windows 为什么即使设置了这个批处理变量也永远不会改变？
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Why does this batch variable never change even when set?
提问by jcao219
@echo off
SET first=0
FOR %%N IN (hello bye) DO (
SET first=1
echo %first%
echo %%N
)
It seems that the variable "first" is always 0. Why?
似乎变量“first”总是0。为什么？
回答by Michael Madsen
With batch files, variables are expanded when their command is read - so that would be as soon as the forexecutes. At that point, it no longer says echo %first%, it literally says echo 0, because that was the value at the point of expansion.
使用批处理文件，变量在读取命令时会被扩展 - 这样就会在for执行时立即展开。那时，它不再说echo %first%，而是字面意思echo 0，因为那是扩张点的价值。
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 enabledelayedexpansionin the beginning of your batch file (just after echo off).
为了解决这个问题，您需要使用延迟扩展，将您的变量名用!而不是%- 括起来，这样就可以了echo !first!。这可能需要您使用 /V 参数启动 cmd.exe，或者setlocal enabledelayedexpansion在批处理文件的开头使用（紧跟在 之后echo off）。
If you type set /?, you'll see a much more detailed explanation of this at the end of the output.
如果您键入set /?，您将在输出的末尾看到更详细的解释。
