【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 00:00:00 extras | 3.4 kB 00:00:00 updates | 3.4 kB 00:00:00 (1/12): base/x86_64/group_gz | 166 kB 00:00:00 (2/12): base/x86_64/primary_db | 6.0 MB 00:00:00 (3/12): base/x86_64/filelists_db | 7.1 MB 00:00:00 (4/12): base/x86_64/other_db | 2.6 MB 00:00:00 (5/12): extras/x86_64/prestodelta | 73 kB 00:00:00 (6/12): extras/x86_64/primary_db | 215 kB 00:00:00 (7/12): extras/x86_64/other_db | 131 kB 00:00:00 (8/12): extras/x86_64/filelists_db | 249 kB 00:00:00 (9/12): updates/x86_64/prestodelta | 945 kB 00:00:00 (10/12): updates/x86_64/filelists_db | 5.2 MB 00:00:00 (11/12): updates/x86_64/other_db | 764 kB 00:00:00 (12/12): updates/x86_64/primary_db | 7.4 MB 00:00:00 Metadata Cache Created