How to output only lines from file 2 that are not in file 1?

Question

*nix-like systems

There are 2 txt files
1.txt

1
2
3
4
5

2.txt

1
3
12
44
55

How can I use diff or another utility to display only lines from file 2 that are not in file 1?

Answer 1

cat 2.txt | while read line ; do if ! grep -q $line 1.txt; then echo $line; fi ; done

For each line from 2.txt, we check if this line is not present in 1.txt, then we display it on the screen.

Answer 2

diff 1.txt 2.txt | grep -E "^>" | sed -e "s/^> //g"