Bash Script to check if rbenv for RVM is installed

check_if_ruby_is_system.sh

#!/bin/bash

command_exists () {
  command "$1" &> /dev/null ;
}

if command_exists rbenv
then
  echo 'rbenv found, no action needed'
elif command_exists rvm
then
  echo 'rvm found, no action needed'
else
  echo 'Did not find rbenv or rvm!'
  echo 'Please install either tool to manage ruby'
  echo 'rbenv: `brew install rbenv ruby-build`'
  echo 'rvm: `\curl -sSL https://get.rvm.io | bash -s stable --ruby`'
  exit 1
fi

How about which rbenv?