batch file to keep one of ten lines

Question

I have a file with n lines. (n above 100 millions)

I want to output a file with only 1 of 10 lines, I can't split the file in ten part and keep only one part, as it must be a little more random. later I have to do a statistical analysis I can't afford to create a strong bias in the data).

I was thinking of reading the file and for each record if the record number mod 10 then output it.

The constraints are:

• it's a windows (likely hardened) computer possibly XP Vista or Windows server 2003.

• no development tools available

• no network,usb,cd-rom. read no external communication.

Therefore I was thinking of windows batch file (I can't assume powershell, and vbscript is likely to have been removed). And at the moment looking at the FOR /F command. Still I am not an expert and I don't know how to achieve this.

Thank you Paul for your answer. I reformat (with Hosam help) the answer to put it in a batch file:

@echo off
setlocal   
findstr/N . inputFile| findstr ^[0-9]*0: >temporaryFile
FOR /F "tokens=1,* delims=: " %%i in (temporaryfile) do echo %%j > outputFile

Thanks quux and Pax for the similar alternative solution. However after a quick test on a larger file Paul's answer is roughly 8 times faster. I guess the evaluation (in SET) is kind of slow, even if the logic seems brilliant.

Answer 1

Ok, I think I've cracked it:

findstr/N . path-to-log-file | findstr ^[0-9]*0:

(use findstr to add the line number to the beginning of the line, then again to print only lines with a line number ending in zero)

So you'll get one line in 10, but with the linenumber and colon prepended to each line

If I can think of a way using command-line tools only of stripping that out, I'll edit this answer :)

Remove the line number and colon with

FOR /F "tokens=1,2* delims=: " %i in (file-with-linenumbers) do echo %j

Paul.