Script de selection en Bash read

#!/bin/bash
function press_enter
{
echo ""
echo -n "Press Enter to continue"
read
clear
}

selection=
until [ "$selection" = "0" ]; do
echo ""
echo "PROGRAM MENU"
echo "1 - display free disk space"
echo "2 - display free memory"
echo ""
echo "0 - exit program"
echo ""
echo -n "Enter selection: "
read selection
echo ""
case $selection in
1 ) df ; press_enter ;;
2 ) free ; press_enter ;;
0 ) exit ;;
* ) echo "Please enter 1, 2, or 0"; press_enter
esac
done