问题：无法执行“apt-get update”（Ubuntu 18.10）
Ubuntu 18.10 宇宙生命终结
我试图升级我的系统版本，但我得到了同样的 apt-get update 命令错误
$ apt update
Ign:1 http://archive.ubuntu.com/ubuntu cosmic InRelease
Ign:2 http://security.ubuntu.com/ubuntu cosmic-security InRelease
Ign:3 http://archive.ubuntu.com/ubuntu cosmic-updates InRelease
Ign:4 http://archive.ubuntu.com/ubuntu cosmic-backports InRelease
Err:5 http://security.ubuntu.com/ubuntu cosmic-security Release
404 Not Found [IP: 91.189.88.173 80]
Err:6 http://archive.ubuntu.com/ubuntu cosmic Release
404 Not Found [IP: 91.189.88.31 80]
Err:7 http://archive.ubuntu.com/ubuntu cosmic-updates Release
404 Not Found [IP: 91.189.88.31 80]
Err:8 http://archive.ubuntu.com/ubuntu cosmic-backports Release
404 Not Found [IP: 91.189.88.31 80]
Reading package lists... Done
E: The repository 'http://security.ubuntu.com/ubuntu cosmic-security Release' …