rbenv functions for the fish shell, by Łukasz Niemier: https://coderwall.com/p/6hja1w

rbenv.fish

function rbenv_shell
  set -l vers $argv[1]

  switch "$vers"
    case '--complete'
      echo '--unset'
      echo 'system'
      command rbenv versions --bare
      return
    case '--unset'
      set -e RBENV_VERSION
      return 1
    case ''
      if [ -z "$RBENV_VERSION" ]
        echo "rbenv: no shell-specific version configured" >&2
        return 1
      else
        echo "$RBENV_VERSION"
        return
      end
    case '*'
      rbenv prefix "$vers" > /dev/null
      set -gx RBENV_VERSION "$vers"
  end
end

function rbenv_lookup
  set -l vers (command rbenv versions -- bare| sort | grep -- "$argv[1]" | tail -n1)

  if [ ! -z "$vers" ]
    echo $vers
    return
  else
    echo $argv
    return
  end
end

function rbenv
  set -l command $argv[1]
  [ (count $argv) -gt 1 ]; and set -l args $argv[2..-1]

  switch "$command"
    case shell
      rbenv_shell (rbenv_lookup $args)
    case local global
      command rbenv $command (rbenv_lookup $args)
    case '*'
      command rbenv $command $args
  end
end