Question: I need to test my under development Saas web application continuously for some months without incurring any monthly costs. Is there any free Linux VPS hosting available ( with root access ) for this purpose?
Answer: There are hardly any free Linux VPS hosting with root access available out there, at least not for commercial use. For your purpose of a free VPS to test something without incurring any cost, you may opt for many trial VPS hosting and then cancel your plan before you incur any charges. You can also switch to another provider if one trial period expires. This may add some switching or migration effort on your side but will solve your purpose of not incurring any costs
Alternatively, there are some hosting servers that give substantial discounts on hosting for educational purposes or for students to test their apps. So if that is the case with you, you can check for some providers offering student discounts.