dd(1)
Name
dd − copy and convert data
Syntax
dd [option = value...]
Description
The dd command copies an input file to an output file with any requested conversions. The dd command is especially suited to I/O on the raw physical devices because it allows reading and writing in arbitrary record sizes.
After completion, dd reports the number of whole and partial input and output blocks.
The dd utility supports EOT handling which allows the use of multiple media. When writing, the utility prompts for the next volume when it encounters the end of the current volume. When reading multivolume archives, the utility requires that the user specify the number of volumes in the tape set.
This utility supports the TA90 style sequential stacker loader device. The device ejects a cartridge when it is taken off line. This utility performs the device ejection on behalf of the user when it encounters a multivolume boundary during write or read operations involving tape. The device then automatically loads the next available tape (if there is one). The utility then attempts to access this tape before prompting the user for a manual reload operation.
Options
Where sizes (n) are given for an option, the number may end with k for kilobytes (1024 bytes), b for blocks (512 bytes), or w for words (2 bytes). Also, two numbers may be separated by the character x to indicate a product.
mv=nThe number of tapes in a multivolume tape set.
if=nameInput file name. The standard input is the default.
of=nameOutput file name. The standard output is the default.
ibs=nInput block size, n bytes. The default is 512 bytes. Some devices do not support greater than 65,535 bytes.
obs=nOutput block size, n bytes. The default is 512 bytes. Some devices do not support greater than 65,535 bytes.
bs=nSet both input and output block size to n bytes, superseding ibs and obs. Also, if bs is specified, the copy is more efficient, since no blocking conversion is necessary.
cbs=nConversion buffer size, n bytes. Use only if ascii, unblock, ebcdic, ibm, or block conversion is specified. For ascii and unblock, n characters are placed into the conversion buffer, any specified character mapping is done, trailing blanks are trimmed and new line added before sending the line to the output. For ebcdic, ibm, or block, characters are read into the conversion buffer, and blanks added to make an output record of size n bytes.
skip=nSkip n input records before starting to copy.
files=nCopy n input files before terminating. This option is useful only when the input is a magnetic tape or similar device.
seek=nSeek n records from beginning of output file before copying.
rbuf=nUse n buffers for reading from those raw devices that support n-buffered I/O. (See Reference Pages Section 4: Special Files to check whether a specific device supports n-buffered I/O.) All n reads are started and each read must complete before the data can be used. This allows an n-buffered read-ahead on supported raw devices.
A default of eight read buffers are used if the read device supports n-buffered I/O and the write device does not.
The rbuf option cannot be used with the wbuf option.
wbuf=nUse n buffers for writing from those raw devices that support n-buffered I/O. (See Reference Pages Section 4: Special Files to check whether a specific device supports n-buffered I/O.) Each write is started but not known to be complete until all n buffers have been used. (This allows an n-buffered write-behind on supported raw devices).
A default of eight write buffers are used if the write device supports n-buffered I/O.
The wbuf option cannot be used with the rbuf option.
count=nCopy only n input records.
conv=asciiConvert EBCDIC to ASCII.
conv=ebcdicConvert ASCII to EBCDIC.
conv=ibmSlightly different map of ASCII to EBCDIC (see Restrictions).
conv=blockConvert variable length records to fixed length.
conv=unblockConvert fixed length records to variable length.
conv=lcaseMap alphabetics to lowercase.
conv=ucaseMap alphabetics to uppercase.
conv=loaderPermits the program to treat the tape drive as a sequential loader device even though it is not represented as such in the system driver tables.
conv=swabSwap every pair of bytes.
conv=noerrorDo not stop processing on an error.
conv=syncPad every input record to ibs.
conv=nomultiDisable multiple tape volumes.
conv=sparseCreate a sparse output file.
conv=... , ...Include several arguments for the conv option, separated by commas (see example below).
Restrictions
The ASCII/EBCDIC conversion tables are taken from the 256-character standard in the Communications of the ACM, November, 1968.
The ibm conversion corresponds to certain IBM print train conventions.
One must specify “conv=noerror,sync” when copying raw disks with bad sectors to ensure that dd stays synchronized.
Do not specify a block size greater than 65535 bytes when performing device I/O with the TK50 device. This device gives an I/O error when a size greater than 65535 is given. If you attempt to give a block size larger than 65535 on other devices and the device returns an I/O error, try using the maximum size of 65535 bytes.
Also, the buffering scheme employed by the VAX tz and st device drivers limits the block size to 16 Kbytes.
Furthermore, when writing to tapes, do not use an odd block size value when specifying the blocking factor. This can result in characters being dropped on every second block written to the tape.
We recommend that users label their tapes to include the values specified in the obs or bs options of the dd command. This ensures the ability to correctly specify the tape blocksize in the ibs or bs options used to read the tape at a later date. The dd command does not attempt to autosize the tape. Thus, if the user accidentally uses a smaller blocksize when reading the tape, the command would appear to succeed (reporting no errors), but the effect would be to truncate the tape records on output.
Using a blocking factor other than the default may lead to higher performance. However, you must select a value that is compatible with the hardware limitations of the source and destination machines.
Examples
The following example shows how to read an EBCDIC tape blocked ten 80-byte EBCDIC card images per record into the ASCII file x:
dd if=/dev/rmt0h of=x ibs=800 cbs=80 conv=ascii,lcase
Note the use of raw magtape. As noted in the Description, the dd command is especially suited to I/O on the raw physical devices because it allows reading and writing in arbitrary record sizes.
Diagnostics
f+p records in(out): numbers of full and partial records read(written)