substract lines

awk.sh

#!/usr/bin/env bash

f1="file1.txt"
f2="file2.txt"

echo "f2 only:"
awk 'FNR==NR{array[$0];next}{if ($0 in array) next;print $0}' "$f1" "$f2"

file1.txt

li
li ne1
li ne2

file2.txt

li
li ne1
li ne3