The accepted answer will also list the files that exist in both directories, but are different. To list only the files that exist in dir1 you can use:

diff -r dir1 dir2 | grep 'Only in' | grep dir1 | awk '{print $4}' > difference1.txt

The accepted answer will also list the files that exist in both directories, but have different content. To list ONLY the files that exist in dir1 you can use:

diff -r dir1 dir2 | grep 'Only in' | grep dir1 | awk '{print $4}' > difference1.txt

Explanation:

• diff -r dir1 dir2 : compare
• grep 'Only in': get lines that contain 'Only in'
• grep dir1 : get lines that contain dir