How to write out all directory names into a text file

Improving you current code, this command should be helpful:

for d in ./*;do [[ -d "$d" ]] && echo "$d" >> dir.txt; done

To remove the ./ from the output:

for d in ./*;do [[ -d "$d" ]] && echo "${d##./}" >> dir.txt; done

You are appending the entries (>>) where you must be overwriting them (>). It is better to use find. To find all subdirectories recursively, use

find . -type d > directories.txt

To find subdirectories only in the current directory, use

find . -type d -maxdepth 1 > directories.txt

-type d instructs find to list only directory files.

What surprises me is the lack of a semicolon (;) in your solution, does your problem persist when you do:

for d in *; do echo $d >> directories.txt; done

? It works for me fine. Also, are you sure you run it in bash? Finally, the ">>" might be the problem, you might be appending directories' names several times.

Not to mention that if you want to list directories only (not files) you could rather do:

for d in `find . -type d -maxdepth 1`; do echo $d >> directories.txt; done

or even simpler:

find . -type d -maxdepth 1 > directories.txt