Filegen

Have you ever felt the need to generate files based on environment variables or yaml files? If your answer is yes, then filegen can be quite helpful for you. If your answer is no, than sorry Ma'am, this gem is not for you.

Installation

Add this line to your application's Gemfile:

gem 'filegen'

And then execute:

$ bundle

Or install it yourself as:

$ gem install filegen

This gem ships with an executable called filegen. It makes the power of ERB available on the commandline.

Usage on Commandline

General advice

Please make sure you have an ERB-template available. It needs to end with .erb! Place in anywhere you like. It's important that the name of variable in the template matches the name of environment variable or yaml-key: wording, case. The lookup is case-sensitive. If you want to write the output to a file you need to redirect stdout with >. Otherwise it will output the content on $stdout.

Variable lookup

If you want to get access to the variable, you need to use the lookup-method.

# look up variable
lookup(<variable>)

The lookup-method will return an empty string '' if a variable cannot be looked up. If a default value is given it will return the default value instead of the empty string.

# look up variable and use default value if variable is undefined
lookup(<variable>, <default_value>)

The default order of data sources to lookup a variable, is: 1st environment variable and 2nd yaml file. The yaml file needs be given as command line argument see below at Generate a file based on YAML file.

The order of data sources can be changed by using:

--data-sources env,yaml
--data-sources yaml,env

A short cut for --data-sources is -d. This option can also be used to ommit a data source. But it makes sense only for the enviroment data source today, because the yaml data source is only added if a yaml file name is given on the commandline

--data-sources yaml

Generate a file based on Environment Variables

The content of template.erb:

Hello my name is: <%= lookup('NAME') %>

After that you can use it with filegen.

NAME=Karl filegen template.erb > file

And get the following result.

Hello my name is: Karl

Generate a file based on YAML file

The content of template.erb:

Hello my name is: <%= lookup('NAME') %>

Additionally you need to create a YAML-file - e.g. names.yaml.

---
NAME: Karl

After that you can use it with filegen.

#short format
filegen -y names.yaml template.erb > file

#long format
filegen --yaml-file names.yaml template.erb > file

And get the following result.

Hello my name is: Karl

Usage within ruby

Hash with Symbol-Keys

require 'filegen'
require 'stringio'

template = <<-EOS
Hi <%= lookup('name') %>
EOS

data = {
  name: 'Karl'
}

generator = Filegen::Rubygen.new
puts generator.run(template, data)

# => Hi Karl

Hash with String-Keys

require 'filegen'
require 'stringio'

template = <<-EOS
Hi <%= lookup('name') %>
EOS

data = {
  'name' => 'Karl'
}

generator = Filegen::Rubygen.new
puts generator.run(template, data)

# => Hi Karl

Use Cases

RPM packaging

Say you would like to package a ruby-based application for a rpm-based distribution. You can build a rpm package for each gem it depends on or build one rpm for the whole application containing all needed gems. After using the first approach at first I switched to the second one at last: No need to care about different version of one rubygem as dependency of different applications.

To make this possible I decided to use a small wrapper which sets all neccessary paths: GEM_PATH, GEM_ROOT and GEM_HOME. After that it executes the ruby application. Because every application resists below a different path, I needed to generate a different wrapper for each application. For this I use this small template:

#!/usr/bin/env bash

export GEM_ROOT='<%= lookup('SOFTWARE_LIB') %>'
export GEM_PATH='<%= lookup('SOFTWARE_LIB') %>'
export GEM_HOME='<%= lookup('SOFTWARE_LIB') %>'

exec <%= lookup('SOFTWARE_BINARY') %> $*

The wrapper is then generated within the rpm spec:

SOFTWARE_BINARY=<path to rubygems bin> SOFTWARE_LIB=<library_path> filegen gem.erb > <wrapper>
chmod +x <wrapper>

Future

• Maybe I will add additional data sources.

Contributing

Please have a look at the {file:CONTRIBUTING.md contribution guide}