It seems that two, completely different kinds of snb to txt conversion exist, although both deal with text export.
How to Export S Notes as text
If you are looking for a way to export just the text (and save it as .txt) from older Galaxy S Note notes (.snb), the best way would be to download the S Note for desktop from Samsung website and use it to export snb to pdf (you have to first import the .snb file via the File ► Import ► S Note files function). Once you have the .pdf file, you can use some of the existing pdf to txt conversion solutions to get the plain text file or just simply copy the text out.
How to convert Shanda Bambooks
You can for example use the popular Calibre ebook management tool to export to Shanda Bambook eBook files This can be achieved with the programs default ebook convert menu.Open SNB file Open TXT file
A free e-book library management application
Linux version of the popular ebook manager
A free open source ebook creator, converter and viewer