pyenv/libexec/pyenv-version-name

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

if [ -z "$PYENV_VERSIONS" ]; then
  PYENV_VERSION_FILE="$(pyenv-version-file)"
  IFS=: PYENV_VERSIONS=($(pyenv-version-file-read "$PYENV_VERSION_FILE" || true))
fi

if [ -z "$PYENV_VERSIONS" ] || [ "$PYENV_VERSIONS" = "system" ] ; then
  echo "system"
  exit
fi

for PYENV_VERSION in "${PYENV_VERSIONS[@]}"; do
  PYENV_VERSION_PATH="${PYENV_ROOT}/versions/${PYENV_VERSION}"
  
  if [ ! -d "$PYENV_VERSION_PATH" ]; then
    echo "pyenv: version \`$PYENV_VERSION' is not installed" >&2
    exit 1
  fi
done

IFS=: echo "${PYENV_VERSIONS[*]}"