[batch DOS CMD summary] extension variables - delay variables CMD /v:on, CMD /v:off, SETLOCAL enabledelayedexpansion, disabledelayedexpansion

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Bat Delay variable of script cmd /v:on_komomon‘s blog The blog of -CSDN Blog _bat Delay variable

One 、“ Delay variable ” The existence background of

The execution process of batch processing is “ From top to bottom , Implementation by item ”, and “ Implementation by item ” Do not represent “ Execute... Line by line ”.

Because one instruction may occupy multiple lines .

In compound statements , The whole compound statement is a complete statement , No matter how many lines the compound statement occupies . Common compound statements are ：for sentence 、if……else sentence 、 Use connector &、|| and && Connected statement , Use the pipe symbol | Connected statement , And enclosed in parentheses 、 A statement block composed of multiple statements .

In non compound statements , If the statement occupies a line position , Then this line of code is a complete statement . 　

Batch processing is performed before executing a statement “ Preprocessing ”, The preprocessing process will identify all variables , Assign existing values to variables directly .

Even in a statement , Operations involving the same variable are preceded and followed , Changed the value of the variable before , This updated value cannot be passed to the following variables , Therefore, the latter variables are the old values when executing the command （ But often we want the updated value obtained by executing the previous command to be directly passed to the following variables ）.

for example , You wrote a script file .

chcp 65001
@echo off 
set school= Xiangtan University 
echo " I am now studying %school%"
echo " After four years , I received the admission notice from Chongqing University "
set school= Chongqing University  & echo  I am going to study in %school% 
pause>nul

The results are as follows . Isn't that weird ？ What should we do according to reason , First put your own school Set as Chongqing University , The output should be “ I am going to study in Chongqing University now ” Well .

The reason is because of the order 【set school= Chongqing University & echo I am going to study in %school% 】 It's a compound statement , During pretreatment, the following school Variable has been assigned “ Xiangtan University ” 了 .

Is there an option or switch , After we opened it , Without changing the original sentence structure , Replace the variable after the compound statement with the latest value executed before .

Yes, there is ！ Of course. ！ What if you want to know what happened , Please listen to the following text ！

Two 、 stay cmd Open the delay variable in the window

stay cmd Window , We carry out orders 【cmd /?】 You can get relevant help information , The following is a key quotation （ All other irrelevant items have been deleted , If you are interested in other parameters, you can print them by yourself ）.

C:\Users\Administrator>cmd /?
 start-up  Windows  A new instance of the command interpreter 

CMD [/A | /U] [/Q] [/D] [/E:ON | /E:OFF] [/F:ON | /F:OFF] [/V:ON | /V:OFF]
    [[/S] [/C | /K] string]

/V:ON    Use  !  Enable deferred environment variables as delimiters 
         Expand . for example ,/V:ON  Will allow  !var!  When executed 
         Extended variables  var.var  The syntax will be used when inputting 
         Extended variables , This is different from  FOR
         Different within the cycle .
/V:OFF   Disable deferred environment extensions .

 Deferred environment variable extension is not enabled by default . you 
 It can be used /V:ON  or  /V:OFF  switch , by  CMD.EXE  And 
 Enable or disable deferred environment variable extensions . you 
 Can be on the machine and / Or enable or disable... On the user login session  CMD.EXE  all 
 Deferred extension of the call , This should be set to use  REGEDIT.EXE  In the registry of 
 One or two  REG_DWORD  value :

    HKEY_LOCAL_MACHINE\Software\Microsoft\Command Processor\DelayedExpansion

         and / or 

    HKEY_CURRENT_USER\Software\Microsoft\Command Processor\DelayedExpansion

 To  0x1  or  0x0. User specific settings 
 Has priority over the machine settings . Command line switch 
 Has priority over registry settings .

 In a batch file ,SETLOCAL ENABLEDELAYEDEXPANSION  or  DISABLEDELAYEDEXPANSION
 Parameter ratio  /V:ON  or  /V:OFF  The switch has priority . see also  SETLOCAL /?
 Get details .

 If deferred environment variable extension is enabled ,
 The exclamation point character can be used at execution time 
 Replace the value of an environment variable .

We can know from the help information , There are roughly two ways to turn on the delay variable switch , Either way, you need to put the percentage sign on both sides of the variable % Change to exclamation point !.

The first one is , stay cmd Open delay variable in , It is also divided into temporary passing of orders 【cmd /v:on】 Or order 【cmd /v】 Open and permanently modify registry variables .

The second kind , Open the delay variable in the batch script , It is also divided into temporary passing of orders 【SETLOCAL ENABLEDELAYEDEXPANSION】 Open and permanently modify registry variables .

Let's try the first temporary modification .

Carry out orders 【cmd /v:on】 after , Then enter the delayed variable expansion environment ,cmd The command code is also added after the window title .

Then execute the order .

set CQ=1
set /a CQ+=1 & echo !CQ!

The results are shown below ,set /a CQ+=1 The output of is 2, And the back echo It also outputs 2, Explain what follows echo The previous one is used set The latest value after execution . 

We can order 【exit】 Or order 【cmd /v:off】 To exit the delayed variable state .

after , Then execute the above command , give the result as follows .

Let's continue to explain how to use the deferred variable mechanism in batch scripts .

3、 ... and 、 Open the delay variable in the batch script

Create a new one bat file , The code is as follows .

chcp 65001
@echo off 
set school= Xiangtan University 
echo " I am now studying %school%"
echo " After four years , I received the admission notice from Chongqing University "
set school= Chongqing University  & echo  I am going to study in %school%
echo  Wrong output this time 
echo " Open the delay variable and try "
setlocal EnableDelayedExpansion
set school= Xiangtan University 
set school= Chongqing University  & echo  I am going to study in !school! 
echo " It's finally right "
echo " Try closing the delay variable again "
setlocal DisableDelayedExpansion
set school= Xiangtan University 
set school= Chongqing University  & echo  I am going to study in %school%
echo " Sure enough, I was wrong again "
pause>nul

After performing , The output is as follows .