Centos8:无法使用yum命令安装

问题：centos 8 使用yum命令报错信息如下：

Errors during downloading metadata for repository 'AppStream':
  - Status code: 404 for http://mirrors.aliyun.com/centos/8/AppStream/x86_64/os/repodata/repomd.xml (IP: 39.96.118.191)
Error: Failed to download metadata for repo 'AppStream': Cannot download repomd.xml: Cannot download repodata/repomd.xml: All mirrors were tried

方案一(不可行)：修改vim CentOS-AppStream.repo中baseurl的地址，报错信息如下：只修改下载源路径不可行，大家别踩坑哦

Errors during downloading metadata for repository 'BaseOS':
  - Status code: 404 for http://mirrors.cloud.aliyuncs.com/centos/8/BaseOS/x86_64/os/repodata/repomd.xml (IP: 100.100.2.148)
Error: Failed to download metadata for repo 'BaseOS': Cannot download repomd.xml: Cannot download repodata/repomd.xml: All mirrors were tried

方案二(实测有效)：执行以下4步即可

1. 运行下面的命令备份文件

rename '.repo' '.repo.bak' /etc/yum.repos.d/*.repo

2. 运行下面命令，下载最新的repo文件

wget https://mirrors.aliyun.com/repo/Centos-vault-8.5.2111.repo -O /etc/yum.repos.d/Centos-vault-8.5.2111.repo
wget https://mirrors.aliyun.com/repo/epel-archive-8.repo -O /etc/yum.repos.d/epel-archive-8.repo #如果后期 yum clean all && yum makecache报错下载xml文件404，则不需要执行这一句

3. 运行以下命令替换repo文件中的链接。

sed -i 's/mirrors.cloud.aliyuncs.com/url_tmp/g'  /etc/yum.repos.d/Centos-vault-8.5.2111.repo &&  sed -i 's/mirrors.aliyun.com/mirrors.cloud.aliyuncs.com/g' /etc/yum.repos.d/Centos-vault-8.5.2111.repo && sed -i 's/url_tmp/mirrors.aliyun.com/g' /etc/yum.repos.d/Centos-vault-8.5.2111.repo
sed -i 's/mirrors.aliyun.com/mirrors.cloud.aliyuncs.com/g' /etc/yum.repos.d/epel-archive-8.repo  #如果后期 yum clean all && yum makecache报错下载xml文件404，则不需要执行这一句

4. 重新创建缓存

yum clean all && yum makecache