Explain XML code

What this XML sample shows

elements and nesting

An XML file is one tree: a single root element, here <catalog>, with every other element inside it, and each opening tag closed in the right order. Tag names are case-sensitive, so <Name>Blue mug</name> stopped the parser with Opening and ending tag mismatch: Name line 7 and name.

attributes

Attributes add small facts to an element, in quotes inside its opening tag: id="mug-blue", inStock="true", currency="USD". The XPath query //product[@inStock="true"]/name/text() picked out Blue mug, and string(/catalog/@updated) read 2026-11-05.

escaping and CDATA

< and & have jobs in XML, so in text they're written < and &: Plates & bowls set reads back as Plates & bowls set. A plain & there stopped the parser with xmlParseEntityRef: no name. Inside <![CDATA[ ... ]]> nothing is read as markup, so the description came back as the text Fits <b>4 people</b>; dishwasher safe.

schemas

A schema (.xsd) says which elements may appear, in what order and with what types. Well-formed isn't the same as valid: with the first <name> removed, the file still parsed, but the schema check said Element 'price': This element is not expected. Expected is ( name ). A price written 12,50 failed with '12,50' is not a valid value of the atomic type 'xs:decimal'.

Good to know

• Checked with xmllint, libxml2 2.15.3: xmllint --noout catalog.xml found no errors, --schema catalog.xsd printed catalog.xml validates, and XPath counted 2 tags and summed the prices to 60.5.
• <?xml version="1.0" encoding="UTF-8"?> is the declaration, and it has to come first. With a comment or even one blank line above it, xmllint stopped with XML declaration allowed only at the start of the document.
• In the schema, id has the type xs:ID, so two products with id="mug-blue" failed with 'mug-blue' is not a valid value of the atomic type 'xs:ID': each ID can be used once per file.

Watch out: A comment can't contain two hyphens in a row. With check with xmllint --schema in the comment on line 2, the whole file failed to parse with Double hyphen within comment, before any element was read.