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The ' dd ' command is one of the original Unix utilities and
should be in everyone's tool box. It can strip headers, extract parts
of binary files and write into the middle of floppy disks; it is used
by the Linux kernel Makefiles to make boot images. It can be used to
copy and convert magnetic tape formats, convert between ASCII and
EBCDIC, swap bytes, and force to upper and lowercase.
For blocked I/O, the dd command has no competition in the standard tool
set. One could write a custom utility to do specific I/O or formatting
but, as dd is already available almost everywhere, it makes sense to
use it.
Like most well-behaved commands, dd reads from its standard input and
writes to its standard output, unless a command line specification has
been given. This allows dd to be used in pipes, and remotely with the
rsh remote shell command.
Unlike most commands, dd uses a keyword=value format for its
parameters. This was reputedly modeled after IBM System/360 JCL, which
had an elaborate DD 'Dataset Definition' specification for I/O devices.
A complete listing of all keywords is available from GNU dd with
# dd --help
For more options check dd man
page
Using dd you can create backups of an entire harddisk or just a parts
of it. This is also usefull to quickly copy installations to similar
machines. It will only work on disks that are exactly the same in disk
geometry, meaning they have to the same model from the same brand.
full hard disk copy
dd if=/dev/hdx of=/dev/hdy
dd if=/dev/hdx of=/path/to/image
dd if=/dev/hdx | gzip > /path/to/image.gz
Hdx could be hda, hdb etc. In the second example gzip is used to
compress the image if it is really just a backup.
Restore Backup of hard disk copy
dd if=/path/to/image of=/dev/hdx
gzip -dc /path/to/image.gz | dd of=/dev/hdx
MBR backup
In order to backup only the first few bytes containing the MBR and the
partition table you can use dd as well.
dd if=/dev/hdx of=/path/to/image count=1 bs=512
MBR restore
dd if=/path/to/image of=/dev/hdx
Add "count=1 bs=446" to exclude the partition table from being
written to disk. You can manually restore the table.
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