判断某变量是否定义

      在批处理中，很多时候可能需要判断某变量是否定义。而普通的变量和特定的隐藏变量的判断方法又有些不一样。故总结如下：
1      对普通的变量来讲，只要判断其值是否为空即可：
      IF "%MyVar%"=="" ECHO MyVar is NOT defined.
      如果变量有双引号（"）, 则变通一下即可：
      IF [%MyVar%]==[] ECHO MyVar is NOT defined.

2      如果command 扩展开关打开时，可以使用 defined 命令来判断变量是否存在。
      如：
            IF DEFINED MyVar (ECHO MyVar IS defined) ELSE (ECHO MyVar is NOT defined)

3      扩展开关关闭时，最安全的判断方式：
      如：
            VERIFY OTHER 2>nul
            SETLOCAL ENABLEEXTENSIONS
            IF ERRORLEVEL 1 ECHO Unable to enable extentsions
            IF DEFINED MyVar (ECHO MyVar is defined) ELSE (ECHO MyVar is NOT defined)
            ENDLOCAL

4      对于 hidden 或者动态的变量如何判断呢？
      如下code 可以测试：

Date

其输出结果将是：

C:\>IF DEFINED Date (ECHO Date IS defined )  ELSE (ECHO Date is NOT defined )
Date IS defined
C:\>ECHO Date = 06/06/2008
Date = 06/06/2008
C:\>SET Date
Environment variable Date not defined
C:\>PAUSE
Press any key to continue . . .
C:\>SET Date=Some Other Value
C:\>IF DEFINED Date (ECHO Date IS defined )  ELSE (ECHO Date is NOT defined )
Date IS defined
C:\>ECHO Date = Some Other Value
Date = Some Other Value
C:\>SET Date
Date=Some Other Value
C:\>PAUSE
Press any key to continue . . .
C:\>SET Date=
C:\>IF DEFINED Date (ECHO Date IS defined )  ELSE (ECHO Date is NOT defined )
Date IS defined



C:\>ECHO Date = 06/06/2008
Date = 06/06/2008
C:\>SET Date
Environment variable Date not defined



如是，在此只能使用如下的判断方法：
SET Date >NUL 2>&1
IF ERRORLEVEL 1 (
ECHO Date still has its dynamic system value
) ELSE (
ECHO Date has been set with a static value
)