Bash loop, print current iteration?

To do this, you would need to increment a counter on each iteration (like you are trying to avoid).

count=0
while read -r line; do
   printf '%d %s\n' "$count" "${line*//}"
   (( count++ ))
done < test.txt

EDIT: After some more thought, you can do it without a counter if you have bash version 4 or higher:

mapfile -t arr < test.txt
for i in "${!arr[@]}"; do
   printf '%d %s' "$i" "${arr[i]}"
done

The mapfile builtin reads the entire contents of the file into the array. You can then iterate over the indices of the array, which will be the line numbers and access that element.