blog/content/2015/05/xml-schema-validation-for-command-line.html

---
title: XML Schema Validation for the command line
description: >
  XML Schema Validation for the command line
created: !!timestamp '2015-05-07'
time: 2:17 PM
tags:
  - xml
  - schema
---

It turns out that unless you use a full fledge XML editor, validating
your XML document against a schema is difficult.  Most tools require you
to specify a single schema file.  If you have an XML document that
contains more than one name space this doesn't work too well as often,
each name space is in a separate schema file.

The XML document has xmlns attributes which use a URI as the identifier.
These URIs are for identifing it, and not a URL, so not able to be used.
In fact, different cases in the URIs specify different name spaces even
in the "host" part, though that is not the case with URLs.  In order for
validators to find the schema, the attribute
[xsi:schemaLocation](http://www.w3.org/TR/xmlschema-1/#schema-loc) is
used to map the name space URIs to the URLs of the schema.

The `xsi:schemaLocation` mapping is very simple.  It is simply a white
space delimited list of URI/URL pairs.  None of the command line tools
that I used uses this attribute to make the schema validation simple.
This includes [xmllint](http://xmlsoft.org/xmllint.html) which uses
the libxml2 library.  I also tried to use the Java XML library
Xerces, but was unable to get it to work.  Xerces did not provide a
simple command line utility, and I couldn't figure out the correct java
command line to invoke the validator class.

My coworker, [Patrick](http://fivetwentysix.com/), found the blog entry,
[Nokogiri XML schema validation with multiple schema files](http://avinmathew.com/nokogiri-xml-schema-validation-with-multiple-schema-files/),
which talks about using `xs:import` to have a single schema file support
multiple name spaces.  With this, we realized that we could finally get
our XML document verified.

As I know shell scripting well, I decided to write a script to automate
creating a unified schema and validate a document.  The tools don't cache
the schema documents, requiring fetching the schema each time you want
to validate the XML document.  We did attempt to write the schema files
to disk, and reuse those, *but* there are issues in that some schemas
reference other resources in them.  If the schema is not retrieved from
the web, these internal resources are not retrieved also, causing errors
when validating some XML documents.

With a little bit of help from `xsltproc` to extract xsi:schemaLocation,
it wasn't to hard to generate the schema document and provide it to
xmllint.

The code ([xmlval.sh](http://www.funkthat.com/~jmg/xmlval.sh)):

``` { .shell .showlines }
#!/bin/sh -

cat <<EOF |
<?xml version="1.0"?>
<xsl:stylesheet version="1.0"
    xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
>

 <xsl:output method="text"/>
 <xsl:template match="/">
  <xsl:value-of select="/*/@xsi:schemaLocation"/>
 </xsl:template>

</xsl:stylesheet>
EOF
    xsltproc - "$1" |
    sed -e 's/ */\
/g' |
    sed -e '/^$/d' |
    (echo '<?xml version="1.0" encoding="UTF-8"?>'
     echo '<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:nospace="nospace" targetNamespace="http://www.example.com/nospace">'
     while :; do
        if ! read a; then
            break
        fi
        if ! read b; then
            break
        fi
        echo '<xs:import namespace="'"$a"'" schemaLocation="'"$b"'"/>'
    done
    echo '</xs:schema>') |
    xmllint --noout --schema - "$1"
```


Though the script looks complicated, it is a straight forward pipeline:

1. Lines 3-16 provide the xslt document to `xsltproc` on line 17 to
   extract schema location attribute.
1. Lines 18-20 replace multiple spaces with new lines and deletes any
   blank lines.  It should probably also handle tabs, but none of the
   documents that I have had tabs.  After this, we now have the odd
   lines containing the URI of the name space, and the even lines
   contain the URL for the schema.
1. Lines 21 and 22 are the header for the new schema document.
1. Lines 23-31 pulls in these line pairs and create the necessary
   `xs:import` lines.
1. Line 32 provides the closing element for the schema document.
1. Line 33 gives the schema document to xmllint for validation.