为centos7配置阿里yum源遇到的问题以及解决方法

【问题背景】

卸载安装的Ambari，之前都是因为卸载不干净。这次重写安装，卸载完之后，发现httpd无法启动，所以想卸载httpd进行重新安装，但是执行命令yum list | grep httpd报错。

【遇到的问题】

配置过程很简单，去https://opsx.alibaba.com/mirror 获得执行命令：

但是我在执行yum makecache时报错，报错信息如下：

Loaded plugins: fastestmirror, langpacks, priorities
Repository epel is listed more than once in the configuration
Cleaning repos: base epel extras updates
Cleaning up everything
Maybe you want: rm -rf /var/cache/yum, to also free up space taken by orphaned data from disabled or removed repos
Cleaning up list of fastest mirrors
[root@hadoop01 yum.repos.d]# yum makecache
Loaded plugins: fastestmirror, langpacks, priorities
Repository epel is listed more than once in the configuration
Determining fastest mirrors
 * base: mirrors.aliyun.com
 * extras: mirrors.aliyun.com
 * updates: mirrors.aliyun.com
http://mirrors.aliyun.com/centos/%24releasever/os/x86_64/repodata/repomd.xml: [Errno 14] HTTP Error 404 - Not Found
Trying other mirror.
To address this issue please refer to the below wiki article 

https://wiki.centos.org/yum-errors

If above article doesn't help to resolve this issue please use https://bugs.centos.org/.

http://mirrors.aliyuncs.com/centos/%24releasever/os/x86_64/repodata/repomd.xml: [Errno 12] Timeout on http://mirrors.aliyuncs.com/centos/$releasever/os/x86_64/repodata/repomd.xml: (28, 'Connection timed out after 30000 milliseconds')
Trying other mirror.
http://mirrors.cloud.aliyuncs.com/centos/%24releasever/os/x86_64/repodata/repomd.xml: [Errno 14] curl#6 - "Could not resolve host: mirrors.cloud.aliyuncs.com; Unknown error"
Trying other mirror.
http://mirrors.aliyuncs.com/centos/%24releasever/os/x86_64/repodata/repomd.xml: [Errno 12] Timeout on http://mirrors.aliyuncs.com/centos/$releasever/os/x86_64/repodata/repomd.xml: (28, 'Connection timed out after 30001 milliseconds')
Trying other mirror.
http://mirrors.aliyuncs.com/centos/%24releasever/os/x86_64/repodata/repomd.xml: [Errno 12] Timeout on http://mirrors.aliyuncs.com/centos/$releasever/os/x86_64/repodata/repomd.xml: (28, 'Connection timed out after 30001 milliseconds')
Trying other mirror.
http://mirrors.aliyuncs.com/centos/%24releasever/os/x86_64/repodata/repomd.xml: [Errno 12] Timeout on http://mirrors.aliyuncs.com/centos/$releasever/os/x86_64/repodata/repomd.xml: (28, 'Connection timed out after 30001 milliseconds')
Trying other mirror.
http://mirrors.aliyuncs.com/centos/%24releasever/os/x86_64/repodata/repomd.xml: [Errno 12] Timeout on http://mirrors.aliyuncs.com/centos/$releasever/os/x86_64/repodata/repomd.xml: (28, 'Connection timed out after 30001 milliseconds')
Trying other mirror.
http://mirrors.aliyuncs.com/centos/%24releasever/os/x86_64/repodata/repomd.xml: [Errno 12] Timeout on http://mirrors.aliyuncs.com/centos/$releasever/os/x86_64/repodata/repomd.xml: (28, 'Connection timed out after 30001 milliseconds')
Trying other mirror.
...

【解决方法】

在网上找了好多帖子，都没能解决我的问题，后来看到是HTTP请求失败，尝试本地访问之前wget下来的 CentOS-Base.repo 发现其中的：

http://mirrors.aliyun.com/centos/$releasever/os/$basearch/

等URL通过http的形式根本无法访问，此时将变量$releasever改为7（我的系统是centos 7），发现可以访问了，顺便将所有的变量$releasever都改为7。感觉这个变量$releasever根本就没起作用呀。

改完之后，执行yum clean all 以及 yum makecache 成功。

oaded plugins: fastestmirror, langpacks
Determining fastest mirrors
 * base: mirrors.aliyun.com
 * extras: mirrors.aliyun.com
 * updates: mirrors.aliyun.com
base                                                                                                                 | 3.6 kB  ::
extras                                                                                                               | 3.4 kB  ::
updates                                                                                                              | 3.4 kB  ::
(/): base/x86_64/group_gz                                                                                         |  kB  ::
(/): base/x86_64/primary_db                                                                                       | 6.0 MB  ::
(/): base/x86_64/filelists_db                                                                                     | 7.1 MB  ::
(/): base/x86_64/other_db                                                                                         | 2.6 MB  ::
(/): extras/x86_64/prestodelta                                                                                    |   kB  ::
(/): extras/x86_64/primary_db                                                                                     |  kB  ::
(/): extras/x86_64/other_db                                                                                       |  kB  ::
(/): extras/x86_64/filelists_db                                                                                   |  kB  ::
(/): updates/x86_64/prestodelta                                                                                   |  kB  ::
(/): updates/x86_64/filelists_db                                                                                 | 5.2 MB  ::
(/): updates/x86_64/other_db                                                                                     |  kB  ::
(/): updates/x86_64/primary_db                                                                                   | 7.4 MB  ::
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