Getting rid of nul, or: How I learnt to love UNC (29 Apr 2006)

Every now and then, some tool on my system runs berserk and starts to generate files called nul. This is a clear indication that there's something going wrong with output redirection in a script, but I still have to figure out exactly what's going on. Until then, I need at least a way to get rid of those files.

Yes, that's right, you cannot delete a file called nul that easily - neither using Windows Explorer nor via the DOS prompt. nul is a very special filename for Windows - it is an alias for the null device, i.e. the bit bucket where all the redirected output goes, all those cries for help from software which we are guilty of ignoring all the time.

UNC path notation to the rescue: To remove a file called nul in, say c:\temp, you can use the DOS del command as follows:

  del \\.\c:\temp\nul

Works great for me. But since I rarely use UNC syntax, I sometimes forget how it looks like. Worse, the syntax requires to specify the full path of the nul file, and I hate typing those long paths. So I came up with the following naïve batch file which does the job for me. It takes one argument which specifies the relative or absolute path of the nul file. Examples:

   rem remove nul file in current dir
   delnul.bat nul
   rem remove nul file in subdir
   delnul.bat foo\nul
   rem remove nul file in tempdir
   delnul.bat c:\temp\nul

For the path completion magic, I'm using the for command which has so many options that my brain hurts whenever I read its documentation. I'm pretty sure one could build a Turing-complete language using just for...

  @echo off
  set fullpath=
  for %%i IN (%1x) DO set fullpath=%%~di%%~pi

  set filename=
  for %%i IN (%1x) DO set filename=%%~ni
  if not "x%filename%" == "xnulx" (echo Usage: %0 [somepath\]nul && goto :eof)
 
  echo Deleting %fullpath%nul...
  del \\.\%fullpath%nul

DelinvFile takes this a lot further; it has a Windows UI and can delete many other otherwise pretty sticky files - nul is not the only dangerous file name; there's con, aux, prn and probably a couple of other magic names which had a special meaning for DOS, and hence also for Windows.