/usr/bin/ruby-standalone
/usr/lib/ruby-standalone/bin/erb
/usr/lib/ruby-standalone/bin/gem
/usr/lib/ruby-standalone/bin/irb
/usr/lib/ruby-standalone/bin/rake
/usr/lib/ruby-standalone/bin/rdoc
/usr/lib/ruby-standalone/bin/ri
/usr/lib/ruby-standalone/bin/ruby2.1
/usr/lib/ruby-standalone/bin/ruby
/usr/lib/ruby-standalone/bin/testrb
You may install this package on Debian with
apt-get
install ruby-standalone