运行centos7中yum命令时提示：
Existing lock /var/run/yum.pid: another copy is running as pid 3936.
Another app is currently holding the yum lock; waiting for it to exit...
The other application is: PackageKit
Memory : 26 M RSS (355 MB VSZ)
Started: Wed Jul 19 23:21:24 2017 - 00:16 ago
State : Uninterruptible, pid: 3936
Another app is currently holding the yum lock; waiting for it to exit...
解决方式：rm -f /var/run/yum.pid
将该文件删除，然后再次运行yum。