How to compare two text files in Linux?

Question

linux

There is a file a.txt with the content:
1
2
3
4
5
And there is a file b.txt with the content :
1
2
3 .txt = c.txt Lines left in c.txt: 4 5

Answer 1

comm -3 a.txt b.txt > c.txt

Answer 2

comm -3 a.txt b.txt > c.txt
But files a.txt and b.txt must be sorted alphabetically

Answer 3

cat a.txt b.txt | uniq-u

Answer 4

diff?