Which cycle is preferable for daily commute to office in India? I have two questions – which cycle and which bag?

I need to travel everyday 20 KM to and fro by office cab. I want to switch to bicycle. 
Could you recommend a good enough bicycle in India at low price? I am planning to buy it on Amazon. 

I also need to carry a backpack (for my lunch box and perhaps a book), I found messenger bag to be good (Why do cyclists prefer messenger bags over backpacks?). But since I'm not a regular cyclist, I want to know if 10KM is a short distance or long and any backpack is good enough for this distance. And when should I prefer backpack to messenger bag, because I think the strap of messenger bag would uncomfortable as it may keep shifting while walking or riding?

And I also may use the bicycle every weekends for going to library which is 15KM far from my home.

I am ready to buy bicycle and backpack soon after I get an answer. Please help me here.
I'd suggest that you start with a used bike – perhaps the Indian craigslist would be a place to start. You could observe what people are using for commuter bikes in your area, maybe even talk with some of them, and get some ideas about what sort of bike would be appropriate for you. A used bike will be less expensive and you'll lose less money if you decide to sell it and get another bike once you learn what works for you.

Regarding the bag, 10 km isn't too far, if you've got a backpack start with that. You'll probably find that the pack is hot on your back and you may prefer something else – but for starters any bag that isn't too uncomfortable to ride with will get you started. For myself, I don't like having any kind of bag on my back when I ride, so I use panniers – but I have the luxury of feeling relatively comfortable leaving them on the bike for short stops. If you want to get off you bike and go into a store or office with minimal fussing with the bike I think a messenger bag or backpack would be a better way to go.