How to Resolve YUM Lock Issue : Another app is currently holding the yum lock; waiting for it to exit...

If you are Linux user or System Administrator then you might face below issue while trying to use YUM command for software package management. It's a very common issue.

The error states that :

Existing lock /var/tmp/yum-vagrant-qt0qu8/x86_64/6/yum.pid: another copy is 
running as pid 5025.
Another app is currently holding the yum lock; waiting for it to exit...
  The other application is: yum
    Memory :  20 M RSS (310 MB VSZ)
    Started: Mon Jun 26 10:49:35 2017 - 00:14 ago
    State  : Traced/Stopped, pid: 5025

This error message tells us two things:

• There is another yum instance running by some processes that are holding the lock.
• The origin of this situation implies that the system does not support YUM concurrent operations.

This is "How To" guide in which we would discuss the solution / workaround of this problem.

How to install EPEL Repository on RHEL/CentOS 7.x/6.x/5.x

This is "How To" guide to show us, how one can configure RHEL/CentOS system to use Extra Packages from EPEL repository. After enabling the EPEL repository, one can easily install standard open source packages and their dependencies by using YUM command.  

In this article, we would discuss below points:
What is EPEL?
Why one should use EPEL repository?
How To enable EPEL repository on RHEL/CentOS 7/6/5?
How To verify if EPEL repository is enabled or not?
How To use EPEL repository?