为什么设置时，此批变量甚至从来没有改变？ [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
)

看来这个变量第一始终是0。为什么？

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

推荐答案

通过批处理文件，变量当他们的命令读取拓展 - 所以这将是很快的为执行。在这一点上，它不再说回声％首％，其字面意思是呼应0 ，因为这是值在膨胀的点。

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，你需要用周围的变量名使用延迟扩展！而不是％ - 所以这将是第一呼应！。这可能需要（后仅用/ V参数启动的cmd.exe，或在批处理文件的开头使用 SETLOCAL enabledelayedexpansion ECHO OFF ）。

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).

如果您键入设置/？，你会在输出的末尾看到这样一个更详细的解释。

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

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