Unix : Compare two TXT file issue and keep the lines which is not unique

Question

I have two TXT files as

#1.txt
1.1.1.1
2.2.2.2
3.3.3.3
4.4.4.4
5.5.5.5

#2.txt
1.1.1.1
2.2.2.2
6.6.6.6
7.7.7.7
8.8.8.8

I made comparison of 1.txt to 2.txt and get the IPs which is not in 2.txt when comparing with 1.txt . I did,

#comm -2 -3 <(sort 1.txt) <(sort 2.txt) > Out.txt

Output

#Out.txt
3.3.3.3
4.4.4.4
5.5.5.5

The Out.txt IPs are the IPs not in 2.txt file.

Now,I would like compare 1.txt against 2.txt and get the one which is not unique from 1.txt .

#1.txt
1,1.1.1.1
2,2.2.2.2
3,3.3.3.3
4,4.4.4.4
5,5.5.5.5

#2.txt
1.1.1.1
2.2.2.2
6.6.6.6
7.7.7.7
8.8.8.8

Expected Result

#Out.txt
3,3.3.3.3
4,4.4.4.4
5,5.5.5.5

How to include IDs in my output result ?

Answer 1

this awk one-liner should work for your example:

awk -F, 'NR==FNR{a[$0];next}!($2 in a)' 2.txt 1.txt