Set the text value of a node using xpath

XPath by itself cannot be used to modify XML documents. But the xmlstarlet command line utility can do it. Example:

xml ed -u "//book[1]/title" -v "Game of Thrones" bookstore.xml

Output:

<?xml version="1.0" encoding="ISO-8859-1"?><bookstore>  <book>    <title lang="eng">Game of Thrones</title>    <price>29.99</price>  </book>  <book>    <title lang="eng">Learning XML</title>    <price>39.95</price>  </book></bookstore>

Note: without [1] in the expression, the result would be:

<?xml version="1.0" encoding="ISO-8859-1"?><bookstore>  <book>    <title lang="eng">Game of Thrones</title>    <price>29.99</price>  </book>  <book>    <title lang="eng">Game of Thrones</title>    <price>39.95</price>  </book></bookstore>

The command above outputs the result to stdout. There is an option (-L or --inplace) that modifies the document in place:

xml ed -L -u "//book[1]/title" -v "Game of Thrones" bookstore.xml 

This option is not mentioned in the documentation for xmlstarlet 1.3.1, but it is shown when executing

xml ed --help

As far as I know, XPath itself cannot be used for that, because it's only a query language, so it's not for creation or modification of XML files. You'll need some other tool for that. If you're into command line tools, maybe check xsltproc, which is a part of libxslt. I'm not an XSLT expert, but a quick look at the linked page gives me a feeling that you can achieve the desired result with <if> and <value-of>. Or you could parse XML with regular expressions, but please don't, I hear it's a slippery slope.