How can I write a shell script to update one octet in binary file (like I do manualy with hexedit program) ?

[KLEIN Stéphane <stephane@harobed.org>]

Hi,

I need to update one octet in a binary file (it's disk image file raw).

To do that, I use hexedit and I do update manualy.

Now, I would like to script this task, I know address of octet to update 
and the new value.

Do you know a tool to do that ?

I've tried :

echo "0xFF" | ascii2binary -b 16 | dd of=/tmp/out bs=1 count=1 seek=3

but the file is cutted after 4 octets.

I know how can I do it in Python or other script language but I search 
one "shell" solution.

Note : my context is I would like to write a script to resize some 
VirtualBox disk image.

Thanks for your help,
Stephane