Use dd to write data starting from a specific location

Question

I want to use dd to write data starting from a specific location (basically skip the first 50000 bytes ) on a disk and start writing after the first 50000 bytes. I tried doing this

dd of=/dev/disk1 if=/dev/random seek=50000

I let the above line run for few mins and then when I cancel it out, I get this

0+6 records in
0+0 records out
0 bytes (0 B) copied, 79.2458 s, 0.0 kB/s

Looks to me nothing got copied. Am I doing anything wrong ?

Answer 1

From dd docs:

‘seek=n’

Skip n ‘obs’-byte blocks in the output file before copying. 
if ‘oflag=seek_bytes’ is specified, n is interpreted as a byte 
count rather than a block count. 

So it looks like you want this:

dd of=/dev/disk1 if=/dev/random obs=50000  seek=1

Or this:

dd of=/dev/disk1 if=/dev/random oflag=seek_bytes  seek=50000

Another thing is that /dev/random will block if kernel pool is empty. You can try /dev/urandom instead, which will use other methods to generate a number without blocking when the pool is empty:

dd of=/dev/disk1 if=/dev/urandom oflag=seek_bytes  seek=50000