某天安装一些常用软件，比如lrzsz的时候出错了
$ sudo apt-get install lrzsz
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Unable to locate package lrzsz
出现这个问题的原因之一是apt的源缺失，运行以下命令即可
sudo add-apt-repository main
sudo add-apt-repository universe
sudo add-apt-repository restricted
sudo add-apt-repository multiverse
sudo apt-get update