Hi, is there hop on hop off bus sightseeing in Xian?

As I know, there are imperial city sightseeing buses in Xi'an that stop at the main sites. The routes are:

Red Line: Shaanxi History Museum – Muslim Quarter – Yongxingfang – Forest of Stone Steles Museum – Giant Wild Goose Pagoda – Shaanxi History Museum

Green Line: Tang Paradise – Giant Wild Goose Pagoda – Shaanxi History Museum – South Square of Giant Wild Goose Pagoda – Grand Tang Dynasty Ever-bright City – Qujiang Pool Park – Tang Paradise

Terracotta Army & Huaqing Pool Line.

The operating time of Red Line and Green Line is 10:00 – 17:30, and Terracotta Army & Huaqing Pool Line is 8:30 - 18:00 departing from Shaanxi History Museum.

Thank you for your prompt answer! And do you know if it is necessarry to buy in advance? Or can I buy directly at the bus? Do you know the name of the company that do these tours?
regards
Sara

You can buy the ticket from the ticket offices set at any hop-on hop off stops, or from the staffs on the bus.

Thank you, and do you know if there is hop on hop off also in Beijing?

Yes, Beijing also has sightseeing buses. The Dang-dang Trolley Sightseeing Buses have both day and night lines to the Tiananmen Square, Forbidden City and Jingshan Park. Beijing Hub of Tour Dispatch also operates 3 tourist bus lines to Tiananmen Square, Badaling Great Wall and Ming Tombs. The Dang-dang Trolley Sightseeing Buses tickets can be bought on the bus, the tickets of tourist lines can be booked at Beijing Hub of Tour Dispatch.