How to define a uniq ruby version on Mac OS High-Sierra?

(apologizes: not a native english)

I upgraded my MacBook Air a few days ago to OS High-sierra and I encounter a lot of problems with my ruby versions.

In particular:

Terminal works with a 2.4 ruby version I can't find anywhere…
Apache server seems to work with a 2.3.3 ruby version I find… outside of the rbenv folder…
My rbenv knows only 2.3.0 and 2.3.1 ruby versions…
I'm not be able to install 2.4 version with rbenv (see below)…
rbenv global 2.3.1 doesn't help at all (nothing changes)…

What could I try to use only the 2.3.1 ruby version?………

Some (maybe) helpfull information:

I don't have a ~/.rvm folder.

Some feedbacks about config, in Terminal:

$> ruby -v
   # => ruby 2.4.2p198 (2017-09-14 revision 59899) [x86_64-darwin17]

$> rbenv versions
   # => 
   * system (set by /Users/philippeperret/.rbenv/version)
   2.3.0
   2.3.1

$> which -a ruby
   # => 
     /Users/philippeperret/.rbenv/shims/ruby
     /usr/local/bin/ruby
     /usr/bin/ruby

$> irb
irb> RUBY_VERSION
     # => "2.4.2"

About homebrew

$> brew config
HOMEBREW_VERSION: 1.4.1
ORIGIN: https://github.com/Homebrew/brew
...
HOMEBREW_PREFIX: /usr/local
CPU: quad-core 64-bit haswell
Homebrew Ruby: 2.3.3 => /System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/bin/ruby
Clang: 9.0 build 900
Git: 2.14.3 => /Applications/Xcode.app/Contents/Developer/usr/bin/git
Curl: 7.54.0 => /usr/bin/curl
...
Ruby: /Users/philippeperret/.rbenv/shims/ruby => /System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/bin/ruby
Java: 1.8.0_45
macOS: 10.13.2-x86_64
Xcode: 9.2
CLT: N/A
X11: 2.7.11 => /opt/X11

I can't figure out why the ruby version 2.3 (.3) is in /System/Library/Frameworks/Ruby.framework/Versions/ rather than in .rbenv folder?… (despite the fact that I know it's a high-sierra installation)

When I run a local (rails-like) web app (in my browser), asking it the ruby version:

$> RUBY_VERSION
# => 2.3.3
$> ruby --version
# => ruby 2.3.3p222 (2016-11-21 revision 56859) [universal.x86_64-darwin17]
$> which -a ruby
# => /usr/bin/ruby

And it can't find some ruby gems (require 'rubygems' doesn't help). I can't install these gems, of course, until I can't choose 2.3.3 version with rbenv in Terminal:

When I run:

$> rbenv global 2.4.2p198

… (which is the Terminal ruby version), it complains:

rbenv: version `2.4.2p198' not installed

When I try:

rbenv install 2.4.2p198

… it complains:

ruby-build: definition not found: 2.4.2p198

See all available versions with `rbenv install --list'.
If the version you need is missing, try upgrading ruby-build:
brew update && brew upgrade ruby-build

(of course I "brew updated" and so on)

I spent the last few days to read documentation and SO's question/answers in vain…

Solution

Without any answer, the only way I found was to remove all ruby versions:

> brew remove --force ruby
> brew uninstall --ignore-dependencies ruby
> brew uninstall --force ruby
> sudo rm -rf /usr/local/lib/ruby
> sudo rm -rf ~/.rbenv
> sudo rm -rf ~/.gem

Now I use 2.3.3 version everywhere.

> ruby -v
# => 2.3.3

In brew config:

Homebrew Ruby: 2.3.3 => /System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/bin/ruby
Ruby: /usr/bin/ruby => /System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/bin/ruby

I can find my gems in /Library/Ruby/Gems/2.3.0/gems:

gem which sass
# => /Library/Ruby/Gems/2.3.0/gems/sass-3.5.4/lib/sass.rb

To install a new gem, super user privileges are required:

sudo gem install <gem name>

Thanks for help. Maybe It helps.