2.3作业 编写一个shell 脚本，判断输入的是否是数字

#!/bin/bash

var=$1
len=${#var}
j=0
for(($j;$j<$len;((j++))))
do
    if [ ${var:$j:1} -eq 0 ]
    then 
    
     echo "第$j位数字是0"
     elif [ ${var:$j:1 } -eq 1 ]
      then 
     echo "第$j位数字是1"
   
     elif [ ${var:$j:1} -eq 2 ]
      then 
     echo "第$j位数字是2"
  
     elif [ ${var:$j:1} -eq 3 ]
      then 
     echo "第$j位数字是3"
  
     elif [ ${var:$j:1} -eq 4 ]
      then 
     echo "第$j位数字是4"
  
     elif [ ${var:$j:1} -eq 5  ]
      then 
     echo "第$j位数字是5"
 
     elif [ ${var:$j:1} -eq 6 ]
      then 
     echo "第$j位数字是6"
     
     elif [ ${var:$j:1} -eq 7 ]
      then 
     echo "第$j位数字是7"
   
     elif [ ${var:$j:1} -eq 8 ]
      then 
     echo "第$j位数字是8"
 
     elif [ ${var:$j:1} -eq 9 ]
      then 
     echo "第$j位数字是9"
  
    else
    echo "input error,please input number"
    exit
    fi
done
    echo "the number is $var"