secret origins: the jpt

On building a JSON tool for macOS without using Python, perl, or Ruby.

In my work as a Macintosh engineer and administrator I’ve noticed macOS has lacked a bundled tool for working with JSON at the command line. Where XML has its xpath, if your shell script needs some JSON chops, it’ll require an external binary like jq or something else scripted in Python, Ruby or perl using their JSON modules. The problem is, those runtimes have been slated for removal from a future macOS. So I took that as challenge to devise a method to query and modify JSON data within shell scripts, that didn’t use one of those deprecated scripting runtimes and didn’t require an external binary dependency either. Could I achieve robust and native JSON parsing on a Mac by simply “living off the land”?

Why not just re-install the runtimes when Apple deprecates them?

“Why limit yourself like this? Just re-install the runtimes and move on”, you may ask. Well, I’d like to think that limitations can inspire creativity but we should also consider there may be some other reasons why Apple is discontinuing the inclusion of those runtimes. Some may say, “Apple Silicon + macOS 11.0 is the perfect time for them to clean house”, to which I’d have to agree that’s a very good reason and likely a factor. Others could say they are looking to tighten the screws to keep out unsigned code: maybe, they do like to glue things shut! But really, I think it’s more akin to the web-plugins of yore like Java and Flash. Apple does not want to be the conduit for deploying 3rd party party runtimes which increase the attack surface of macOS. This seems like the most reasonable of the explanations. So, if you accept that Apple is attempting to reduce attack surface, why increase it by re-installing Python, Ruby, or perl, just so a transient script (like a Jamf Extension Attribute) can parse a JSON file? My answer to that, is you don’t. You play the hand you’re dealt. Game on!

Looking for truffles (in a very small back yard)

Despite having another project (shui) that can output and invoke Applescript from within a shell scripts for generating user interfaces, I definitely knew that Applescript was not the way to go. Apple however, added to the languages Open Scripting Architecture (OSA) supports back in 2014 with OS X Yosemite (10.10), they added Javascript along with a bridge to the Cocoa Objective-C classes and they called it JXA: Javascript for Automation. This seemed like a promising place, so I started playing with osascript and figured out how to load files and read /dev/stdin using JXA, and while looking for an answer for garbled input from stdin I came upon a Japanese blog that mentioned jsc the JavaScriptCore binary which resides in the /System/Library/Frameworks/ JavaScriptCore.framework. Arigato! Pay dirt! πŸ€‘ jsc does exactly what we need it to do: It can interpret Javascript passed as an argument, can access the filesystem and read from /dev/stdin, and best of all is in non-Private System level Framework that exists all the way back to OS X 10.4! Just the kind of foundation on which to build the tool.

Homesteading jsc

The existence of jsc goes back all the way to OS X Tiger and it’s functionality has evolved over the years. In order to have a consistent experience in jpt from macOS 10.4 – 11.x+ a few polyfills had to be employed for missing functions, along with a few other workarounds regarding file loading, printing and exit codes (or lack thereof). Once those were addressed the jsc proved to be a highly optimized Javascript environment that’s blazingly fast. It spans 13 macOS releases and is even present in many Linux distros out-of-the-box (Ubuntu and CentOS) and can even be run on Windows when the Linux Subsytem is installed.

With the host environment sorted, I began working with the original JSONPath code by Stefan Goessner as the query language. I didn’t know about JSON Pointer yet so this strange beast was all I knew! It worked out really. I went full throttle into developing the “swiss army knife” of JSON tools. I really leaned into the Second System Effect, as described in the Mythical Man-month, it’s when you put every doodad, gizmo and doo-hicky in your 2nd product (my first simple JSON pretty-printer built in JS on jsc). Eventually though, after I reached a feature plateau, I came back around to the address the quirks of the original JSONPath code. I ended up rewriting signifigant chunks of JSONPath and released it as it’s own project: brunerd JSONPath. But I digress, let’s get back to the JSON Power Tool.

jpt: powers and abilities

At it’s most basic, jpt will format or “pretty print” any JSON given. jpt can also handle data retrieval from JSON document using either JSONPath or JSON Pointer syntax. JSONPath, while not a standard, is a highly expressive query language akin to XPath for XML, with poweful features like recursive search, filter expressions, slices, and unions. JSON Pointer on the other hand is narrower in focus, succint and easily expressed, and standardized but it does not offer any of the interogative features that I feel make JSONPath so intriguing. Finally, jpt can also modify values in a JSON document using standardized JSON Patch operations like: add, remove, replace, copy, move, and test, as well as the also standardized JSON Merge Patch operation. Altogether, the jpt can format, retrieve and alter JSON documents using only a bit of outer shell script plus a lot more Javascript on any macOS since 10.4! πŸ˜…

Where can I get the jpt?

Stop by the project’s GitHub page at: https://github.com/brunerd/jpt

There you will find the full source and also a minified version of the jpt for inclusion within your shell scripts. Since it’s never compiled you can always peer inside and learn from it, customize it, modify it, or just tinker around with it (usually the best teacher).

Future Plans

There will undoubtedly be continued work on the jpt. Surely there are less than optimal routines, un-idiomatic idioms, edge cases not found, and features yet to be realized. But as far as the core functionality goes though, it’s fairly feature complete in my opinion. Considering that one of my top 10 StrengthsFinder qualities is “Maximizer”, the odds are pretty good, I’ll keep honing the jpt‘s utility, size (smaller), and sharing more articles with examples on the kinds of queries and data alteration operations the jpt can so perform. Stay tuned!

New Projects: jpt and shui, Now Available

Between March and October 2020 I had some great ideas for command line Mac utilities the MacAdmin could apprecite and I had the time to devote to their realization. I’m excited to present these two open source projects, available on GitHub: jpt and shui. I hope they can add richness to your shell scripts’ presentation and capabilities without requiring additional external dependancies.

jpt – the “JSON Power Tool” is a Javascript and shell script polyglot that leverages jsc, the JavascriptCore binary that is standard on every Mac since 10.4 and since the jpt is purposefully written in ES5 to maintain maximum compatibility, why yes, this tool does run on both PPC and Intel Macs all the way back to OS X Tiger and then all the way forward to the latest 11.0 macOS Big Sur! Many Linux distros like CentOS and Ubuntu come with jsc pre-installed also, even Windows with the Linux Subsystem installed can run jsc and therefore can run the jpt!

What you can do with the jpt? Query JSON documents using either the simple yet expressive JSONPath syntax or the singular and precise JSON Pointer (RFC6901) syntax. The output mode is JSON but additional creative output modes can render JSONPaths, JSON Pointer paths, or even just the property names with their “constructor” types (try -KC with -J or -R) Textual output can be encoded in a variety of formats (hex/octal/URI encoding, Unicode code points, etc…), data can be modified using both JSON Patch (RFC6902) operations (add, replace, remove, copy, move, test) and also JSON Merge Patch (RFC7386) operations. JSON can be worked with in new ways, try -L for “JSONPath Object Literal” output to see what I mean. Or you simply feed jsc a file to pretty-print (stringify) to /dev/stdout. I’ll be writing more about this one for sure.
Github project page: jpt
Tagged blog posts: scripting/jpt

shui – first-class Applescript dialog boxes in your shell scripts without needing to remember esoteric Applescript phrasings! If you think it’s odd for code to have possessive nouns and are more comfortable in shell, you’re not alone. shui can be embedded in either bash or zsh scripts but it can also output Applescript if you really want to know how the sausage is made or want to embed in your script without shui. Hopefully shui will let you forget those awkward Applescript phrasing and focus on your shell script’s features and functionality. It uses osascript to execute the Applescript and launchctl to invoke osascript in the correct user context so user keyboard layouts are respected (vs. root runs). Check out the project page for demo videos and then give shui a try.
Project page: shui
Tagged blog posts: scripting/shui