ERB Templates as Standalone Executables

January 24, 2018

The following is a technique that can convert ERB files (Embeded Ruby) to executable files like any other ruby/python/shell scripts, the difference is that ERB is more suitable in cases where you have some output that needs to be filled with data, like rails view files, or a text that needs to report number of files/dependencies, in this post I will try to explore this idea.

first I created a file with this content

Today is : <%= %>

You can render the file with erb as it’s command line tool to render erb files and output the result to the terminal.

erb date.erb

will output something similar to the following

Today is : 2018-01-24 21:08:20 +0100

As Linux looks for shebang in every executable file, then if this erb file given the executable permission and prepended by a shebang for erb it will be executed nearly the same way but without explicitly specifying erb in the terminal, so first rename the file

mv date.erb dateprinter

and give it the executable permission

chmod +x dateprinter

now prepend it with a shebang for erb, it should look like this now

#!/usr/bin/env erb
Today is : <%= %>

now it could be executed as follows


the only difference is that the output will have the shebang line also, as ERB doesn’t parse it and consider it part of the template file.

#!/usr/bin/env erb
Today is : 2018-01-24 21:08:20 +0100

if you put the previous file in a directory that is added to your shell $PATH you can execute it from anywhere by it’s name like any other executable file.


So in that case your shell will look for executable file in the path, will find the dateprinter file, it’ll try to figure out how to execute it with exec so it’ll inspect for magic bytes, it’ll find the shebang, so it’ll pass the file to the appropriate interpreter erb and erb in that case will execute it and hand you the output.

now lets do something useful with this idea, lets create a template that print the number of dependencies for a rails application.

When you execute bundle install the last 2 lines looks like this

Bundle complete! 27 Gemfile dependencies, 109 gems now installed.
Use `bundle info [gemname]` to see where a bundled gem is installed.

so to get the appropriate line we need to cut the output with head and tail as follows

bundle --local | tail -n2 | head -n1

so the returned output will be similar to this

Bundle complete! 27 Gemfile dependencies, 109 gems now installed.

now with ruby we can match the numbers in any string and extract them

stats = `bundle --local | tail -n2 | head -n1`
numbers = stats.scan(/[0-9]+/)
direct_dep = numbers.first
indirect_dep = numbers.last

so an ERB file as the following can print out these stats, I added some sprinkles on top

#!/usr/bin/env erb
Project Name: <%= File.basename(Dir.pwd).capitalize %>
<% stats = `bundle --local | tail -n2 | head -n1`.scan(/[0-9]+/) %>
Direct Dependencies: <%= stats.first %>
Indirect Dpendencies: <%= stats.last %>
Direct Initializers:  <%= Dir.glob('config/initializers/*.rb').count %>
Initializers in Development Env: <%= `rake initializers`.lines.count %>
Initializers in Production Env: <%= `RAILS_ENV=production rake initializers`.lines.count %>
Controllers: <%= Dir.glob('app/controllers/**/*_controller.rb').count %>
Models: <%= Dir.glob('app/models/**/*.*').reject{|f| f.include?('concern') }.count %>
Views: <%= Dir.glob('app/views/**/*.*').count %>

def files_for(ext)
  Dir.glob('**/*.' + ext)

def size_for(ext)
  files_for(ext).map{|f| File.size(f) }.inject(:+).to_i
JS: <%= files_for('js').count %> Files, <%= size_for('js') / 1024 %> KB
SCSS: <%= files_for('scss').count %> Files, <%= size_for('scss') / 1024 %> KB
PNG: <%= files_for('png').count %> Files, <%= size_for('png') / 1024 %> KB
JPG: <%= files_for('jpg').count %> Files, <%= size_for('jpg') / 1024 %> KB

The final output will look like this

#!/usr/bin/env erb
Project Name: Web

Direct Dependencies: 27
Indirect Dependencies: 109
Direct Initializers:  5
Initializers in Development Env: 119
Initializers in Production Env: 117
Controllers: 10
Models: 16
Views: 65

JS: 15 Files, 2569 KB
SCSS: 30 Files, 86 KB
PNG: 42 Files, 596 KB
JPG: 0 Files, 0 KB

Numbers will be different for your project.

Generating Graphs

The following is my approach

  1. a template that is interpreted by ERB to generate DOT format
  2. pass the output to dot command to generate another format like pdf or svg

So lets try to have the same data visualized, first a simple graph, lets name it rails-graph

#!/usr/bin/env erb
digraph graphname {
     a -> b -> c;
     b -> d;

executing this file with

rails-graph | dot -Tpng > graph.png

You should see the following

basic graph

Now lets put more nodes and numbers to this graph

#!/usr/bin/env erb
stats = `bundle --local | tail -n2 | head -n1`.scan(/[0-9]+/)
ini = Dir.glob('config/initializers/*.rb').count
prod_ini = `RAILS_ENV=production rake initializers`.lines.count
dev_ini = `rake initializers`.lines.count
dev_mwares = `rake middleware`.lines.count
prod_mwares = `RAILS_ENV=production rake middleware`.lines.count

m = Dir.glob('app/models/**/*.*').reject{|f| f.include?('concern') }.count
v = Dir.glob('app/views/**/*.*').count
c = Dir.glob('app/controllers/**/*_controller.rb').count
routes = `rake routes`.lines.count - 1

def files_for(ext)
  Dir.glob('**/*.' + ext)
def count_for(ext)
def size_for(ext)
  files_for(ext).map{|f| File.size(f) }.inject(:+).to_i
digraph graphname {
        direct_gems [label="Direct gems <%= stats.first %>"]
        indirect_gems [label="Indirect gems <%= stats.last %>"]

        initializers [label="<%= ini  %> initializers"]
        dev_initializers [label="<%= dev_ini %> Development initializers"]
        prod_initializers [label="<%= prod_ini %> Production initializers"]

        { rank=same; initializers dev_initializers prod_initializers }

        dev_middlewares [label="<%= dev_mwares %> Development middlewares"]
        prod_middlewares [label="<%= prod_mwares %> Production middlewares"]

        controllers [label="Controllers: <%= c %>"]
        models [label="Models: <%= m %>"]
        views [label="Views: <%= v %>"]

        routes [label="Routes <%= routes %>"]

        indirect_gems -> direct_gems -> initializers -> routes
        direct_gems -> dev_initializers -> dev_middlewares -> routes
        direct_gems -> prod_initializers -> prod_middlewares -> routes

        routes -> controllers

        controllers -> models
        controllers -> views

        js [label="JS: <%= count_for('js') %> Files, <%= size_for('js') / 1024 %> KB"]
        scss [label="SCSS: <%= count_for('scss') %> Files, <%= size_for('scss') / 1024 %> KB"]
        png [label="PNG: <%= count_for('png') %> Files, <%= size_for('png') / 1024 %> KB"]
        jpg [label="JPG: <%= count_for('jpg') %> Files, <%= size_for('jpg') / 1024 %> KB"]

        views -> assets
        assets -> js
        assets -> scss
        assets -> png
        assets -> jpg


The result will be as follows: basic graph