Concise way to “minify” XML using command line tools?

[Buddah]

I have a bunch of XML files I'm using for user interface and string translation in my project, each of which have the following structure:\[code\]<?xml version="1.0" encoding="UTF-8" ?><messages> <message id="x"> <!-- Text node or arbitrary XHTML markup in here --> </message> <message id="y"> <!-- Text node or arbitrary XHTML markup in here --> </message> <message id="z"> <!-- Text node or arbitrary XHTML markup in here --> </message> ...</messages>\[/code\]As part of my build process I'd like to "minify" these files into a single XML file, whereby each \[code\]<message>\[/code\] tag and all of its children are embedded within a \[code\]<messages>\[/code\] tag.The current solution I have is using grep to rip out the XML prolog, opening messages tag and closing messages tag from each file, and concatenating the result to a new file, after concatenating the XML prolog and opening messages, then finally concatenating the closing messages tag. This solution is... rather messy and error prone.So, how can I use any command-line XML tools to automate this process? Could I use something like xmlpatterns and/or XSL transforms?Side question: how would I verify that each \[code\]<message>\[/code\] tag has an ID attribute, and that all ID attribute values in the final document are unique? I know I can do the first part by means of a DTD, but is the second also in the realm of DTDs or would I need to do something else?