The syntax

installs the latest version of the uglifier gem (which handles file compression for the asset pipeline) as long as it’s greater than version 1.2.3—even if it’s, say, version 7.2. Meanwhile, the code

installs the gem coffee-rails (also needed by the asset pipeline) as long as it’s lower than version 3.3. In other words, the >= notation always performs upgrades, whereas the ˜> 3.2.2 notation only performs upgrades to minor point releases (e.g., from 3.1.1 to 3.1.2), but not to major point releases (e.g., from 3.1 to 3.2). Unfortunately, experience shows that even minor point releases often break things, so for the Rails Tutorial we’ll err on the side of caution by including exact version numbers for virtually all gems. (The only exception is gems that are in release candidate or beta stage as of this writing; for those gems, we’ll use ˜> so that the final versions will be loaded oncethey’redone.)        

Once you’ve assembled the proper Gemfile, install the gems using bundle install:

(If you’re running OS X and you get an error about missing Ruby header files (e.g., ruby.h) at this point, you may need to install Xcode. These are developer tools that came with your OS X installation disk, but to avoid the full installation I recommend the much smaller Command Line Tools for Xcode.12) The bundle install command might take a few moments, but when it’s done our application will be ready to run. Note: This setup is fine for the first app, but it isn’t ideal. Chapter 3 covers a more powerful (and slightly more advanced) method for installing Ruby gems with Bundler.