：
https://mirrors.ustc.edu.cn/help/ubuntu.html
清华的版本与源的生成页面：
https://mirror.tuna.tsinghua.edu.cn/help/ubuntu/
Ubuntu版本对照：
版本 开发代号 中译 发布日期 支持结束时间 内核版本
桌面版 服务器版
4.10 Warty Warthog 多疣的疣猪 2004-10-20 2006-04-30 2.6.8
5.04 Hoary Hedgehog 白发的刺猬 2005-04-08 2006-10-31 2.6.10
5.10 Breezy Badger 活泼的獾 2005-10-13 2007-04-13 2.6.12
6.06 LTS Dapper Drake 整洁的公鸭 2006-06-01 2009-07-14 2011-06-01 2.6.15
6.10 Edgy Eft 尖利的小蜥蜴 2006-10-26 2008-04-25 2.6.17
7.04 Feisty Fawn 烦躁不安的鹿 2007-04-19 2008-10-19 2.6.20
7.10 Gutsy Gibbon 胆大的长臂猿 2007-10-18 2009-04-18 2.6.22
8.04 LTS Hardy Heron 坚强的鹭 2008-04-24 2011-05-12 2013-05-09 2.6.24
8.10 Intrepid Ibex 无畏的羱羊 2008-10-30 2010-04-30 2.6.27
9.04 Jaunty Jackalope 活泼的鹿角兔 2009-04-23 2010-10-23 2.6.28
9.10 Karmic Koala 幸运的树袋熊 2009-10-29 2011-04-30 2.6.31
10.04 LTS Lucid Lynx 清醒的山猫 2010-04-29 2013-05-09 2015-04-30 2.6.32
10.10 Maverick Meerkat 标新立异的狐獴 2010-10-10 2012-04-10 2.6.35
11.04 Natty Narwhal 敏捷的独角鲸 2011-04-28 2012-10-28 2.6.38
11.10 Oneiric Ocelot 有梦的虎猫 2011-10-13 2013-05-09 3.0
12.04 LTS Precise Pangolin 精准的穿山甲 2012-04-26[39] 2017-04-28[40] 3.2[41]
12.10 Quantal Quetzal 量子的格查尔鸟 2012-10-18 2014-05-16[42] 3.5[43]
13.04 Raring Ringtail 铆足了劲的环尾猫熊 2013-04-25 2014-01-27[44] 3.8[45]
13.10 Saucy Salamander 活泼的蝾螈 2013-10-17[46] 2014-07-17[47] 3.11
14.04 LTS Trusty Tahr 可靠的塔尔羊 2014-04-17[48] 2019-04 3.13
14.10 Utopic Unicorn 乌托邦的独角兽 2014-10-23[49] 2015-07-23[50] 3.16[51]
15.04 Vivid Vervet 活泼的长尾黑颚猴 2015-04-23[52] 2016-02-04[53] 3.19[54]
15.10 Wily Werewolf 老谋深算的狼人 2015-10-22[55] 2016-07-28[56] 4.2[57]
16.04 LTS Xenial Xerus 好客的非洲地松鼠 2016-04-21[58] 2021-04 4.4[59]
16.10 Yakkety Yak 喋喋不休的牦牛 2016-10-13[60] 2017-07-20 4.8
17.04 Zesty Zapus 热情的美洲林跳鼠 2017-04-13[61] 2018-01-13 4.10[62]
17.10 Artful Aardvark 巧妙的土豚 2017-10-19[63] 2018-07-19 4.13[64]
18.04 LTS Bionic Beaver[65][66] 仿生的海狸 2018-04-26[67] 2028-04[68] 4.15
18.10 Cosmic Cuttlefish 宇宙的墨鱼 2018-10-18[69] 2019-07 4.18[70]
19.04 Disco Dingo 迪斯可的澳洲野犬 2019-04-18[71] 2020-01 TBA
