echo "The current process ID is $$."

if [ "$!" != "" ]
then
    echo "The ID of the last-run background process is $!."
else
    echo "No background process ID stored in" '$!'
fi

# Now, run something in the background.
ls > /dev/null &
if [ "$!" != "" ]
then
    echo "The ID of the last-run background process is $!."
else
    echo "No background process ID stored in" '$!'
fi