I tried to install Rails but I kept getting the error Rails is not currently installed on this system.

I couldn’t figure it out, until I did the following steps:

Install rvm

brew install gpg2
gpg --recv-keys 409B6B1796C275462A1703113804BB82D39DC0E3 7D2BAF1CF37B13E2069D6956105BD0E739499BDB

Then I ran

curl -sSL https://get.rvm.io | bash -s stable --rails

This installs the latest stable version of Ruby and Rails

Then I ran (Bash / Zsh)

source /Users/flavio/.rvm/scripts/rvm

and the rails command worked fine.

On Fish, instead of the above command I had to run:

curl -L --create-dirs -o ~/.config/fish/functions/rvm.fish https://raw.github.com/lunks/fish-nuggets/master/functions/rvm.fish

and

echo "rvm default" >> ~/.config/fish/config.fish