man-pages/man1/dd.1

.\" Copyright Andries Brouwer, Ragnar Hojland Espinosa and A. Wik, 1998.
.\"
.\" This file may be copied under the conditions described
.\" in the LDP GENERAL PUBLIC LICENSE, Version 1, September 1998
.\" that should have been distributed together with this file.
.\"
.TH DD 1 1998-11 "GNU fileutils 4.0"
.SH NAME
dd \- convert and copy a file
.SH SYNOPSIS
.B dd
.B [\-\-help] [\-\-version]
.BI [if= file ]
.BI [of= file ]
.BI [ibs= bytes ]
.BI [obs= bytes ]
.BI [bs= bytes ]
.BI [cbs= bytes ]
.BI [skip= blocks ]
.BI [seek= blocks ]
.BI [count= blocks ]
.B "[conv={ascii, ebcdic, ibm, block, unblock, lcase, ucase, swab, noerror, notrunc, sync}]"
.SH DESCRIPTION
.B dd
copies a file (from standard input to standard output, by
default) using specific input and output blocksizes,
while optionally performing conversions on it.
.PP
It reads the input one block at a time, using the specified input
block size (the default is 512 bytes).
If the
.BI bs= bytes
option was given, and no conversion other than
.BR 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
.B sync
conversion was specified, in which case the data is padded with NULs
(or spaces, see below).
.PP
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.
.PP
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.)
.SH OPTIONS
.TP
.BI "if=" file
Read from
.I file
instead of standard input.
.TP
.BI "of=" file
Write to
.I file
instead of standard output.  Unless
.B conv=notrunc
is given,
.B dd
truncates
.I file
to zero bytes (or the size specified with
.BR seek= ")."
.TP
.BI "ibs=" bytes
Read
.I bytes
bytes at a time. The default is 512.
.TP
.BI "obs=" bytes
Write
.I bytes
bytes at a time. The default is 512.
.TP
.BI "bs=" bytes
Both read and write
.I bytes
bytes at a time.  This overrides
.B ibs
and
.BR obs .
(And setting
.B bs
is not equivalent with setting both
.B ibs
and
.B obs
to this same value, at least when no conversion other than
.BR sync ,
.B noerror
and
.B notrunc
is specified, since it stipulates that each input block
shall be copied to the output as a single block
without aggregating short blocks.)
.TP
.BI "cbs=" bytes
Specify the conversion block size for
.B block
and
.BR unblock .
.TP
.BI "skip=" blocks
Skip
.I blocks
.BR ibs -byte
blocks in the input file before copying.
.TP
.BI "seek=" blocks
Skip
.I blocks
.BR obs -byte
blocks in the output file before copying.
.TP
.BI "count=" blocks
Copy
.I blocks
.BR ibs -byte
blocks from the input file, instead of everything
until the end of the file.
.TP
.BI "conv=" CONVERSION "[," CONVERSION "]..."
Convert the file as specified by the
.I CONVERSION
argument(s).  (No spaces around any comma(s).)
.RS
.PP
Conversions:
.PP
.TP
.B ascii
Convert EBCDIC to ASCII.
.TP
.B ebcdic
Convert ASCII to EBCDIC.
.TP
.B ibm
Convert ASCII to alternate EBCDIC.
.TP
.B block
For each line in the input, output
.B cbs
bytes, replacing the input newline with a space and padding
with spaces as necessary.
.TP
.B unblock
Replace trailing spaces in each
.BR cbs -sized
input block with a newline.
.TP
.B lcase
Change uppercase letters to lowercase.
.TP
.B ucase
Change lowercase letters to uppercase.
.TP
.B swab
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 1003.2b, PASC interpretations 1003.2 #3 and #4]
.TP
.B noerror
Continue after read errors.
.TP
.B notrunc
Do not truncate the output file.
.TP
.B sync
Pad every input block to size of
.B ibs
with trailing zero bytes.
.RE
.SH "GNU STANDARD OPTIONS"
.TP
.B "\-\-help"
Print a usage message on standard output and exit successfully.
.TP
.B "\-\-version"
Print version information on standard output, then exit successfully.
.TP
.B "\-\-"
Terminate option list.
.SH ENVIRONMENT
The variables LANG, LC_ALL, LC_CTYPE and LC_MESSAGES have the
usual meaning.
.SH "CONFORMING TO"
POSIX 1003.2
.SH EXAMPLE
Often a tape drive will not accept arbitrarily sized blocks, and
.B 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 nulls added at the end.
.SH BUGS
Commands like `dd if=myfile of=/dev/fd0 bs=1k seek=172' fail
on some systems because
.B dd
tries to truncate the output file, but truncation of a block device
is not possible. In such cases, add the `conv=notrunc' option.
.SH NOTES
This page describes
.B dd
as found in the fileutils-4.0 package;
other versions may differ slightly.