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 Common options. dd accepts the following operands.
- ‘if=file’
- Read from file instead of standard input.
- ‘of=file’
- Write to file instead of standard output. Unless
‘conv=notrunc’ is given, dd truncates file to zero
bytes (or the size specified with ‘seek=’).
- ‘ibs=bytes’
- Set the input block size to bytes.
This makes dd read bytes per block.
The default is 512 bytes.
- ‘obs=bytes’
- Set the output block size to bytes.
This makes dd write bytes per block.
The default is 512 bytes.
- ‘bs=bytes’
- Set both input and output block sizes to bytes.
This makes dd read and write bytes per block,
overriding any ‘ibs’ and ‘obs’ settings.
In addition, if no data-transforming conv option is specified,
input is copied to the output as soon as it's read,
even if it is smaller than the block size.
- ‘cbs=bytes’
- 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=blocks’
- Skip blocks ‘ibs’-byte blocks in the input file before copying.
- ‘seek=blocks’
- Skip blocks ‘obs’-byte blocks in the output file before copying.
- ‘count=blocks’
- Copy blocks ‘ibs’-byte blocks from the input file, instead
of everything until the end of the file.
- ‘status=noxfer’
- Do not print the overall transfer rate and volume statistics
that normally make up the third status line when dd exits.
- ‘conv=conversion[,conversion]...’
- Convert the file as specified by the conversion argument(s).
(No spaces around any comma(s).)
Conversions:
- ‘ascii’
- Convert EBCDIC to ASCII,
using the conversion table specified by POSIX.
This provides a 1:1 translation for all 256 bytes.
- ‘ebcdic’
- Convert ASCII to EBCDIC.
This is the inverse of the ‘ascii’ conversion.
- ‘ibm’
- 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.
- ‘block’
- For each line in the input, output ‘cbs’ bytes, replacing the
input newline with a space and padding with spaces as necessary.
- ‘unblock’
- Remove any trailing spaces in each ‘cbs’-sized input block,
and append a newline.
The ‘block’ and ‘unblock’ conversions are mutually exclusive.
- ‘lcase’
- Change uppercase letters to lowercase.
- ‘ucase’
- Change lowercase letters to uppercase.
The ‘lcase’ and ‘ucase’ conversions are mutually exclusive.
- ‘swab’
- 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).
- ‘sync’
- 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.
The following “conversions” are really file flags
and don't affect internal processing:
- ‘excl’
- Fail if the output file already exists; dd must create the
output file itself.
- ‘nocreat’
- Do not create the output file; the output file must already exist.
The ‘excl’ and ‘nocreat’ conversions are mutually exclusive.
- ‘notrunc’
- Do not truncate the output file.
- ‘noerror’
- Continue after read errors.
- ‘fdatasync’
- Synchronize output data just before finishing. This forces a physical
write of output data.
- ‘fsync’
- Synchronize output data and metadata just before finishing. This
forces a physical write of output data and metadata.
- ‘iflag=flag[,flag]...’
- Access the input file using the flags specified by the flag
argument(s). (No spaces around any comma(s).)
- ‘oflag=flag[,flag]...’
- 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.
- ‘append’
- 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.
- ‘cio’
- Use concurrent I/O mode for data. This mode performs direct I/O
and drops the POSIX requirement to serialize all I/O to the same file.
A file cannot be opened in CIO mode and with a standard open at the
same time.
- ‘direct’
- Use direct I/O for data, avoiding the buffer cache.
Note that the kernel may impose restrictions on read or write buffer sizes.
For example, with an ext4 destination file system and a linux-based kernel,
using ‘oflag=direct’ will cause writes to fail with
EINVAL if the
output buffer size is not a multiple of 512.
- ‘directory’
-
Fail unless the file is a directory. Most operating systems do not
allow I/O to a directory, so this flag has limited utility.
- ‘dsync’
- 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.
- ‘sync’
- Use synchronized I/O for both data and metadata.
- ‘nocache’
- Discard the data cache for a file.
When count=0 all cache is discarded,
otherwise the cache is dropped for the processed
portion of the file. Also when count=0
failure to discard the cache is diagnosed
and reflected in the exit status.
Here as some usage examples:
# Advise to drop cache for whole file
dd if=ifile iflag=nocache count=0
# Ensure drop cache for the whole file
dd of=ofile oflag=nocache conv=notrunc,fdatasync count=0
# Drop cache for part of file
dd if=ifile iflag=nocache skip=10 count=10 of=/dev/null
# Stream data using just the read-ahead cache
dd if=ifile of=ofile iflag=nocache oflag=nocache
- ‘nonblock’
- Use non-blocking I/O.
- ‘noatime’
- 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.
- ‘noctty’
- 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.
- ‘nofollow’
- Do not follow symbolic links.
- ‘nolinks’
- Fail if the file has multiple hard links.
- ‘binary’
- Use binary I/O. This option has an effect only on nonstandard
platforms that distinguish binary from text I/O.
- ‘text’
- Use text I/O. Like ‘binary’, this option has no effect on
standard platforms.
- ‘fullblock’
- Accumulate full blocks from input. The
read system call
may return early if a full block is not available.
When that happens, continue calling read to fill the remainder
of the block.
This flag can be used only with iflag.
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 Block size).
Any block size you specify via ‘bs=’, ‘ibs=’, ‘obs=’, ‘cbs=’
should not be too large—values larger than a few megabytes
are generally wasteful or (as in the gigabyte..exabyte case) downright
counterproductive or error-inducing.
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:
disk=/dev/rdsk/c0t1d0s2
tape=/dev/rmt/0
# 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 normally or is killed by the
SIGINT signal, 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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