[ruby] cmd

# rvm
rvm list // list installed interpreters 
rvm list known // list available interpreters
rvm install <version> // install ruby interpreter
rvm use <version> // switch to specified ruby interpreter
rvm remove <version> // remove ruby interpreter (alias: delete)
rvm get <version> // upgrade rvm: stable, master
ls -al ~/.rvm/

# ruby
ruby -h // -h <=> --help
ruby -v // print version number, then turn on verbose mode

# gem
gem -h // -h <=> --help
gem -v // -v <=> --version
gem install <package> // in gem, a package is a gem
gem list // list installed gems