Xml Zone Tip: Command-line Xml Processing

Working with XML Documents from the Shell


David Mertz, Ph.D.
Line commander, Gnosis Software, Inc.
April, 2003

Most of the time, processing XML documents utilizes heavy-duty APIs and custom applications. However, the tradition of using small tools with I/O piped between them is a very fine one on Unix-like platforms. XML need not be entirely left out of quick-and-dirty processing with one-liners that is especially useful during development and debugging cycles.

Introduction

As much as I hate to say it, XML tools simply have not reached the level of convenience of the text utilities available at a Unix-like command-line. For line-oriented, whitespace- or comma-delimited text files, it is quite amazing what you can accomplish with clever combinations of sed, grep, xargs, wc, cut, pipes, and short shell scripts.

In my opinion, it is not that XML is inherently resistent to the modular treatment flat text files enjoy. We just need to learn from experience the best ways to componentize XML tools. For example, in writing this tip, I had a few realistic sample tasks in mind; but what I found was that even those tools that have command-line facilities have not yet learned to "play nice" with each other. Working with multiple tools is not intractable, it just requires a little bit of wrapping.

A fact to note is that quite a few people have written versions--in various programming languages--of similar simple tools. Each version behaves a bit differently, but they tend to accomplish the same overall task. For this tip, I look at the tools xml_indexer, xmlcat, and xpath--the first two come from my Gnosis Utilities, the last is a Perl module written by Matt Sergeant (get it from CPAN).

Finding Words In Xml Prose

I have written previously about xml_indexer, which creates an index of the words within XML documents by their XPath. For example, you can index then search an XML document with:

Indexing and searching on XPaths

% xml_indexer chap.xml
% indexer events were
/Users/dqm/chap.xml::/chapter/sect1[2]/sect2[1]/para[1]
/Users/dqm/chap.xml::/chapter/sect1[2]/sect2[4]/para[3]
1 files matched wordlist: ['events', 'mostly']
Processed in 0.062 seconds (SlicedZPickleIndexer)

These commands display the elements within the XML document chap.xml that contain the words events and were (not necessarily in order or proximity). If other XML documents were added to the index, matching occurrences in them would appear also. New searches are almost instantaneous, even if multiple documents are indexed, by the way.

While it tells you a little bit to know that words occur at particular XPaths within particular documents, the point of a search is usually to see (or further process) the actual content matches. For that, you need to employ a command-line xpath tool; I have installed Perl's XML:XPath, whose behavior I like.

You can cut-and-paste discovered XPaths into the tool xpath. E.g.:

Manually looking at an XPath

% xpath chap.xml '/chapter/sect1[2]/sect2[4]/para[3]'
Found 1 nodes:
-- NODE --
<para>It is not particularly remarkable that...
...
</para>

This points to a nice modularity in the tools. Moreover, if the XPath passed to xpath had wildcards in it, it might have matched more than just the one node. Unfortunately, the output of indexer does not have quite the right form to pipe to xpath, to automate looking at the nodes with matched words: indexer separates the filename from the XPath with "::", and xpath only looks at one XPath at a time. We can do better.

A First Little Shell Script

There might be a way to manage the above "impedence mismatch" using clever combinations of xargs, apply, pipes, and the like. But I found it easier to write a short (and reusable) shell script:

find-xml-elements

#!/bin/sh
for hit in `indexer $@ 2> /dev/null`
do
  echo $hit | sed 's/::/ /' > loc.tmp
  cat loc.tmp | xargs xpath 2> /dev/null
  echo
done
rm loc.tmp

As with other well-designed command-line tools, indexer and xpath send informational messages to STDERR, the actual results to STDOUT. For my script, I am not interested in the STDERR messages. Now I can find all the nodes in which a list of words occur as easily as:

Searching XML elements for words

% find-xml-elements events were
<para>Lest we forget some events in a recent decade...
...
Salem and by HUAC.</para>
<para>It is not particularly remarkable that...
...
being uncovered.</para>

So far, so good. What our search outputs is a series of XML snippets, where each top-level element contains the searched words. However, the result is generally not quite a well-formed XML document, since it is multiply-rooted.

Comparing Xml Document And Extracting Text

One difficulty in analyzing XML data is that XML documents can contain variations in formatting that are irrelevant to their semantic content. Some whitespace is "ignorable", the order of attributes is discarded during parsing, empty elements may be either self-closed or have an end-tag, and entities can be encoded in a few different ways. In truth, even much whitespace that is non-ignorable from a parser's perspective is nonetheless insignificant from an application point-of-view; "pretty" newlines and indenting are useful for people, and many applications (optionally) perform such stylistic formatting.

There are a rather large number of tools that have been written to compare XML documents in a semantically useful way. Most of them have chosen the obvious name xmldiff, or something close to it (use Google to find versions for various programming languages). Underlying such a comparison of XML documents is a canonicalization of the layout of each document. Once inflexible algorithmic decisions have been made about the exact rendering of an XML document, semantically similar documents are easier to compare with generic tools like diff.

I use a Python script I wrote called xmlcat. The tool is not complicated--it acts much like the standard cat utility, but canonicalizes XML documents along the way. In a chance to use my favorite word, I can note that the operation of xmlcat is idempotent. The reason I like xmlcat over similar tools like xmlpp (see Resources) is that it adds an option inspired by the web browser lynx. If you pass the --dump argument to xmlcat, it outputs only the textual content of an XML document, eliminating the tags (using vertical whitespace is a moderately pretty way). For data-oriented XML, this capability is of little use, but for marked-up prose, it is handy.

A Second Shell Script For Viewing Text

If you search XML documents of prose for content words, most likely you are interested in the content more than you are the markup. Filtering with xmlcat --dump is exactly the trick to remove unwanted XML tags. However, directly piping the output of find-xml-elements to xmlcat is not quite right, since the output of find-xml-elements is not quite an entire well-formed XML documents (it is fragments, as noted). A short shell script solves the problem:

find-xml-text

#!/bin/sh
for hit in `indexer $@ 2> /dev/null`
do
  echo $hit | sed 's/::/ /' > loc.tmp
  cat loc.tmp | xargs xpath 2> /dev/null | xmlcat --dump
  echo
done
rm loc.tmp

The output from find-xml-text plays nice with standard text utilites. For example, I would like to display all the paragraphs that contain some search terms, but remove any left indent from their lines and limit line-length:

Searching XML element text for words

% find-xml-text events were | sed 's/^ *//' | fmt -w 70
Lest we forget some events in a recent decade...
...
...those in Salem and by HUAC.

It is not particularly remarkable...
...
...being uncovered.

Resources

Kip Hampton wrote a worthwhile article last year looking at Perl tools for command-line XML processing:

http://www.xml.com/lpt/a/2002/04/17/perl-xml.html

The Perl tools xmldiff (compare XML documents) and xmlpp (XML pretty printer) can be found at:

http://software.decisionsoft.com/tools.html

Gnosis Utilies includes several of the utilities discussed in this article, download it from:

http://gnosis.cx/download/Gnosis_Utils-current.tar.gz

XML Matters #10 discusses full text indexing of XML documents by XPath:

http://www-106.ibm.com/developerworks/xml/library/x-matters10.html

About The Author

Picture of Author David Mertz uses a wholly unstructured brain to write about structured document formats. David may be reached at mertz@gnosis.cx; his life pored over athttp://gnosis.cx/publish/. And buy his book: http://gnosis.cx/TPiP/.