看一下自己的ubuntu版本：
命令：lsb_release -a
输出：No LSB modules are available. Distributor ID: Ubuntu Description: Ubuntu 22.04 LTS Release: 22.04 Codename: jammy
一、安装gcc7
1.添加镜像sudo vim /etc/apt/sources.list
2.添加deb [arch=amd64] http://archive.ubuntu.com/ubuntu focal main universe
3.更新镜像sudo apt-get update
4.安装gcc7sudo apt-get -y install gcc-7
5.查看是否安装成功gcc-7 --version
6.配置update-alternativesupdate-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-7 7

二、安装gcc5
1.添加镜像sudo vim /etc/apt/sources.list
2.添加deb http://dk.archive.ubuntu.com/ubuntu/ xenial main
deb http://dk.archive.ubuntu.com/ubuntu/ xenial universe
3.添加缺失的公钥

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 40976EAF437D05B5
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 3B4FE6ACC0B21F32

4.安装gcc5sudo apt install gcc-5
5.配置update-alternativesupdate-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-5 5