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Requirement

I want to write a file at various offsets into the partition.

Partition /dev/part2 is mounted at /mypart

I tried the commands below:

dd if=/dev/urandom of=/mypart/aaa bs=1024 seek=0     count=15000
dd if=/dev/urandom of=/mypart/aaa bs=1024 seek=15000 count=15000
dd if=/dev/urandom of=/mypart/aaa bs=1024 seek=30000 count=15000

Are they doing what I want to do?  Are they writing a file to the partition at offset 0, 15000K and 30000K?

At what offset is the file written if I omit seek from dd?

dd if=/dev/urandom of=/mypart/aaa bs=1024            count=15000
G-Man Says 'Reinstate Monica'
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Ankur Agarwal
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2 Answers2

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You can't "write a file" at an "offset into the partition" using dd this way -- you are just writing data into a file named "aaa" within the mounted file system on that partition.

"seek=" will indeed cause dd to lseek to the given position before beginning its writes -- that means that it will simply create a file called /mypart/aaa and lseek the given number of blocks into that file before writing.

If you omit "seek=", dd will write starting at the beginning of the file named "aaa".

Perry
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  • I see. If I write directly to the device file at an offset, I basically destroy the filesystem on the partition. I tried that already. So how do I write data to a file at a particular offset in the file system? See my original requirement above. – Ankur Agarwal Mar 07 '12 at 01:10
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    You're asking how to do something you can't do. Of course if you just write to some arbitrary location in the partition you will destroy it, and there is no API for telling a file system to write at an arbitrary location -- you don't even know if that location is available for data to be written in it and not, say, allocated for metadata. I must confess that I have no idea why you would want to be able to do this. –  Mar 07 '12 at 01:15
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    If you have a filesystem on a device (like partition), you should think that the fs "owns" that device and you shouldn't touch it except by going through the fs. – XTL Mar 07 '12 at 08:25
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Re: "are they doing what I want to do"

try

strace dd if=/dev/urandom of=/mypart/aaa bs=1024 seek=15000 count=15000

and see what system calls are being made.

Kaz
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