Updated the script Jan 6, 2010 to add mkdir INSTALL_ROOT (excellent suggestion), use recent versions of ruby and rubygems (the old script was broken for me), and pull the versions into variables so it is easier to tweak this later.
It might also help to note that you will need the zlib and openssl development packages, and if you plan to use the console you almost certainly want readline as well, and all of that before you build ruby. On Debian these were libssl-dev, zlib1g-dev, and libreadline5-dev.
Some people are concerned with keeping their /usr hierarchy clean. Some people have no root access to the system that they are on. Some people don’t like using /usr/local. These people still want to install and run Ruby and Rails, but it will be best for them if it is done in a way that will keep gem working as it should.
I like to tell these people, “Oh it should be easy,” but I had to really make sure. There is one little trick, which is to make sure you really are using the right ruby when you install rubygems and then the right gem when you install Rails (and forever after).
As much for my own benefit as for anyone else’s, I put together a script so that I don’t forget. This should work on any UNIX that has the build dependencies for Ruby. That includes Linux of flavors Redhat, Ubuntu, Debian, and Slackware for all you Google fans out there. (The guy whose problems prompted me to put this together was using Ubuntu.) I like Debian but have found that Rails changes way too fast for Debian packages to be a viable way to install it. Gem is much better and standard in the Rails community, so you should do a local install of Ruby from source and then install rubygems using that version.
Without further ado, here’s the script that will save you time. You will need to edit it some unless you want /opt/rails. Personally I use /usr/local. It will also leave a build subdirectory in whatever location you use. It is your option to remove it after the fact. I would do so myself.
tar zxvf $RUBY_FILE
tar zxvf $RUBYGEMS_FILE
# This is crucial. rubygems setup really cares where ruby ran from, and if
# you have a system-wide ruby then it might get confused.
# Of course you will also want to make sure you include this wherever you
# normally set your PATH.
gem install rails