How to Create Rails 6 application from within a Rails 5 environment - ruby-on-rails

Question

I'm playing around with a microservice automation pipeline that builds out a bunch of templated code.
Currently, the pipeline code is automatically running via guard and generates a shell script to create databases, create new rails application and other types of assets.
I run ruby & rails within RbEnv and the script that I am executing works fine from the command line, it creates a brand new Rails 6 application.
But, when I run it from guard which has an existing Rails 5.2 environment Application, the new application is built as a Rails 5.2 app.
I have tried overriding certain settings, but it still picks up the environment that my code is running in.
When I run the script from the command line:
echo 'SHOULD CREATE A RAILS APP using VERSION 6.0.0 on RUBY 2.6.3'
gem install rails -v 6.0.0
echo 'rails -v'
rails -v
echo 'rbenv local'
rbenv local
echo 'rbenv version'
rbenv version
echo 'rbenv versions'
rbenv versions
rails _6.0.0_ new . -d postgresql --force --skip-action-mailer --skip-action-mailbox --skip-action-text --skip-spring --skip-test --skip-bundle --skip-webpack-install
I get the following output log
Successfully installed rails-6.0.0
1 gem installed
SHOULD CREATE A RAILS APP using VERSION 6.0.0 on RUBY 2.6.3
rails -v
Rails 6.0.0
rbenv local
2.6.3
rbenv version
2.6.3 (set by /Users/myname/dev/myapp/.ruby-version)
rbenv versions
system
2.4.1
2.4.6
2.5.3
* 2.6.3 (set by /Users/myname/dev/myapp/.ruby-version)
With following GEM file
ruby '2.6.3'
# Bundle edge Rails instead: gem 'rails', github: 'rails/rails'
gem 'rails', '~> 6.0.0'
# Use postgresql as the database for Active Record
gem 'pg', '>= 0.18', '< 2.0'
# Use Puma as the app server
And confirmation via the last line that the application is Rails 6
remove config/initializers/new_framework_defaults_6_0.rb
When I run the same script from ruby using variations on system and fork { exec { } }
Dir.chdir File.dirname(output_file) do
# fork { exec("bash #{output_file}") }
system "bash #{output_file}"
# system "/usr/local/bin/zsh #{output_file}"
end
I get Rails 5.2
rails -v
Rails 5.2.3
rbenv local
rbenv: no local version configured for this directory
rbenv version
2.5.3 (set by RBENV_VERSION environment variable)
rbenv versions
system
2.4.1
2.4.6
* 2.5.3 (set by RBENV_VERSION environment variable)
2.6.3

Answer 1

I don't really understand why you'd want to do this in ruby, it seems like you're trying to reinvent something like what docker or kubernetes already has solved pretty well.
But I suspect the problem with your script is the bash session doesn't have the rbenv stubs you want. I would find where they are on your computer or server's filesystem and then specify exact paths to them when running commands like gem, rails, or bundler.