Existing lock /var/run/yum.pid: another copy is running

This is a very common error which you might come across in Red Hat Linux distro while trying to run yum command

Error:
# yum install bind
Loaded plugins: fastestmirror, refresh-packagekit, security
Existing lock /var/run/yum.pid: another copy is running as pid 3054.
Another app is currently holding the yum lock; waiting for it to exit...
The other application is: yum
Memory : 49 M RSS (343 MB VSZ)
Started: Fri Mar 14 03:32:19 2014 - 00:10 ago
State : Sleeping, pid: 3054
Reason
As you see from the error it says yum command is locked by another application. yum command can be run only one at a time in one machine. In case you try to run it on multiple terminals then you will get the above pop up error unless the lock is released by the application which is using yum.

Solution:
Look out for the process id and the user who is using the yum command

# ps aux | grep yum

root      3054  0.5  4.9 351016 49860 pts/1    S+   03:32   0:00 /usr/bin/python /usr/bin/yum install bind bind-chroot

root      3083  5.4  2.5 341876 25596 ?        S    03:34   0:01 /usr/bin/python /usr/share/PackageKit/helpers/yum/yumBackend.py refresh-cache yes

root      3087  0.0  0.0 103244   860 pts/2    S+   03:35   0:00 grep yum

So in my case 3054 PID is used by deepak user. You can either notify user deepak and check the time of how long his application will lock yum

or else you can go ahead and kill the PID
# kill -9 3054

Next try to run the command yum from your console. It should run like Milkha Singh (just kidding..).