一段Shell脚本的语法问题

#!/bin/bash
#Reverse Order

echo "Reverse Order and print."

if [ $# -ne 1 ]
then
  echo "Usage $0 number"
  echo "    I will find reverse order of given number."
  echo "    For e.g. $0 123, I will print 321."
  exit 1
fi

n=$1
rev=0
sd=0

while [ $n -gt 0 ]
do
  sd=`expr $n % 10`
  rev=`expr $rev /* 10 + $sd`
  n=`expr $n / 10`

  echo "$sd $rev $n"
done

echo "Reversed number is $rev"

结果打印如下：

Reverse Order and print.
expr: syntax error
3  872
expr: syntax error
2  87
expr: syntax error
7  8
expr: syntax error
8  0
Reversed number is

为什么执行后会是有语法错误？？？
[解决办法]
`expr $rev /* 10 + $sd` ？
[解决办法]
rev=`expr $rev /* 10 + $sd` 改为

rev=`expr $rev \* 10 + $sd`

是运算符出错导致的。