我习惯于使用“pdfjoin”，这是基于 Debian 的较旧发行版（8 及更早版本）中的一个小实用程序。当我迁移到 Ubuntu 18.04 时，我意识到该命令不再可用。它去了哪里？
这与 Ubuntu 18.04 LTS 有关。我试图通过删除不需要的 Linux 内核来节省空间，但看起来它仍然会安装其他“未签名”的内核映像。为什么是这样？
这是日志：
root@ubuntu18:~# apt-get remove linux-image-4.18.0-15-generic
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following additional packages will be installed:
linux-image-unsigned-4.18.0-15-generic
Suggested packages:
fdutils linux-hwe-doc-4.18.0 | linux-hwe-source-4.18.0 linux-hwe-tools
The following packages will be REMOVED:
linux-image-4.18.0-15-generic linux-modules-extra-4.18.0-15-generic
The following NEW packages will be installed:
linux-image-unsigned-4.18.0-15-generic
0 upgraded, 1 newly installed, 2 to remove and 8 not upgraded.
Need to get 8,219 kB of archives.
After this operation, 168 MB disk space …