Useful uses of dd(1).

There remain some useful applications of dd. These may of course be achieved by other mechanisms, but typically less conveniently.

1. Read a specific number of blocks or bytes from a source:

  dd if=/dev/hda of=/root/mbr bs=512 count=1

This will make a copy of, say, your master boot record (first 512 bytes of your first disk drive) and stash it in your /root directory.

2. Read from specific bytes of a file

  dd if=mydata skip=1k bs=32 count=1

Reads 32 bytes after the first 1024 (1k) bytes of "mydata".

3. Write to specific bytes of a file

  dd if=source of=target seek=10k bs=512 count=1 conv=notrunc

That should write 512 bytes from "source" beginning 10k into "target". (I've not tested this, you should verify.)

4. Create a sparse file. Sparse files appear to have a nonzero size, but take up no space on disk, until data is actually written to them. These are often used as "inflating" dynamic filesystem images for virtual machines.

  dd if=/dev/zero of=sparsefile bs=1 count=0 seek=20000M # Create 20 GB sparse file

5. Case conversions. Sure, you could use tr(1), but where's the sport?

  dd if=MixEdCaSE of=lcase conv=lcase   # Convert to lower case
  dd if=MixEdCaSE of=ucase conv=ucase   # Convert to upper case

6. ASCII / EBCDIC conversions

  dd if=ebcdic of=ascii conv=ascii   # ebcdic -> ascii
  dd if=ascii of=ebcdic conv=ebcdic  # ascii -> ebcdic

When reading to or from IBM data tapes, you might find blocking / unblocking conversions useful. I've done this, but it's so long ago that I don't trust my memory on that any more. Odds are good you'll not have to worry about this.

There are other useful applications as well, though these are not typically encountered very often. Do feel free to explore and attempt these on safe media.