Windows批处理文件：什么是变量扩展，又是什么意思EnableDelayedExpansion？ [英] Windows Batch files: what is variable expansion, and what does EnableDelayedExpansion mean?

问题描述

什么是变量扩展是什么意思？这是否意味着简单的变量定义，还是别的什么？

What is meant by "variable expansion"? Does it mean simply "variable definition", or something else?

当我说会发生什么 SETLOCAL EnableDelayedExpansion ？谷歌不明确。

What happens when I say setLocal EnableDelayedExpansion? Google wasn't clear.

推荐答案

• 变量的扩展的手段取代它的价值包含在％或！变量


• 的％正常％扩张发生的只有一次 在执行前行的。这意味着％变量％扩张具有的相同的值的任何事项，如果该行是在为命令）。


• 的！延迟！执行扩展的每次的该行执行。

• Variable expansion means replace a variable enclosed in % or ! by its value.
• The %normal% expansion happen just once, before a line is executed. This means that a %variable% expansion have the same value no matters if the line is executed several times (like in a for command).
• The !delayed! expansion is performed each time that the line is executed.

请参阅下面的例子：

@echo off
setlocal EnableDelayedExpansion
set "var=Original"
set "var=New" & echo Normal: "%var%", Delayed: "!var!"

输出：

Normal: "Original", Delayed: "New"

还有一句：

@echo off
setlocal EnableDelayedExpansion
set "var1=Normal"
set "var2=Delayed"
for /L %%i in (1,1,10) do (
   set "var1=%var1% %%i"
   set "var2=!var2! %%i"
)
echo Normal:  "%var1%"
echo Delayed: "%var2%"

输出：

Normal:  "Normal 10"
Delayed: "Delayed 1 2 3 4 5 6 7 8 9 10"

普通扩张不一定是缺点，而是取决于它被使用的具体情况。例如，在任何其他的编程语言，来交换两个变量的值则需要第三个的帮助，但是在批量它可以在只有一行来完成：

Normal expansion is not necessarily a disadvantage, but depends on the specific situation it is used. For example, in any other programming languages, to exchange the value of two variables you need the aid of a third one, but in Batch it can be done in just one line:

set "var1=%var2%" & set "var2=%var1%"

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