我尝试从 16.04.1 升级到 16.04.3，当前系统版本为 16.04.1
menghan@menghan:~$ uname -ra
Linux menghan 4.10.0-42-generic #46~16.04.1-Ubuntu SMP Mon Dec 4 15:57:59 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux
然后，我尝试 sudo apt-get update
最后，我得到了
menghan@menghan:~$ sudo apt-get update
.........
.........
........
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Reading package lists... Done
W: http://archive.canonical.com/dists/precise/Release.gpg: Signature by key 630239CC130E1A7FD81A27B140976EAF437D05B5 uses weak digest algorithm (SHA1)
W: The repository 'http://ppa.launchpad.net/bumblebee/stable/ubuntu xenial 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://ppa.launchpad.net/texlive-backports/ppa/ubuntu xenial 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://ppa.launchpad.net/bumblebee/stable/ubuntu/dists/xenial/main/binary-amd64/Packages 404 Not Found
E: Failed to fetch http://ppa.launchpad.net/texlive-backports/ppa/ubuntu/dists/xenial/main/binary-amd64/Packages 404 Not Found
E: Some index files failed to download. They have been ignored, or old ones used instead.
最终，没有任何更新：
menghan@menghan:~$ sudo apt-get upgrade
Reading package lists... Done
Building dependency tree
Reading state information... Done
Calculating upgrade... Done
The following packages have been kept back:
bumblebee bumblebee-nvidia
0 to upgrade, 0 to newly install, 0 to remove and 2 not to upgrade.
我知道我的笔记本电脑比较坚固，无法在其上正常运行 Ubuntu，因为它有 bbswitch、nividia 驱动程序、bumbble-bee（从记忆中，当我尝试安装它们时，其中一个东西坏了，所以我不得不使用旧版本）....
我的直觉是其中之一无法正确更新，导致最终什么都没有更新......