To install Ruby under Ubuntu or Debian, you need to get several packages. You can use a command like this:

$ sudo apt-get install ruby ruby1.8 ruby1.8-dev rdoc ri irb

Obtain RubyGems and install it manually. You can get a tarball here. Then untar it:

$ tar zxvf rubygems-0.9.0.tgz 

And install it:

$ cd rubygems-0.9.0/
$ sudo ruby setup.rb

For the websearch.rb Reby script, you will need to get this gem:

$ sudo gem install rubyful_soup

It will prompt you about a dependency; just answer yes.

Install required dependency htmltools? Yn 

$ cd /home/username/eggdrop/scripts
$ wget http://purepistos.net/eggdrop/reby/reby-0.7.4.tar.gz
$ tar zxvf reby-0.7.4.tar
$ mv reby-0.7.4 reby

$ cd reby
$ cp reby.conf.sample reby.conf
$ nano reby.conf

Edit the file to suit, including host, port, eggdrop username and password. Exit your editor and save. In Nano: Ctrl o; ctrl x.

Edit eggdrop.conf and comment out the following lines (by adding a # symbol in front of them):

unbind dcc n tcl *dcc:tcl
unbind dcc n set *dcc:set

Restart your eggdrop.

$ cd /home/username/eggdrop/scripts/reby
$ ruby reby.rb

Check reby.log for errors. Type !reby in your IRC channel to test it.

Further documentation and files can be found here. Reby's home channel is #mathetes on the FreeNode IRC network (irc.freenode.net).