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.
As part of the GNU sys-apps/coreutils, is installed in any standard any Gentoo GNU/Linux system.
In the event that coreutils goes missing:
emerge --ask sys-apps/coreutils
By default dd takes input from stdin, optionally manipulates the data, and writes to stdout.
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'
Some common tasks where dd is used:
This should work with any live media as long as the memory stick /dev/sdX is large enough.
Any data on the memory stick will be lost.
dd if=/home/myLiveCD.iso of=/dev/sdX bs=8M
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
Master boot record backup
To backup the master boot record (MBR), copy only the first 512 bytes:
dd if=/dev/sdX of=/root/mbr.bin bs=512 count=1
count: The number of blocks to copy.
This is the complete MBR with the partition layout.
Hard disk backup
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:
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:
dd if=/dev/sda conv=sync,noerror bs=64k status=progress > /mnt/MaxiTux/sda_backup.img
To restore a backup:
dd if=/mnt/MaxiTux/sda_backup.img bs=8192 conv=sync,noerror of=/dev/sda status=progress
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:
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=noerror: Continue if a read error occurs.
- dcfldd — an enhanced dd tool that includes additional features for forensics and security.
- ddrescue — a tool provided by GNU to retrieve data from failing (block) storage devices like disk drives, CDROMs, or memory sticks, etc.
- pv — a command line tool to view verbose information about data streamed/piped through it.