图1
1、在 CentOS 7.6 64位 下执行命令：yum makecache ，以更新 yum 源 的缓存，报错：There are no enabled repos，如图1
[root@iZ8vb0tg9av3a17vlbfi28Z ~]# yum makecache
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
There are no enabled repos.
Run "yum repolist all" to see the repos you have.
To enable Red Hat Subscription Management repositories:
subscription-manager repos --enable <repo>
To enable custom repositories:
yum-config-manager --enable <repo>
2、决定重新安装 7.6 的 release 包，先到阿里云源的网站，查找到对应的 release 包，复制其链接，如图2
3、依次执行如下命令，如图3
wget https://mirrors.aliyun.com/centos/7.6.1810/os/x86_64/Packages/centos-release-7-6.1810.2.el7.centos.x86_64.rpm rpm -ivh --replacepkgs --replacefiles centos-release-7-6.1810.2.el7.centos.x86_64.rpm yum update
4、执行命令：yum makecache ，以更新 yum 源 的缓存，未再报错，符合预期，如图4
[root@iZ8vb0tg9av3a17vlbfi28Z ~]# yum makecache Loaded plugins: fastestmirror Loading mirror speeds from cached hostfile * base: mirrors.tuna.tsinghua.edu.cn * extras: mirrors.huaweicloud.com * updates: mirrors.huaweicloud.com base | 3.6 kB 00:00 extras | 3.4 kB 00:00 updates | 3.4 kB 00:00 (1/8): extras/7/x86_64/prestodelta | 47 kB 00:00 (2/8): extras/7/x86_64/filelists_db | 231 kB 00:00 (3/8): extras/7/x86_64/other_db | 118 kB 00:00 (4/8): updates/7/x86_64/prestodelta | 358 kB 00:00 (5/8): updates/7/x86_64/other_db | 351 kB 00:00 (6/8): updates/7/x86_64/filelists_db | 2.2 MB 00:00 (7/8): base/7/x86_64/other_db | 2.6 MB 00:00 (8/8): base/7/x86_64/filelists_db | 7.1 MB 00:04 Metadata Cache Created
