另一种临时代理

export http_proxy=http://<proxy>:<port>
export https_proxy=http://<proxy>:<port>
sudo -E add-apt-repository ppa:linaro-maintainers/toolchain

https://askubuntu.com/questions/53146/how-do-i-get-add-apt-repository-to-work-through-a-proxy

如果没有代理配置文件就创建一个
sudo touch /etc/apt/apt.conf.d/proxy.conf

编辑配置
sudo vim /etc/apt/apt.conf.d/proxy.conf

Acquire::http::Proxy "http://[username]:[password]@ [proxy-web-or-IP-address]:[port-number]";
Acquire::https::Proxy "http://[username]:[password]@ [proxy-web-or-IP-address]:[port-number]";

输入 sudo apt update 测试

参考
https://linuxiac.com/how-to-use-apt-with-proxy/
https://phoenixnap.com/kb/ubuntu-proxy-settings

设置清华源

Ubuntu 的软件源配置文件是 /etc/apt/sources.list。将系统自带的该文件做个备份，将该文件替换为下面内容，即可使用选择的软件源镜像。

# 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/ jammy main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ jammy main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ jammy-updates main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ jammy-updates main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ jammy-backports main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ jammy-backports main restricted universe multiverse

# deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ jammy-security main restricted universe multiverse
# # deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ jammy-security main restricted universe multiverse

deb http://security.ubuntu.com/ubuntu/ jammy-security main restricted universe multiverse
# deb-src http://security.ubuntu.com/ubuntu/ jammy-security main restricted universe multiverse

# 预发布软件源，不建议启用
# deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ jammy-proposed main restricted universe multiverse
# # deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ jammy-proposed main restricted universe multiverse

参考
https://zhuanlan.zhihu.com/p/251009600
https://mirrors.tuna.tsinghua.edu.cn/help/ubuntu/