【批处理】求取字符串长度

@echo off 
set /p str=请输入任意长度字符串：
if not defined str goto :eof
echo 您输入了：%str%
set num=0
:label
set /a num+=1
set str=%str:~0,-1%
if defined str goto :label
echo 字符串长度：%num%
pause