pyenv/libexec/rbenv-version-name

#!/usr/bin/env bash
set -e

# Read the first non-whitespace word from the specified file.
read_version_file() {
  local words version
  while read -a words; do
    version="${words[0]}"
    if [ -n "$version" ]; then
      echo "$version"
      break
    fi
  done < "$1"
}

DEFAULT_PATH="${HOME}/.rbenv/default"

if [ -z "$RBENV_VERSION" ]; then
  RBENV_VERSION_FILE="$(rbenv-version-origin)"

  if [ -n "$RBENV_VERSION_FILE" ]; then
    RBENV_VERSION="$(read_version_file "$RBENV_VERSION_FILE")"
  else
    echo system > "$DEFAULT_PATH"
    RBENV_VERSION=system
  fi
fi

if [ "$RBENV_VERSION" = "system" ]; then
  echo "$RBENV_VERSION"
  exit
fi

RBENV_VERSION_PATH="${HOME}/.rbenv/versions/${RBENV_VERSION}"

if [ -d "$RBENV_VERSION_PATH" ]; then
  echo "$RBENV_VERSION"
else
  echo "rbenv: version \`$RBENV_VERSION' is not installed" >&2
  exit 1
fi