Middleman is distributed using the RubyGems package manager. This means you will need both the Ruby language runtime installed and RubyGems to begin using Middleman.
Mac OS X comes prepackaged with both Ruby and Rubygems, however, some of the Middleman's dependencies need to be compiled during installation and on OS X that requires Xcode Command Line Tools. Xcode can be installed from the terminal:
$ xcode-select --install
Once you have Ruby and RubyGems up and running, execute the following from the command line:
$ gem install middleman
This will install Middleman, its dependencies and the command-line tools for using Middleman.
The installation process will add one new command to your environment, with 3 useful features:
$ middleman init $ middleman server $ middleman build
The uses of each of these commands will be covered in the next section, Start a New Site.