Diff, show modified line from right file only

With GNU diffutils package's diff this will output only lines from file b which either were modified or newly inserted:

diff --unchanged-line-format= --old-line-format= --new-line-format='%L' a b

You have to add one more option :

grep -vf file1 file2

awk 'FNR==NR{old[$0];next};!($0 in old)' old.txt new.txt