The least painful way depends on your other requirements. If the file is simple then simple line-oriented shell utilities you are familiar with should usually suffice. If you need to do anything which requires any real understanding of XML structures (nesting, sibling and similar relationships, conditionals etc) then it probably makes sense to get at least some introductory familiarity to some proper XML tool. There are several formalisms to choose from and many tools which implement them; personally, I've been able to get things done with xsltproc after the first shock of trying to understand what got into the heads of the people who came up with the specifications for this.