NAME

dd − convert and copy a file

SYNOPSIS

dd [option=value] ... 

DESCRIPTION

Dd copies the specified input file to the specified output with possible 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. 

option values

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 particularly 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 copying

count=n copy only n input records

conv=ascii convert EBCDIC to ASCII

ebcdicconvert ASCII to EBCDIC

ibmslightly different map of ASCII to EBCDIC

lcasemap alphabetics to lower case

ucasemap alphabetics to upper case

swabswap every pair of bytes

noerrordo not stop processing on an error

syncpad 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 respectively; 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, converted 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 output blocks. 

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 record sizes. 

To skip over a file before copying from magnetic tape do

(dd of=/dev/null; dd of=x) </dev/rmt0

SEE ALSO

cp(1), tr(1)

DIAGNOSTICS

f+p records in(out): numbers of full and partial records read(written)

RESTRICTIONS

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. 

CAUTION

Image copying disks using dd requires careful selection of the block size (bs, ibs, obs).  Using the default block size of 512 bytes (1b) will always work, however this is not very efficient.  In most cases the optimum block size is the number of 512 byte sectors per track.  For example: to copy an RL02 disk to another RL02 disk use ’bs=20b’, or to copy an RP06 to an RM03 use ’ibs=22b obs=32b’.  If the size of the input disk is larger that the size of the output disk a portion of the last record will not be transferred.  An disk image copy using dd will always terminate with the message ’No such device or address’, this indicates that the end of the disk has been reached.