我想知道为什么upgrade有时不想升级系统的某些部分，而dist-upgrade确实如此。这是运行后的示例apt-get upgrade：
apt-get upgrade：
rimmer@rimmer-Lenovo-IdeaPad-S10-2:~$ sudo apt-get upgrade
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages have been kept back:
linux-generic linux-headers-generic linux-image-generic
0 upgraded, 0 newly installed, 0 to remove and 3 not upgraded.
与apt-get dist-upgrade：
rimmer@rimmer-Lenovo-IdeaPad-S10-2:~$ sudo apt-get dist-upgrade
Reading package lists... Done
Building dependency tree
Reading state information... Done
Calculating upgrade... Done
The following NEW packages will be installed:
linux-headers-3.0.0-13 linux-headers-3.0.0-13-generic
linux-image-3.0.0-13-generic …PPA 似乎一直处于离线状态。每当我使用时sudo apt-get update，都会显示此错误：
W: Failed to fetch http://ppa.launchpad.net/ 404 Not Found
如何修复这些错误？
我在终端中运行了这个命令：
sudo apt-get update
更新以以下错误报告结束：
W: A error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://extras.ubuntu.com precise Release: The following signatures were invalid: BADSIG 16126D3A3E5C1192 Ubuntu Extras Archive Automatic Signing Key <ftpmaster@ubuntu.com>
W: GPG error: http://ppa.launchpad.net precise Release: The following signatures were invalid: BADSIG 4C1CBC1B69B0E2F4 Launchpad PPA for Jonathan French
W: Failed to fetch http://extras.ubuntu.com/ubuntu/dists/precise/Release
这是什么意思，为什么会这样？
我访问的所有密钥服务器都超时了。我需要在不检查公钥签名的情况下安装软件包。有没有办法绕过所有签名检查/忽略所有签名错误或愚蠢地认为签名通过了？
为什么会这样？我上周更新了我的机器。
W: GPG error: http://repo.mysql.com trusty InRelease: The following signatures were invalid: KEYEXPIRED 1487236823 KEYEXPIRED 1487236823 KEYEXPIRED 1487236823
几分钟前，我运行了以下命令，它一如既往地工作正常：
sudo apt update
sudo apt upgrade
然后大约 10 分钟后，我再次运行相同的命令：
sudo apt-get update
Ign http: //fi.archive.ubuntu.com trusty InRelease
Get:1 http://fi.archive.ubuntu.com trusty-updates InRelease [64.4 kB]
Ign http: //dl.google.com stable InRelease
Get:2 http://fi.archive.ubuntu.com trusty-backports InRelease [64.5 kB]
Ign http: //fi.archive.ubuntu.com trusty-updates InRelease
E: GPG error: http: //fi.archive.ubuntu.com trusty-updates InRelease:
The following signatures were invalid: NODATA 2
我尝试阅读 askubuntu 和 stackoverflow 中的每篇文章，但到目前为止一无所获。以下是唯一对我来说听起来合理但并没有解决问题的事情：
sudo mv lists lists.old
sudo mkdir -p lists/partial
sudo apt-get clean
sudo apt-get update
任何建议表示赞赏。
更新：即使我的 Thunderbird 在加载后立即崩溃，所以我重新启动计算机并apt-get update …