Gemfile parser: outputs the same Gemfile with the version of each gem used in the current project. Does not handle version of gems from a git repo.

parse_gemfile.rb

require 'active_support/core_ext/object/try.rb'

trap("INT") { puts "Exiting..."; exit }

FILENAME = ARGV[0]

if FILENAME.nil? || FILENAME.empty?
  puts 'Usage:'
  puts 'ruby parse_gemfile.rb path-to-your-Gemfile'
  exit
end

file =
  begin
    File.open(FILENAME)
  rescue Errno::ENOENT => e
    puts "Could not open file #{FILENAME}" ; exit
  end

file.each_line do |l|
  gem = l.match(/^(\s*)gem\s?['"]([\-a-zA-z]+)['"](,\s*['"](.*[-\.0-9a-zA-Z]+)['"])?(.*)/)

  # Puts the line if it does not declare a gem
  (puts l ; next) unless gem

  # Skips gems with already a version specified
  actual_version = gem[4]
  (puts l ; next) if actual_version

  spaces, gem_name, end_of_line = gem[1], gem[2], gem[5]

  output  = `bundle show #{gem_name}`
  version = output.match(/.*-([0-9\.]+)/).try(:[], 1) or nil

  if version
    puts %|#{spaces}gem '#{gem_name}', '~> #{version}'#{end_of_line}|
  else
    puts l
  end
end

Could be enhanced parsing gem version from the Gemfile.lock instead of calling bundle show.