什么是安装源？
安装源类似于一个软件仓库，绝大部分系统软件和工具都可以从软件仓库中自动下载和安装。
Ubuntu安装后默认的官方源是美国的服务器，因此访问速度很慢。所以需要切换成国内的
切换安装源
- 安装源位置：/etc/apt/sources.list
- 使用gedit(或者vim等)命令，打开sources.list文件，修改安装源 gedit /etc/apt/sources.list
- 修改保存后，更新源 apt-get update
国内源：
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