why is this dd command incorrect?

[Member stevesr0]

**N.B. I figured out HOW to make it work, so this post might be useful to others. The question is why do I have to use this form of the “of=destination/backup.img”, since there are many examples of copying from /dev/sda1 to /dev/sdb2…

Hi all,

I am playing with the dd command in VirtualBox.

I created two partitions (/dev/sda1 mounted to media/target and /dev/sda2 mounted to /media/destination).

On my first attempt, I was able to create a backup.img file of /dev/sda1 in /dev/sda2.

Tonight, attempting to repeat the dd command (after adding a second test file to /dev/sda1),
it failed.

I UNmounted both partitions, and used the command
"$ sudo dd if=/dev/sda1 of=/dev/sda2/sda1backup.img".

However, I got the error message
dd: failed to open '/dev/sda2/sda1backup.img': Not a directory

instead of a second backup.img file being created.

The command only works if I mount the destination partition and rewrite the dd command as

sudo dd if=/dev/sda1 of=/media/destination/backup2.img.

I thought it was possible to dd between partitions (/dev/sda1 to /dev/sda2) so I don’t understand when you have to use the directory location the partition is mounted upon and when you use the /dev location itself.

stevesr0

• This topic was modified 5 months, 2 weeks ago by stevesr0.

[Forum Admin dolphin_oracle]

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you aren’t writing a partition, you are writing a file, which means you need the mounted file system, which is not at /dev/sda2/ its at /media/destination/, as you discovered.

[Member stevesr0]

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Hi dolphin_oracle,

Thanks for comment.

Yeh. This morning it struck me that I have heard that (a) everything in Linux is a file and (b) a directory is a special kind of file.

But I didn’t realize that “regular” files can’t exist outside of a directory.

I guess when you copy a partition to a partition, that involves (re)creating the DIRECTORY and FILE structures, so there is no problem for dd.

stevesr0

[Forum Admin dolphin_oracle]

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when you are copying a partition to a partition, you are copying from one file to another

/dev/whatever to /dev/whatever2

which are both files, in the linux sense.

But you can’t write a file inside a file with dd, which is what your original command was trying to do. you write files inside mounted file systems. /dev being the mount point for a file system, but /dev/sdXY is not, your notion of file to file holds true.

you can see your mounted filesystems with the “df” command.

• This reply was modified 5 months, 2 weeks ago by dolphin_oracle.
• This reply was modified 5 months, 2 weeks ago by dolphin_oracle.

[Member stevesr0]

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Hi dolphin_oracle,

I never thought of a partition as a file (although everything in linux is a file <g>).

That makes perfectly good (and clear) sense.

It is interesting that you have to use the mounted “directory” name of a partition in order for dd to be able to create an image file of a partition, but dd can put a copy of a partition into another partition (mounted) using its partition name not its mounted directory name.

Thanks.

stevesr0

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