DD(1) General Commands Manual DD(1)
dd - convert and copy a file
dd [option=value] ...
Dd copies the specified input file to the specified output with possi-
ble conversions. The standard input and output are used by default.
The input and output block size may be specified to take advantage of
raw physical I/O.
if= input file name; standard input is default
of= output file name; standard output is default
ibs=n input block size n bytes (default 512)
obs=n output block size (default 512)
bs=n set both input and output block size, superseding ibs
and obs; also, if no conversion is specified, it is par-
ticularly efficient since no copy need be done
cbs=n conversion buffer size
skip=n skip n input records before starting copy
files=n copy n files from (tape) input
seek=n seek n records from beginning of output file before
count=n copy only n input records
conv=ascii convert EBCDIC to ASCII
ebcdic convert ASCII to EBCDIC
ibm slightly different map of ASCII to EBCDIC
lcase map alphabetics to lower case
ucase map alphabetics to upper case
swab swap every pair of bytes
noerror do not stop processing on an error
sync pad every input record to ibs
... , ... several comma-separated conversions
Where sizes are specified, a number of bytes is expected. A number may
end with k, b or w to specify multiplication by 1024, 512, or 2 respec-
tively; a pair of numbers may be separated by x to indicate a product.
Cbs is used only if ascii or ebcdic conversion is specified. In the
former case cbs characters are placed into the conversion buffer, con-
verted to ASCII, and trailing blanks trimmed and new-line added before
sending the line to the output. In the latter case ASCII characters
are read into the conversion buffer, converted to EBCDIC, and blanks
added to make up an output record of size cbs.
After completion, dd reports the number of whole and partial input and
For example, to read an EBCDIC tape blocked ten 80-byte EBCDIC card
images per record into the ASCII file x:
dd if=/dev/rmt0 of=x ibs=800 cbs=80 conv=ascii,lcase
Note the use of raw magtape. Dd is especially suited to I/O on the raw
physical devices because it allows reading and writing in arbitrary
To skip over a file before copying from magnetic tape do (dd
of=/dev/null; dd of=x) </dev/rmt0
f+p records in(out): numbers of full and partial records read(written)
The ASCII/EBCDIC conversion tables are taken from the 256 character
standard in the CACM Nov, 1968. The `ibm' conversion, while less
blessed as a standard, corresponds better to certain IBM print train
conventions. There is no universal solution.
Newlines are inserted only on conversion to ASCII; padding is done only
on conversion to EBCDIC. These should be separate options.