The dd command copies and converts files to enable them to be transferred from one type of media to another. The dd command has various operands, or actions, to perform.
| Operand | Used To |
|---|---|
| if={file name} | Specify the file from which data will be read. |
| of={file name} | Specify the file to which data will be written. |
| bs={bytes} | Specify the total block size to read and write, in bytes. Bytes can also be formatted in a more human-friendly way, such as 50M to specify 50 megabytes and 10G to specify 10 gigabytes. |
| count={blocks} | Specify the number of blocks to be written to the output file from the input file. |
| status={level} | Specify the level of information to print to standard error – none to suppress everything except error messages, noxfer to suppress total transfer statistics, progress to display transfer statistics periodically. |
Syntax
The syntax of the dd command is:
# dd [options] [operands]
Using dd for backups
You can use dd to perform a full backup of a storage partition. The following example copies data from /dev/sda1 to /dev/sdb2:
# dd if=/dev/sda of=/dev/sdb
Using dd, you can also create an image of a drive and then clone a second drive with it:
# dd if=/dev/sda of=drive_image.iso # dd if=drive_image.iso of=/dev/sdb
dd Command Examples
1. To copy a file:
# dd if=old.txt of=old.txt
2. To read and write specified bytes of a file at a time:
# dd bs=1024 if=old.txt of=old.txt
3. To convert a file to a specified format:
# dd bs=1024 if=old.txt of=new.txt conv=ascii (from EBCDIC to ASCII) # dd bs=1024 if=old.txt of=new.txt conv=ebcdic (from ASCII to EBCDIC) # dd bs=1024 if=old.txt of=new.txt conv=ibm (from ASCII to alternate EBCDIC) # dd bs=1024 if=old.txt of=new.txt conv=block (pad oldline-terminated records with spaces to cbs-size) # dd bs=1024 if=old.txt of=new.txt conv=unblock (replace trailing spaces in cbs-size records with oldline) # dd bs=1024 if=old.txt of=new.txt conv=lcase (change upper case to lower case) # dd bs=1024 if=old.txt of=new.txt conv=excl (fail if the output old already exists) # dd bs=1024 if=old.txt of=new.txt conv=notrunc (do not truncate the output old) # dd bs=1024 if=old.txt of=new.txt conv=ucase (change lower case to upper case) # dd bs=1024 if=old.txt of=new.txt conv=swab (swap every pair of input bytes) # dd bs=1024 if=old.txt of=new.txt conv=noerror (continue after read errors) # dd bs=1024 if=old.txt of=new.txt conv=sync (pad every input block with NULs to ibs-size) # dd bs=1024 if=old.txt of=new.txt conv=fdatasync (physically write output old data before finishing) # dd bs=1024 if=old.txt of=new.txt conv=fsync (likewise, but also write metadata)
4. To copy only specified number of blocks:
# dd count=1024 if=old.txt of=new.txt
5. To copy a file with specifying a flag:
# dd if=old.txt of=new.txt oflag=append # dd if=old.txt iflag=directory of=new.txt oflag=directory # dd if=old.txt iflag=noatime of=new.txt oflag=append
6. To convert specified bytes at a time:
# dd cbs=1024 if=old.txt of=new.txt
7. To read specified number of bytes at a time:
# dd ibs=1024 if=old.txt of=new.txt
8. To write specified number of bytes at a time:
# dd obs=1024 if=old.txt of=new.txt
9. To skip specified number of blocks at start of o/p:
# dd seek=1024 if=old.txt of=new.txt
10. To skip specified number of blocks at start of i/p:
# dd skip=1024 if=old.txt of=new.txt
11. To suppress the transfer statistics:
# dd status=noxfer if=old.txt of=new.txt
12. To erase the partition table for a device:
# dd if=/dev/zero of=/dev/sda4 bs=512 count=1