Type ruby –version you should have that if you didn’t update ruby before :

jdasilva@jdasilva:~$ ruby --version
ruby 1.8.7 (2010-01-10 patchlevel 249) [i486-linux]

This version is not completely compatible with rails 3
(according to http://guides.rails.info/3_0_release_notes.html)

So first thing, we will install the ruby 1.8.7 enterprise edition that you can find here :
http://www.rubyenterpriseedition.com/download.html

Now if you should have this (you may have to open a new terminal):

jdasilva@jdasilva:~$ ruby --version
ruby 1.8.7 (2010-04-19 patchlevel 253) [i686-linux], MBARI 0x8770, Ruby Enterprise Edition 2010.02

Ruby is ready, let’s install rails 3 now

jdasilva@jdasilva:~$ sudo gem install rails --pre
[sudo] password for jdasilva:
Successfully installed activesupport-3.0.0.beta4
Successfully installed builder-2.1.2
Successfully installed i18n-0.4.1
Successfully installed activemodel-3.0.0.beta4
Successfully installed rack-test-0.5.4
Successfully installed rack-mount-0.6.9
Successfully installed tzinfo-0.3.22
Successfully installed abstract-1.0.0
Successfully installed erubis-2.6.6
Successfully installed actionpack-3.0.0.beta4
Successfully installed arel-0.4.0
Successfully installed activerecord-3.0.0.beta4
Successfully installed activeresource-3.0.0.beta4
Successfully installed mime-types-1.16
Successfully installed polyglot-0.3.1
Successfully installed treetop-1.4.8
Successfully installed mail-2.2.5
Successfully installed actionmailer-3.0.0.beta4
Successfully installed thor-0.13.8
Successfully installed railties-3.0.0.beta4
Successfully installed bundler-0.9.26
Successfully installed rails-3.0.0.beta4
22 gems installed

Now let’s check we have the good version installed

jdasilva@jdasilva:~$ rails --version
Rails 3.0.0.beta4