A set of scripts for installing Ruby and Rails, via rbenv, in an entirely self-contained manner, i.e. with no global/system gems beyond bundler. Once you've run the install script, run `source developer_start.sh` then `rails new .` to generator your new Rails app. You'll need to run `source developer_start.sh` for each new shell where you'll be …

developer_start.sh

# Usage: source developer_start.sh
#
# If you execute this script rather than sourcing it, the binstubs will not be
# prepended to your path.

# Install gems in local folder (with binstubs) in lieu of using a gemset.
path=".bundle"
binstubs="$path/bin"
bundle install --path=$path --binstubs=$binstubs

# Put Bundler binstubs on the path.
# Using a full path because relative paths are a security threat. See
# http://superuser.com/questions/156582/why-is-not-in-the-path-by-default.
export PATH="$PWD/$binstubs:$PATH"

Gemfile

source 'https://rubygems.org'

gem 'rails', '4.0.0.beta1'

install_ruby.sh

#!/bin/bash
#
# Install rbenv, ruby-build (as an rbenv extension), local Ruby and Bundler.
# Also ensures local documentation generation is skipped to reduce execution
# time and disk usage. Tested on Ubuntu 12.04.

# Install compilation essentials plus git for source code fetching.
sudo apt-get install -y build-essential git-core

# Install rbenv per https://github.com/sstephenson/rbenv.
git clone git://github.com/sstephenson/rbenv.git ~/.rbenv
profile=~/.profile
echo 'export PATH="$HOME/.rbenv/bin:$PATH"' >> $profile
echo 'eval "$(rbenv init -)"' >> $profile
source $profile

# Install ruby-build per https://github.com/sstephenson/ruby-build.
git clone git://github.com/sstephenson/ruby-build.git ~/.rbenv/plugins/ruby-build

# Install global Ruby version, skipping local documentation generation.
ruby_version=2.0.0-p0
CONFIGURE_OPTS="--disable-install-doc" rbenv install $ruby_version
rbenv local $ruby_version

# Ensure gem installations skip local documentation generation.
cat <<GEMRC >~/.gemrc 
install: --no-rdoc --no-ri 
update: --no-rdoc --no-ri
GEMRC

gem install bundler

echo
echo 'Run `exec $SHELL -l` to use rbenv in the current shell.'