SETLOCAL 和 ENABLEDELAYEDEXPANSION 如何工作？

发布于 2024-11-19 14:53:19 字数 211 浏览 1 评论 0原文

我注意到在大多数脚本中，两者通常位于同一行，如下所示：

SETLOCAL ENABLEDELAYEDEXPANSION

这两个实际上是单独的命令并且可以写在单独的行上吗？

如果设置 ENABLEDELAYEDEXPANSION 是在脚本的第一行设置并且直到脚本末尾才禁用，是否会对脚本产生不利影响？

原文

I notice in most scripts, the two are usually in the same line as so:

SETLOCAL ENABLEDELAYEDEXPANSION

Are the two in fact separate commands and can be written on separate lines?

Will setting ENABLEDELAYEDEXPANSION have an adverse effect on a script if it is set on the first lines of the script and not disabled until the end of the script?

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薄凉少年不暖心 2024-11-26 14:53:20

我想你应该明白什么是延迟扩展。现有的答案并不能（充分）解释它。恕我直言。

输入 SET /? 很好地解释了这一点：

延迟环境变量扩展对于解决问题很有用
当前扩展的局限性，当一行
文本是在读取时读取的，而不是在执行时读取的。下面的例子
演示了立即变量扩展的问题：
设置 VAR=之前
如果“%VAR%”==“之前”(
    设置VAR=之后
    if "%VAR%" == "after" @echo 如果你看到这个，它就起作用了
）
永远不会显示该消息，因为两个 IF 语句中都有 %VAR%
当读取第一个 IF 语句时被替换，因为它在逻辑上
包括 IF 的主体，它是一个复合语句。所以如果
复合语句内部实际上是在比较“before”与
“之后”永远不会相等。同样，下面的例子
将无法按预期工作：
<前><代码>设置列表=
对于 (*) 中的 %i，请设置 LIST=%LIST% %i
回显%列表%
因为它不会在当前目录中建立文件列表，
而是将 LIST 变量设置为最后找到的文件。
同样，这是因为当 FOR 时 %LIST% 仅扩展一次
语句被读取，此时 LIST 变量为空。所以
我们正在执行的实际 FOR 循环是：
对于 (*) 中的 %i 设置 LIST= %i
它只是不断将 LIST 设置为找到的最后一个文件。
延迟环境变量扩展允许您使用不同的
字符（感叹号）来扩展环境变量
执行时间。如果启用延迟变量扩展，则上述
示例可以编写如下以按预期工作：
设置 VAR=之前
如果“%VAR%”==“之前”(
    设置VAR=之后
    如果“！VAR！” ==“after”@echo 如果你看到这个，它就起作用了
）

设置列表=
对于 (*) 中的 %i，请设置 LIST=!LIST! ％我
回显%列表%

另一个示例是此批处理文件：

@echo off
setlocal enabledelayedexpansion
set b=z1
for %%a in (x1 y1) do (
 set b=%%a
 echo !b:1=2!
)

这会打印 x2 和 y2：每个 1 都会被 2 替换。

没有 setlocalenabledelayedexpansion，感叹号就是这样，所以它会回显 !b:1=2! 两次。

由于读取（块）语句时会扩展普通环境变量，因此扩展 %b:1=2% 使用 b 之前的值循环：z2（但未设置时y2）。

I think you should understand what delayed expansion is. The existing answers don't explain it (sufficiently) IMHO.

Typing SET /? explains the thing reasonably well:

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%

Another example is this batch file:

@echo off
setlocal enabledelayedexpansion
set b=z1
for %%a in (x1 y1) do (
 set b=%%a
 echo !b:1=2!
)

This prints x2 and y2: every 1 gets replaced by a 2.

Without setlocal enabledelayedexpansion, exclamation marks are just that, so it will echo !b:1=2! twice.

Because normal environment variables are expanded when a (block) statement is read, expanding %b:1=2% uses the value b has before the loop: z2 (but y2 when not set).

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