How can I merge two files?

Question

I have two files

File 1:

7118
7457
7591
7539
8001

File 2:

5003
5008
5011
5026
5028
5029
5031

Output t

Answer 1

cat file1 | (exec 3< file2; while read A && read B <&3; do echo "$A,$B"; done)