pyenv/libexec/pyenv-version-file-read

#!/usr/bin/env bash
# Usage: pyenv version-file-read <file>
set -e
[ -n "$PYENV_DEBUG" ] && set -x

VERSION_FILE="$1"

if [ -e "$VERSION_FILE" ]; then
  # Read the first non-whitespace word from the specified version file.
  # Be careful not to load it whole in case there's something crazy in it.
  words=( $(cut -b 1-1024 "$VERSION_FILE") )
  versions=("${words[@]}")

  if [ -n "$versions" ]; then
    OLDIFS="$IFS"
    { IFS=:
      echo "${versions[*]}"
    }
    IFS="$OLDIFS"
    exit
  fi
fi

exit 1