The same difference in three formats

Answer

diff v1.txt v2.txt diff -u v1.txt v2.txt diff -e v1.txt v2.txt diff -b v1.txt v2.txt

Default ed-script form, unified form, ed form ready to run, then -b which treats runs of whitespace as equal and ignores trailing whitespace.

S&K 3e ch6 §6.6