RPM and YUM trick: synchronize packages installed on two hosts

The question was asked in IRC today:

hello folks, is there any way to install packages from a list written by yum list installed? I've two CentOS 6.3 hosts and I like to get them with the same packages installed (also versions)

Here is a quick (and accurate) answer:

1. Take and test backups to taste
2. Run:
   

   rpm -qa --qf '%{name} \n' | sort > MANIFEST

   Note: that is a backslash n -- the html markup makes it hard to see the distinction
3. Then copy that MANIFEST to the second unit and run:
   

   yum -y install `cat MANIFEST`

   Note: and here, backtick around the cat to get a sub-shell
4. Finish by running:
   

   yum -y update

   on each unit

For extra credit, re-run the MANIFEST creator on each unit, and use diff to find any variances