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11.2 dd: Convert and copy a file

dd copies a file (from standard input to standard output, by default) with a changeable I/O block size, while optionally performing conversions on it. Synopses:

dd [operand]…
dd option

The only options are ‘--help’ and ‘--version’. See section Common options. dd accepts the following operands.


Read from file instead of standard input.


Write to file instead of standard output. Unless ‘conv=notrunc’ is given, dd truncates file to zero bytes (or the size specified with ‘seek=’).


Set the input block size to bytes. This makes dd read bytes per block.


Set the output block size to bytes. This makes dd write bytes per block.


Set both input and output block sizes to bytes. This makes dd read and write bytes per block, overriding any ‘ibs’ and ‘obs’ settings.


Set the conversion block size to bytes. When converting variable-length records to fixed-length ones (‘conv=block’) or the reverse (‘conv=unblock’), use bytes as the fixed record length.


Skip blocksibs’-byte blocks in the input file before copying.


Skip blocksobs’-byte blocks in the output file before copying.


Copy blocksibs’-byte blocks from the input file, instead of everything until the end of the file.


Convert the file as specified by the conversion argument(s). (No spaces around any comma(s).)



Convert EBCDIC to ASCII, using the conversion table specified by POSIX. This provides a 1:1 translation for all 256 bytes.


Convert ASCII to EBCDIC. This is the inverse of the ‘ascii’ conversion.


Convert ASCII to alternate EBCDIC, using the alternate conversion table specified by POSIX. This is not a 1:1 translation, but reflects common historical practice for ‘~’, ‘[’, and ‘]’.

The ‘ascii’, ‘ebcdic’, and ‘ibm’ conversions are mutually exclusive.


For each line in the input, output ‘cbs’ bytes, replacing the input newline with a space and padding with spaces as necessary.


Replace trailing spaces in each ‘cbs’-sized input block with a newline.

The ‘block’ and ‘unblock’ conversions are mutually exclusive.


Change uppercase letters to lowercase.


Change lowercase letters to uppercase.

The ‘lcase’ and ‘ucase’ conversions are mutually exclusive.


Swap every pair of input bytes. GNU dd, unlike others, works when an odd number of bytes are read—the last byte is simply copied (since there is nothing to swap it with).


Continue after read errors.


Do not create the output file; the output file must already exist.


Fail if the output file already exists; dd must create the output file itself.

The ‘excl’ and ‘nocreat’ conversions are mutually exclusive.


Do not truncate the output file.


Pad every input block to size of ‘ibs’ with trailing zero bytes. When used with ‘block’ or ‘unblock’, pad with spaces instead of zero bytes.


Synchronize output data just before finishing. This forces a physical write of output data.


Synchronize output data and metadata just before finishing. This forces a physical write of output data and metadata.


Access the input file using the flags specified by the flag argument(s). (No spaces around any comma(s).)


Access the output file using the flags specified by the flag argument(s). (No spaces around any comma(s).)

Here are the flags. Not every flag is supported on every operating system.


Write in append mode, so that even if some other process is writing to this file, every dd write will append to the current contents of the file. This flag makes sense only for output. If you combine this flag with the ‘of=file’ operand, you should also specify ‘conv=notrunc’ unless you want the output file to be truncated before being appended to.


Use direct I/O for data, avoiding the buffer cache.


Fail unless the file is a directory. Most operating systems do not allow I/O to a directory, so this flag has limited utility.


Use synchronized I/O for data. For the output file, this forces a physical write of output data on each write. For the input file, this flag can matter when reading from a remote file that has been written to synchronously by some other process. Metadata (e.g., last-access and last-modified time) is not necessarily synchronized.


Use synchronized I/O for both data and metadata.


Use non-blocking I/O.


Do not update the file's access time. Some older file systems silently ignore this flag, so it is a good idea to test it on your files before relying on it.


Do not assign the file to be a controlling terminal for dd. This has no effect when the file is not a terminal. On many hosts (e.g., GNU/Linux hosts), this option has no effect at all.


Do not follow symbolic links.


Fail if the file has multiple hard links.


Use binary I/O. This option has an effect only on nonstandard platforms that distinguish binary from text I/O.


Use text I/O. Like ‘binary’, this option has no effect on standard platforms.

These flags are not supported on all systems, and ‘dd’ rejects attempts to use them when they are not supported. When reading from standard input or writing to standard output, the ‘nofollow’ and ‘noctty’ flags should not be specified, and the other flags (e.g., ‘nonblock’) can affect how other processes behave with the affected file descriptors, even after dd exits.

The numeric-valued strings above (bytes and blocks) can be followed by a multiplier: ‘b’=512, ‘c’=1, ‘w’=2, ‘xm’=m, or any of the standard block size suffixes like ‘k’=1024 (see section Block size).

Use different dd invocations to use different block sizes for skipping and I/O. For example, the following shell commands copy data in 512 KiB blocks between a disk and a tape, but do not save or restore a 4 KiB label at the start of the disk:


# Copy all but the label from disk to tape.
(dd bs=4k skip=1 count=0 && dd bs=512k) <$disk >$tape

# Copy from tape back to disk, but leave the disk label alone.
(dd bs=4k seek=1 count=0 && dd bs=512k) <$tape >$disk

Sending an ‘INFO’ signal to a running dd process makes it print I/O statistics to standard error and then resume copying. In the example below, dd is run in the background to copy 10 million blocks. The kill command makes it output intermediate I/O statistics, and when dd completes, it outputs the final statistics.

$ dd if=/dev/zero of=/dev/null count=10MB & pid=$!
$ kill -s INFO $pid; wait $pid
3385223+0 records in
3385223+0 records out
1733234176 bytes (1.7 GB) copied, 6.42173 seconds, 270 MB/s
10000000+0 records in
10000000+0 records out
5120000000 bytes (5.1 GB) copied, 18.913 seconds, 271 MB/s

On systems lacking the ‘INFO’ signal dd responds to the ‘USR1’ signal instead, unless the POSIXLY_CORRECT environment variable is set.

An exit status of zero indicates success, and a nonzero value indicates failure.

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