Mike Slinn

Debugging Jekyll Plugins with an IDE

Published 2022-02-21. Last modified 2024-02-21.
Time to read: 5 minutes.

This page is part of the jekyll collection.

This article is the last in a three-part series. The first article describes how to install and use the jekyll_bootstrap5_tabs plugin. The second article describes how the Jekyll plugin was constructed. The plugin is built and published as an open-source Ruby gem. This article is dedicated to demonstrating a straightforward way of debugging Jekyll plugins, which are always written in Ruby.

Many open-source projects fall far short of their true potential because no-one bothers to tell the story, completely and thoughtfully, in depth. Hopefully this article will in some small way improve Jekyll’s circumstance in the F/OSS world.

The Jekyll documentation provides absolutely no commentary on how to debug plugins. Perhaps the Jekyll developers felt the information would be obvious to sufficiently experienced programmers. Because so many debugging possibilities exist, most of which recently became obsolete, I did not find it obvious. Hopefully this article will clear things up.


The Setting Up a Ruby Development Environment and Essential Visual Studio Code Extensions for Ruby articles describe the necessary preparations.

These Instructions Work Everywhere

The instructions in this article should work on every OS that Jekyll runs on.

Many problems with debugging Jekyll plugins are related to the plugin’s need to receive parameters from Jekyll. We do not need to dig into Jekyll itself to get this sorted out; all we need to do is to find where the gem you want to debug was installed, and set breakpoints after it receives parameters. The information that the IDE will then present to you will far exceed what the Jekyll plugin documentation provides.

Plugins Run In the Jekyll Address Space

The most important thing to know about debugging Jekyll plugins is that they run in the same address space as Jekyll itself. That means to debug a plugin you must actually debug the Jekyll process, and it will load the most recently created version of your plugin.


If you set a breakpoint on the installed plugin's source code, execution will halt when the plugin is loaded or invoked. You will be able to see the call stack and the variables at each level of the call stack. This is super helpful!


For the purposes of this article, I assume that you have a Jekyll plugin that you want to debug. It does not matter if you wrote the plugin or not. All that matters is that the plugin was installed. Throughout this article, I will refer to this gem as “the subject gem”. I wrote this article to figure out how to debug my subject gem, which is jekyll_bootstrap5_tabs, so you will see references to that gem in this article whenever I discuss what needs to be done with your subject gem.

Move to your Jekyll project directory, this is where we will work. For me, that meant:

$ cd $jekyll_bootstrap5_tabs

If you are curious where the environment variable above was defined, when my Bash shells start, they source $work/.evars. The following lines are within:

export work=/var/work
export jekyll=$work/jekyll
export jekyll_flexible_include_plugin=$jekyll/jekyll-flexible-include-plugin
export jekyll_bootstrap5_tabs=$jekyll/jekyll_bootstrap5_tabs
export jekyll_template=$sites/jekyll_template

That is part of a directory structure I maintain across several machines.

Jekyll Launcher

Jekyll is provided as a Ruby gem. When the Jekyll gem is installed, it also creates a launcher at /usr/local/bin/jekyll, which is actually a small Ruby program that loads the gem.

# This file was generated by RubyGems.
# The application 'jekyll' is installed as part of a gem, and
# this file is here to facilitate running it.

require 'rubygems'


version = ">= 0.a"

str = ARGV.first
if str
  str = str.b[/\A_(.*)_\z/, 1]
  if str and Gem::Version.correct?(str)
    version = str

if Gem.respond_to?(:activate_bin_path)
load Gem.activate_bin_path('jekyll', 'jekyll', version)
gem "jekyll", version
load Gem.bin_path("jekyll", "jekyll", version)

Jekyll is best debugged when launched via the normal means, that is, via /usr/local/bin/jekyll. The launcher figures out where the Jekyll gem resides, and loads it.

Locating the Subject Gem

Debugging an installed gem requires that the gem be located before it can be debugged. If you need to debug Ruby code that was not installed you can skip this step.

You need to know where the subject gem was installed to set breakpoints in it. From within a Jekyll project that uses the subject gem, discover the location of the Jekyll entry points within the gem as follows:

$ bundle info jekyll_bootstrap5_tabs
  * jekyll_bootstrap5_tabs (1.1.0)
        Summary: Jekyll plugin for Bootstrap 5 tabs
        Homepage: https://mslinn.com/blog/2022/02/13/jekyll-gem.html
        Source Code: https://github.com/mslinn/jekyll_bootstrap5_tabs
        Path: /home/mslinn/gems/gems/jekyll_bootstrap5_tabs-1.1.0 

The jekyll_bootstrap5_tabs gem is located at /home/mslinn/gems/gems/jekyll_bootstrap5_tabs-1.1.0.

Now we need to find the entry point for the plugin, which is where Jekyll invokes the functionality of the plugin. If we search for Liquid:: we'll find the source files containing the entry points.

$ grep -rl 'Liquid::' \

Now we know we need to open /home/mslinn/gems/gems/jekyll_bootstrap5_tabs-1.1.0/lib/jekyll_bootstrap5_tabs.rb in Visual Studio to set breakpoints.

Here is a one-line command that will tell you that same information:

$ grep -rl 'Liquid::' \
  "$( bundle info jekyll_bootstrap5_tabs | \
    grep Path | \
    awk '{print $2}' )"

Setup Debugging

Now it is time to launch Jekyll under control of rdbg.

You can specify all of the normal Jekyll options for local usage, as shown below. They might impact the responsiveness of the debug session, but are workable nonetheless.

$ bundle exec rdbg \
  -O \
  --port=0 \
  -- \
    /usr/local/bin/jekyll serve \
      --livereload_port 25721 \
      --force_polling \
      --host \
      --port 4444 \
      --future \
      --incremental \
      --livereload \
      --drafts \
DEBUGGER: Debugger can attach via TCP/IP (
DEBUGGER: wait for debugger connection... 

Jekyll does not run any slower under the Ruby debugger.

Above you see 2-stage launches expressed as single command lines. The backslashes are line continuation characters, which is why 'one command line' actually spans a dozen or so lines.

  1. Stage 1: debugger parameters
  2. Stage 2: Jekyll parameters

I took a screen shot of the above command line and added a few comments:

I made the following run configuration for Visual Studio Code to launch a debug process. Note that this is not the entire contents of .vscode/launch.json, just the one run configuration.

.vscode/launch.json Entry
  "args": [
    "--livereload_port", "35732",
    "--host", "",
    "--port", "4444",
  "cwd": "${workspaceRoot}/demo",
  "debugPort": "0",
  "name": "Debug Demo",
  "request": "launch",
  "script": "${workspaceRoot}/binstub/jekyll",
  "type": "rdbg",

The steps to establish a debugging session are listed below. If you would like to try this out on a project that is already setup for this, download the jekyll_plugin_support project.

  1. Set a breakpoint in Visual Studio Code (I will discuss this next).
  2. Attach the Visual Studio debug client using the run configuration called Debug Demo.

Breakpoint Types

Visual Studio Code supports several types of breakpoints. This article discusses only two types: location-based breakpoints and function breakpoints. The Microsoft documentation explain how breakpoints work in detail; I won't repeat that information here.

The overhead of breakpoints can be reduced.

Execution Break - Ta-Da!

When VS Code hits a breakpoint, the call stack and variables are shown, and the editor displays the line that the debugger stopped at.

Now for the ta-da! moment. If you are not a programmer, this will go over with a thud. Since you made it this far, you are either a masochist, or a programmer.


Take a good long look at the variables in the following screenshot. You can see what has been passed from Jekyll to the plugin easily here. This information is really hard to come by any other way.


context.instance_variable_get('@scopes'), an array of hashes, has only one entry: 'nowMillis': '1645386226'. That was a Liquid variable that I had set in the page that contained a reference to the Bootstrap 5 tabs plugin.


The debugger paused at the start of TabsBlock#render. The variable self has type JeklyyBootstrap5Tabs::TabsBlock, and it looks like this:

The Jekyll tag was: {% tabs test pretty %}:

  • self.@markup is a string with value test pretty (with a space at the end).
  • self.@pretty_print is true.
  • self.@tab_name is a string with value test.
  • self.tag_name is a string with value tabs.

To Learn More

For more about creating Jekyll plugins, please see the The jekyll_plugin_support project.

About the Author

I, Mike Slinn, have been working with Ruby for a long time now. Back in 2005, I was the product marketing manager at CodeGear (the company was formerly known as Borland) for their 3rd Rail IDE. 3rd Rail supported Ruby and Ruby on Rails at launch.

In 2006, I co-chaired the Silicon Valley Ruby Conference on behalf of the SD Forum in Silicon Valley. As you can see, I have the t-shirt. I was the sole chairman of the 2007 Silicon Valley Ruby Conference.

Several court cases have come my way over the years in my capacity as a software expert witness. The court cases featured questions about IP misappropriation for Ruby on Rails programs. You can read about my experience as a software expert if that interests you.

I currently enjoy writing Jekyll plugins in Ruby for this website and others, as well as Ruby utilities.

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