How to recursively list subdirectories in Bash without using “find” or “ls” commands?

Question

I know you can use the find command for this simple job, but I got an assignment not to use find or ls and do the job. How can I do th

Answer 1

Since it is for bash, it is a surprise that this hasn't been already said:
(globstar valid from bash 4.0+)

shopt -s globstar nullglob dotglob
echo **/*/

That's all.
The trailing slash / is there to select only dirs.

Option globstar activates the ** (search recursivelly). Option nullglob removes an * when it matches no file/dir. Option dotglob includes files that start wit a dot (hidden files).