<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def valid_env_vars() -> bool: """Validate that required env vars exist. :returns: True if required env vars exist. .. versionadded:: 0.0.12 """

    for envvar in _REQUIRED_ENV_VARS:
        try:
            _check_env_var(envvar)
        except KeyError as ex:
            LOG.error(ex)
            sys.exit(1)
    return True