Here is a list of commands that I have run to try and install ruby 1.9.3-p0 with rbenv but every time I run ruby --version I get 1.8.23. I'm not sure why this is?
Anderss-MacBook-Pro:anchor anderskitson$ ruby --version
ruby 1.8.7 (2012-02-08 patchlevel 358) [universal-darwin12.0]
Anderss-MacBook-Pro:anchor anderskitson$ rbenv install 1.9.3-p0
Downloading yaml-0.1.4.tar.gz...
-> http://dqw8nmjcqpjn7.cloudfront.net/36c852831d02cf90508c29852361d01b
Installing yaml-0.1.4...
Installed yaml-0.1.4 to /Users/anderskitson/.rbenv/versions/1.9.3-p0
Downloading ruby-1.9.3-p0.tar.gz...
-> http://dqw8nmjcqpjn7.cloudfront.net/8e2fef56185cfbaf29d0c8329fc77c05
Installing ruby-1.9.3-p0...
Installed ruby-1.9.3-p0 to /Users/anderskitson/.rbenv/versions/1.9.3-p0
Downloading rubygems-1.8.23.tar.gz...
-> http://dqw8nmjcqpjn7.cloudfront.net/178b0ebae78dbb46963c51ad29bb6bd9
Installing rubygems-1.8.23...
Installed rubygems-1.8.23 to /Users/anderskitson/.rbenv/versions/1.9.3-p0
Anderss-MacBook-Pro:anchor anderskitson$ ruby --version
ruby 1.8.7 (2012-02-08 patchlevel 358) [universal-darwin12.0]
Anderss-MacBook-Pro:anchor anderskitson$ rbenv global 1.9.3-p0
Anderss-MacBook-Pro:anchor anderskitson$ ruby --version
ruby 1.8.7 (2012-02-08 patchlevel 358) [universal-darwin12.0]
Anderss-MacBook-Pro:anchor anderskitson$ rbenv version
1.9.3-p0 (set by /Users/anderskitson/.rbenv/version)
Anderss-MacBook-Pro:anchor anderskitson$
UPDATE
I get this out put for rbenv global
Anderss-MacBook-Pro:anchor anderskitson$ $ rbenv global 1.9.3-p0
-bash: $: command not found
You need to set the ruby version you wish your shell to use with rbenv global after installing the ruby version with rbenv.
$ rbenv global 1.9.3-p0
See these docs for more.