#!/bin/bash

while true
do
    read -p "please input a number:" number
    ((number + 0)) > /dev/null
    [ $? -ne 0 ] && echo "input error, please input again!" && continue || break 

done
number=$((number*2))
for i in `seq 1 2 $number`
do
    for x in `seq $(((number-i)/2))`
    do 
        echo -n " "
    done
    for j in `seq $i`
    do 
        echo -n "*"
    done
    echo 
done