Firstly, I have installed rails via:

sudo gem install rails

Everything seems to install correctly, but when I type the rails command (rails s, rails -v, etc), I get this error:

Rails is not currently installed on this system. To get the latest version, simply type:
    $ sudo gem install rails
You can then rerun your "rails" command.

The result of which rails is /usr/bin/rails
I tried many different ways that I found on the Internet.
And finally, that works for me:

gem update --system

then,type sudo gem install rails again

Oh yeah