Zeroing a file from command-line

Solution 1:

Is there a built-in command in Windows to fill a file with zero / NULL bytes?

Yes. You can use fsutil for this:

> fsutil file setzerodata /?
Usage : fsutil file setzerodata offset=<val> length=<val> <filename>
   offset : File offset, the start of the range to set to zeroes
   length : Byte length of the zeroed range
   Eg : fsutil file setzerodata offset=100 length=150 C:\Temp\sample.txt

To zero fill a complete file you will need to use an offset of 0 and you will need to know the file length.

Can we use batch file that would compute the size automatically?

Of course.

Use the following batch file (zero.cmd):

@echo off
setlocal enabledelayedexpansion
for %%a in (%1) do (
  fsutil file setzerodata offset=0 length=%%~za %%a
)
endlocal

Usage:

• You can pass a single filename as an argument: zero test.txt or a wildcard: zero *.txt

Example:

> type test.txt
abc
foo$
foo
bar

> zero test.txt
Zero data is changed

> type test.txt

>

Further Reading

• An A-Z Index of the Windows CMD command line
• A categorized list of Windows CMD commands
• fsutil - File and Volume specific commands, Hardlink management, Quota management, USN, Sparse file, Object ID and Reparse point management
• parameters - A command line argument (or parameter) is any value passed into a batch script.

Solution 2:

Not exactly a solution, but if you have CygWin installed, you just do (example for a 500000000 bytes file) :

dd if=/dev/zero of=MyFile.zero bs=1 count=500000000

This is the same method you would use at any Unix-like operating system.