Formatting with libxo

Most unix commands emit text output aimed at humans. It is designed to be parsed and understood by a user. Humans are gifted at extracting details and pattern matching in such output. Often programmers need to extract information from this human-oriented output. Programmers use tools like grep, awk, and regular expressions to ferret out the pieces of information they need. Such solutions are fragile and require maintenance when output contents change or evolve, along with testing and validation.

Modern tool developers favor encoding schemes like XML and JSON, which allow trivial parsing and extraction of data. Such formats are simple, well understood, hierarchical, easily parsed, and often integrate easier with common tools and environments. Changes to content can be done in ways that do not break existing users of the data, which can reduce maintenance costs and increase feature velocity.

In addition, modern reality means that more output ends up in web browsers than in terminals, making HTML output valuable.

libxo allows a single set of function calls in source code to generate traditional text output, as well as XML and JSON formatted data. HTML can also be generated; “<div>” elements surround the traditional text output, with attributes that detail how to render the data.

A single libxo function call in source code is all that’s required:

xo_emit("Connecting to {:host}.{:domain}...\n", host, domain);

  Connecting to
  "host": "my-box",
  "domain": ""
   <div class="line">
     <div class="text">Connecting to </div>
     <div class="data" data-tag="host"
     <div class="text">.</div>
     <div class="data" data-tag="domain"
     <div class="text">...</div>

Encoding Styles

There are four encoding styles supported by libxo:

  • TEXT output can be display on a terminal session, allowing compatibility with traditional command line usage.
  • XML output is suitable for tools like XPath and protocols like NETCONF.
  • JSON output can be used for RESTful APIs and integration with languages like Javascript and Python.
  • HTML can be matched with a small CSS file to permit rendering in any HTML5 browser.

In general, XML and JSON are suitable for encoding data, while TEXT is suited for terminal output and HTML is suited for display in a web browser (see xohtml).

Text Output

Most traditional programs generate text output on standard output, with contents like:

36      ./src
40      ./bin
90      .

In this example (taken from du source code), the code to generate this data might look like:

printf("%d\t%s\n", num_blocks, path);

Simple, direct, obvious. But it’s only making text output. Imagine using a single code path to make TEXT, XML, JSON or HTML, deciding at run time which to generate.

libxo expands on the idea of printf format strings to make a single format containing instructions for creating multiple output styles:

xo_emit("{:blocks/%d}\t{:path/%s}\n", num_blocks, path);

This line will generate the same text output as the earlier printf call, but also has enough information to generate XML, JSON, and HTML.

The following sections introduce the other formats.

XML Output

XML output consists of a hierarchical set of elements, each encoded with a start tag and an end tag. The element should be named for data value that it is encoding:


XML is the W3C standard for encoding data.

JSON Output

JSON output consists of a hierarchical set of objects and lists, each encoded with a quoted name, a colon, and a value. If the value is a string, it must be quoted, but numbers are not quoted. Objects are encoded using braces; lists are encoded using square brackets. Data inside objects and lists is separated using commas:

items: [
    { "blocks": 36, "path" : "./src" },
    { "blocks": 40, "path" : "./bin" },
    { "blocks": 90, "path" : "./" }

HTML Output

HTML output is designed to allow the output to be rendered in a web browser with minimal effort. Each piece of output data is rendered inside a <div> element, with a class name related to the role of the data. By using a small set of class attribute values, a CSS stylesheet can render the HTML into rich text that mirrors the traditional text content.

Additional attributes can be enabled to provide more details about the data, including data type, description, and an XPath location:

<div class="line">
  <div class="data" data-tag="blocks">36</div>
  <div class="padding">      </div>
  <div class="data" data-tag="path">./src</div>
<div class="line">
  <div class="data" data-tag="blocks">40</div>
  <div class="padding">      </div>
  <div class="data" data-tag="path">./bin</div>
<div class="line">
  <div class="data" data-tag="blocks">90</div>
  <div class="padding">      </div>
  <div class="data" data-tag="path">./</div>