为什么即使设置了这个批处理变量也永远不会改变? [英] Why does this batch variable never change even when set?

问题描述

@echo off
SET first=0
FOR %%N IN (hello bye) DO (
SET first=1
echo %first%
echo %%N
)

好像变量first"总是0，为什么?

It seems that the variable "first" is always 0. Why?

推荐答案

对于批处理文件，当读取命令时变量会被扩展 - 这样会在 for 执行后立即展开.那时，它不再说 echo %first%，而是字面上的说 echo 0，因为那是扩展点的值.

With batch files, variables are expanded when their command is read - so that would be as soon as the for executes. At that point, it no longer says echo %first%, it literally says echo 0, because that was the value at the point of expansion.

要解决这个问题，您需要通过用 ! 而不是 % 包围变量名称来使用延迟扩展 - 这样就可以 echo !first!.这可能需要您使用/V 参数启动 cmd.exe，或者在批处理文件的开头使用 setlocal enabledelayedexpansion(就在 echo off 之后).

To get around that, you need to use delayed expansion by surrounding your variable name with ! instead of % - so that would be echo !first!. This may require you to start cmd.exe with the /V parameter, or use setlocal enabledelayedexpansion in the beginning of your batch file (just after echo off).

如果你输入 set/?，你会在输出的末尾看到更详细的解释.

If you type set /?, you'll see a much more detailed explanation of this at the end of the output.

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