dd

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Warning
There is a reason dd is sometimes humorously called disk destroyer! Incorrect use of the dd command can wipe any drive connected to the system. Always backup any data you're not willing to lose before using the command.

dd is a utility used to copy raw data from a source into sink, where source and sink can be a block device, file, or piped input/output. Because of its flexibility dd can be used for a variety of purposes ranging from writing installation media to a backup and recovery tool of last resort.

The dd utility is specified by the POSIX standard. On Gentoo, dd is provided by GNU coreutils, sys-apps/coreutils, installed via the @system set.

Installation

Emerge

In the event that coreutils goes missing:

root #emerge --ask sys-apps/coreutils

Usage

By default dd takes input from stdin, optionally manipulates the data, and writes to stdout.

Invocation

user $dd --help

Expand

Usage: dd [OPERAND]...
  or:  dd OPTION
Copy a file, converting and formatting according to the operands.

  bs=BYTES        read and write up to BYTES bytes at a time (default: 512);
                  overrides ibs and obs
  cbs=BYTES       convert BYTES bytes at a time
  conv=CONVS      convert the file as per the comma separated symbol list
  count=N         copy only N input blocks
  ibs=BYTES       read up to BYTES bytes at a time (default: 512)
  if=FILE         read from FILE instead of stdin
  iflag=FLAGS     read as per the comma separated symbol list
  obs=BYTES       write BYTES bytes at a time (default: 512)
  of=FILE         write to FILE instead of stdout
  oflag=FLAGS     write as per the comma separated symbol list
  seek=N          (or oseek=N) skip N obs-sized output blocks
  skip=N          (or iseek=N) skip N ibs-sized input blocks
  status=LEVEL    The LEVEL of information to print to stderr;
                  'none' suppresses everything but error messages,
                  'noxfer' suppresses the final transfer statistics,
                  'progress' shows periodic transfer statistics

N and BYTES may be followed by the following multiplicative suffixes:
c=1, w=2, b=512, kB=1000, K=1024, MB=1000*1000, M=1024*1024, xM=M,
GB=1000*1000*1000, G=1024*1024*1024, and so on for T, P, E, Z, Y.
Binary prefixes can be used, too: KiB=K, MiB=M, and so on.
If N ends in 'B', it counts bytes not blocks.

Each CONV symbol may be:

  ascii     from EBCDIC to ASCII
  ebcdic    from ASCII to EBCDIC
  ibm       from ASCII to alternate EBCDIC
  block     pad newline-terminated records with spaces to cbs-size
  unblock   replace trailing spaces in cbs-size records with newline
  lcase     change upper case to lower case
  ucase     change lower case to upper case
  sparse    try to seek rather than write all-NUL output blocks
  swab      swap every pair of input bytes
  sync      pad every input block with NULs to ibs-size; when used
            with block or unblock, pad with spaces rather than NULs
  excl      fail if the output file already exists
  nocreat   do not create the output file
  notrunc   do not truncate the output file
  noerror   continue after read errors
  fdatasync  physically write output file data before finishing
  fsync     likewise, but also write metadata

Each FLAG symbol may be:

  append    append mode (makes sense only for output; conv=notrunc suggested)
  direct    use direct I/O for data
  directory  fail unless a directory
  dsync     use synchronized I/O for data
  sync      likewise, but also for metadata
  fullblock  accumulate full blocks of input (iflag only)
  nonblock  use non-blocking I/O
  noatime   do not update access time
  nocache   Request to drop cache.  See also oflag=sync
  noctty    do not assign controlling terminal from file
  nofollow  do not follow symlinks

Sending a USR1 signal to a running 'dd' process makes it
print I/O statistics to standard error and then resume copying.

Options are:

      --help        display this help and exit
      --version     output version information and exit

GNU coreutils online help: <https://www.gnu.org/software/coreutils/>
Report any translation bugs to <https://translationproject.org/team/>
Full documentation <https://www.gnu.org/software/coreutils/dd>
or available locally via: info '(coreutils) dd invocation'

Examples

Some common tasks where dd is used:

Boot stick

This should work with any live media as long as the memory stick /dev/sdX is large enough.

Warning
Any data on the memory stick will be lost.

root #dd if=/home/myLiveCD.iso of=/dev/sdX bs=8M status=progress && sync

if: Defines the source.
of: Defines the sink.
bs: Defines the block size (amount of data read/written at a time). The default is 512 bytes but most modern devices can read/write much faster. It is possible to define different sizes for source and sink using ibs and obs.
status: Defines the level of status information printed.
sync: Synchronizes write caches to make the stick removal safe.

Master boot record backup

To backup the master boot record (MBR), copy only the first 512 bytes:

root #dd if=/dev/sdX of=/root/mbr.bin bs=512 count=1

count: The number of blocks to copy.

Note
This is the complete MBR with the partition layout.

Hard disk backup

Note
Using dd as a disk backup is generally not recommended except when a perfect image is needed. This will include unused space in the image making it larger than the data contained inside the image. The storage medium must be as large or larger than the source disk.

To backup a complete hard disk or partition, it is necessary to boot the computer with into a Live CD environment (such as the Gentoo Minimal or Gentoo Admin disk).

The following example will backup a computer drive on /dev/sda to an external USB drive. To be able to mount that USB drive read/write, this example will use its label:

root #ls /dev/disk/by-label

'Gentoo\x20amd64\x20AdminCD\x2020201230T21'   MaxiTux

root #

mkdir /mnt/MaxiTux

root #

mount /dev/disk/by-label/MaxiTux /mnt/MaxiTux

Not every file system includes a label. Using UUID values or verified disk paths are alternatives.

To create a backup:

root #dd if=/dev/sda conv=sync,noerror bs=64k status=progress > /mnt/MaxiTux/sda_backup.img

To restore a backup:

root #dd if=/mnt/MaxiTux/sda_backup.img bs=8192 conv=sync,noerror of=/dev/sda status=progress

Input manipulation

As an example, convert any upper case character in a file to lowercase and reverse the input per line, then pipe the output to less to display the file:

user $dd if=/etc/portage/make.conf conv=swab,lcase,noerror | less

conv=swab: Revert the input per line by swapping any input byte (writing backwards).
conv=lcase: Convert any upper case letter to lower case. To convert lower case to upper case use conv=ucase.
conv=noerror: Continue if a read error occurs.