1. yum lock
错误代码如下：
[root@localhost ~]# yum -y install epel-release
Loaded plugins: fastestmirror, langpacks
Existing lock /var/run/yum.pid: another copy is running as pid 44478.
Another app is currently holding the yum lock; waiting for it to exit...
The other application is: PackageKit
Memory : 26 M RSS (436 MB VSZ)
Started: Wed Oct 16 18:47:32 2019 - 00:25 ago
State : Sleeping, pid: 44478
解决办法：
rm -rf /var/run/yum.pid
