Find the files existing in one directory but not in the other

This should do the job:

diff -rq dir1 dir2

Options explained (via diff(1) man page):

• -r - Recursively compare any subdirectories found.
• -q - Output only whether files differ.

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

Explanation:

• diff -r dir1 dir2 shows which files are only in dir1 and those only in dir2 and also the changes of the files present in both directories if any.

• diff -r dir1 dir2 | grep dir1 shows which files are only in dir1

• awk to print only filename.