Comparing two unsorted lists in linux, listing the unique in the second file

grep -Fxv -f first-file.txt second-file.txt

Basically looks for all lines in second-file.txt which don’t match any line in first-file.txt. Might be slow if the files are large.

Also, once you sort the files (Use sort -n if they are numeric), then comm should also have worked. What error does it give? Try this:

comm -23 second-file-sorted.txt first-file-sorted.txt