服务器上安装了最新的Ubuntu Server 18.04，代号为bionic。使用apt-get命令安装软件时，有时候速度比较慢，有时候会失败。因此考虑用国内的镜像源更换下apt-get的默认源。
1、ubuntu 查看版本号：
lsb_release -a
输出：
Distributor ID: Ubuntu
Description: Ubuntu 18.04.5 LTS
Release: 18.04
Codename: bionic
其中Codename表示代号。
2、备份系统自带源
mv /etc/apt/sources.list /etc/apt/sources.list.bak
3、修改/etc/apt/sources.list文件，加入想替换的国内源，我使用的是阿里云的源
vi /etc/apt/sources.list
由于服务器的版本是18.04，因此源地址的代号部分都是bionic，如果是其他版本，可以替换为对应的版本，具体版本代号可以网上查找。
4、替换源
**##阿里源**
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
**##163源**
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
**##清华源**
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic main restricted universe multiverse\
deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic main restricted universe multiverse\
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-updates main restricted universe multiverse\
deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-updates main restricted universe multiverse\
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-backports main restricted universe multiverse\
deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-backports main restricted universe multiverse\
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-security main restricted universe multiverse\
deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-security main restricted universe multiverse\
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-proposed main restricted universe multiverse\
deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-proposed main restricted universe multiverse
**##中科大源**
deb https://mirrors.ustc.edu.cn/ubuntu/ bionic main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ bionic main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ bionic-updates main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ bionic-updates main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ bionic-backports main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ bionic-backports main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ bionic-security main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ bionic-security main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ bionic-proposed main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ bionic-proposed main restricted universe multiverse
5、更新生效
apt-get upgrade
done!
