Ubuntu20使用apt安装gcc-5（使用Ubuntu阿里云镜像）

添加 xenial 到 /etc/apt/sources.list

参考 https://askubuntu.com/questions/1235819/ubuntu-20-04-gcc-version-lower-than-gcc-7 这篇文章，

核心是 Add xenial to /etc/apt/sources.list

但是，我之前参考 https://developer.aliyun.com/mirror/ubuntu?spm=a2c6h.13651102.0.0.3e221b11BmMBfZ 配置过国内镜像

所以，sudo vim /etc/apt/sources.list 内容是这样的：

deb https://mirrors.aliyun.com/ubuntu/ focal main restricted universe multiverse
deb-src https://mirrors.aliyun.com/ubuntu/ focal main restricted universe multiverse

deb https://mirrors.aliyun.com/ubuntu/ focal-security main restricted universe multiverse
deb-src https://mirrors.aliyun.com/ubuntu/ focal-security main restricted universe multiverse

deb https://mirrors.aliyun.com/ubuntu/ focal-updates main restricted universe multiverse
deb-src https://mirrors.aliyun.com/ubuntu/ focal-updates main restricted universe multiverse

# deb https://mirrors.aliyun.com/ubuntu/ focal-proposed main restricted universe multiverse
# deb-src https://mirrors.aliyun.com/ubuntu/ focal-proposed main restricted universe multiverse

deb https://mirrors.aliyun.com/ubuntu/ focal-backports main restricted universe multiverse
deb-src https://mirrors.aliyun.com/ubuntu/ focal-backports main restricted universe multiverse

然后，我就在文件末尾追加了：

deb https://mirrors.aliyun.com/ubuntu/ xenial main
deb-src https://mirrors.aliyun.com/ubuntu/ xenial main

deb https://mirrors.aliyun.com/ubuntu/ xenial universe
deb-src https://mirrors.aliyun.com/ubuntu/ xenial universe

xenial 这个选项在 ubuntu 16.04 配置下是默认的

安装gcc-5

接下来就可以顺利执行一下安装命令了

geekziyu@geekziyu-ubuntu-1:~$ sudo apt update
geekziyu@geekziyu-ubuntu-1:~$ sudo apt install g++-5 gcc-5

切换GCC默认版本到gcc-5

geekziyu@geekziyu-ubuntu-1:~$ sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-5 5
geekziyu@geekziyu-ubuntu-1:~$ sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-5 5

如果有多个gcc版本，输入以下命令，再输入数字手动配置正确的 gcc/g++ 版本

geekziyu@geekziyu-ubuntu-1:~$ sudo update-alternatives --config gcc
geekziyu@geekziyu-ubuntu-1:~$ sudo update-alternatives --config g++

最后使用 gcc -v 和 g++ -v 查看当前版本是否正确