__init_ruby.fish

function __init_ruby
  set -xg ARCHFLAGS '-arch x86_64' # build gemses on Snowy

  set -l lastruby (cat ~/.lastruby ^/dev/null)
  if test -z $lastruby
    set lastruby system
  end

  rvm $lastruby
end

fish shell.sh

# Install gems locally, and only locally, without gem complaining
# about path access, and ensuring access to executables.
# 
# Goes in config.fish

__init_ruby

gistfile2.sh

# Install gems locally, and only locally, without gem complaining
# about path access, and ensuring access to executables.
# 
# Goes in .bash_profile or .bash_login.

export GEM_PATH="$HOME/.gem/ruby/1.8"
export GEM_HOME="$HOME/.gem/ruby/1.8"
export PATH="$HOME/.gem/ruby/1.8/bin:$PATH"

rvm.fish

function rvm
  set -l gems ''
  set -l ruby ''

  if test ! $argv[1]
    set argv[1] '<nothing>'
  end

  switch $argv[1]
    case system
      set gems $HOME/.gem-system
      set ruby /usr/bin/ruby

    # for other rubies add somthing like the following.
    # the important principle is that I don't want to mix gems between versions
    # cause its bad for space-time.

    # case r19
    #   set gems $HOME/.gem-1.9 
    #   set ruby ~/homebrew/.../ruby19/bin/ruby # whatever the path is

    case '*'
      echo 'hey, ruby needs to be one of [system]'
      return
  end

  echo $argv[1] > ~/.lastruby

  set -xg GEM_HOME $gems
  set -xg GEM_PATH $gems
  set -xg PATH $gems/bin (dirname $ruby) $PATH
end

Why & more info

http://ujihisa.blogspot.com/2009/06/rubygems-best-practice.html