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In what situations should variable delays be used? What is the format? What is the role of variable delays?

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Re namejm:

Regarding environment variable delayed expansion, you can check some instructions using set /?. However, considering its crude translation level, it is recommended to first switch to English by chcp 437 to view the original English instructions before checking. Since the text has been described very detailedly and there are several code examples, it should not be difficult to understand. Only some supplements are made here.

In many visible official documents, the behavior of using a pair of percent signs to close an environment variable to complete the replacement of its value is called "expansion" (expansion), which is actually a first-party concept, referred to from the perspective of the command interpreter. From our user's perspective, it can be regarded as reference (Reference), call (Call) or get (Get).

And the behavior of the command interpreter to expand environment variables is roughly as follows: First, read a complete statement of the command line. After some preliminary preprocessing, before the command is interpreted and executed, it will match the string closed with percent signs. If a matching environment variable is found in the environment space, its value will be used to replace the original string and the percent sign itself. If no match is obtained, an empty string will be used for replacement. This process is the "expansion" of the environment variable, which still belongs to the preprocessing category of the command line.

And a "complete statement" is interpreted in the NT command interpreter CMD as statements containing statement blocks such as "for if else" and compound statements connected by "& | && ||" and so on.

Therefore, when CMD reads a for statement, all statements closed with a pair of parentheses after it will be read together, and necessary preprocessing work will be completed, including the expansion of environment variables. Therefore, before all statements in the for are executed, all environment variables have been replaced with the values set before the for, thus becoming a string constant, and no longer a variable. No matter how those environment variables are modified in the for, what is really affected is the environment variable space, not the inside of the for statement.

And in order to be able to perceive the dynamic changes of environment variables inside the for statement, CMD has designed the delayed environment variable expansion feature, that is, when CMD reads a complete statement, it will not immediately perform the variable expansion behavior, but will expand again before a single statement is executed, that is, this expansion behavior is "delayed".

The delayed environment variable expansion feature is off by default in CMD. There are currently two methods to turn it on: one is CMD /v:off ( There is a mistake here, it should be CMD /v:on——note by namejm ), which will open a new command line shell. Before using exit to exit this shell, the expansion feature is always effective, which is often used in the command line environment; the second is setlocal EnableDelayedExpansion, which will limit the modification of environment variables to the local space. After endlocal, the expansion feature and the previous modifications to environment variables will disappear together, which is often used in batch processing statements.

Last edited by namejm on 2007-3-17 at 09:01 PM ]

Thanks to willsort for the detailed explanation. The Chinese help for `set /?` was a bit hard to understand. There were a few technical terms that were tough. After reading willsort's explanation, I realized that "expansion" is actually common actions like referencing and calling. It's like a light - bulb moment.

I was used to posting for help in the "DOS疑难解答 & 问题讨论（解答室）" section. After the new version was launched, I didn't adjust right away and made a mistake in posting to the wrong place, causing trouble for the moderators. I'm really sorry about that. But I feel a bit wronged about "deducting 2 points of punitive points for title violation". It seems my title didn't cause ambiguity or was too broad. If it's an additional punishment for posting in the wrong place, I have no objection, but the term "title violation" doesn't seem appropriate. This is just my personal opinion. I hope the moderators can give some advice.

Re namejm:

Thanks for carefully pointing out the typo in the moderation record. It has been corrected now. This version of the moderation record template has been in use for more than a month, and this is the first time someone has pointed out the typo. At the same time, moved by your sincerity, I will award 16 points of points to the reply on floor 3. At the same time, I ask other moderators in the Batch Processing Room to deduct 16 points of points from the reply on floor 2, because I can't rate my own reply.

Hehe, I posted two wrong posts before, totaling 16 points deducted. Now I'm covered by the moderator's mercy and all are made up. Since it's the template's reason, it's not the moderator's fault. I request other moderators not to deduct points from willsort, and deeply admire willsort's responsible attitude of daring to blame oneself.

Both of you are so sincere, so I really admire you, junior brother.

It seems to have a bit of understanding.

There is also the case where when you need to use a variable inside another variable, you can use variable delay. For example:
set a=1000
set b=dd
set a%b%=9000
set c=!a%b%!

Regarding environment variable delayed expansion, you can view some instructions using set /?. However, considering its poor translation level, it is recommended to first switch to English by chcp 437 to view the original English instructions. Since the text has been very detailed and there are several code examples, it should not be difficult to understand. Here, only a few supplements are made.

Can your Chinese be better?

May I ask what doubts the person above has? You can say it out and everyone will help you.

Whenever you need to call the variables you originally defined again after the for statement, you need to use variable delay. I finally solved a problem. Hehe

Should be understood. Thanks!

What is delay, finally I know. Thanks willsort!

？？？
++++++++++Points

Bump up~ Too high-tech, can't understand