Exiting out of a FOR loop in a batch file? - batch-file

Question

Why does this batch file never break out of the loop?
For /L %%f In (1,1,1000000) Do #If Not Exist %%f Goto :EOF
Shouldn't the Goto :EOF break out of the loop?
Edit:
I guess I should've asked more explicitly... how can I break out of the loop?

Answer 1

Based on Tim's second edit and this page you could do this:
#echo off
if "%1"=="loop" (
for /l %%f in (1,1,1000000) do (
echo %%f
if exist %%f exit
)
goto :eof
)
cmd /v:on /q /d /c "%0 loop"
echo done
This page suggests a way to use a goto inside a loop, it seems it does work, but it takes some time in a large loop. So internally it finishes the loop before the goto is executed.

Answer 2

You could simply use echo on and you will see that goto :eof or even exit /b doesn't work as expected.
The code inside of the loop isn't executed anymore, but the loop is expanded for all numbers to the end.
That's why it's so slow.
The only way to exit a FOR /L loop seems to be the variant of exit like the exsample of Wimmel, but this isn't very fast nor useful to access any results from the loop.
This shows 10 expansions, but none of them will be executed
echo on
for /l %%n in (1,1,10) do (
goto :eof
echo %%n
)

Answer 3

My answer
Use nested for loops to provide break points to the for /l loop.
for %%a in (0 1 2 3 4 5 6 7 8 9) do (
for %%b in (0 1 2 3 4 5 6 7 8 9) do (
for /l %%c in (1,1,10) do (
if not exist %%a%%b%%c goto :continue
)
)
)
:continue
Explanation
The code must be tweaked significantly to properly use the nested loops. For example, what is written will have leading zeros.
"Regular" for loops can be immediately broken out of with a simple goto command, where for /l loops cannot. This code's innermost for /l loop cannot be immediately broken, but an overall break point is present after every 10 iterations (as written). The innermost loop doesn't have to be 10 iterations -- you'll just have to account for the math properly if you choose to do 100 or 1000 or 2873 for that matter (if math even matters to the loop).
History
I found this question while trying to figure out why a certain script was running slowly. It turns out I used multiple loops with a traditional loop structure:
set cnt=1
:loop
if "%somecriteria%"=="finished" goto :continue
rem do some things here
set /a cnt += 1
goto :loop
:continue
echo the loop ran %cnt% times
This script file had become somewhat long and it was being run from a network drive. This type of loop file was called maybe 20 times and each time it would loop 50-100 times. The script file was taking too long to run. I had the bright idea of attempting to convert it to a for /l loop. The number of needed iterations is unknown, but less than 10000. My first attempt was this:
setlocal enabledelayedexpansion
set cnt=1
for /l %%a in (1,1,10000) do (
if "!somecriteria!"=="finished" goto :continue
rem do some things here
set /a cnt += 1
)
:continue
echo the loop ran %cnt% times
With echo on, I quickly found out that the for /l loop still did ... something ... without actually doing anything. It ran much faster, but still slower than I thought it could/should. Therefore I found this question and ended up with the nested loop idea presented above.
Side note
It turns out that the for /l loop can be sped up quite a bit by simply making sure it doesn't have any output. I was able to do this for a noticeable speed increase:
setlocal enabledelayedexpansion
set cnt=1
#for /l %%a in (1,1,10000) do #(
if "!somecriteria!"=="finished" goto :continue
rem do some things here
set /a cnt += 1
) > nul
:continue
echo the loop ran %cnt% times

Answer 4

you do not need a seperate batch file to exit a loop using exit /b if you are using call instead of goto like
call :loop
echo loop finished
goto :eof
:loop
FOR /L %%I IN (1,1,10) DO (
echo %%I
IF %%I==5 exit /b
)
in this case, the "exit /b" will exit the 'call' and continue from the line after 'call'
So the output is this:
1
2
3
4
5
loop finished

Answer 5

So I realize this is kind of old, but after much Googling, I couldn't find an answer I was happy with, so I came up with my own solution for breaking a FOR loop that immediately stops iteration, and thought I'd share it.
It requires the loop to be in a separate file, and exploits a bug in CMD error handling to immediately crash the batch processing of the loop file when redirecting the STDOUT of DIR to STDIN.
MainFile.cmd
ECHO Simple test demonstrating loop breaking.
ECHO.
CMD /C %~dp0\LOOP.cmd
ECHO.
ECHO After LOOP
PAUSE
LOOP.cmd
FOR /L %%A IN (1,1,10) DO (
ECHO %%A
IF %%A EQU 3 DIR >&0 2>NUL )
)
When run, this produces the following output. You'll notice that both iteration and execution of the loop stops when %A = 3.
:>MainFile.cmd
:>ECHO Simple test demonstrating loop breaking.
Simple test demonstrating loop breaking.
:>ECHO.
:>CMD /C Z:\LOOP.cmd
:>FOR /L %A IN (1 1 10) DO (
ECHO %A
IF %A EQU 3 DIR 1>&0 2>NUL
)
:>(
ECHO 1
IF 1 EQU 3 DIR 1>&0 2>NUL
)
1
:>(
ECHO 2
IF 2 EQU 3 DIR 1>&0 2>NUL
)
2
:>(
ECHO 3
IF 3 EQU 3 DIR 1>&0 2>NUL
)
3
:>ECHO.
:>ECHO After LOOP
After LOOP
:>PAUSE
Press any key to continue . . .
If you need to preserve a single variable from the loop, have the loop ECHO the result of the variable, and use a FOR /F loop in the MainFile.cmd to parse the output of the LOOP.cmd file.
Example (using the same LOOP.cmd file as above):
MainFile.cmd
#ECHO OFF
ECHO.
ECHO Simple test demonstrating loop breaking.
ECHO.
FOR /F "delims=" %%L IN ('CMD /C %~dp0\LOOP.cmd') DO SET VARIABLE=%%L
ECHO After LOOP
ECHO.
ECHO %VARIABLE%
ECHO.
PAUSE
Output:
:>MainFile.cmd
Simple test demonstrating loop breaking.
After LOOP
3
Press any key to continue . . .
If you need to preserve multiple variables, you'll need to redirect them to temporary files as shown below.
MainFile.cmd
#ECHO OFF
ECHO.
ECHO Simple test demonstrating loop breaking.
ECHO.
CMD /C %~dp0\LOOP.cmd
ECHO After LOOP
ECHO.
SET /P VARIABLE1=<%TEMP%\1
SET /P VARIABLE2=<%TEMP%\2
ECHO %VARIABLE1%
ECHO %VARIABLE2%
ECHO.
PAUSE
LOOP.cmd
#ECHO OFF
FOR /L %%A IN (1,1,10) DO (
IF %%A EQU 1 ECHO ONE >%TEMP%\1
IF %%A EQU 2 ECHO TWO >%TEMP%\2
IF %%A EQU 3 DIR >&0 2>NUL
)
Output:
:>MainFile.cmd
Simple test demonstrating loop breaking.
After LOOP
ONE
TWO
Press any key to continue . . .
I hope others find this useful for breaking loops that would otherwise take too long to exit due to continued iteration.

Answer 6

As jeb noted, the rest of the loop is skipped but evaluated, which makes the FOR solution too slow for this purpose. An alternative:
set F=1
:nextpart
if not exist "%F%" goto :EOF
echo %F%
set /a F=%F%+1
goto nextpart
You might need to use delayed expansion and call subroutines when using this in loops.

Answer 7

It is impossible to get out of a FOR /L before it completes all iterations.
I have debugged the execution of a FOR /L by the cmd.exe process.
Microsoft could document it better and save us all this effort.
Facts:
The loop is a simple while (TRUE) and the break only happens when the iteration limit is reached.
When an EXIT /b or a GOTO is encountered, no more commands are executed until the end of the iterations.
When an EXIT is encountered, the cmd.exe process is terminated.
Tests:
12 seconds
FOR /L %%G in (1,1,5000000) do (ECHO Only once & GOTO :EOF)
7 seconds
FOR /L %%G in (1,1,5000000) do (ECHO Only once & EXIT /b)
0 seconds, but this terminates the cmd.exe process
FOR /L %%G in (1,1,5000000) do (ECHO Only once & EXIT)

Answer 8

Assuming that the OP is invoking a batch file with cmd.exe, to properly break out of a for loop just goto a label;
Change this:
For /L %%f In (1,1,1000000) Do If Not Exist %%f Goto :EOF
To this:
For /L %%f In (1,1,1000000) Do If Not Exist %%f Goto:fileError
.. do something
.. then exit or do somethign else
:fileError
GOTO:EOF
Better still, add some error reporting:
set filename=
For /L %%f In (1,1,1000000) Do(
set filename=%%f
If Not Exist %%f set tempGoto:fileError
)
.. do something
.. then exit or do somethign else
:fileError
echo file does not exist '%filename%'
GOTO:EOF
I find this to be a helpful site about lesser known cmd.exe/DOS batch file functions and tricks: https://www.dostips.com/

Answer 9

Did a little research on this, it appears that you are looping from 1 to 2147483647, in increments of 1.
(1, 1, 2147483647): The firs number is the starting number, the next number is the step, and the last number is the end number.
Edited To Add
It appears that the loop runs to completion regardless of any test conditions. I tested
FOR /L %%F IN (1, 1, 5) DO SET %%F=6
And it ran very quickly.
Second Edit
Since this is the only line in the batch file, you might try the EXIT command:
FOR /L %%F IN (1, 1, 2147483647) DO #IF NOT EXIST %%F EXIT
However, this will also close the windows cmd prompt window.