从我遇到这个问题的日子开始。当我运行sudo apt update它说
1 package can be upgraded. Run 'apt list --upgradable' to see it.
和sudo apt list --upgradable -a输出是
[NAME]/unknown 3.1.2-2.1 amd64 [upgradable from: 3.0.1-19.1]
[NAME]/now 3.0.1-19.1 amd64 [installed,upgradable to: 3.1.2-2.1]
但是sudo apt upgrade说没有什么可以升级的
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
我什至不明白它是否取决于 Ubuntu 17.04 64 位版本或软件包存储库。更奇怪的是，sudo apt-get update它没有显示我有 1 个要升级的软件包，而是sudo apt-get upgrade说
The following packages have been kept back:
[NAME]
解决方案：经过多次尝试（均失败），我决定重新安装 Ubuntu，现在一切正常。