while i=$( expr ${i-0} ${op-+} 1 ); do
    j=$i
    t=$( expr 2 \* $i - 1 )
    printf %$( expr \( $( expr 2 \* ${1-6} - 1 ) - $t \) / 2 )s
    while test $(( j-- )) -gt 0; do printf '* '; done
    echo
    test $i = ${1-6} && op=-
done