Files – How to Write One File into Another Using dd

ddfiles

I have an empty file (only zeroes are in it) of size 9,0KB and I need to write another file (with size 1,1KB) to it, but the first file must not lose its size or the rest of its contents. So if the whole file is 00000000000000... now, I need to write second file in it and leave the zeroes as they are. I have tried to use dd, but I haven't succeed – file resizes.

dd if=out/one.img of=out/go.img

Does anybody know how can I do it?

Best Answer

To overwrite the start of the destination file without truncating it, give the notrunc conversion directive:

$ dd if=out/one.img of=out/go.img conv=notrunc

If you wanted the source file's data appended to the destination, you can do that with the seek directive:

$ dd if=out/one.img of=out/go.img bs=1k seek=9

This tells dd that the block size is 1 kiB, so that the seek goes forward by 9 kiB before doing the write.

You can also combine the two forms. For example, to overwrite the second 1 kiB block in the file with a 1 kiB source:

$ dd if=out/one.img of=out/go.img bs=1k seek=9 conv=notrunc

That is, it skips the first 1 kiB of the output file, overwrites data it finds there with data from the input file, then closes the output without truncating it first.

Related Solutions

Bash – How to echo output of multiple commands into md5sum

If you really want to hash the data use a subshell:

( dd_1 ; dd_2 ; dd_3 ) | md5sum

Otherwise I would suggest using Hojat's answer - hash it piecewise and do something with those "subhashes".

Shell – Prevent already running process to write to an existing file

At least on my version of Linux, it looks like you may be able to use mandatory locks. I've only tested it with /dev/null, but I can't see any reason why it wouldn't work with other devices like your frame buffers:

As root:

mount -t tmpfs -o mand locked-fb some-dir
cp -a /dev/fb0 some-dir/fb0-for-process-A
cp -a /dev/fb0 some-dir/fb0-for-process-B
chmod g+s some-dir/fb0-for-process-[AB] # enable mand-lock

Then for instance, using perl and the File::FcntlLock module (or do it directly in C):

#! /usr/bin/perl
use File::FcntlLock;

$l = new File::FcntlLock;
$l->l_type(F_RDLCK);

open my $fba, '<', 'some-dir/fb0-for-process-A' or die$!;

sleep 10; # writing OK
$l->lock($fba, F_SETLK); # writing blocked on fb0-for-process-A

sleep 10;
exit; # lock now released (or do an explicit unlock)

Have one process open the fb0 device via the fb0-for-process-A file, and the other one via fb0-for-process-B and apply locking to both files to decide which process may write at a given time.

Best Answer

Related Solutions

Bash – How to echo output of multiple commands into md5sum

Shell – Prevent already running process to write to an existing file

Related Question