两步解决yum无法安装软件问题：Cannot find a valid baseurl for repo: centos-sclo-rh/x86_64

报错信息：

[root@iZwz946ibli8ikuyqgtc58Z ~]# yum install rh-redis5-redis
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
Could not retrieve mirrorlist http://mirrorlist.centos.org?arch=x86_64&release=7&repo=sclo-rh error was
14: curl#6 - "Could not resolve host: mirrorlist.centos.org; Unknown error"


 One of the configured repositories failed (Unknown),
 and yum doesn't have enough cached data to continue. At this point the only
 safe thing yum can do is fail. There are a few ways to work "fix" this:

     1. Contact the upstream for the repository and get them to fix the problem.


▽
     2. Reconfigure the baseurl/etc. for the repository, to point to a working
        upstream. This is most often useful if you are using a newer
        distribution release than is supported by the repository (and the
        packages for the previous distribution release still work).

     3. Run the command with the repository temporarily disabled
            yum --disablerepo= ...

     4. Disable the repository permanently, so yum won't use it by default. Yum
        will then just ignore the repository until you permanently enable it
        again or use --enablerepo for temporary usage:

            yum-config-manager --disable <repoid>
        or
            subscription-manager repos --disable=<repoid>

     5. Configure the failing repository to be skipped, if it is unavailable.
        Note that yum will try to contact the repo. when it runs most commands,
        so will have to try and fail each time (and thus. yum will be be much
        slower). If it is a very temporary problem though, this is often a nice
        compromise:

            yum-config-manager --save --setopt=<repoid>.skip_if_unavailable=true

Cannot find a valid baseurl for repo: centos-sclo-rh/x86_64

出错原因： 上述错误信息表明无法检索镜像列表，原因是 http://mirrorlist.centos.org 已经停止服务了，因此我们需要更改scl源的baseUrl。

解决方案：
1、进入/etc/yum.repos.d目录，删除目录下面所有的软件包，删除命令如下：

sudo rm -rf *

2、下载安装阿里云镜像仓库：两条命令都行

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

sudo wget -O /etc/yum.repos.d/CentOS-Base.repo https://mirrors.aliyun.com/repo/Centos-7.repo

3、下载安装阿里云epel.repo：

sudo curl -o /etc/yum.repos.d/epel.repo https://mirrors.aliyun.com/repo/epel-7.repo

sudo wget -O /etc/yum.repos.d/epel.repo https://mirrors.aliyun.com/repo/epel-7.repo

4、清除缓存：

yum clean all 

5、生成缓存：

yum makecache
yum makecache fast

6、安装gcc-c++测试：

sudo yum -y install gcc-c++

7、结果展示：安装成功