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Ubuntu 的软件源配置文件是 /etc/apt/sources.list。在修改之前，先将系统自带的该文件做个备份，将该文件替换为清华源官网提供的内容，即可使用 TUNA 的软件源镜像。最重要的是使用清华源可以走ipv6~
清华源官网：https://mirror.tuna.tsinghua.edu.cn/help/ubuntu/
- 首先需要使用root用户
sudo -s
- 进行源码备份.
cp /etc/apt/sources.list /etc/apt/sources.list.bak
- 进行源列表的修改
vi /etc/apt/sources.list
将sources.list中的内容提换如下
# 默认注释了源码镜像以提高 apt update 速度，如有需要可自行取消注释
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
# 预发布软件源，不建议启用
# 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
如图所示：
4. 查看是否已经修改
cat /etc/apt/sources.list
可以看到已经修改成功~
5. 执行更新列表
sudo apt-get update
如图所示：
至此，已经将源更换为清华源~~