It's Easy to Build: Custom docs project | Part 3 - ERB templating

In this third part of the series - It's Easy to Build: Custom docs project, we'll learn how to use ERB templating engine to insert our HTML code generated into a standard HTML file.

It's Easy to Build: Custom docs project | Part 3 - ERB templating

Hello there, welcome to part 3 of my series to build a custom docs project. Since I'm a kind of DRY person, you should see part 1 of this series to see the introduction and problem statement of this project. You can also find Part 2 of this series here(part 2 is optional).

In this tutorial, we'll learn about what ERB templating is and how we can inject the processed markdown code and inject it into the HTML file.

From our part 1 of series, we saw that our index.html didn't have a valid HTML structure like <head>, <body> tags were not present. Also, we can't deploy this file to production as there are no Meta tags or SEO tags. We'll solve this issue in this tutorial.

Getting Started

Image from Pixabay

What is ERB?
ERB stands for eRuby or Embedded RuBy and has a .erb file extension.

It is a templating system that embeds Ruby into a text document. It is often used to embed Ruby code in an HTML document. -- Wikipedia

In other words, we can write ruby code in our files(text, HTML, among others), and this way, we can bring programming features(like variables, loops, etc.) in our regular files.

All the ruby code in ERB files are in this format: <%= "Hello" %>, i.e. between the <%= %>. The = sign shows that you want to evaluate code + SHOW this as output. If you use <% "Hello" %>, this will evaluate the code but doesn't show anything as output. We use <% %> for looping or flow control. See below for an example.

Why is this useful?
You can add ruby code dynamically to the files. I can explain it better in a form of an example. Say you want to write <p> tag with I'm just a P tag 5 times. In normal HTML you would do something like this:

<!-- index.html -->
<div class="container">
  <p>I'm just a P tag</p>
  <p>I'm just a P tag</p>
  <p>I'm just a P tag</p>
  <p>I'm just a P tag</p>
  <p>I'm just a P tag</p>

But with ERB, you can use the DRY principle for the above.

<!-- index.html.erb -->
<div class="container">
  <% 5.times do %>
    <%= "<p>I'm just a P tag</p>" %>
  <% end %>

The above code will do the same, but it respects the DRY principle. Also, see the difference between <% %> and <%= %>.


In part 1 of the series, we only did 1 task: Convert our markdown to HTML, then inject this HTML to index.html. It looked something like this:

So like I said, we can't deploy this to production. So let's make it fancier.


Let's start writing the code:

  1. Create a new file named template.html.erb:

    $ touch template.html.erb
  2. The template.html.erb file will be like our template, we'll add all our static content here + variables for dynamic content:

    <!DOCTYPE html>
    <html lang="en">
      <meta charset="UTF-8">
      <meta name="viewport" content="width=device-width, initial-scale=1.0">
      <title>Docs Project</title>
      <!-- Other Meta tags -->
      <div class="container">
        <%= body_content %>
  3. In the above code, everything is like regular HTML, but we also injected a <%= body_content %> ERB code. This is where we'll insert all our HTML code generated from our markdown.

  4. From part 1, we have this in our config.rb file:

    require 'redcarpet'
    hello_md_text_file ='./', 'r')
    index_html_file ='./index.html', 'w+')
    md_renderer_options = {}
    md_extensions = {}
    md_renderer =
    markdown =, md_extensions)
    md_text =
    md_to_html_text = markdown.render(md_text)
    index_html_file.puts md_to_html_text
    • In short, what above code does it, It reads the Markdown file, converts that markdown to HTML, then injects this HTML in index.html file.
  5. Now, let's modify our config.rb file, see the comments:

    require 'redcarpet'
    hello_md_text_file ='./', 'r')
    index_html_file ='./index.html', 'w+')
    ## Import the template file
    ## We only give it READ permission
    template_file ='./template.html.erb', 'r')
    md_renderer =
    markdown =, md_extensions)
    md_text =
    md_to_html_text = markdown.render(md_text)
    ## Now let's read the contents of template file
    template_content =
    ## This variable will be injected
    body_content = md_to_html_text
    ## Create a new instance of ERB with the template_content
    erb_renderer =
    ## Now, parse the ERB template, and replace
    ## body_content with the HTML generated code
    final_html_output = erb_renderer.result()
    ## Finally, inject all the HTML code in `index.html` file
    index_html_file.puts final_html_output
    ## Close all files
    • The above code is simple; everything is explained in the code comments above, and to summarize:
    • First, we import and read our template file and save it in our variable template_content
    • We then Initialize a new instance of ERB with the template_content
    • Then we parse this ERB instance. This part will replace all our variables in template.html.erb with appropriate content.
    • We save all the above parsing in final_html_output variable.
    • Then finally, we inject final_html_output content into our index.html file.
  6. Running the script:

    • Run the script by using this command:
      $ ruby config.rb
    • Now, in the index.html file, you'll see all your template + markdown to HTML converted code.
  7. You can now upload this generated index.html file to production. This will have valid HTML structure, Meta tags and SEO tags that you mention in template.html.erb file.

Summary and conclusion

Let's take a quick look at what we learned today:

  1. First of all, I introduced myself, so don't forget to connect with me on Twitter or elsewhere.
  2. Since I'm a DRY person, I referred to my part 1 article where you can see the introduction and problem statement.
  3. Then we got the introduction to ERB and how it's templating works with a cute example.
  4. We then created our template file with a variable that makes it dynamic.
  5. We then wrote our ruby code logic to solve our problem, i.e., replace the variable dynamically to inject our converted HTML code. Then place this HTML to our index.html file.
  6. Finally, we took a quick look at this very summary...

That's it and let there be the end. 🙏