DD(1) General Commands Manual DD(1)

dd - convert and copy a file

dd [--help] [--version] [if=file] [of=file] [ibs=bytes] [obs=bytes] [bs=bytes] [cbs=bytes] [skip=blocks] [seek=blocks] [count=blocks] [conv={ascii, ebcdic, ibm, block, unblock, lcase, ucase, swab, noerror, notrunc, sync}]

dd copies a file (from standard input to standard output, by default) using specific input and output blocksizes, while optionally performing conversions on it.

It reads the input one block at a time, using the specified input block size (the default is 512 bytes). If the bs=bytes option was given, and no conversion other than sync, noerror, or notrunc was specified, it writes the amount of data read (which could be smaller than what was requested) in a separate output block. This output block has precisely the same length as was read unless the sync conversion was specified, in which case the data is padded with null bytes (NUL, '\0'; or with spaces, see below).

Otherwise, the input, read one block at a time, is processed and the resulting output is collected and written in blocks of the specified output block size. The final output block may be shorter.

The numeric-valued options below (bytes and blocks) can be followed by a multiplier: `k'=1024, `b'=512, `w'=2, `c'=1 (`w' and `c' are GNU extensions; `w' should never be used - it means 2 in System V and 4 in 4.2BSD). Two or more of such numeric expressions can be multiplied by putting `x' in between. The GNU fileutils-4.0 version also allows the following multiplicative suffixes in the specification of blocksizes (in bs=, cbs=, ibs=, obs=): M=1048576, G=1073741824, and so on for T, P, E, Z, Y. A `D' suffix makes them decimal: kD=1000, MD=1000000, GD=1000000000, etc. (Note that for ls, df, du the size of M etc. is determined by environment variables, but for dd it is fixed.)

Read from file instead of standard input.
Write to file instead of standard output. Unless conv=notrunc is given, dd truncates file to zero bytes (or the size specified with seek=).
Read bytes bytes at a time. The default is 512.
Write bytes bytes at a time. The default is 512.
Both read and write bytes bytes at a time. This overrides ibs and obs. (And setting bs is not equivalent with setting both ibs and obs to this same value, at least when no conversion other than sync, noerror and notrunc is specified, since it stipulates that each input block shall be copied to the output as a single block without aggregating short blocks.)
Specify the conversion block size for block and unblock.
Skip blocks ibs-byte blocks in the input file before copying.
Skip blocks obs-byte blocks in the output file before copying.
Copy blocks ibs-byte blocks from the input file, instead of everything until the end of the file.
Convert the file as specified by the CONVERSION argument(s). (No spaces around any comma(s).)

Conversions:

Convert EBCDIC to ASCII.
Convert ASCII to EBCDIC.
Convert ASCII to alternate EBCDIC.
For each line in the input, output cbs bytes, replacing the input newline with a space and padding with spaces as necessary.
Replace trailing spaces in each cbs-sized input block with a newline.
Change uppercase letters to lowercase.
Change lowercase letters to uppercase.
Swap every pair of input bytes. If an odd number of bytes are read the last byte is simply copied (since there is nothing to swap it with). [POSIX.2b, PASC interpretations 1003.2 #3 and #4]
Continue after read errors.
Do not truncate the output file.
Pad every input block to size of ibs with trailing null bytes ('\0').

Print a usage message on standard output and exit successfully.
Print version information on standard output, then exit successfully.
--
Terminate option list.

The variables LANG, LC_ALL, LC_CTYPE and LC_MESSAGES have the usual meaning.

POSIX.2

Often a tape drive will not accept arbitrarily sized blocks, and dd would get an I/O error for the last fragment of data that does not fill an entire block. Use `dd if=myfile of=/dev/mytape conv=sync' to get everything on tape. Of course, reading it back will now produce a slightly larger file, with null bytes added at the end.

Commands like `dd if=myfile of=/dev/fd0 bs=1k seek=172' fail on some systems because dd tries to truncate the output file, but truncation of a block device is not possible. In such cases, add the `conv=notrunc' option.

This page describes dd as found in the fileutils-4.0 package; other versions may differ slightly.

1998-11 GNU fileutils 4.0

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