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Does dd act at the filesystem level or go to the raw device directly?

Bardi Harborow
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zhoucengchao
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  • dd works on device level. – Pilot6 Sep 15 '15 at 09:01
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    It depends on what you mean exactly. Technically it always reads the filesystem, or it wouldn't be able to determine, for one, the permissions of the regular file / device file or whatever is put to work on. – kos Sep 15 '15 at 09:44

1 Answers1

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dd works on whatever you chose it to work on.

You can take the terminal streams (stdin and stdout), or anything represented by a file descriptor as in- and output of dd.

And as almost everything on Linux has a file descriptor, you can use files, directories, character devices (e.g. /dev/null, /dev/random), block devices (e.g. partitions/file systems like /dev/sda1 or directly entire disks like /dev/sda).

As you can see, dd is a pretty powerful and versatile tool, but make sure you use it with caution as it's also known as "disk destroyer" if you accidentally use it with unintended arguments.

If you're interested in more information about dd, read its manpage (run man dd) or look for example at Answer to "dd vs cat — is dd still relevant these days?" (Unix&Linux.SE) or What does the command name "dd" stand for?

Community
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Byte Commander
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  • Is dd powerful in itself or mostly takes advantage of the fact that as almost everything on Linux has a file descriptor? – A.L Sep 15 '15 at 13:37
  • @A.L Good question, I don't really know. You could look at its source code probably if you're interested. But does it matter? – Byte Commander Sep 15 '15 at 13:40
  • No, that was just nitpicking about the assumption dd is a pretty powerful and versatile tool. – A.L Sep 15 '15 at 13:41
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    Check your command 5 times and write it on a piece of paper and check it another 5 times. Mixing up IF and OF is instant destruction of potentially a lot of data. – Nelson Sep 15 '15 at 13:52
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    dd has some nice features, but the most time it is used in ways which would work equally well with cp, cat, the shell ... in this answer some things which are hard without dd are listed. But dd if=/dev/sda of=/mnt/ext/backup can better be done with cat /dev/sda > /mnt/ext/backup for example – Josef Sep 15 '15 at 14:21
  • @Josef Thanks for the interesting comment, I included your link to my post. – Byte Commander Sep 15 '15 at 14:25
  • +1 for mentioning Disk Destroyer... ;-) – Fabby Sep 18 '15 at 11:03