How can a zero byte text file generate a hash when hashed with sha1sum, sha256sum etc? What data are the programs hashing to generate a hash value?
Ta

Hash algorithms read the input and process it, no matter if there's data at all. This is a valid and wanted behaviour and is even used to verify if a certain implementation is correct. This leads to "nullhashes" for all major algorithms. To sum it up: da39a3ee5e6b4b0d3255bfef95601890afd80709 is the sha1hash for an empty file everywhere, the same is true with the nullhashes of other alrogrithms. 


All hash algorithms in Quick Hash are Merkle–Damgård constructions. As those, they pad the message to a mutiplie of the block size by appending a 1 bit, as many 0 bits as needed and finally the message length. This allows hashing messages of arbitrary length, including zerolength messages. 

