Now, as I think about it, ruby is not like FHS

    └── gems
        ├── irb-1.2.3
        │   ├── exe
        │   │   └── irb
        │   └── lib
        │       ├── irb
        │       └── irb.rb
        └── rack-2.2.2
            ├── bin
            │   └── rackup
            └── lib
                ├── rack
                └── rack.rb
    

vs

    ├── bin
    │   ├── irb
    │   └── rackup
    └── ruby
        ├── irb
        ├── irb.rb
        ├── rack
        └── rack.rb
    

FHS approach does not support multiple versions, gems approach requires declaring version in code or Gemfile, playing with PATH, considerable slowdown https://github.com/Shopify/bootsnap