web-server.rb

require 'socket'

class RequestParser
  def parse(request)
    method, path, version = request.lines[0].split
    
    {
      path: path,
      method: method,
      headers: parse_headers(request)
    }
  end

  def parse_headers(request)
    headers = {}
    
    request.lines[1..-1].each do |line|
      return headers if line == "\r\n"
      
      header, value = line.split
      header        = normalize(header)
      
      headers[header] = value
    end
  end
    
  def normalize(header)
    header.gsub(":", "").downcase.to_sym
  end
end

class Response
  attr_reader :code

  def initialize(code:, data: "")
    @response =
    "HTTP/1.1 #{code}\r\n" +
    "Content-Length: #{data.size}\r\n" +
    "\r\n" +
    "#{data}\r\n"
    
    @code = code
  end
  
  def send(client)
    client.write(@response)
  end
end

class ResponseBuilder
  SERVER_ROOT = "/tmp/web-server/"
  
  def prepare(request)
    if request.fetch(:path) == "/"
      respond_with(SERVER_ROOT + "index.html")
    else
      respond_with(SERVER_ROOT + request.fetch(:path))
    end
  end
  
  def respond_with(path)
    if File.exists?(path)
      send_ok_response(File.binread(path))
    else
      send_file_not_found
    end
  end

  def send_ok_response(data)
    Response.new(code: 200, data: data)
  end
  
  def send_file_not_found
    Response.new(code: 404)
  end
end

server  = TCPServer.new('localhost', 8080)

loop {
  client  = server.accept
  request = client.readpartial(2048)
  
  request  = RequestParser.new.parse(request)
  response = ResponseBuilder.new.prepare(request)
  
  puts "#{client.peeraddr[3]} #{request.fetch(:path)} - #{response.code}"
  
  response.send(client)
  client.close
}

There are a few problems with this code:
if File.exists?(path) == X
if File.exist?(path) == 2714 FE0F

Also the SERVER_ROOT = "/tmp/web-server/" assumes the person knows to create a folder called /tmp/ and then /web-server/. The simple path would be to just do this:
SERVER_ROOT = ""
OR
SERVER_ROOT = "/"

The former works, but the latter won't the way you set up the code. Not a criticism just articulating that for people who see the the / and prefer it and don't realize there then needs to be other changes for that option to work.

Ahh last thing. You infer the creation of an HTML file and that this will then render it, but it won't actually render it. It'll present the code, but not the rendering of that code. The content_type needs to be added to the the initialize method and the @ response:

class Response
  attr_reader :code

  def initialize(code:, data: "", content_type: 'text/html') # content_type argument was added as well.
    @response =
    "HTTP/1.1 #{code}\r\n" +
    "Content-Type: #{content_type}\r\n" +.  #See this line is new
    "Content-Length: #{data.size}\r\n" +
    "\r\n" +
    "#{data}\r\n"
    
    @code = code
  end
  
  def send(client)
    client.write(@response)
  end
end

This was helpful and I learned a lot. The bug fixes were not a deterrent as it forced me to understand more and look up some things, which is why I sought out your article in the first place. Win win. Just adding this help in case someone is more junior and struggling more and how to proceed to solve the issues is less obvious to them. Blocking isn't good, struggling is good...at least I think that way.

Thanks again!