parse erb templates from command line (standalone, cli, json)

README.md

Goal

• provide cli command to render erb template
• template params (bindings) to be provided using json format
• use inputs from file (template.erb, params.json)
• use inputs from cli options / parameters
• render output to stdout OR to file

Why ?

• helps creating build & deploy pipelines

Usage

Help:

$ ruby json_erb_render.rb --help

Examples:

$ ruby json_erb_render.rb -i ./example/foo.erb -d ./example/config.json

$ ruby json_erb_render.rb -i ./example/foo.erb -d ./example/config.json -o ./out.txt

$ ruby json_erb_render.rb -t "FOO_BAR=<%= @config['foo'] %>" -d ./example/config.json

$ ruby json_erb_render.rb -t "FOO_BAR=<%= @config['foo'] %>" -p "{"foo":"bar"}"

$ ruby json_erb_render.rb -t "FOO_BAR=<%= @config['foo'] %>" -p "{"foo":"bar"}" -o ./out.txt

Docker

• no need to install ruby

• just run against official ruby image

  $ docker run --rm -it
  -e "$$PATH=$(PATH)"
  --volume "$(PWD)":"/src" -w /src
  ruby:2.3
  ruby /src/example/json_erb_render.rb -i /src/example/foo.erb -d /src/example/config.json

Limitations

• no proper validations implemented, ruby will throw its exceptions if sth. is wrong

erb_render.rb

#!/usr/bin/env ruby

# usage:
# ruby json_erb_render.rb OPTIONS

# help:
# ruby json_erb_render.rb --help

# examples:
# ruby json_erb_render.rb -i ./example/foo.erb -d ./example/config.json
# ruby json_erb_render.rb -i ./example/foo.erb -d ./example/config.json -o ./out.txt
# ruby json_erb_render.rb -t "FOO_BAR=<%= @config['foo']['bar'] %>" -d ./example/config.json
# ruby json_erb_render.rb -t "FOO_BAR=<%= @config['foo']['bar'] %>" -p "{\"foo\":\"bar\"}"
# ruby json_erb_render.rb -t "FOO_BAR=<%= @config['foo']['bar'] %>" -p "{\"foo\":\"bar\"}" -o ./out.txt

require 'erb'
require 'json'
require 'yaml'
require 'pathname'
require 'optparse'
require 'ostruct'

options = OpenStruct.new
OptionParser.new do |opt|
  opt.on('-i', '--input_file TEMPLATE_FILE', 'The template file being processed. e.g.: ./example/foo.erb') { |o| options['input_file'] = o }
  opt.on('-t', '--template_text TEMPLATE_TEXT', 'The template text being processed. e.g.: "FOO_BAR=<%= @config[\'foo\'][\'bar\'] %>"') { |o| options['template_text'] = o }
  opt.on('-d', '--data_file DATA_FILE', 'The config file (json) being processed. e.g.: ./example/config.json') { |o| options['data_file'] = o }
  opt.on('-p', '--template_params TEMPLATE_PARAMS', 'The template params (json) being processed. e.g.: -p "{\"foo\":\"bar\"}"') { |o| options['template_params'] = o }
  opt.on('-o', '--output_file SINK_FILE', 'The output file. if not specified, output gets rendered to stdout. e.g.: ./out.txt') { |o| options['output_file'] = o }
end.parse!




data_file = options['data_file']
data_text = options['template_params']

template_file = options['input_file']
template_text = options['template_text']

output_file = options['output_file']

class TemplateBuilder
  attr_accessor :config

  def initialize(config)
    @config = config
  end

  # Expose private binding() method.
  def get_binding
    binding()
  end

end

if template_file == nil
  template=template_text
else
  template=File.read(template_file)
end

if data_file == nil
  params=JSON.parse(data_text)
else
  if File.extname(data_file) == '.json'
    puts "Detected json"
    params=JSON.parse(File.read(data_file))
  elsif [ '.yaml', '.yml'].include? File.extname(data_file)
    puts "Detected yaml"
    params=YAML.load_file(data_file)
  else
    raise "Unkown file extension for #{data_file}"
  end
end


renderer = ERB.new(template)
builder = TemplateBuilder.new(params)
result = renderer.result(builder.get_binding)

# Extract strings '<%= -%>' and '<%= %>'
variables = template.scan(/<%= ?([^>]+) -?%>/).flatten
# Ensure there is a value at @config or ENV for all templates
# Because we are using eval() here, which is super dangerous, verfiy only 
# variables that start with @config or ENV are evaluated
variables.each do |variable|
  if variable.start_with?("@config")
    variable.sub!("@config", "")
    @config2 = builder.instance_variable_get("@config")
    puts "WARNING: Undefined variable: @config#{variable}" unless eval("@config2#{variable}")
  elsif variable.start_with?("ENV")
    puts "WARNING: Undefined variable: #{variable}" unless eval(variable)
  else
    raise "Variable '#{variable}' not allowed"
  end
end

if output_file==nil
  puts result
else
  output = File.new(output_file, 'w')
  output.puts(result)
  output.close
end