Changing a batch file when its running

jeb's example is a lot of fun, but it is very dependent on the length of the text that is added or deleted. I think the counter-intuitive results are what rein meant when he said "If you modify it before then it will start doing strange things (repeating commands etc..)".

I've modified jeb's code to show how dynamic code of varying length can be freely modified at the beginning of an executing batch file as long as appropriate padding is in place. The entire dynamic section is completely replaced with each iteration. Each dynamic line is prefixed with a non interfering ;. This conveniently allows FOR /F to strip the dynamic code because of the implicit EOL=; option.

Instead of looking for a particular line number, I look for a specific comment to locate where the dynamic code begins. This is easier to maintain.

I use lines of equal signs to harmlessly pad the code to allow for expansion and contraction. Any combination of the following characters could be used: comma, semicolon, equal, space, tab and/or newline. (Of course the padding cannot begin with a semicolon.) The equal signs within the parentheses allow for code expansion. The equal signs after the parentheses allow for code contraction.

Note that FOR /F strips empty lines. This limitation could be overcome by using FINDSTR to prefix each line with the line number and then strip out the prefix within the loop. But the extra code slows things down, so it's not worth doing unless the code is dependent on blank lines.

@echo off
setlocal DisableDelayedExpansion
echo The starting filesize is %~z0
:loop
echo ----------------------
::*** Start of dynamic code ***
;set value=1
::*** End of dynamic code ***
echo The current value=%value%
::
::The 2 lines of equal signs amount to 164 bytes, including end of line chars.
::Putting the lines both within and after the parentheses allows for expansion
::or contraction by up to 164 bytes within the dynamic section of code.
(
  call :changeBatch
  ==============================================================================
  ==============================================================================
)
================================================================================
================================================================================
set /p "quit=Enter Q to quit, anything else to continue: "
if /i "%quit%"=="Q" exit /b
goto :loop
:changeBatch
(
  for /f "usebackq delims=" %%a in ("%~f0") do (
    echo %%a
    if "%%a"=="::*** Start of dynamic code ***" (
      setlocal enableDelayedExpansion
      set /a newValue=value+1, extra=!random!%%9
      echo ;set value=!newValue!
      for /l %%n in (1 1 !extra!) do echo ;echo extra line %%n
      endlocal
    )
  )
) >"%~f0.tmp"
::
::The 2 lines of equal signs amount to 164 bytes, including end of line chars.
::Putting the lines both within and after the parentheses allows for expansion
::or contraction by up to 164 bytes within the dynamic section of code.
(
  move /y "%~f0.tmp" "%~f0" > nul
  ==============================================================================
  ==============================================================================
)
================================================================================
================================================================================
echo The new filesize is %~z0
exit /b

The above works, but things are much easier if the dynamic code is moved to a subroutine at the end of the file. The code can expand and contract without limitation, and without the need for padding. FINDSTR is much faster than FOR /F at removing the dynamic portion. Dynamic lines can be safely be prefixed with a semicolon (including labels!). Then the FINDSTR /V option is used to exclude lines that begin with a semicolon and the new dynamic code can simply be appended.

@echo off
setlocal DisableDelayedExpansion
echo The starting filesize is %~z0

:loop
echo ----------------------
call :changeBatch
call :dynamicCode1
call :dynamicCode2
echo The current value=%value%
set /p "quit=Enter Q to quit, anything else to continue: "
if /i "%quit%"=="Q" exit /b
goto :loop

:changeBatch
(
  findstr /v "^;" "%~f0"
  setlocal enableDelayedExpansion
  set /a newValue=value+1, extra=!random!%%9
  echo ;:dynamicCode1
  echo ;set value=!newValue!
  echo ;exit /b
  echo ;
  echo ;:dynamicCode2
  for /l %%n in (1 1 !extra!) do echo ;echo extra line %%n
  echo ;exit /b
  endlocal
) >"%~f0.tmp"
move /y "%~f0.tmp" "%~f0" > nul
echo The new filesize is %~z0
exit /b

;:dynamicCode1
;set value=33
;exit /b
;
;:dynamicCode2
;echo extra line 1
;exit /b

I just tried it, and against my intuition, it picked up the new commands at the end (on Windows XP)

I created a batch file containing

echo Hello
pause
echo world

I ran the file, and while it was paused, added

echo Salute

Saved it and pressed enter to contine the pause, all three prompts were echoed to the console.

So, go for it!

The command interpreter remembers the line position byte offset it's at in the batch file. You will be fine as long as you modify the batch file after the current executing line position byte offset at the end of the most recently parsed line of code.

If you modify it before then it will start doing strange things (repeating commands etc..).