20 comment

1. I don’t think this works correctly unless the “count” argument is changed to something greater than 0.

   Try something like this:

   dd if=/dev/zero of=10Gtest bs=1M count=10000

   1. I don’t think you have read and tested this yourself.

      If using “count=0”, as the article suggests, you will still have the file of the specified size.

      However, the output will show something like “0+0 records in” and “0+0 records out”.

      Then if you check the file size:

      ls -al /path/to/test.img

      it should show the proper size.

      It’s even easier, using the format that the commenter Indie suggest:

      dd if=/dev/zero of=/path/to/disk.img bs=1 count=0 seek=1G

      1. JW is correct. Without a count setting – its generating a 0 byte file.

         ls -alh will show a file size; but compare that to df -h – and you’ll see it doesn’t actually consume any space on disk

      2. Bullshit the 10g one above doesnt create the proper file size at all. Check with the du command it replys the file size is 0.

         1. I think the commenter “Indie” has already clarified this in a previous comment (in agreement with the author).

            @JW, @sonassi, @stfu_and_die
            If you think you’re right, why don’t tell the author of the article to either remove or edit the article? or write your own article?

            Indie August 26, 2011 at 3:02 pm

            The ls command will tell you the size of the file while du will tell you the Disk Usage. When you create the empty disk image with the above command it doesn’t write any data so it doesn’t actually allocate any blocks to the file.

            If you type “stat disk.img” you’ll see that it’s using 0 blocks and thus no disk space is being consumed by the file, it’s only when you then write data to it that the blocks will then be allocated.

            1. The FAQ has been updated with new and accurate information. However, I kept dd command based method for historical reasons only. I recommend using fallocate command on Linux.

               Hope this helps!

2. can I supress the output of dd command?

   1. Yes, send it to /dev/null.

      dd args >/dev/null
      OR
      dd args >/dev/null 2>&1

3. Slightly easier version is to use a suffix for the size instead of trying to do the maths.

   dd if=/dev/zero of=disk.img bs=1 count=0 seek=1G

   to create a 1G file.

4. Yes this is work but there is concern !!!

   When do ls -lh disk.img file its showing as 1G size but when u run the command

   du -h disk.img it shows 0 bytes of size…..

   Which one we can believe…i dont know…please clarify if you know about this…

   Jai

5. The ls command will tell you the size of the file while du will tell you the Disk Usage. When you create the empty disk image with the above command it doesn’t write any data so it doesn’t actually allocate any blocks to the file.

   If you type “stat disk.img” you’ll see that it’s using 0 blocks and thus no disk space is being consumed by the file, it’s only when you then write data to it that the blocks will then be allocated.

6. I have 2 tb harddrive and i want to clone only 1 tb to my other drive?

   what command should i do

   1. Hi, the question is old – but I’m happy to provide a nice answer – maybe this basic advice will help other people as well.
      This isn’t exactly possible with dd. If you’d only dd 1tb of the 2tb drive and you’re lucky to be able to mount it, you’ll end up seeing correct filenames – but {all,most,many} files will be corrupted entirely.
      I’d rather suggest you:
      ? format your second hard-drive
      ? look which files/directories are using how much space, recommended tool is ncdu
      ? create a list of files you want to copy and use rsync to copy the files

7. How to check if the created file is empty, is there a way to check that?

   1. ls -l

8. Using /dev/zero will create a file for you, but the free space seems to remain the same on the disk. If you’re looking to make a filler file as I am in /var/lib/mysql so when the database fills up, I still have some space to work with.. you’ll want to substitute /dev/zero to /dev/random or something like that.

   Here’s my attempt at creating a 1G filler file.
   df output before:
   /dev/mapper/VG01-LV_mysql
   30959612 27519608 3439984 89% /var/lib/mysql
   command:
   dd if=/dev/zero of=1gig-filler.img bs=1000 count=0 seek=$[1000*1000*1]
   0+0 records in
   0+0 records out
   0 bytes (0 B) copied, 3e-05 seconds, 0.0 kB/s

   file:
   -rw——- 1 root root 1000000000 Feb 13 12:26 1gig-filler.img

   df output after (space is slightly different because the db is being used):
   /dev/mapper/VG01-LV_mysql
   30959612 27521072 3438520 89% /var/lib/mysql

   I don’t see any use for using /dev/zero in this scenario. I’ve used it to wipe a disk clean… dd if=/dev/zero of=/dev/sda will clear a disk, but this file doesn’t take up real space.

9. Actually it might be the count=0 part that is making it not use any real space.. I can’t seem to get df to show me the space actively being used even with /dev/random.. I know I’ve done this successfully before.

10. How to delete that test file that is being created?

    1. Use rm command:

       rm fileName-here

11. What if I wanted to create a large file but with random content as I need to perform some verification on the file and have a file full of NULLs doesn’t really help.

    Thanks in advance

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