def require_env(repository_ctx, variable):
    value = repository_ctx.os.environ.get(variable)
    if not value:
        fail("{} must be set.".format(variable))
    return value


def execute(repository_ctx, arguments):
    result = repository_ctx.execute(arguments)
    if result.return_code != 0:
        fail("command failed: {}\n{}".format(" ".join(arguments), result.stderr))
    return result.stdout.strip()