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

I've read many guides and forum posts describing how to use dd, but one thing I've noticed is that people always use different values for the bs=, count= and seek= switches.

Please can someone explain what these switches do exactly (the man page isn't very detailed), and explain what the best settings for them are for different tasks, such as creating files from /dev/random or /dev/zero, and overwriting partitions and external drives.

Best Answer

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.

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The fastest way to write all sectors on a drive

If your goal is to really "zero" the drive, then I bet the fastest you can get is to issue a low-level secure erase command using hdparm (see here for step-by-step instructions).

Note two things:

As hdparm manpage vividly states, the operation is "DANGEROUS".
On the other hand, it may also repair bad blocks.

‘seek’ argument in command dd

dd is designed to copy blocks of data from an input file to an output file. The dd block size options are as follows, from the man page:

ibs=expr
    Specify the input block size, in bytes, by expr (default is 512).
obs=expr
    Specify the output block size, in bytes, by expr (default is 512).
bs=expr
    Set both input and output block sizes to expr bytes, superseding ibs= and obs=.

The dd seek option is similar to the UNIX lseek() system call¹. It moves the read/write pointer within the file. From the man page:

seek=n
    Skip n blocks (using the specified output block size) from the beginning of the output file before copying.

Ordinary files in UNIX have the convenient property that you do not have to read or write them starting at the beginning; you can seek anywhere and read or write starting from there. So bs=4096 seek=7 means to move to a position 7*4096 bytes from the beginning of the output file and start writing from there. It won't write to the portion of the file that is between 0 and 7*4096 bytes.

Areas of ordinary files that are never written to at all aren't even allocated by the underlying filesystem. These areas are called holes and the files are called sparse files. In your example, file_with_holes will have a 7*4096-byte hole at the beginning. (h/t @frostschutz for pointing out that dd truncates the output file by default.)

It is OK to read these unallocated areas; you get a bunch of zeroes.

[1] back when dd was written, the analogous system call was seek().

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‘seek’ argument in command dd

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