How to determine whether yum have changed something from a script

Here's the case. I need to create a script which basically runs yum -y
update, and after runs another script whose action depends on what yum did.

yum errors can be checked from the return code, but it seems that a
successful update and no update at all returns the same return code : 0.

Is there a scriptable-way to test whether yum did anything, other than
grepping the string "No Packages marked for Update" from yum output?

Regards,

Fajar

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