Redirect Output

Hi,

I would like to list files: ls *.hdf

But I would like a copy of the output directed to the screen, but also APPENDED to a text file: test.txt

I have tried:

ls *.hdf | tee test.txt

However, that will just write over everything already existing in test.txt. How can I append the output of the ls command to test.txt, but also print a copy to the screen?

ls *.hdf |tee -a test.txt

-a option "append"