DosTips.com

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>myfile.txt echo Line1
>>myfile.txt echo Line2
>>myfile.txt echo.
>>myfile.txt echo Line4

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(
  echo Line1
  echo Line2
  echo(
  echo Line4
) > myFile.txt

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@echo off
>myFile.txt more +10 %0
for /f %%z in ("myFile.txt") do if not "%%~zz"=="23" (echo:Wrong myFile.txt!&pause&exit)

:: Your code is here
:: Your code is here

goto :eof

:: 10th line of batch
Line1
Line2

Line4

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:: Generate name of temporary data file.
   set vDatFile="%vAppDir%DTF_FTP_%random%.dat"
:: Generate name of temporary working file.
   set vTmpFile="%vAppDir%DTF_FTP_%random%.tmp"
:: Generate name / location of log file.
   set vLogFile="%vAppDir%DTF_FTP.log"

   set /a "vCntr=0"

:: Delete logfile entries older than 52 runs ago, which should be about a year.
:: First, find the starting positions of the individual log entries in the log file
:: and put all the header lines into a data file along with the line positions where
:: they can be found.  Skip 2 lines for the header generated by FIND.
   find /n "*************** DTF FTP Start" %vLogFile% | more /s +2 > %vDatFile% rem (Yay MORE!!!  Thanks to !K!)
:: (First you find out how to use the pipe, then it becomes indispensable!)

:: Count the number of log headers deposited in the vDatFile file and put 
:: it into the vCntr variable.
:: Set the vCntr variable = the difference between the current number of log 
:: entries and the number of log entries expected over a year.  
:: Many thanks to Brett Batie 
:: (http://brett.batie.com/scripting/count-number-of-lines-in-a-file-using-dos/)
:: for the counting algorithym. 
   findstr /l /n "[" %vDatFile% | find /c ":"> %vTmpFile%
   set /p vCntr=<%vTmpFile% &del /q %vTmpFile%
   set /a vCntr-=52

:: Check to see if vCntr is > 0.  If not, skip log modification.
   if %vCntr% LEQ 0 (goto SkipLog)

:: If there are over a year's worth of logs in the logfile, delete earlier
:: log entries.
:: Set up the vCntr variable as a SKIP parameter for a FOR loop using the
:: calculated extra number of log entries as the SKIP parameter value.  
:: Thanks to dbenham (http://www.dostips.com/forum/viewtopic.php?f=3&t=2490)
:: for the explanation on how to do this.
   set vCntr=skip=%vCntr%
:: Access the vDatFile file and set the vCntr variable = the line number
:: of the beginning log entry which will not be deleted.  Subtract 1 from
:: that number to preserve the log header.
   for /f "usebackq %vCntr% delims=[] tokens=1,2,*" %%a in (%vDatFile%) do (
      set "vCntr=%%a"
      set /a vCntr-=1
      goto Break2
   )
   :Break2

:: Transfer the logfile, minus the earlier log entries, to the vDatFile file.
:: Tremendous amount of thanks to !K for this solution!
   more +%vCntr% %vLogFile% > %vDatFile%
:: Finally, copy over the old logfile with the revised log.
   copy %vDatFile% %vLogFile%

   :SkipLog