本文转载于 https://blog.csdn.net/trokey/article/details/84908838
安装好CenOS7后，自带的yum不能直接使用，使用会出现如下问题：
原因是没有配置yum源，修改/etc/yum.repos.d/CenOS-Base.repo文件内容如下：
# CentOS-Base.repo
#
# The mirror system uses the connecting IP address of the client and the
# update status of each mirror to pick mirrors that are updated to and
# geographically close to the client. You should use this for CentOS updates
# unless you are manually picking other mirrors.
#
# If the mirrorlist= does not work for you, as a fall back you can try the
# remarked out baseurl= line instead.
#
# [base]
name=CentOS-$releasever - Base
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os&infra=$infra
#baseurl=http://mirror.centos.org/centos/$releasever/os/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7 #released updates
[updates]
name=CentOS-$releasever - Updates
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=updates&infra=$infra
#baseurl=http://mirror.centos.org/centos/$releasever/updates/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7 #additional packages that may be useful
[extras]
name=CentOS-$releasever - Extras
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=extras&infra=$infra
#baseurl=http://mirror.centos.org/centos/$releasever/extras/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7 #additional packages that extend functionality of existing packages
[centosplus]
name=CentOS-$releasever - Plus
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=centosplus&infra=$infra
#baseurl=http://mirror.centos.org/centos/$releasever/centosplus/$basearch/
gpgcheck=1
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
编辑命令如下：
vi /etc/yum.repos.d/CentOS-Base.repo
保存命令
按ESC键 跳到命令模式，然后：
:w 保存文件但不退出vi
:w file 将修改另外保存到file中，不退出vi
:w! 强制保存，不推出vi
:wq 保存文件并退出vi
:wq! 强制保存文件，并退出vi
q: 不保存文件，退出vi
:q! 不保存文件，强制退出vi
:e! 放弃所有修改，从上次保存文件开始再编辑
或者先删除原有的文件
rm -f /etc/yum.repos.d/CentOS-Base.repo
然后重新下载阿里的
wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo
清理缓存
yum clean all
ps:如果上述方法没有解决，尝试下面：
删除yum.repos.d目录下所有文件
rm -f /etc/yum.repos.d/*
然后重新下载阿里的
wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo
清理缓存
yum clean all
测试下载安装
yum install gcc
结果如下：