dd — Convert and copy a file

Description

dd reads and writes data by blocks. It can convert data between formats. It is frequently used for such devices as tapes that have discrete block sizes, or for fast multisector reads from disks. dd performs conversions to accommodate nonprogrammable terminals, which require deblocking, conversion to and from EBCDIC, and fixed-length records.

dd processes the input data as follows:

1. dd reads an input block.
2. If this input block is smaller than the specified input block size, dd pads it to the specified size with null bytes. When you also specify a block or unblock conversion, dd uses spaces instead of null bytes.
3. If you specified bs=s and requested no conversion other than sync or noerror, dd writes the padded (if necessary) input block to the output as a single block and omits the remaining steps.
4. If you specified the swab conversion, dd swaps each pair of input bytes. If there is an odd number of input bytes, dd does not attempt to swap the last byte.
5. dd performs all remaining conversions on the input data independently of the input block boundaries. A fixed-length input or output record may span these boundaries.
6. dd gathers the converted data into output blocks of the specified size. When dd reaches the end of the input, it writes the remaining output as a block (without padding if conv=sync is not specified). As a result, the final output block might be shorter than the output block size.

Options

bs=size

Sets both input and output block sizes to size bytes. You can suffix this decimal number with w, b, k, or x number, to multiply it by 2, 512, 1024, or number, respectively. You can also specify size as two decimal numbers (with or without suffixes) separated by x to indicate the product of the two values. Processing is faster when ibs and obs are equal, since this avoids buffer copying. The default block size is 1B. bs=size supersedes any settings of ibs=size or obs=size.

If you specify bs=size and you request no other conversions than noerror, notrunc, or sync, dd writes the data from each input block as a separate output block; if the input data is less than a full block and you did not request sync conversion, the output block is the same size as the input block.

cbs=size

Sets the size of the conversion buffer used by various conv options.

conv=conversion[, conversion, …]

conversion can be any one of the following:

ascii

Converts EBCDIC input to ASCII for output; it is provided for compatibility purposes only.

To copy a file and convert between a shell code page and ASCII, use iconv, not dd.

block

Converts variable-length records to fixed-length records. dd treats the input data as a sequence of variable-length records (each terminated by a newline or an EOF character) independent of the block boundaries. dd converts each input record by first removing any newline characters and then padding (with spaces) or truncating the record to the size of the conversion buffer. dd reports the number of truncated records on standard error (stderr). You must specify cbs=size with this conversion.

Note: When working with double-byte characters, dd truncates the record after the last complete double-byte character that will fit in the conversion buffer. dd then pads the record with spaces if it is shorter than the conversion buffer size.

convfile

Uses convfile as a translation table if it is not one of the conversion formats listed here and it is the name of a file of exactly 256 bytes.

You can perform multiple conversions at the same time by separating arguments to conv with commas; however, some conversions are mutually exclusive (for example, ucase and lcase).

Note:

1. When you specify one or more of the character set conversions (ascii, ebcdic, ibm, or convfile), dd assumes that all characters are single-byte characters, regardless of the locale. Do not use these conversions with double-byte character sets.
2. When working with DBCS text, dd treats the input and output files as character strings and handles DBCS characters correctly (no splitting and retaining of proper shift states). This happens only if any of the conversion options (block, unblock, ucase, or lcase) are specified. Otherwise, DBCS strings can be corrupted with the seek, count, or iseek processing.

ebcdic

Converts ASCII input to EBCDIC for output; it is provided for compatibility purposes only.

To copy a file and convert between a shell code page and ASCII, use iconv, not dd.

ibm

Like ebcdic, converts ASCII to EBCDIC; it is provided for compatibility purposes only.

To copy a file and convert between code page 01047 (used in the z/OS shell) and ASCII, use iconv, not dd.

lcase

Converts uppercase input to lowercase.

noerror

Ignores errors on input.

notrunc

Does not truncate the output file. dd preserves blocks in the output file that it does not explicitly write to.

swab

Swaps the order of every pair of input bytes. If the current input record has an odd number of bytes, this conversion does not attempt to swap the last byte of the record.

sync

Specifies that dd is to pad any input block shorter than ibs to that size with NUL bytes before conversion and output. If you also specified block or unblock, dd uses spaces instead of null bytes for padding.

ucase

Converts lowercase input to uppercase.

unblock

Converts fixed-length records to variable-length records by reading a number of bytes equal to the size of the conversion buffer, deleting all trailing spaces, and appending a newline character. You must specify cbs=size with this conversion.

count=n

Copies only n input blocks to the output.

ibs=size

Sets the input block size in bytes. You specify it in the same way as with the bs option.

if=file

Reads input data from file. If you don't specify this option, dd reads data from standard input (stdin).

imsg=string

Displays string when all data has been read from the current volume, replacing all occurrences of %d in string with the number of the next volume to be read. dd then reads and discards a line from the controlling terminal.

iseek=n

seeks to the nth block of the input file. The distinction between this and the skip option is that iseek does not read the discarded data. There are some devices, however, such as tape drives and communication lines, on which seeking is not possible, so only skip is appropriate.

obs=size

Sets the output block size in bytes. You specify it in the same way as the bs value. The size of the destination should be a multiple of the value chosen for size. For example, if you choose obs=10K, the destination's size should be a multiple of 10K.

of=file

Writes output data to file. If you don't specify this option, dd writes data to standard output (stdout). dd truncates the output file before writing to it, unless you specified the seek=n operand. If you specify seek=n, but do not specify conv=notrunc, dd preserves only those blocks in the output file over which it seeks. If the size of the seek plus the size of the input file is less than the size of the output file, this can result in a shortened output file.

omsg=string

Displays string when dd runs out of room while writing to the current volume. Any occurrences of %d in string are replaced with the number of the next volume to be written. dd then reads and discards a line from the controlling terminal.

seek=n

Initially seeks to the nth block of the output file.

Note: Use caution when working with DBCS characters and the seek option. Seeking into the output file that contains DBCS characters can cause the DBCS string in the output file to be corrupted. Be sure that the seek count is not aligned with an existing DBCS string in the output file. Otherwise, part of the existing DBCS string either is written over with single-byte data or has extra shift codes from the input file's DBCS data.

skip=n

Reads and discards the first n blocks of input.

Localization

See Localization for more information.

Exit values

0

Successful completion

1

Failure due to any of the following:

• I/O errors on read/write
• Incorrect command-line option
• Incorrect arguments to a conversion

2

Failure resulting in a usage message such as:

• An option that should contain = does not
• Unknown or incorrect command-line option

Messages

Portability

POSIX.2, X/Open Portability Guide, UNIX systems.

The conv=convfile, iseek, imsg, and omsg options plus the w suffix described in the bs= option are all extensions of the POSIX standard.

Related information

cp, cpio, iconv, mv, tr