title: Equivalent in yum of apt-get update
body: <p>In Debian derivatives, before installing software from apt using <code>apt-get install xxx</code>, it is necessary to run <code>apt-get update</code>.  This refreshes the package lists so that dependencies are resolved correctly, etc.</p>
  
  <p>When using an RHEL instance, out of habit, I ran <code>yum update</code> before installing software.  However, this was only to find that <code>yum update</code> <em>upgraded</em> all of my packages instead of, or possibly in addition to, updating package lists locally as in <code>apt-get update</code>.</p>
  
  <p>Is it necessary to update yum's package lists before running <code>yum install xxx</code>?  If so, how do you do it?</p>