The double percentage means match the LONGEST pattern FROM THE END of the variable.

If we tried the previous command using this new construct:

echo ${MAGIC%%r*a}
            

we should end up with:

ab
            

Why is it leaving the ab? Because it's matching the longest match of an 'r', followed by any number of characters ( * ) followed by an 'a', removing the matched pattern, echo'ing the remainder.

So where is this sort of thing used? Well perhaps you've got a long path:

SCRIPT=/home/hamish/scripts/testing/myscript.sh
            

Let's say we want to extract the path but not the script name itself. Then we would type:

THE_PATH=${SCRIPT%/*}
            

This construct would mean: from the end of the string, match the shortest pattern of a forward slash followed immediately by any number of characters, resulting in:

echo $THE_PATH
            

or

/home/hamish/scripts/testing