Introduction to XML

XML (Extensible Markup Language) is a text-based format for storing and transporting structured data. It is both human-readable and machine-parseable.

XML uses nested elements delimited by opening and closing tags. A well-formed XML document has exactly one root element:

<?xml version="1.0" encoding="UTF-8"?>
<library>
  <book id="1">
    <title>Learning XML</title>
    <author>Jane Smith</author>
    <year>2023</year>
  </book>
  <book id="2">
    <title>XML in Practice</title>
    <author>John Doe</author>
    <year>2024</year>
  </book>
</library>

XML was designed to be self-descriptive. Unlike HTML, you define your own tag names to suit your data. It is widely used in configuration files (Maven, Spring, Android), SOAP web services, RSS/Atom feeds, SVG graphics, and document formats such as DOCX and ODT.

Validate whether a file is well-formed from the command line using xmllint (part of libxml2):

sudo apt install libxml2-utils
xmllint --noout library.xml && echo "Well-formed"