pyenv/libexec/rbenv-whence

#!/usr/bin/env bash
set -e
[ -n "$RBENV_DEBUG" ] && set -x

if [ "$1" = "--path" ]; then
  print_paths="1"
  shift
else
  print_paths=""
fi

whence() {
  local command="$1"
  rbenv-versions --bare | while read version; do
    path="$(rbenv-prefix "$version")/bin/${command}"
    if [ -x "$path" ]; then
      [ "$print_paths" ] && echo "$path" || echo "$version"
    fi
  done
}

RBENV_COMMAND="$1"
if [ -z "$RBENV_COMMAND" ]; then
  echo "usage: rbenv whence [--path] COMMAND" >&2
  exit 1
fi

result="$(whence "$RBENV_COMMAND")"
[ -n "$result" ] && echo "$result"