mjhjmtu

Can any one give me a simple example? The partition size is 4 MB but the data inside is 3.1 MB. I need to dd only the 3.1 MB.

dd if=/dev/block/partition of=/tmp/3.1mb.img bs= skip=

I'm confused about bs and skip. Hopefully an example can clarify it.

The dd on my version of Android appears to be similar to that of most linux distributions in its operation. So for documentation that explains its features I'd look at the dd man page (you can just google it).

The man page has this to say about bs, count, and skip:

 bs=BYTES
 read and write up to BYTES bytes at a time (default: 512);
 overrides ibs and obs

 count=N
 copy only N input blocks

 skip=N skip N ibs-sized blocks at start of input

If you're looking for an exact number of bytes, you need to figure out
if you want 3.1MB or 3.1MiB. That is to say are your megabytes 1000*1000 bytes or 1024*1024 bytes? I'll assume the later in the following examples:

1. 
   

   dd if=/dev/block/partition of=/tmp/3.1mb.img count=6349

   
   
   

   So as above the default blocksize for dd is 512 bytes. Thus the following will copy out a little more than 3.1MiB starting from the beginning of the partition. Its a little more because 3.1MiB does not compute to a whole number of bytes (3.1*1024*1024 = 3250585.6). 6349 is used because 3.1 * 1024*1024 / 512 (blocksize) = 6348.8 and then I round up to make sure at least 3.1MiB of data is retrieved. Keep in mind that dd only reads and writes data in a multiple of blocksize, so in this case rounding up adds an extra 102 bytes. This shows you some of the things to think about if you are concerned about how many bytes you're actually getting.

   
   


2. 
   

   dd if=/dev/block/partition of=/tmp/3.1mb.img bs=1 count=3250586

   
   
   

   If you want an exact number of bytes which you do not know to be a multiple of some number, you can set the block size to 1 and the count to the exact number of bytes you want.

   
   


3. 
   

   dd if=/dev/block/partition of=/tmp/3.1mb.img skip=1K count=6349

   
   
   

   Suppose that the data you want to copy is half a megabyte (512KiB, or 512*1024 bytes) into the partition. Then skip can be used as above. 1K is used because the K suffix in dd mean 1024 bytes. Since the block size is the default 512, that means we are skipping 512KiB or 512 * 1024 bytes into the partition. Skip means that a certain number of blocks from the beginning of the input file (partition in our case) will be skipped from the beginning to get to the position in the input file where dd will begin reading.

   
   


4. 
   

   dd if=/dev/block/partition of=/tmp/3.1mb.img bs=1 skip=524288 count=3250586

   
   
   

   To get an exact number of bytes while skipping an in initial 512KiB we could rewrite the previous example as the one above.

   
   

Keep in mind that, since you are looking to copy a certain number of bytes, the count option should always be given. Otherwise, dd will read until it can no longer do so, which may read a lot more than you want.

Simple arithmetic:

dd if=/dev/block/partition of=/tmp/partition.img bs=1K count=3100

1 kilobyte times 3100 = 3.1 megabytes, more or less.

Are you sure the partition in question is so very small though?