Comment by dabiged | Hacker News Reader

Comment by dabiged | Hacker News Reader

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I only found out because I use dd 1000's of times each day and have had to prototype some really esoteric data pipelines using it. I have even compiled a few custom versions to do various tasks.

One other nuggets of wisdom which is very poorly documented in the man page: if you have a long running dd process (I often have 12+ hr dd processes reading 10+TB tapes) you can send a USR1 signal to check the progress. Very useful to check if your tape drive has had a hardware failure.

Information on both this and the byte counts are in my man page (GNU coreutils 8.30 on Linux Mint 20) around lines 130.

I knew about USR1 and it's in the man for me too, but I don't see anything about skip_bytes / count_bytes flags, which is strange.

Debian testing, coreutils 9.1.

I am running Coreutils 8.3 on Linux mint 20.

It is also in Coreutils 8.22 in RHEL7.

https://man7.org/linux/man-pages/man1/dd.1.html

Edit: I just found this in the coreutils 9.0 changelog:

  dd now counts bytes instead of blocks if a block count ends in "B".
  For example, 'dd count=100KiB' now copies 100 KiB of data, not
  102,400 blocks of data.  The flags count_bytes, skip_bytes and
  seek_bytes are therefore obsolescent and are no longer documented,
  though they still work.

Thanks for the pointer.

So you can simply do something like:

   dd if=foo skip=nB count=mB bs=4M of=bar

To get m bytes from offset n

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