installed ruby using apt-get install ruby 2.0.0 succeeded but not using correct ruby version

If you're new to linux I'd recommend using something like RVM (Ruby Version Manager) to install ruby. It makes it easier to switch ruby versions and manage multiple gemsets.

To install RVM with the latest (stable) ruby:

\curl -L https://get.rvm.io | bash -s stable --ruby

then check which rubies are installed by using

rvm list

you can then switch ruby versions using

rvm use 2.0.0 --default

with the --default flag overriding any system ruby.

Update
If you really don't want to use RVM, then use

sudo apt-get install checkinstallwget -c http://ftp.ruby-lang.org/pub/ruby/2.0/ruby-2.0.0-p0.tar.gztar -xzf ruby-2.0.0-p0.tar.gzcd ruby-2.0.0-p0./configure   makesudo checkinstall -y \  --pkgversion 2.0.0-p0 \  --provides "ruby-interpreter"

checkinstall will package the source, making it easier to remove in the future

You'll then need to add the Ruby binaries to your path, by editing the env file:

sudo nano /etc/environment

add /usr/local/ruby/bin

PATH="/usr/local/ruby/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"

then run

source /etc/environment

to reload the file, and check your ruby version with

ruby -v

You didn't actually install ruby 2.x.x with that apt-get command. The normal repositories have ruby 1.8 and ruby 1.9.1 in them, currently.

There shouldn't be a space in your apt-get command either. With that command you would've installed ruby 1.9.1 (which is the same thing as saying apt-get install ruby). The 2.0.0 would have been interpreted as a package name.