def expand_env_var(env_var: str) -> str:
    """Expands potentially nested env var by repeatedly applying `expandvars` and
    `expanduser` until interpolation stops having any effect.
    """
    while True:
        interpolated = os.path.expanduser(os.path.expandvars(str(env_var)))
        if interpolated == env_var:
            return interpolated
        else:
            env_var = interpolated
Expands potentially nested env var by repeatedly applying `expandvars` and
`expanduser` until interpolation stops having any effect.
src/bentoml/_internal/configuration/__init__.py