DD(8) Maintenance Commands and Procedures DD(8)

NAME

dd - convert and copy a file

SYNOPSIS

dd [operand=value] ...

DESCRIPTION

The dd utility copies the specified input file to the specified output with
possible conversions. The standard input and output are used by default.
The input and output block sizes may be specified to take advantage of raw
physical I/O. Sizes are specified in bytes; a number may end with k, b, or
w to specify multiplication by 1024, 512, or 2, respectively. Numbers may
also be separated by x to indicate multiplication.

The dd utility reads the input one block at a time, using the specified
input block size. dd then processes the block of data actually returned,
which could be smaller than the requested block size unless the input flag
fullblock is used. dd applies any conversions that have been specified and
writes the resulting data to the output in blocks of the specified output
block size.

cbs is used only if ascii, asciib, unblock, ebcdic, ebcdicb, ibm, ibmb, or
block conversion is specified. In the first two cases, cbs characters are
copied into the conversion buffer, any specified character mapping is done,
trailing blanks are trimmed, and a NEWLINE is added before sending the line
to output. In the last three cases, characters up to NEWLINE are read into
the conversion buffer and blanks are added to make up an output record of
size cbs. ASCII files are presumed to contain NEWLINE characters. If cbs
is unspecified or 0, the ascii, asciib, ebcdic, ebcdicb, ibm, and ibmb
options convert the character set without changing the input file's block
structure. The unblock and block options become a simple file copy.

After completion, dd reports the number of whole and partial input and
output blocks.

OPERANDS

The following operands are supported:

if=file

Specifies the input path. Standard input is the default.

of=file

Specifies the output path. Standard output is the default. If
the seek=expr conversion is not also specified, the output file
will be truncated before the copy begins, unless conv=notrunc is
specified. If seek=expr is specified, but conv=notrunc is not,
the effect of the copy will be to preserve the blocks in the
output file over which dd seeks, but no other portion of the
output file will be preserved. (If the size of the seek plus the
size of the input file is less than the previous size of the
output file, the output file is shortened by the copy.)

ibs=n

Specifies the input block size in n bytes (default is 512).

obs=n

Specifies the output block size in n bytes (default is 512).

bs=n

Sets both input and output block sizes to n bytes, superseding
ibs= and obs=. If no conversion other than sync, noerror, and
notrunc is specified, each input block is copied to the output as
a single block without aggregating short blocks.

cbs=n

Specifies the conversion block size for block and unblock in bytes
by n (default is 0). If cbs= is omitted or given a value of 0,
using block or unblock produces unspecified results.

This option is used only if ASCII or EBCDIC conversion is
specified. For the ascii and asciib operands, the input is
handled as described for the unblock operand except that
characters are converted to ASCII before the trailing SPACE
characters are deleted. For the ebcdic, ebcdicb, ibm, and ibmb
operands, the input is handled as described for the block operand
except that the characters are converted to EBCDIC or IBM EBCDIC
after the trailing SPACE characters are added.

files=n

Copies and concatenates n input files before terminating (makes
sense only where input is a magnetic tape or similar device).

skip=n

Skips n input blocks (using the specified input block size) before
starting to copy. On seekable files, the implementation reads the
blocks or seeks past them. On non-seekable files, the blocks are
read and the data is discarded.

iseek=n

Seeks n blocks from beginning of input file before copying
(appropriate for disk files, where skip can be incredibly slow).

oseek=n

Seeks n blocks from beginning of output file before copying.

seek=n

Skips n blocks (using the specified output block size) from
beginning of output file before copying. On non-seekable files,
existing blocks are read and space from the current end-of-file to
the specified offset, if any, is filled with null bytes. On
seekable files, the implementation seeks to the specified offset
or reads the blocks as described for non-seekable files.

ostride=n

Writes every nth block (using the specified output block size)
when writing output. Skips n - 1 blocks after writing each
record.

istride=n

Reads every nth block (using the specified input block size) when
reading input. Skips n - 1 blocks after reading each record.

stride=n

Reads every nth block (using the specified input block size) when
reading input. Skips n - 1 blocks after reading each record.
Also writes every nth block (using the specified output block
size) when writing output. Skips n - 1 blocks after writing each
record.

count=n

Copies only n input blocks.

conv=value[,value...]

Where values are comma-separated symbols from the following list:

ascii Converts EBCDIC to ASCII.


asciib Converts EBCDIC to ASCII using BSD-compatible character
translations.

ebcdic Converts ASCII to EBCDIC. If converting fixed-length
ASCII records without NEWLINEs, sets up a pipeline with dd
conv=unblock beforehand.

ebcdicb Converts ASCII to EBCDIC using BSD-compatible character
translations. If converting fixed-length ASCII records
without NEWLINEs, sets up a pipeline with dd conv=unblock
beforehand.

ibm Slightly different map of ASCII to EBCDIC. If converting
fixed-length ASCII records without NEWLINEs, sets up a
pipeline with dd conv=unblock beforehand.

ibmb Slightly different map of ASCII to EBCDIC using
BSD-compatible character translations. If converting
fixed-length ASCII records without NEWLINEs, sets up a
pipeline with dd conv=unblock beforehand.

The ascii (or asciib), ebcdic (or ebcdicb), and ibm (or ibmb)
values are mutually exclusive.

block Treats the input as a sequence of NEWLINE-terminated or
EOF-terminated variable-length records independent of the
input block boundaries. Each record is converted to a
record with a fixed length specified by the conversion
block size. Any NEWLINE character is removed from the
input line. SPACE characters are appended to lines that
are shorter than their conversion block size to fill the
block. Lines that are longer than the conversion block
size are truncated to the largest number of characters
that will fit into that size. The number of truncated
lines is reported.

unblock Converts fixed-length records to variable length. Reads a
number of bytes equal to the conversion block size (or the
number of bytes remaining in the input, if less than the
conversion block size), delete all trailing SPACE
characters, and append a NEWLINE character.

The block and unblock values are mutually exclusive.

lcase Maps upper-case characters specified by the LC_CTYPE
keyword tolower to the corresponding lower-case character.
Characters for which no mapping is specified are not
modified by this conversion.

ucase Maps lower-case characters specified by the LC_CTYPE
keyword toupper to the corresponding upper-case character.
Characters for which no mapping is specified are not
modified by this conversion.

The lcase and ucase symbols are mutually exclusive.

swab Swaps every pair of input bytes. If the current input
record is an odd number of bytes, the last byte in the
input record is ignored.

noerror Does not stop processing on an input error. When an input
error occurs, a diagnostic message is written on standard
error, followed by the current input and output block
counts in the same format as used at completion. If the
sync conversion is specified, the missing input is
replaced with null bytes and processed normally.
Otherwise, the input block will be omitted from the
output.

notrunc Does not truncate the output file. Preserves blocks in
the output file not explicitly written by this invocation
of dd. (See also the preceding of=file operand.)

sync Pads every input block to the size of the ibs= buffer,
appending null bytes. (If either block or unblock is also
specified, appends SPACE characters, rather than null
bytes.)

iflag=value[,value ...]

Where values are comma-separated symbols from the following list
which affect the behavior of reading from the input file:

fullblock Accumulate full blocks of input.

oflag=value[,value ...]

Where values are comma-separated symbols from the following list
which affect the behavior of writing the output file:

dsync The output file is opened with the O_DSYNC flag set. All
data writes will be synchronous. For more information on
O_DSYNC see fcntl.h(3HEAD).

sync The output file is opened with the O_SYNC. All data and
metadata writes will be synchronous. For more information
on O_SYNC see fcntl.h(3HEAD).

status=type
Changes the way that dd outputs information about transfers. Only
one type may be specified. The following are valid values for
type:

none The program will not output any information and statistics
about the transfer. This includes when dd receives a
signal such as SIGINFO and SIGUSR1. Only errors will be
emitted.

noxfer When printing statistical information such as when the
program terminates or in response to a signal, only print
information about the number of records and none of the
statistics about the transfer (total bytes, total time,
and throughput).

progress Once a second, output the normal dd transfer statistics
that include the total number of bytes transferred, time
elapsed, and average throughput.

If operands other than conv= and oflag= are specified more than once, the
last specified operand=value is used.

For the bs=, cbs=, ibs=, and obs= operands, the application must supply an
expression specifying a size in bytes. The expression, expr, can be:
1. a positive decimal number

2. a positive decimal number followed by k, specifying
multiplication by 1024

3. a positive decimal number followed by M, specifying
multiplication by 1024*1024

4. a positive decimal number followed by G, specifying
multiplication by 1024*1024*1024

5. a positive decimal number followed by T, specifying
multiplication by 1024*1024*1024*1024

6. a positive decimal number followed by P, specifying
multiplication by 1024*1024*1024*1024*1024

7. a positive decimal number followed by E, specifying
multiplication by 1024*1024*1024*1024*1024*1024

8. a positive decimal number followed by Z, specifying
multiplication by 1024*1024*1024*1024*1024*1024*1024

9. a positive decimal number followed by b, specifying
multiplication by 512

10. two or more positive decimal numbers (with or without k or b)
separated by x, specifying the product of the indicated values.

All of the operands will be processed before any input is read.

SIGNALS

When dd receives either SIGINFO or SIGUSR1, dd will emit the current input
and output block counts, total bytes written, total time elapsed, and the
number of bytes per second to standard error. This is the same information
format that dd emits when it successfully completes. Users may send
SIGINFO via their terminal. The default character is ^T, see stty(1) for
more information.

For SIGINT, dd writes status information to standard error before exiting.
dd takes the standard action for all other signals.

USAGE

See largefile(7) for the description of the behavior of dd when
encountering files greater than or equal to 2 Gbyte (2^31 bytes).

EXIT STATUS

The following exit values are returned:

0 The input file was copied successfully.

>0 An error occurred.

If an input error is detected and the noerror conversion has not been
specified, any partial output block will be written to the output file, a
diagnostic message will be written, and the copy operation will be
discontinued. If some other error is detected, a diagnostic message will
be written and the copy operation will be discontinued.

EXAMPLES

Example 1 Copying from one tape drive to another

The following example copies from tape drive 0 to tape drive 1, using a
common historical device naming convention.

% dd if=/dev/rmt/0h of=/dev/rmt/1h

Example 2 Stripping the first 10 bytes from standard input

The following example strips the first 10 bytes from standard input:

% dd ibs=10 skip=1

Example 3 Reading a tape into an ASCII file

This example reads an EBCDIC tape blocked ten 80-byte EBCDIC card images
per block into the ASCII file x:

% dd if=/dev/tape of=x ibs=800 cbs=80 conv=ascii,lcase

Example 4 Using conv=sync to write to tape

The following example uses conv=sync when writing to a tape:

% tar cvf -. | compress | dd obs=1024k of=/dev/rmt/0 conv=sync

DIAGNOSTICS

f+p records in(out) numbers of full and partial blocks read(written)

ENVIRONMENT VARIABLES

See environ(7) for descriptions of the following environment variables that
affect the execution of dd: LANG, LC_ALL, LC_CTYPE, LC_MESSAGES, and
NLSPATH.

INTERFACE STABILITY

Standard

SEE ALSO

cp(1), sed(1), tr(1), fcntl.h(3HEAD), attributes(7), environ(7),
largefile(7), standards(7)

NOTES

Do not use dd to copy files between file systems having different block
sizes.

Using a blocked device to copy a file will result in extra nulls being
added to the file to pad the final block to the block boundary.

When dd reads from a pipe, using the ibs=X and obs=Y operands, the output
will always be blocked in chunks of size Y. When bs=Z is used, the output
blocks will be whatever was available to be read from the pipe at the time.

When using dd to copy files to a tape device, the file size must be a
multiple of the device sector size (for example, 512 Kbyte). To copy files
of arbitrary size to a tape device, use tar(1) or cpio(1).

illumos October 24, 2021 illumos