gem Command Reference | RubyGems Manuals

gem cleanup clean the old junk from my machine:

Usage: gem install GEMNAME [options] Options: -v, --version VERSION Specify version of gem to install -l, --local Restrict operations to the LOCAL domain (default) -r, --remote Restrict operations to the REMOTE domain -b, --both Allow LOCAL and REMOTE operations -i, --install-dir DIR -d, --[no-]rdoc Generate RDoc documentation for the gem on install -f, --[no-]force Force gem to install, bypassing dependency checks -t, --[no-]test Run unit tests prior to installation -w, --[no-]wrappers Use bin wrappers for executables Not available on dosish platforms -P, --trust-policy POLICY Specify gem trust policy. --ignore-dependencies Do not install any required dependent gems -y, --include-dependencies Unconditionally install the required dependent gems Common Options: --source URL Use URL as the remote source for gems -p, --[no-]http-proxy [URL] Use HTTP proxy for remote operation...

Full article: http://docs.rubygems.org/read/chapter/10#page77