You’re generally better off using the ROOT file format rather than ASCII text which another tool then needs to read again… If you “insist” in doing this you can simply use tree->Scan() and redirect the output, or write a little script that prints the column values that you care about.
@eguiraud that mightbe a nice feature for RDataFrame, too: Print() printing all or the given column values, maybe for every n-th entry, or the first n entries. Especially with Define() etc this could be nice!