there are no enabled repos.
mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.backup
cd /etc/yum.repos.d/
wget http://mirrors.163.com/.help/CentOS7-Base-163.repo
yum clean all
yum makecache
安装nginx时yum install nginx报错：CentOS No package nginx available.
先安装epel:yum install epel-release
然后再安装nginx