Is there an assymetrical diff option or command on GNU/linux?

is there a version of diff that runs

diff a.txt b.txt

and shows only what a.txt has in excess of b.txt, but not anything else?
that is, only lines in a.txt that are not in b.txt

(I am running linux mint, but I presume any relevant command will be available)

>Solution :

Yes.

diff --changed-group-format='%>' --unchanged-group-format='' /tmp/a /tmp/b