Tente isso usando bc
#!/bin/bash
h=0
l=0
reads=0
divider=6
while read user_input; do
((reads++))
((user_input == 1 && h++))
((user_input == 2 && n++))
((user_input == 3 && h--))
if ((reads%52 == 0 && divider > 1)); then
((divider--))
fi
echo "True Count $(bc <<< "scale=3; ($h+$l)/$divider") High cards $h/120 Null Cards $n/72 Low Cards $l/120"
done