How to verify that hard drive is filled with zeroes in Linux?

Question

I have hard drive filled with zeroes.

How to check if all bits on hard drive are zeros using bash?

score 40 · Accepted Answer · edited Apr 15 '20 at 03:12

od will replace runs of the same thing with *, so you can easily use it to scan for nonzero bytes:

$ sudo od /dev/disk2 | head
0000000    000000  000000  000000  000000  000000  000000  000000  000000
*
234250000

You can combine this with either dd or pv to get a progress display while the scan is taking place:

$ sudo dd if=/dev/disk2 bs=1M status=progress | od | head

# or:

$ sudo pv /dev/disk2 | od | head

score 9 · Answer 2 · edited Jun 27 '19 at 00:56

I've written a short C++ program to do so, source available here.

To build it:

wget -O iszero.cpp https://gist.github.com/BobVul/5070989/raw/2aba8075f8ccd7eb72a718be040bb6204f70404a/iszero.cpp
g++ -o iszero iszero.cpp

To run it:

dd if=/dev/sdX 2>/dev/null | ./iszero

It will output the position and value of any nonzero bytes. You can redirect this output to a file with >, e.g.:

dd if=/dev/sdX 2>/dev/null | ./iszero >nonzerochars.txt

You might want to try changing BUFFER_SIZE for better efficiency. I'm not sure what an optimum value might be. Note that this also affects how often it prints progress, which will affect speed somewhat (printing output to the console is slow). Add 2>/dev/null to get rid of progress output.

I am aware this is not using standard bash, nor even builtins, but it should not require any extra privileges. @Hennes' solution is still faster (I haven't really optimised anything - this is the naïve solution); however, this little program can give you a better idea of just how many bytes your wiper has missed, and in what location. If you disable the progress output, it'll still be faster than most consumer hard drives can read (>150 MB/s), so that's not a big issue.

A faster version with less verbose output is available here. However, it is still a little slower than @Hennes' solution. This one, however, will quit on the first nonzero character it encounters so it is potentially much faster if there's a nonzero near the beginning of the stream.

Adding source to post to keep answer better self-contained:

#include <cstdio>

#define BUFFER_SIZE 1024

int main() {
    FILE* file = stdin;
    char buffer[BUFFER_SIZE];
    long long bytes_read = 0;
    long long progress = 0;
    long long nonzero = 0;

    while (bytes_read = fread(buffer, 1, BUFFER_SIZE, file)) {
        for (long long i = 0; i < bytes_read; i++) {
            progress++;
            if (buffer[i] != 0) {
                nonzero++;
                printf("%lld: %x\n", progress, buffer[i]);
            }
        }
        fprintf(stderr, "%lld bytes processed\r", progress);
    }

    fprintf(stderr, "\n");

    int error = 0;
    if (error = ferror(file)) {
        fprintf(stderr, "Error reading file, code: %d\n", error);
        return -1;
    }

    printf("%lld nonzero characters encountered.\n", nonzero);
    return nonzero;
}

score 7 · Answer 3 · answered Sep 26 '13 at 23:08

Expanding on Gordon's answer, pv provides an indication of how far along the process is:

$ sudo pv -tpreb /dev/sda | od | head
0000000 000000 000000 000000 000000 000000 000000 000000 000000
*
9.76GiB 0:06:30 [25.3MiB/s] [=================>               ] 59% ETA 0:04:56

score 6 · Answer 4 · edited Jun 20 '25 at 08:39

This seems to be a bit inefficient solution, but if you need to check only once:

dd if=/dev/sdX | tr --squeeze-repeats "\000" "T"

Using dd to read from disk sdX. (replace the X with the drive you want to read from),
Then translating all unprintable zero bytes to something we can handle.

Next we either count the bytes we can handle and check if it is the right number (use wc -c for that), or we skip counting and use the -s or --squeeze-repeats to squeeze all multiple occurrences to a single char.

Thus dd if=/dev/sdX | tr --squeeze-repeats "\000" "T" should print only a single T.

If you want to do this regularly then you want something more efficient.

If you want to do this only once then this kludge may verify that your normal wiper is working and that you can trust it.

Editor's notes:

The single T or whatever character you choose, will appear at the very beginning of your next shell prompt, it is a bit hard to spot at first.
With large drives like 4TB, one will likely want to monitor progress.
Both of the above points can be dealt with something similar to this, it's - at core - the same solution, but better in some ways, the resulting T char will be placed in zero-test file, you can wait for that with e.g. tail -f zero-test:
```
pv -Wptearb < /dev/sdX | tr --squeeze-repeats "\000" "T" > zero-test
```

Beardy · Answer 5 · 2014-04-28T23:52:33.813

To check only, you'll see any blocks that don't match listed

sudo badblocks -sv -t 0x00 /dev/sdX

Or use badblocks to write them as well as check:

sudo badblocks -svw -t 0x00 /dev/sdX

The default destrucive test is my secure erase of choice

sudo badblocks -svw /dev/sdX

If anyone can retrieve anything after filling the drive with alternating 0s and 1s, then their complement, then all 1s, then all 0s, with every pass verified it worked, good luck to them!

Makes a good pre-deployment check on new drives too

man badblocks

for other options

Not saying it's fast, but it works...

score 2 · Answer 6 · answered Dec 09 '19 at 19:10

Wanted to post this clever solution from a similar but earlier question, posted by a user who hasn't logged in for a while:

There is a device /dev/zero on a Linux system that always gives zeroes when read.

So, how about comparing your hard drive with this device:
cmp /dev/sdX /dev/zero
If all is well with zeroing out your hard drive it will terminate with:
cmp: EOF on /dev/sdb
telling you that the two files are the same until it got to the end of the hard drive. If there is a non-zero bit on the hard drive cmp will tell you where it is in the file.

If you have the pv package installed then:
pv /dev/sdX | cmp /dev/zero
will do the same thing with a progress bar to keep you amused while it checks your drive (the EOF will now be on STDIN rather than sdX though).

score 2 · Answer 7 · edited Oct 01 '13 at 13:38

2

Best of both worlds. This command will skip bad sectors:

sudo dd if=/dev/sdX conv=noerror,sync | od | head

Use kill -USR1 <pid of dd> to see progress.

edited Oct 01 '13 at 13:38

John Bensin

1,645

answered Oct 01 '13 at 13:15

jiveformation

21

Tino · Answer 8 · 2014-05-10T18:44:53.860

Some time ago I was curious about AIO. The result was a sample test program which happens to check for sectors (512 byte blocks) which are NUL. You can see this as a variant of a sparse file-regions detector. I think the source says it all.

If the entire file/drive is NUL output looks like 0000000000-eof. Note that there is a trick in the program, function fin() is not called at line 107 on purpose to give the shown output.
Not heavily tested, so may contain bugs
The code is a bit longer, as AIO is not as straight forward as other ways,
however AIO is probably the fastest way to keep a drive busy reading, because the NUL compare is done while the next data block is read in. (We could squeeze out a few more milliseconds by doing overlapping AIO, but I really do not think this is worth the effort.)
It always returns true if the file is readable and everything worked. It does not return false if the file is non-NUL.
It assumes that the file size is a multiple of 512. There is a bug on the last sector, however on a file entirely NUL it still works, as the memory buffers already contain NUL. If somebody thinks this needs a fix, in line 95 the memcmp(nullblock, buf+off, SECTOR) could read memcmp(nullblock, buf+off, len-off<SECTOR : len-off : SECTOR). But the only difference is, that the "end reporting" perhaps is a bit random (not for a file which is entirely NUL).
The changed memcmp() also fixes another issue on platforms, which do not NUL alloc()ed memory, because the code does not do it. But this only might be seen by files less than 4 MiB, but checknul probably is plain overkill for such a small task ;)

HTH

/* Output offset of NUL sector spans on disk/partition/file
 *
 * This uses an AIO recipe to speed up reading,
 * so "processing" can take place while data is read into the buffers.
 *
 * usage: ./checknul device_or_file
 *
 * This Works is placed under the terms of the Copyright Less License,
 * see file COPYRIGHT.CLL.  USE AT OWN RISK, ABSOLUTELY NO WARRANTY.
 */

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>

#include <malloc.h>
#include <aio.h>

#define SECTOR  512
#define SECTORS 40960
#define BUFFERLEN   (SECTOR*SECTORS)

static void
oops(const char *s)
{
  perror(s);
  exit(1);
}

static void *
my_memalign(size_t len)
{
  void      *ptr;
  static size_t pagesize;

  if (!pagesize)
    pagesize = sysconf(_SC_PAGESIZE);
  if (len%pagesize)
    oops("alignment?");
  ptr = memalign(pagesize, len);
  if (!ptr)
    oops("OOM");
  return ptr;
}

static struct aiocb aio;

static void
my_aio_read(void *buf)
{
  int   ret;

  aio.aio_buf = buf;
  ret = aio_read(&aio);
  if (ret<0)
    oops("aio_read");
}

static int
my_aio_wait(void)
{
  const struct aiocb    *cb;
  int           ret;

  cb = &aio;
  ret = aio_suspend(&cb, 1, NULL);
  if (ret<0)
    oops("aio_suspend");
  if (aio_error(&aio))
    return -1;
  return aio_return(&aio);
}

static unsigned long long   nul_last;
static int          nul_was;

static void
fin(void)
{
  if (!nul_was)
    return;
  printf("%010llx\n", nul_last);
  fflush(stdout);
  nul_was   = 0;
}

static void
checknul(unsigned long long pos, unsigned char *buf, int len)
{
  static unsigned char  nullblock[SECTOR];
  int           off;

  for (off=0; off<len; off+=SECTOR)
    if (memcmp(nullblock, buf+off, SECTOR))
      fin();
    else
      {
        if (!nul_was)
          {
            printf("%010llx-", pos+off);
            fflush(stdout);
            nul_was = 1;
          }
        nul_last    = pos+off+SECTOR-1;
      }
}

int
main(int argc, char **argv)
{
  unsigned char *buf[2];
  int       fd;
  int       io, got;

  buf[0] = my_memalign(BUFFERLEN);
  buf[1] = my_memalign(BUFFERLEN);

  if (argc!=2)
    oops("Usage: checknul file");
  if ((fd=open(argv[1], O_RDONLY))<0)
    oops(argv[1]);

  aio.aio_nbytes    = BUFFERLEN;
  aio.aio_fildes    = fd;
  aio.aio_offset    = 0;

  io = 0;
  my_aio_read(buf[io]);
  while ((got=my_aio_wait())>0)
    {
      unsigned long long    pos;

      pos   = aio.aio_offset;

      aio.aio_offset += got;
      my_aio_read(buf[1-io]);

      checknul(pos, buf[io], got);

      io    = 1-io;
    }
  if (got<0)
    oops("read error");
  printf("eof\n");
  close(fd);
  return 0;
}

Stephen Elliott · Answer 9 · 2025-04-06T07:12:59.970

This is a fresh approach. (I rewrote my previous answer completely.):

From the man page for cmp:

If a FILE is '-' or missing, read standard input.

Hence I can get the advantage of using a large block size (to get high throughput in the checking) using also the dd command, to pipe its output the the cmp command saving a lot of time versus printing the output from /dev/sdb directly to the display:

dd if=/dev/sdb bs=$((8*1024)) status=progress | cmp - /dev/zero

Right now on my 262322 MB Crucial_CT275MX300SSD1 SSD, I am getting a bandwidth of 267196481536 bytes (267 GB, 249 GiB) copied, 547 s, at 488 MB/s which is very adequate. I could use badblocks, but I am more familiar with setting the block size with dd (using bs=$((8*1024)) which is 8192 bytes, which is nearly optimal).

How to verify that hard drive is filled with zeroes in Linux?

9 Answers9

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