source: vendor/bash/3.1/examples/functions/fact

# Who said shells can't use recursion?  Here is a factorial function.
# You call it with a number as an argument, and it returns the factorial
# of that number.

fact ()
{
    local num=$1;
    if [ "$num" = 1 ] ; then
        echo 1
        return ;
    fi;
    echo $(( $num * $(fact $(( $num - 1 )) ) ))
}