dd - convert and copy a file
The dd utility copies the specified input file to the speci-
fied 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
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. 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.
The following operands are supported:
Specifies the input path. Standard input is the
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 por-
tion 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
obs=n Specifies the output block size in n bytes (default is
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 omit-
ted or given a value of 0, using block or unblock pro-
duces 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.
Copies and concatenates n input files before terminat-
ing (makes sense only where input is a magnetic tape
or similar device).
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
Seeks n blocks from beginning of input file before
copying (appropriate for disk files, where skip can be
Seeks n blocks from beginning of output file before
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.
Copies only n input blocks.
Where values are comma-separated symbols from the fol-
ascii Converts EBCDIC to ASCII.
Converts EBCDIC to ASCII using BSD-compatible charac-
Converts ASCII to EBCDIC. If converting fixed-length
ASCII records without NEWLINEs, sets up a pipeline
with dd conv=unblock beforehand.
Converts ASCII to EBCDIC using BSD-compatible charac-
ter translations. If converting fixed-length ASCII
records without NEWLINEs, sets up a pipeline with dd
ibm Slightly different map of ASCII to EBCDIC. If convert-
ing 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 con-
verted 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 conver-
sion 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.
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 char-
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 char-
acter. 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 char-
acter. 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.
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 out-
put block counts in the same format as used at comple-
tion. If the sync conversion is specified, the missing
input is replaced with null bytes and processed nor-
mally. Otherwise, the input block will be omitted from
Does not truncate the output file. Preserves blocks in
the output file not explicitly written by this invoca-
tion 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
If operands other than conv= 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 mul-
tiplication by 1024
3. a positive decimal number followed by b, specifying mul-
tiplication by 512
4. two or more positive decimal numbers (with or without k
or b) separated by x, specifying the product of the indi-
All of the operands will be processed before any input is
See largefile(5) for the description of the behavior of dd
when encountering files greater than or equal to 2 Gbyte ( 2
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.
example% 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 stan-
example% 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:
example% 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:
example% tar cvf - . | compress | dd obs=1024k of=/dev/rmt/0 conv=sync
See environ(5) for descriptions of the following environment
variables that affect the execution of dd: LANG, LC_ALL,
LC_CTYPE, LC_MESSAGES, and NLSPATH.
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.
See attributes(5) for descriptions of the following attri-
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
| Availability | SUNWcsu |
| Interface Stability | Standard |
cp(1), sed(1), tr(1), attributes(5), environ(5), large-
f+p records in(out)
numbers of full and partial blocks read(written)
Do not use dd to copy files between file systems having dif-
ferent 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
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 what-
ever 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 dev-
ice, use tar(1) or cpio(1).
For SIGINT, dd writes status information to standard error
before exiting. It takes the standard action for all other
Man(1) output converted with