Linux – dd with obs and seek makes file of unexpected size

ddlinux

I'm learning to use dd by experimentally playing with its arguments. I would like to create a 10-byte file. I thought the following would work:

dd if=/dev/zero of=./foo count=1 bs=1 obs=9 seek=1

…because of these comments from the man page:

   obs=BYTES
          write BYTES bytes at a time (default: 512)
   seek=N skip N obs-sized blocks at start of output

…but it does not; it creates a 2-byte file:

>ls -l foo
-rw-rw-r-- 1 user user 2 Mar 28 16:05 foo

My workaround has been:

dd if=/dev/zero of=./foo count=1 bs=1 obs=1 seek=9

But for my learning, I'd like to understand why the first version does not work. Thank you.

Best Answer

Your command dd if=/dev/zero of=./foo count=1 bs=1 obs=9 seek=1 creates a two-byte file rather than a 10-byte file because of poorly-defined interaction between bs and obs. (Call this a program bug if you like, but it's probably better defined as a documentation bug.) You are supposed to use either bs or ibs and obs.

Empirically it appears that bs overrides obs, so what gets executed is dd if=/dev/zero of=./foo count=1 bs=1 seek=1, which creates a two-byte file as you have seen.

If you had used dd if=/dev/zero of=./foo count=1 ibs=1 obs=9 seek=1 you would have got a 10-byte file as expected.

As an alternative, if you want to create an empty file that doesn't take any data space on disk you can use the counter-intuitively named truncate command:

truncate --size=10 foo

Related Solutions

The difference between ‘bs’, ‘count’ and ‘seek’ in dd command

I really don't know how to explain this better than the manpage does.

bs= sets the blocksize, for example bs=1M would be 1MiB blocksize.

count= copies only this number of blocks (the default is for dd to keep going forever or until the input runs out). Ideally blocks are of bs= size but there may be incomplete reads, so if you use count= in order to copy a specific amount of data (count*bs), you should also supply iflag=fullblock.

seek= seeks this number of blocks in the output, instead of writing to the very beginning of the output device.

So, for example, this copies 1MiB worth of y\n to position 8MiB of the outputfile. So the total filesize will be 9MiB.

$ yes | dd bs=1M count=1 seek=8 iflag=fullblock of=outputfile
$ ls -alh outputfile
9.0M Jun  3 21:02 outputfile
$ hexdump -C outputfile
00000000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00800000  79 0a 79 0a 79 0a 79 0a  79 0a 79 0a 79 0a 79 0a  |y.y.y.y.y.y.y.y.|
*
00900000

Since you mention /dev/random and overwriting partitions... it will take forever since /dev/random (as well as /dev/urandom) is just too slow. You could just use shred -v -n 1 instead, that's fast and usually available anywhere.

Dd, seek with one block size, write with another block size

Solution:

dd if='input_file.bin'           \
   of='/dev/sd{X}'               \
   bs={desired write block size} \
   seek={start offset in bytes}  \
   count={write size in bytes}   \
   oflag=seek_bytes              \
   iflag=count_bytes

From the man page:

count_bytes
    treat 'count=N' as a byte count (iflag only)

...

seek_bytes
    treat 'seek=N' as a byte count (oflag only)

This does seem to slow down the transfer a bit, but at least puts it in MB/s, instead of kB/s. Also, be sure to check the man page on your system, as it seems the ones available on the web (i.e. googling 'man dd') don't include these options.

Best Answer

Related Solutions

The difference between ‘bs’, ‘count’ and ‘seek’ in dd command

Dd, seek with one block size, write with another block size

Related Question