gistfile1.txt

# if you want the switch to include gem (slaved) remove the separate gem alternative command.
sudo update-alternatives --remove-all gem
# install ruby1.8 & friends with priority 500 
# so this will be the default "auto" choice 
sudo update-alternatives --install /usr/bin/ruby ruby /usr/bin/ruby1.8 500 \
                    --slave   /usr/share/man/man1/ruby.1.gz ruby.1.gz /usr/share/man/man1/ruby1.8.1.gz \
                    --slave   /usr/bin/ri ri /usr/bin/ri1.8 \
                    --slave   /usr/bin/irb irb /usr/bin/irb1.8 \
                    --slave   /usr/bin/rake rake /usr/bin/rake1.8 \
                    --slave   /usr/bin/gem gem /usr/bin/gem1.8 \
                    --slave   /etc/bash_completion.d/gem bash_completion_gem /etc/bash_completion.d/gem1.8 \
                    --slave   /usr/share/man/man1/gem.1.gz gem.1.gz /usr/share/man/man1/gem1.8.1.gz

# install ruby1.9 & friends with priority 400 
sudo update-alternatives --install /usr/bin/ruby ruby /usr/bin/ruby1.9.1 400 \
                    --slave   /usr/share/man/man1/ruby.1.gz ruby.1.gz /usr/share/man/man1/ruby1.9.1.1.gz \
                    --slave   /usr/bin/ri ri /usr/bin/ri1.9.1 \
                    --slave   /usr/bin/irb irb /usr/bin/irb1.9.1 \
                    --slave   /usr/bin/rake rake /usr/bin/rake1.9.1 \
                    --slave   /usr/bin/gem gem /usr/bin/gem1.9.1 \
                    --slave   /etc/bash_completion.d/gem bash_completion_gem /etc/bash_completion.d/gem1.9.1 \
                    --slave   /usr/share/man/man1/gem.1.gz gem.1.gz /usr/share/man/man1/gem1.9.1.1.gz