背景
有时使用apt-get安装依赖库或软件时，看到15KB~40KB的速度下载，真的是恨铁不成刚，为什么不能快一些呢；明明电脑下载时都有1M~3M的，感到无奈，一直在等待安装。如果排除是电脑本身网络速度慢问题，那很有可能是安装依赖库或软件是外网的，需要从外网那里获取；这样自然就很慢啦。
修改成国内软件源（apt-get）
1. 备份原始文件
sudo cp /etc/apt/sources.list /etc/apt/sources.list2
2.修改 sources.list 文件的权限
sudo chmod 777 /etc/apt/sources.list
3. 修改文件并添加国内源
vi /etc/apt/sources.list
不喜欢用 vi 编译器的可以使用 gedit
gedit /etc/apt/sources.list
4.注释原文件内的源并添加如下地址
不同的系统版本对应的软件源也是不一样的，之前被坑了；以为ubuntu系统下，使用的软件源应该都一样的，后来发现替换源后，部分软件无法安装，经过调查发现不同的系统版本对应的不同的软件源；
比如同一个品牌的软件源（网易163），也会区分是ubuntu18、ubuntu16还是ubuntu14系统的。
#阿里云 （推荐）Ubuntu18
deb http://mirrors.aliyun.com/ubuntu/ bionic main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ bionic-security main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ bionic-proposed main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ bionic-backports main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-security main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-proposed main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-backports main restricted universe multiverse
#网易163（这个速度比较快）Ubuntu18
deb http://mirrors.163.com/ubuntu/ bionic main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ bionic-security main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ bionic-updates main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ bionic-proposed main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ bionic-backports main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ bionic main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ bionic-security main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ bionic-updates main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ bionic-proposed main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ bionic-backports main restricted universe multiverse
#阿里云 （推荐）Ubuntu16
deb http://mirrors.aliyun.com/ubuntu/ xenial main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ xenial-security main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ xenial-updates main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ xenial-proposed main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ xenial-backports main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ xenial main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ xenial-security main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ xenial-updates main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ xenial-proposed main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ xenial-backports main restricted universe multiverse
#网易163（这个速度比较快）Ubuntu16
deb http://mirrors.aliyun.com/ubuntu/ xenial main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ xenial-security main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ xenial-updates main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ xenial-proposed main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ xenial-backports main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ xenial main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ xenial-security main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ xenial-updates main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ xenial-proposed main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ xenial-backports main restricted universe multiverse
#网易163（这个速度比较快）Ubuntu14
deb http://mirrors.163.com/ubuntu/ trusty main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ trusty-security main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ trusty-updates main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ trusty-proposed main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ trusty-backports main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ trusty main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ trusty-security main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ trusty-updates main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ trusty-proposed main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ trusty-backports main restricted universe multiverse
软件源-分析
每一行的开头的deb或者deb-src，分别表示通过.deb文件、源文件的方式进行安装。URL后面的5个用空格隔开的字符串，分别对应相应的目录结构。
1）当我们打开URL，比如Ubuntu14 的网易软件源的，http://mirrors.163.com/ubuntu/ 会看到如下图的目录结构：
2）进入dists目录，可以发现trusty、trusty-security、trusty-updates、trusty-proposed、trusty-backports 五个目录：
3）trusty、trusty-security、trusty-updates、trusty-proposed、trusty-backports 后面四个字段对应子目录：
deb http://mirrors.163.com/ubuntu/ trusty main restricted universe multiverse
细心对比后会发现以下规律：
| 系统版本 | Ubuntu14 | Ubuntu16 | Ubuntu18 |
| 软件源--关键字 | trusty | xenial | bionic |
5,更新源
sudo apt-get update
6. 更新软件( 可选)
sudo apt-get dist-upgrade
sudo apt-get upgrade
由于包与包之间存在各种依赖关系。upgrade只是简单的更新包，不管这些依赖，它不和添加包，或是删除包。而dist-upgrade可以根据依赖关系的变化，添加包，删除包。
upgrade:系统将现有的Package升级,如果有相依性的问题,而此相依性需要安装其它新的Package或影响到其它Package的相依性时,此Package就不会被升级,会保留下来. dist-upgrade:可以聪明的解决相依性的问题,如果有相依性问题,需要安装/移除新的Package,就会试着去安装/移除它. (所以通常这个会被认为是有点风险的升级)
希望对你有帮助；
如果有更新或不对的地方欢迎指正。