Bootstrap script (script/bootstrap) from a small rails project I'm working on. Designed to be quick to run when nothing changed. Intended to be used in CI builds and on dev machines.

gistfile1.txt

$ time script/bootstrap                                                                                                                                  
Bundler missing, installing.
Fetching: bundler-1.1.3.gem (100%)
Successfully installed bundler-1.1.3
1 gem installed
Gemfile changed, bundling.
Schema changed, updating databases.

real    0m9.163s
user    0m3.762s
sys     0m2.413s

$ time script/bootstrap 

real    0m0.054s
user    0m0.033s
sys     0m0.011s

script-bootstrap.rb

#!/usr/bin/env ruby
def check(key)
  cache_file = "tmp/.#{key}"
  if File.exists?(cache_file)
    old_version = File.read(cache_file)
  else
    old_version = nil
  end

  version = yield(old_version)
  File.open(cache_file, 'w') { |f| f.puts(version || 'cached') }
end

check(:bundler) do |old_version|
  break if old_version
  unless system("source .rvmrc; which bundle > /dev/null")
    puts "Bundler missing, installing."
    system("gem install bundler --no-ri --no-rdoc") || exit(1)
  end 
end

check(:gemfile) do |old_version|
  # check both Gemfile and Gemfile.lock as there might be local changes that hasn't been bundled yet
  version = File.read("Gemfile") + File.read("Gemfile.lock")
  if version != old_version
    puts "Gemfile changed, bundling."
    system("bundle 1> /dev/null") || exit(1)
  end
  version
end

check(:schema) do |old_version|
  version = File.readlines("db/schema.rb").find { |line| line.include?("define(:version") }
  if version != old_version
   puts "Schema changed, updating databases."
   system("rake db:migrate db:test:prepare") || exit(1)
  end
  version
end

Tip: Add system('script/bootstrap') to script/rails before "APP_PATH".