Overwriting the middle of a file from a block device *without* truncating file?

Question

I have a Linux (Kubuntu) system with a large (~20Gb) file which I would like to mess with. Specifically, I want to copy the entire contents of another large file (~12Gb) to the middle of this file, and keep the bits of the file which are not overwritten.

This is what I want to happen, with the text between the pipes representing file contents:

Before:

outfile: |abcdefghijklmnopqrstuvwxyz|
infile:  |123456789|

After:

outfile: |abcdef123456789pqrstuvwxyz|
(infile is here ^^^^^^^^^)

I tried this:

dd if=infile of=outfile seek=162

(162 is the number of sectors I want to offset by in outfile)

But this is what happened:

outfile: |abcdef123456789|
(missing part of outfile ^)

What is the best way to do this?

Answer 1

You need to specify conv=notrunc to dd. From the manpage

Any regular output file is truncated unless the notrunc conversion value is specified. If an initial portion of the output file is seeked past (see the oseek operand), the output file is trun- cated at that point.