Supported environment variables in YAML config files (closes snowplow…

…/snowplow#1215)

config/config.yml.sample

@@ -1,6 +1,7 @@
 aws:
-  access_key_id: ADD HERE
-  secret_access_key: ADD HERE
+  # Credentials can be hardcoded or set in environment variables
+  access_key_id: <%= ENV['AWS_SNOWPLOW_ACCESS_KEY'] %>
+  secret_access_key: <%= ENV['AWS_SNOWPLOW_SECRET_KEY'] %>
   s3:
     region: ADD HERE
     buckets:

lib/snowplow-emr-etl-runner/cli.rb

@@ -16,6 +16,7 @@
 require 'optparse'
 require 'yaml'
 require 'contracts'
+require 'erb'
 
 module Snowplow
   module EmrEtlRunner
@@ -135,14 +136,16 @@ def self.load_config(config_file, optparse)
 
         # A single hyphen indicates that the config should be read from stdin
         if config_file == '-'
-          config = YAML.load($stdin.readlines.join)
+          config = $stdin.readlines.join
         elsif File.file?(config_file)
-          config = YAML.load_file(config_file)
+          config = File.new(config_file).read
         else
           raise ConfigError, "Configuration file '#{config_file}' does not exist, or is not a file\n#{optparse}"
         end
 
-        recursive_symbolize_keys(config)
+        erb_config = ERB.new(config).result(binding)
+
+        recursive_symbolize_keys(YAML.load(erb_config))
       end
 
       # Load the enrichments directory into an array