To install any ruby gem which has a command-line interface on Ubuntu 8.10, you have to add a path to your PATH environment variable. In your .bashrc file, add the following line:

export PATH=$PATH:/var/lib/gems/1.8/bin

Also worth noting is the fact that the default ruby interpreter on 8.10 is back to the 1.8 branch: it’s 1.8.7 (1.9 was the default on 8.04 iirc). 1.9  also be installed right besides 1.8.

$ ruby --version
ruby 1.8.7 (2008-08-11 patchlevel 72) [i486-linux]
$ ruby1.9 --version
ruby 1.9.0 (2008-06-20 revision 17482) [i486-linux]

Neither comes installed by default, however. You must install them explicitly.

sudo aptitude install ruby irb rubygems

While I’m at it, why not mention that rubygems 1.2.0 is installed by default. It doesn’t want to update to 1.3.0 with the usual “gem update –system” command. Since it’s not my main machine I didn’t investigate further, but the suggestion is to use apt-get or aptitude. The repos don’t seem to be up to date with 1.3.0, but rather with a version named something like 1.3.0really1.2.0.