understand for why MRUBY_CONFIG use ternary operator .

mrb_config_watch.rb

require 'pp'
# encoding: utf-8
# Build description.
# basic build file for mruby
p MRUBY_ROOT = File.dirname(File.expand_path(__FILE__))
p MRUBY_BUILD_HOST_IS_CYGWIN = RUBY_PLATFORM.include?('cygwin')

# load build systems
#p load "#{MRUBY_ROOT}/tasks/ruby_ext.rake"
#p load "#{MRUBY_ROOT}/tasks/mruby_build.rake"
#p load "#{MRUBY_ROOT}/tasks/mrbgem_spec.rake"

# load configuration file
 MRUBY_CONFIG = (ENV['MRUBY_CONFIG'] && ENV['MRUBY_CONFIG'] != '') ? ENV['MRUBY_CONFIG'] : "#{MRUBY_ROOT}/build_config.rb"

 puts "================="

pp (ENV['MRUBY_CONFIG'] && ENV['MRUBY_CONFIG'] != '')

pp (ENV['MRUBY_CONFIG'])