Use dd to extract a precise portion of a file?

Asked Jun 18 '18 at 18:49

Active Sep 21 '21 at 07:42

Viewed 1.6k times

2

I have a 588Ko file, and I want to extract bytes from 0x7E8D6 to 0x8AD5D. I tried :
dd if=file of=result bs=50311 count=1 skip=518358

50311 stands for 0x8AD5D - 0x7E8D6
518358 stands for 0x7E8D6 (from where I want to cut)

dd tells me that it can't skip to the specified offset. What can I do? Is there any other utility to do it?

edited Jun 18 '18 at 18:55

asked Jun 18 '18 at 18:49

Nark

185

dd iflag=skip_bytes,count_bytes skip=518358 count=50311 or dd iflag=fullblock bs=518358 skip=1 count=1 and then truncate. – frostschutz Jun 18 '18 at 19:00
See https://unix.stackexchange.com/questions/121794/read-the-middle-of-a-large-file – Ole Tange Jun 18 '18 at 19:48

2 Answers2

5

Use dd to extract a precise portion of a file?

Lets adjust your syntax a little bit.

bs=1

Set the block size to one because of any count or skip you want to be in single blocks or bytes.

skip=518358

Skip this many blocks or bytes at the beginning of the stream.

count=50311

Copy this many blocks or bytes from the stream.

dd if=/path/file of=/path/result bs=1 count=50311 skip=518358

edited Sep 21 '21 at 07:42

Community

1

answered Jun 18 '18 at 19:03

jc__

2,605

3

skip and count are both expressed in blocks. You're asking to skip 518358 × 50311 bytes.

dd can let errors go undetected, so it's best avoided anyway.

With most Unix variants (including BusyBox but not OpenSBD), you can pass a number of bytes to head and tail. Just remember that tail counts from 1.

<file tail -c "$((0x7E8D6 + 1))" | head -c "$((0x8AD5D - 0x7E8D6))" >result

answered Jun 18 '18 at 18:57

Gilles 'SO- stop being evil'

829,060

Thanks a lot. May you please explain what is the first < for? – Nark Jun 18 '18 at 21:46
@Nofix It's an input redirection. If you're reading from a file, you can pass its name as an argument to tail instead. – Gilles 'SO- stop being evil' Jun 19 '18 at 22:29