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给n1 安装的是 debian stretch（是debian9，一个更老的稳定版，详情请见 https://www.debian.org/releases/index.zh-cn.html 目前我推荐使用的版本是 debian10 buster）

在更换了中科大的源之后(修改 /etc/apt/sources.list,修改前将原始的文件备份到 /etc/apt/sources.list.bak)

/etc/apt/sources.list

deb https://mirrors.ustc.edu.cn/debian/ stretch main contrib non-free
deb-src https://mirrors.ustc.edu.cn/debian/ stretch main contrib non-free

deb https://mirrors.ustc.edu.cn/debian/ stretch-updates main contrib non-free
deb-src https://mirrors.ustc.edu.cn/debian/ stretch-updates main contrib non-free

deb https://mirrors.ustc.edu.cn/debian/ stretch-backports main contrib non-free
deb-src https://mirrors.ustc.edu.cn/debian/ stretch-backports main contrib non-free

deb https://mirrors.ustc.edu.cn/debian-security/ stretch/updates main contrib non-free
deb-src https://mirrors.ustc.edu.cn/debian-security/ stretch/updates main contrib non-free

下面这是原始的 /etc/apt/sources.list.bak

deb http://httpredir.debian.org/debian stretch main contrib non-free
#deb-src http://httpredir.debian.org/debian stretch main contrib non-free

deb http://httpredir.debian.org/debian stretch-updates main contrib non-free
#deb-src http://httpredir.debian.org/debian stretch-updates main contrib non-free

deb http://httpredir.debian.org/debian stretch-backports main contrib non-free
#deb-src http://httpredir.debian.org/debian stretch-backports main contrib non-free

deb http://security.debian.org/ stretch/updates main contrib non-free
#deb-src http://security.debian.org/ stretch/updates main contrib non-free

在执行 apt update 遇到如下错误

GPG error: https://mirrors.ustc.edu.cn/debian stretch-backports InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 648ACFD622F3D138 NO_PUBKEY 0E98404D386FA1D9
E: The repository 'https://mirrors.ustc.edu.cn/debian stretch-backports InRelease' is not signed

解决办法

分别执行

gpg --keyserver keyserver.ubuntu.com --recv 648ACFD622F3D138

gpg --keyserver keyserver.ubuntu.com --recv 0E98404D386FA1D9

gpg --export --armor 648ACFD622F3D138 | sudo apt-key add -

gpg --export --armor 0E98404D386FA1D9 | sudo apt-key add -

可以使用 && 连接，使用一行命令执行: gpg --keyserver keyserver.ubuntu.com --recv 648ACFD622F3D138 && gpg --keyserver keyserver.ubuntu.com --recv 0E98404D386FA1D9 && gpg --export --armor 648ACFD622F3D138 | sudo apt-key add - && gpg --export --armor 0E98404D386FA1D9 | sudo apt-key add -

再次执行 apt update 就ok了