You can clone with
apt-get -y update
apt-get -y install build-essential zlib1g-dev libssl-dev libreadline5-dev libyaml-dev
tar -xvzf ruby-1.9.3-p125.tar.gz
gem install chef ruby-shadow --no-ri --no-rdoc
Heads up, the name of readline's packaged was changed to libreadline-gplv2-dev (or lib64readline-gplv2-dev when using 64bit) from libreadline5-dev on ubuntu 12.04 LTS. And that means without readline, the dependencies for the build-essential will fail and the gcc compiler will not be installed.
You can also use libreadline6-dev instead of libreadline5-dev in 12.04.
Don't you want apt-get -y upgrade after the update? Sometimes, ubuntu asks for a reboot afterwards (if you look at the post-login message).
apt-get -y upgrade
@bluemont, apt-get upgrade will upgrade all the packages in the system, not sure if you want this
I have written a Gist for setting this up on Ubuntu 12.04 32 Bit systems. Check it out at: Ruby & Chef Install
Gist for setting this up on Ubuntu 12.04 LTS 64 Bit here