dd(1)

NAME

dd — convert, reblock, translate, and copy a (tape) file

SYNOPSIS

dd [option = value] ...

DESCRIPTION

dd copies the specified input file to the specified output file with possible conversions. The standard input and output are used by default. Input and output block size can be specified to take advantage of raw physical I/O. Upon completion, dd reports the number of whole and partial input and output records.

Options

dd recognizes the following option=value pairs:

if=file

Input file name; default is standard input.

of=file

Output file name; default is standard output. The output file is created using the same owner and group used by creat().

ibs=n

Input block size is n bytes; default is 512.

obs=n

Output block size is n bytes; default is 512.

bs=n

Set both input and output block size to the same size, superseding ibs and obs. This option is particularly efficient if no conversion (conv option) is specified, because no in-core copy is necessary.

cbs=n

Conversion buffer size is n bytes.

skip=n

Skip n input blocks before starting copy.

iseek=n

Skip n input blocks before starting copy. (This is an alias for the skip option.)

seek=n

Skip n blocks from beginning of output file before copying.

oseek=n

Skip n blocks from beginning of output file before copying. (This is an alias for the seek option.)

count=n

Copy only n input blocks.

files=n

Copy and concatenate n input files. This option should be used only when the input file is a magnetic tape device.

conv=value [,value ...]

Where values are comma-separated symbols from the following list.

ascii

Convert EBCDIC to ASCII.

ebcdic

Convert ASCII to EBCDIC.

ibm

Convert ASCII to EBCDIC using an alternate conversion table.

The ascii, ebcdic, and ibm values are mutually exclusive.

block

Convert each newline-terminated or end-of-file-terminated input record to a record with a fixed length specified by cbs. Any newline character is removed, and space characters are used to fill the block to size cbs. Lines that are longer than cbs are truncated; the number of truncated lines (records) is reported (see DIAGNOSTICS below).

The block and unblock values are mutually exclusive.

unblock

Convert fixed-length input records to variable-length records. For each input record, cbs bytes are read, trailing space characters are deleted, and a newline character is appended.

lcase

Map upper-case input characters to the corresponding lower-case characters.

The lcase and ucase values are mutually exclusive.

ucase

Map lower-case input characters to the corresponding upper-case characters.

swab

Swap every pair of input bytes.

noerror

Do not stop processing on an input error. If the sync conversion symbol is also specified, missing input is replaced with null bytes and processed normally; otherwise, the input block is omitted from the output.

notrunc

Do not truncate existing output file. Blocks in the output file not overwritten by this invocation of dd are preserved.

sync

Pad every input block to size ibs. If block or unblock is also specified, pad with space characters; otherwise, pad with null bytes.

Where sizes are required, n indicates a numerical value in bytes. Numbers can be specified using the forms:

n: for n bytes
nk: for n Kbytes (n × 1024),
nb: for n blocks (n × 512), or
nw: for n words (n × 2).

To indicate a product, use x to separate number pairs.

The cbs option is used when block, unblock, ascii or ebcdic conversion is specified. In case of ascii, cbs characters are placed into the conversion buffer, converted to ASCII, trailing blanks are trimmed, and a newline is added before sending the line to the output. In case of ebcdic, ASCII characters are read into the conversion buffer, converted to EBCDIC, and blanks are added to make up an output block of size cbs.

EXTERNAL INFLUENCES

International Code Set Support

Single- and multi-byte character code sets are supported.

Environment Variables

The following environment variables affect execution of dd:

LANG determines the locale when LC_ALL and a corresponding variable (beginning with LC_) do not specify a locale.

LC_ALL determines the locale used to override any values set by LANG or any environment variables beginning with LC_.

The LC_CTYPE variable determines the locale for the interpretation of sequences of bytes of text data as characters (single-byte/multi-byte characters, upper-case/lower-case characters).

The LC_MESSAGES variable determines the language in which messages are written.

RETURN VALUE

Exit values are:

0: Successful completion.
>0: Error condition occurred.

DIAGNOSTICS

Upon completion, dd reports the number of input and output records:

f+p records in: Number of full and partial blocks read.
f+p records out: Number of full and partial blocks written.

When conv=block is specified and there is at least one truncated block, the number of truncated records is also reported:

n truncated records

EXAMPLES

Read an EBCDIC tape blocked ten 80-byte EBCDIC card images per block into an ASCII file named x:

dd if=/dev/rmt/c0t0d0BEST of=x ibs=800 cbs=80 conv=ascii,lcase 

Note the use of the raw magnetic tape device file. dd is especially suited to I/O on raw physical devices because it allows reading and writing in arbitrary block sizes.

WARNINGS

Some devices, such as 1/2-inch magnetic tapes, are incapable of seeking. Such devices may be positioned prior to running dd by using mt(1) or some other appropriate command. The skip, seek, iseek and oseek options do work for such devices. However, skipping blocks using these options is slow on devices that cannot seek, since the blocks must actually be read to get to the desired position on the tape.

ASCII and EBCDIC conversion tables are taken from the 256-character ACM standard, Nov, 1968. The ibm conversion, while less widely accepted as a standard, corresponds better to certain IBM print train conventions. There is no universal solution.

Newline characters are inserted only on conversion to ASCII; padding is done only on conversion to EBCDIC. These should be separate options.

If if or of refers to a raw disk, bs should always be a multiple of the sector size of the disk. By default, bs is 512 bytes. If the sector size of the disk is different from 512 bytes, bs should be specified using a multiple of sector size. The character special (raw) device file should always be used for devices.

It is entirely up to the user to insure there is enough room in the destination file, file system and/or device to contain the output since dd cannot pre-determine the required space after conversion.

STANDARDS CONFORMANCE

dd: SVID2, SVID3, XPG2, XPG3, XPG4, POSIX.2