在Linux输入一些yum命令时出现如下错误：

Loaded plugins: fastestmirror
http://mirrors.163.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 knowledge base article 

https://access.redhat.com/articles/1320623

If above article doesn't help to resolve this issue please create a bug on https://bugs.centos.org/



 One of the configured repositories failed (CentOS-$releasever - Base - 163.com),
 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=base ...

     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 base
        or
            subscription-manager repos --disable=base

     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=base.skip_if_unavailable=true

failure: repodata/repomd.xml from base: [Errno 256] No more mirrors to try.
http://mirrors.163.com/centos/$releasever/os/x86_64/repodata/repomd.xml: [Errno 14] HTTP Error 404 - Not Found

是因为yum已经更新了的原因导致的

http://mirrors.163.com/centos/
进官网可以看到已经更新到7.x了

进入7可以看到有repodata/repomd.xml

打开CentOS7-Base.repo（名字可能不一样），一般在 ./etc/yum.repos.d文件夹下

vim /etc/yum.repos.d/CentOS7-Base.repo

将其中baseurl中的$releasever全改为7，如下图：

保存

输入命令yum makecache 即可。