A Composer global command that installs the latest official Codeigniter framework.
composer global require "dsv/codeigniter-installer"
Once installed, you can run the command:
codeigniter new <app_name>
Also you can specify the CI version with the second param, so the installer can search for an alternate application version:
codeigniter new <app_name> 3.0.6
Make sure to place the ~/.composer/vendor/bin directory in your bash PATH so the codeigniter executable can be located by your system.