How do I find all the lines that are in one file and not in another?

Question

Say I got:

aaa
bbb
ccc

and:

aaa
ccc
ddd

and I would like:

bbb

How would I get it?

Answer 1

Please try below command:

comm is used for Compare sorted files FILE1 and FILE2 line-by-line.

-23 suppresses column 2 and 3.

$ comm -23 fil1.txt fil2.txt  
bbb

Answer 2

Using grep -vf:

grep -vFxf file2 file1
bbb