Installs all the required gems in the specified file or directory.

gems.rb

#!/usr/bin/env ruby

def installed?(gem_name)
  require gem_name
  true
rescue LoadError
  false
end

def required_gems(file_name)
  File.readlines(file_name).map { |line|
    line.match(/^require.*(?=['"](.+)['"])(?:"\1"|'\1')/)
  }.compact.map { |e| e[1] }.uniq
end

def gems_in_path(path, recursive = false)
  if File.file? path
    required_gems path
  elsif File.directory? path
    Dir.chdir path
    Dir["#{recursive ? '**/' : ''}*.{rb,ru}"].map{ |file_name| required_gems(file_name) }.flatten.uniq
  else
    abort "\e[31mFile not found: #{path}\e[0m"
  end.reject { |e| installed? e }
end

def install(gems)
  if gems.size == 0
    puts 'All the gems are already installed.'
  else
    puts "Following gem" + (gems.size > 1 ? 's' : '') + ' need to be installed:'
    gems.each { |e| puts e }
    print 'Install? [Y/n]: '
    install = ['', 'y', 'yes'].include? STDIN.gets.chomp.downcase
    `gem install #{gems.join(' ')}` if install
  end
end

recursive = ARGV.delete('-r')
abort 'Usage: gemper [-r] file|directory' if ARGV.size == 0

install gems_in_path(ARGV[0], recursive)

I have a repo called gemper, which advances this idea.