一、卸载自带软件包

rpm -qa | grep yum | xargs rpm -e --nodeps

二、保证上网正常

ping www.baidu.com

三、进入以下网站下载对应的包

1、阿里云网络源地址：https://mirrors.aliyun.com/centos/7/os/x86_64/Packages/

2、网易163网络源地址：http://mirrors.163.com/

3、CentOS网络源地址：http://centos.ustc.edu.cn/centos/

使用CTRL+F输入关键字查找对应的安装包即可。

四、找到安装包后下载下来

若出现404错误去重新查询最新rpm包的链接

wget https://mirrors.aliyun.com/centos/7/os/x86_64/Packages/yum-metadata-parser-1.1.4-10.el7.x86_64.rpm
wget https://mirrors.aliyun.com/centos/7/os/x86_64/Packages/yum-utils-1.1.31-45.el7.noarch.rpm
wget https://mirrors.aliyun.com/centos/7/os/x86_64/Packages/yum-3.4.3-158.el7.centos.noarch.rpm
wget https://mirrors.aliyun.com/centos/7/os/x86_64/Packages/yum-plugin-fastestmirror-1.1.31-45.el7.noarch.rpm
wget https://mirrors.aliyun.com/centos/7/os/x86_64/Packages/python-urlgrabber-3.10-8.el7.noarch.rpm

五、强制安装rpm包

--force --nodeps忽略依赖检测的强制安装

rpm -ivh python-urlgrabber-3.10-8.el7.noarch.rpm --force --nodeps
rpm -ivh yum-3.4.3-158.el7.centos.noarch.rpm --force --nodeps
rpm -ivh yum-metadata-parser-1.1.4-10.el7.x86_64.rpm --force --nodeps
rpm -ivh yum-plugin-fastestmirror-1.1.31-45.el7.noarch.rpm --force --nodeps
rpm -ivh yum-utils-1.1.31-45.el7.noarch.rpm --force --nodeps

六、查看yum相关的包是否已安装

[root@localhost ~]# rpm -qa |grep yum
yum-utils-1.1.31-45.el7.noarch
yum-3.4.3-158.el7.centos.noarch
yum-plugin-fastestmirror-1.1.31-45.el7.noarch
yum-metadata-parser-1.1.4-10.el7.x86_64
[root@localhost ~]# rpm -qa |grep python-urlgrabber
python-urlgrabber-3.10-8.el7.noarch

七、修改repo

1、下载源文件

curl -o /etc/yum.repos.d/CentOS-7.repo http://mirrors.aliyun.com/repo/Centos-7.repo

2、需要把CentOS-7.repo文件中的$releasever全部替换为7

cd /etc/yum.repos.d/
vim CentOS-7.repo

3、在vim中执行:%s/$releasever/7/g快速替换。保存退出。

:%s/$releasever/7/g

八、清空重载yum

yum clean all
yum update

参考链接：
https://blog.csdn.net/feng_zhiyu/article/details/80464336
https://www.jianshu.com/p/fd00c304ae5d

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