使用yum按照程序报错如下：
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. Disable the repository, 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>
4. 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 retrieve metalink for repository: epel/x86_64. Please verify its path and try again
解决办法之一：
vim /etc/yum.repos.d/epel.repo
PS:没有vim，可以用vi
将：
[epel] name=Extra Packages for Enterprise Linux 7 - $basearch #baseurl=http://download.fedoraproject.org/pub/epel/7/$basearch mirrorlist=https://mirrors.fedoraproject.org/metalink?repo=epel-7&arch=$basearch
改为：
[epel] name=Extra Packages for Enterprise Linux 7 - $basearch baseurl=http://download.fedoraproject.org/pub/epel/7/$basearch #mirrorlist=https://mirrors.fedoraproject.org/metalink?repo=epel-7&arch=$basearch
网上说还要执行下面几个命令，本人没有执行yum也得到了恢复：
yum clean all
yum makecache
yum update
yum install -y lsof