Use `dd` to cut file end part

Question

There is probably simple trick to do this, but I can't figure from man page.

How do I cut last 1MB from file with undetermined size for example, by using dd?

Answer 1

Well, assuming you have stat and bash, you can get the file size with:

stat -c %s your_file

If you want to extract the last $amount bytes for that file with dd, you could:

dd if=your_file of=extracted_part \
   bs=1 count=$amount \
   skip=$(( $(stat -c %s your_file) - $amount ))

But the saner approach would be to use tail:

tail -c $(( 1024*1024 )) your_file > target_file

Answer 2

dd --help
Usage: dd [OPERAND]...
  or:  dd OPTION
Copy a file, converting and formatting according to the operands.

  bs=BYTES        read and write BYTES bytes at a time (also see ibs=,obs=)
  cbs=BYTES       convert BYTES bytes at a time
  conv=CONVS      convert the file as per the comma separated symbol list
  count=BLOCKS    copy only BLOCKS input blocks
  ibs=BYTES       read BYTES bytes at a time (default: 512)
  if=FILE         read from FILE instead of stdin
  iflag=FLAGS     read as per the comma separated symbol list
  obs=BYTES       write BYTES bytes at a time (default: 512)
  of=FILE         write to FILE instead of stdout
  oflag=FLAGS     write as per the comma separated symbol list
  seek=BLOCKS     skip BLOCKS obs-sized blocks at start of output
  skip=BLOCKS     skip BLOCKS ibs-sized blocks at start of input
  status=noxfer   suppress transfer statistics

BLOCKS and BYTES may be followed by the following multiplicative suffixes:
c =1, w =2, b =512, kB =1000, K =1024, MB =1000*1000, M =1024*1024, xM =M
GB =1000*1000*1000, G =1024*1024*1024, and so on for T, P, E, Z, Y.

If the file size is exactly 10MB, 1024*10 10240K This will leave the last 1024K. You should specify the block size you are working with using the ibs and obs options.

1M = 1024K 
1024*9 = 9216  

dd if=/10/MB/file of=/9/MB/file count=9216K ibs=1K obs=1K
dd if=/10/MB/file of=/9/MB/file count=9M ibs=1M obs=1M

You can also skip the first 1MB of a file, using the skip option to read to the end of the file skipping the first 1MB.

dd if=/10/MB/file of=/9/MB/file skip=1M ibs=1M obs=1M

Using the seek option you can write a to a specific place in your output file. Say you want to keep the first 1MB and over write the last 8MB.

dd if=/10/MB/file of=/9/MB/file skip=1M seek=1M count=8M ibs=1M obs=1M

You probably need to get some details about your file size to make sure you get the right amount of data in and out.

ls -s --block-size 1K ./my/10MB/file

man ls

       --block-size=SIZE
              use SIZE-byte blocks.  See SIZE format below

       -s, --size
              print the allocated size of each file, in blocks

       SIZE  may  be (or may be an integer optionally followed by) one of fol‐
       lowing: KB 1000, K 1024, MB 1000*1000, M 1024*1024, and so on for G, T,
       P, E, Z, Y.