single-user only rubygems

setup runtime:

bash: (~/.bashrc)

export GEM_HOME=$HOME/.gem/ruby/1.8
export GEM_PATH=$GEM_HOME

fish: (~/.config/fish/config.fish)

set GEM_HOME $HOME/.gem/ruby/1.8
set GEM_PATH $GEM_HOME

setup config:

echo "gemhome: $GEM_PATH
gempath: - $GEM_PATH" > ~/.gemrc

now system gem would not mess-up your user gem.