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 NULs (or 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
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
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
Specify the conversion block size for block and
Skip blocksibs-byte blocks in the input file
Skip blocksobs-byte blocks in the output
file before copying.
Copy blocksibs-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).)
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
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 1003.2b, 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
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 nulls
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.