Example of bad XML in a DocBook 5 book and how to detect it with the xmllint command

use-xmllint-example

# First we'll parse an invalid document

$ xmllint --noout --xinclude  ./Virtual-Disk-Operations.xml
./Virtual-Disk-Operations.xml:13: parser error : Opening and ending tag mismatch: bar line 12 and foo
    </foo>
          ^
./Virtual-Disk-Operations.xml:64: parser error : Opening and ending tag mismatch: foo line 11 and info
  </info>
         ^
./Virtual-Disk-Operations.xml:79: parser error : Opening and ending tag mismatch: info line 6 and book
</book>
       ^
./Virtual-Disk-Operations.xml:80: parser error : Premature end of data in tag book line 2

^

$ echo $?
1

# We can see above that an invalid document returns non-zero


# Now we'll parse the same document but with valid syntax:

$ xmllint --noout --xinclude  ./Virtual-Disk-Operations.xml

$ echo $?
0

Virtual-Disk-Operations.xml

<?xml version="1.0" encoding="utf-8"?>
<book xmlns="http://docbook.org/ns/docbook"
      xmlns:xi="http://www.w3.org/2001/XInclude" xml:id="invalid-syntax-example"
      xmlns:xl="http://www.w3.org/1999/xlink" version="5.0">

  <info>
    <title>Test for Developer Blog</title>

    <subtitle>From the basics to the advanced</subtitle>

    <foo>
      <!-- The following element has no closing element, this is invalid XML -->
      <bar>
    </foo>

  <!-- more of your stuff goes here... -->

  </info>
</book>