[windows] While loop in batch

I have a trick for doing this!

I came up with this method because while on the CLI, it's not possible to use the methods provided in the other answers here and it had always bugged me.

I call this the "Do Until Break" or "Infinite" Loop:

Basic Example

FOR /L %L IN (0,0,1) DO @(
 ECHO. Counter always 0, See "%L" = "0" - Waiting a split second&ping -n 1 127.0.0.1>NUL )

This is truly an infinite loop!

This is useful for monitoring something in a CMD window, and allows you to use CTRL+C to break it when you're done.

Want to Have a counter?

Either use SET /A OR You can modify the FOR /L Loop to do the counting and still be infinite (Note, BOTH of these methods have a 32bit integer overflow)

SET /A Method:

FOR /L %L IN (0,0,1) DO @(
 SET /A "#+=1"&ECHO. L Still equals 0, See "%L = 0"!  - Waiting a split second &ping -n 1 127.0.0.1>NUL  )

Native FOR /L Counter:

FOR /L %L IN (-2147483648,1,2147483648) DO @(
 ECHO.Current value of L: %L - Waiting a split second &ping -n 1 127.0.0.1>NUL  )

Counting Sets of 4294967295 and Showing Current Value of L:

FOR /L %L IN (1,1,2147483648) DO @(
 (
  IF %L EQU 0 SET /A "#+=1">NUL
 )&SET /A "#+=0"&ECHO. Sets of 4294967295 - Current value of L: %L - Waiting a split second &ping -n 1 127.0.0.1>NUL )

However, what if:

• You're interested in reviewing the output of the monitor, and concerned that I will pass the 9999 line buffer limit before I check it after it has completed.
• You'd like to take some additional actions once the Thing I am monitoring is finished.

For this, I determined how to use a couple methods to break the FOR Loop prematurely effectively turning it into a "DO WHILE" or "DO UNTIL" Loop, which is otherwise sorely lacking in CMD.

NOTE: Most of the time a loop will continue to iterate past the condition you checked for, often this is a wanted behavior, but not in our case.

Turn the "infinite Loop" into a "DO WHILE" / "DO UNTIL" Loop

UPDATE: Due to wanting to use this code in CMD Scripts (and have them persist!) as well as CLI, and on thinking if there might be a "more Correct" method to achieve this I recommend using the New method!

New Method (Can be used inside CMD Scripts without exiting the script):

FOR /F %%A IN ('
  CMD /C "FOR /L %%L IN (0,1,2147483648) DO @( ECHO.%%L & IF /I %%L EQU 10 ( exit /b  ) )"
') DO @(
  ECHO %%~A
)

At CLI:

FOR /F %A IN ('
  CMD /C "FOR /L %L IN (0,1,2147483648) DO @( ECHO.%L & IF /I %L EQU 10 ( exit /b  ) )"
') DO @(
  ECHO %~A
)

Original Method (Will work on CLI just fine, but will kill a script.)

FOR /L %L IN (0,1,2147483648) DO @(
  ECHO.Current value of L: %L - Waiting a split second &ping -n 1 127.0.0.1>NUL&(
    IF /I %L EQU 10 (
      ECHO.Breaking the Loop! Because We have matched the condition!&DIR >&0
    )
  )
) 2>NUL

Older Post Stuff

Through chance I had hit upon some ways to exit loops prematurely that did not close the CMD prompt when trying to do other things which gave me this Idea.

NOTE:

While ECHO.>&3 >NUL had worked for me in some scenarios, I have played with this off and on over the years and found that DIR >&0 >NUL was much more consistent.

I am re-writing this answer from here forward to use that method instead as I recently found the old note to myself to use this method instead.

Edited Post with better Method Follows:

DIR >&0 >NUL

The >NUL is optional, I just prefer not to have it output the error.

I prefer to match inLine when possible, as you can see in this sanitized example of a Command I use to monitor LUN Migrations on our VNX.

for /l %L IN (0,0,1) DO @(
   ECHO.& ECHO.===========================================& (
     [VNX CMD] | FINDSTR /R /C:"Source LU Name" /C:"State:" /C:"Time " || DIR >&0 >NUL
   ) & Ping -n 10 1.1.1.1 -w 1000>NUL )

Also, I have another method I found in that note to myself which I just re-tested to confirm works just as well at the CLI as the other method.

Apparently, when I first posted here I posted an older iteration I was playing with instead of the two newer ones which work better:

In this method, we use EXIT /B to exit the For Loop, but we don't want to exit the CLI so we wrap it in a CMD session:

FOR /F %A IN ('CMD /C "FOR /L %L IN (0,1,10000000) DO @( ECHO.%L & IF /I %L EQU 10 ( exit /b  ) )" ') DO @(ECHO %~A)

Because the loop itself happens in the CMD session, we can use EXIT /B to exit the iteration of the loop without losing our CMD Session, and without waiting for the loop to complete, much the same as with the other method.

I would go so far as to say that this method is likely the "intended" method for the sort of scenario where you want to break a for loop at the CLI, as using CMD session is also the only way to get Delayed expansion working at the CLI for your loops, and the behavior and such behavior is clearly an intended workflow to leave a CMD session.

IE: Microsoft clearly made an intentional effort to have CMD Exit /B For loops behave this way, while the "Intended" way of doing this, as my other method, relies on having accidentally created just the right error to kick you out of the loop without letting the loop finish processing, which I only happenstantially discovered, and seems to only reliably work when using the DIR command which is fairly strange.

So that said, I think it's probably a better practice to use Method 2:

FOR /F %A IN ('CMD /C "FOR /L %L IN (0,1,10000000) DO @( ECHO.%L & IF /I %L EQU 10 ( exit /b  ) )" ') DO @(ECHO %~A)

Although I suspect Method 1 is going to be slightly faster:

FOR /L %L IN (0,1,10000000) DO @( ECHO.%L & IF /I %L EQU 10 ( DIR >&) >NUL ) )

And in either case, both should allow DO-While loops as you need for your purposes.