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File: coreutils.info, Node: dd invocation, Next: install invocation, Prev: cp invocation, Up: Basic operations
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'. *Note Common
options::. 'dd' accepts the following operands, whose syntax was
inspired by the DD (data definition) statement of OS/360 JCL.
'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=N'
Skip N 'ibs'-byte blocks in the input file before copying. If
'iflag=skip_bytes' is specified, N is interpreted as a byte count
rather than a block count.
'seek=N'
Skip N 'obs'-byte blocks in the output file before copying. if
'oflag=seek_bytes' is specified, N is interpreted as a byte count
rather than a block count.
'count=N'
Copy N 'ibs'-byte blocks from the input file, instead of everything
until the end of the file. if 'iflag=count_bytes' is specified, N
is interpreted as a byte count rather than a block count. Note if
the input may return short reads as could be the case when reading
from a pipe for example, 'iflag=fullblock' will ensure that
'count=' corresponds to complete input blocks rather than the
traditional POSIX specified behavior of counting input read
operations.
'status=LEVEL'
Transfer information is normally output to stderr upon receipt of
the 'INFO' signal or when 'dd' exits. Specifying LEVEL will adjust
the amount of information printed, with the last LEVEL specified
taking precedence.
'none'
Do not print any informational or warning messages to stderr.
Error messages are output as normal.
'noxfer'
Do not print the final transfer rate and volume statistics
that normally make up the last status line.
'progress'
Print the transfer rate and volume statistics on stderr, when
processing each input block. Statistics are output on a
single line at most once every second, but updates can be
delayed when waiting on I/O.
'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.
'sparse'
Try to seek rather than write NUL output blocks. On a file
system that supports sparse files, this will create sparse
output when extending the output file. Be careful when using
this option in conjunction with 'conv=notrunc' or
'oflag=append'. With 'conv=notrunc', existing data in the
output file corresponding to NUL blocks from the input, will
be untouched. With 'oflag=append' the seeks performed will be
ineffective. Similarly, when the output is a device rather
than a file, NUL input blocks are not copied, and therefore
this option is most useful with virtual or pre zeroed devices.
'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'. This flag is
useful with pipes for example as they may return short reads.
In that case, this flag is needed to ensure that a 'count='
argument is interpreted as a block count rather than a count
of read operations.
'count_bytes'
Interpret the 'count=' operand as a byte count, rather than a
block count, which allows specifying a length that is not a
multiple of the I/O block size. This flag can be used only
with 'iflag'.
'skip_bytes'
Interpret the 'skip=' operand as a byte count, rather than a
block count, which allows specifying an offset that is not a
multiple of the I/O block size. This flag can be used only
with 'iflag'.
'seek_bytes'
Interpret the 'seek=' operand as a byte count, rather than a
block count, which allows specifying an offset that is not a
multiple of the I/O block size. This flag can be used only
with 'oflag'.
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 (N and BYTES) 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 (*note 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.
To process data that is at an offset or size that is not a multiple
of the I/O block size, you can use the 'skip_bytes', 'seek_bytes' and
'count_bytes' flags. Alternatively the traditional method of separate
'dd' invocations can be used. 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
Note also the 'status=progress' option which periodically updates the
last line of the transfer statistics above.
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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