Parsing two input files and comparing line existence

Question

I need to take two text files and check to see if a line that exists in File A doesn't exist anywhere in File B.

Each line in File A that does not exist in File B should be copied to an output log.

A friend of mine suggested SED but I've never used it before, so how would I go about doing this?

Answer 1

It sounds like the lines are intended to be unique and order doesn't matter, so try this:

sort fileA > fileA.sort
sort fileB > fileB.sort
diff fileA.sort fileB.sort | sed -n "/^</{s/< //;p}"

Answer 2

Lines that only exist in fileA:

comm -23 <(sort fileA) <(sort fileB) > output.txt

All lines unique to fileA will be saved in the file output.txt.