使用感叹号的Windows批处理文件语法

在检查Axis2二进制发行版中axis2server.bat文件的详细信息时，我看到其中一行包含如下文本：

FOR %%c in ("%AXIS2_HOME%\lib\*.jar") DO set AXIS2_CLASS_PATH=!AXIS2_CLASS_PATH!;%%c

下面带两个感叹号的部分是什么意思

!AXIS2_CLASS_PATH!

---

名称中包含%的平均变量，不确定是什么！标记批处理文件中的平均值。

当

启用延迟扩展

和

更改

或

设置循环中的变量时，则
！变数语法允许您在循环中使用变量

缺点是
成为延迟扩展的有害字符。
正如foxidrive提到的，这与延迟扩展相关。通过在cmd提示符中运行
帮助集
，可以找到更多信息，该提示符有以下说明：

Finally, support for delayed environment variable expansion has been
added.  This support is always disabled by default, but may be
enabled/disabled via the /V command line switch to CMD.EXE.  See CMD /?

Delayed environment variable expansion is useful for getting around
the limitations of the current expansion which happens when a line
of text is read, not when it is executed.  The following example
demonstrates the problem with immediate variable expansion:

    set VAR=before
    if "%VAR%" == "before" (
        set VAR=after
        if "%VAR%" == "after" @echo If you see this, it worked
    )

would never display the message, since the %VAR% in BOTH IF statements
is substituted when the first IF statement is read, since it logically
includes the body of the IF, which is a compound statement.  So the
IF inside the compound statement is really comparing "before" with
"after" which will never be equal.  Similarly, the following example
will not work as expected:

    set LIST=
    for %i in (*) do set LIST=%LIST% %i
    echo %LIST%

in that it will NOT build up a list of files in the current directory,
but instead will just set the LIST variable to the last file found.
Again, this is because the %LIST% is expanded just once when the
FOR statement is read, and at that time the LIST variable is empty.
So the actual FOR loop we are executing is:

    for %i in (*) do set LIST= %i

which just keeps setting LIST to the last file found.

Delayed environment variable expansion allows you to use a different
character (the exclamation mark) to expand environment variables at
execution time.  If delayed variable expansion is enabled, the above
examples could be written as follows to work as intended:

    set VAR=before
    if "%VAR%" == "before" (
        set VAR=after
        if "!VAR!" == "after" @echo If you see this, it worked
    )

    set LIST=
    for %i in (*) do set LIST=!LIST! %i
    echo %LIST%

由于变量扩展的工作方式，请参见@AlexK。谢谢，这很有帮助。如果你能补充一个答案，我想把这个问题标记为已回答。这回答了这个问题，但你能对此进行一点扩展吗？也许是一个简单的例子？