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Commands Reference, Volume 2
dd Command
Purpose
Converts and copies a file.
Syntax
dd [ bs=BlockSize ][ cbs=BlockSize ] [ conv= [ ascii | block
| ebcdic | ibm | unblock ] [ lcase | ucase ]
[ iblock ] [ noerror ] [ swab ] [ sync ]
[ oblock ] [ notrunc ] ] [ count=InputBlocks ] [ files=InputFiles ] [ fskip=SkipEOFs ] [ ibs=InputBlockSize ] [ if=InFile ] [ obs=OutputBlockSize ][ of=OutFile ] [ seek=RecordNumber ] [ skip=SkipInputBlocks ][ span=yes|no ]
dd [ Option=Value ]
Description
The dd command reads the InFile parameter or standard input, does the specified
conversions, then copies the converted data to the OutFile parameter or standard output. The input and output block size can
be specified to take advantage of raw physical I/O.
Note
The term Block refers to the quantity of data read or written
by the dd command in one operation and is not necessarily
the same size as a disk block.
Where sizes are specified, a number of bytes is expected. A number ending
with w, b, or k specifies multiplication by 2, 512, or 1024 respectively; a pair of
numbers separated by an x or an * (asterisk) indicates a product. The count parameter expects the number
of blocks, not the number of bytes, to be copied.
The character-set mappings associated with the conv=ascii and conv=ebcdic flags are complementary operations. These flags map
between ASCII characters and the subset of EBCDIC characters found on most
workstations and keypunches.
Use the cbs parameter
value if specifying any of the block, unblock, ascii, ebcdic, or ibm conversions. If unblock or ascii parameters are specified, then the dd command performs a fixed-length to variable-length conversion. Otherwise
it performs a conversion from variable-length to fixed-length. The cbs parameter determines the fixed-length.
Attention: If the cbs parameter value is specified smaller
than the smallest input block, the converted block is truncated.
After it finishes, the dd command reports the number
of whole and partial input and output blocks.
Notes:
- Usually, you need only write access to the output file. However, when
the output file is not on a direct-access device and you use the seek flag, you also need read access to the file.
- The dd command inserts new-line characters only
when converting with the conv=ascii or conv=unblock flags set; it pads only when converting
with the conv=ebcdic, conv=ibm, or conv=block flags set.
- Use the backup, tar, or cpio command instead of the dd
command whenever possible to copy files to tape. These commands are designed
for use with tape devices. For more information on using tape devices, see
the rmt special file.
- The block size values specified with the bs, ibs and obs flags must always be a multiple
of the physical block size for the media being used.
- When the conv=sync flag
is specified, the dd command pads any partial input
blocks with nulls. Thus, the dd command inserts nulls
into the middle of the data stream if any of the reads do not receive a full
block of data (as specified by the ibs flag). This
is a common occurence when reading from pipes.
- If the bs flag is specified by itself and no conversions
other than sync, noerror or notrunc are specified, then the data from each input block
will be written as a separate output block; if the read returns less than
a full block and sync is not specified, then the resulting
output block will be the same size as the input block. If the bs flag is not specified, or a conversion other than sync, noerror or notrunc
is specified, then the input will be processed and collected into fullsized
output blocks until the end of input is reached.
Spanning across devices
The dd can be made to span across devices if the
input file is larger than the output device physical size.
Note
Care has to be taken when specifying the block size bs as exact multiple of the physical size of the device because improper
block size will result in data inconsistency, or overlap.
The spanning of dd across devices will not occur
if either one of the InFile or the OutFile parameter is stdin or stdout.
Spanning will occur in such a way that dd will prompt
for next device during write if the output device is full. During read from
the input device, dd will prompt for next device if
the data is completely read from the input device even when the device has
not reached the end. In this case it would be required to press 'n' to quit.
Flags
bs=BlockSize |
Specifies both the input and output block size, superseding the ibs and obs flags. The block size values specified with the bs flag must always be a multiple of the physical block
size for the media being used. |
cbs=BlockSize |
Specifies the conversion block size for variable-length to fixed-length
and fixed-length to variable-length conversions, such as conv=block. |
count=InputBlocks |
Copies only the number of input blocks specified by the InputBlocks variable. |
conv= Conversion,.... |
Specifies one or more conversion options. Multiple conversions
should be separated by commas. The following list describes the possible
options:
- ascii
- Converts EBCDIC to ASCII. This option is incompatible with the ebcdic, ibm, block, and unblock
options.
- block
- Converts variable-length records to fixed-length. The length is determined
by the conversion block size (cbs). This option is incompatible with the
ascii, ebcdic, ibm, and unblock options.
- ebcdic
- Converts ASCII to standard EBCDIC. This option is incompatible with
the ascii, ibm, block, and unblock options.
- ibm
- Converts ASCII to an IBM version of EBCDIC. This option is incompatible
with the ascii, ebcdic, block, and unblock options.
- iblock, oblock
- Minimize data loss resulting from a read or write error on direct
access devices. If you specify the iblock variable
and an error occurs during a block read (where the block size is 512 or the
size specified by the ibs=InputBlockSize variable), the dd
command attempts to reread the data block in smaller size units. If the dd command can determine the sector size of the input
device, it reads the damaged block one sector at a time. Otherwise, it reads
it 512 bytes at a time. The input block size ( ibs)
must be a multiple of this retry size. This option contains data loss associated
with a read error to a single sector. The oblock conversion
works similarly on output.
- lcase
- Makes all alphabetic characters lowercase.
- noerror
- Does not stop processing on an error.
- notrunc
- Does not truncate the output file. Instead, blocks not explicitly
written to output are preserved.
- ucase
- Makes all alphabetic characters uppercase.
- swab
- Swaps every pair of bytes.
- sync
- Pads every input block to the ibs value.
- unblock
- Converts fixed-length blocks to variable-length. The length is determined
by the conversion block size (cbs). This option is incompatible with the ascii, ebcdic, ibm, and block options.
|
files=InputFiles |
Copies the number of files specified by the InputFiles variable value of input files before ending (makes sense only where
input is a magnetic tape or similar device). |
fskip=SkipEOFs |
Skips past the number of end-of-file characters specified by the SkipEOFs variable before starting to copy; this SkipEOFs variable is useful for positioning on multifile
magnetic tapes. |
ibs=InputBlockSize |
Specifies the input-block size; the default is 512 bytes or one block.
The block-size values specified with the ibs flag
must always be a multiple of the physical block size for the media being
used. |
if=InFile |
Specifies the input file name; standard input is the default. |
obs=OutputBlockSize |
Specifies the output-block size; the default is 512 bytes or one block.
The block size values specified with the obs flag must
always be a multiple of the physical block size for the media being used. |
of=OutFile |
Specifies the output file name; standard output is the default. |
seek=RecordNumber |
Seeks the record specified by the RecordNumber
variable from the beginning of output file before copying. |
skip=SkipInputBlocks |
Skips the specified SkipInputBlocks value of
input blocks before starting to copy. |
span=yes|no |
Allows spanning across devices if specified yes and works as default if specified as no.
See Spanning Across Devices, for more information.. |
Exit Status
This command returns the following exit values:
0 |
The input file was copied successfully. |
>0 |
An error occurred. |
Examples
- To convert an ASCII text file to EBCDIC, type:
dd if=text.ascii of=text.ebcdic conv=ebcdic
This command converts
the text.ascii file to EBCDIC representation,
storing the EBCDIC version in the text.ebcdic
file.
Note
When you specify the conv=ebcdic parameter, the dd command converts
the ASCII ^ (circumflex) character to an unused EBCDIC character (9A hexadecimal),
and the ASCII ~ (tilde) to the EBCDIC ^ (NOT symbol).
- To convert the variable-length record ASCII file /etc/passwd to a file of 132-byte fixed-length EBCDIC records, type:
dd if=/etc/passwd cbs=132 conv=ebcdic of=/tmp/passwd.ebcdic
- To convert the 132-byte-per-record EBCDIC file to variable-length ASCII
lines in lowercase, type:
dd if=/tmp/passwd.ebcdic cbs=132 conv=ascii of=/tmp/passwd.ascii
- To convert the variable-length record ASCII file /etc/passwd to a file of 132-byte fixed-length records in the IBM version of EBCDIC,
type:
dd if=/etc/passwd cbs=132 conv=ibm of=/tmp/passwd.ibm
- To copy blocks from a tape with 1KB blocks to another tape using 2KB
blocks, type:
dd if=/dev/rmt0 ibs=1024 obs=2048 of=/dev/rmt1
- To use the dd command as a filter, type:
ls -l | dd conv=ucase
This command displays a long listing of the current directory in uppercase.
Note
The performance of the
dd
command and
cpio command to the 9348 Magnetic Tape Unit
Model 12 can be improved by changing the default block size. To change the
block size, use the
chdev
command in the following way:
chdev -l Device_name -a block_size=32k
- To perform efficient transfers to 3.5-inch 1.4MB diskette using 36 blocks
of 512 bytes, type:
dd if=Filename of=/dev/rfd0 bs=36b conv=sync
This command
writes the value of the Filename parameter to the
diskette device a cylinder at a time. The conv=sync is required when reading from disk and when the file size is not a
multiple of the diskette block size. Do not try this if the input to the dd command is a pipe instead of a file, it will pad most
of the input with nulls instead of just the last block.
- To copy blocks from a input file with block size set to 720b blocks into
a 1.44MB size diskette type:
dd if=testfile of=/dev/fd0 bs=720b conv=sync
Note
If the input file is larger than the physical
size of the output device then dd will prompt you for another device.
- To copy blocks from a input file with block size set to 32k blocks to
a tape type:
dd if=inputfile of=/dev/rmt0 bs=32k conv=sync
- To copy blocks of data from tape to a file in the current directory with
block size set to 32k blocks type as follows:
dd if=/dev/rmt0 of=outfile bs=32k conv=sync
- To copy blocks from an input file with block size set to 720b, onto a
1.44MB size diskette, enter:
dd if=testfile of=/dev/fd0 bs=720b conv=sync span=yes
Note
If the input file is larger than the physical size of
the output device, then dd will prompt you for another
device.
- To copy blocks from an input file with block size set to 32k, to a tape,
enter:
dd if=inputfile of=/dev/rmt0 bs=32k conv=sync span=yes
- To copy blocks of data from tape with block size set to 32k, to a file
in the current directory, enter:
dd if=dev/rmt0 of=outfile bs=32k conv=sync span=yes
Files
/usr/bin/dd |
Contains the dd command. |
Related Information
The backup, cp, cpio, tar, tr command.
The rmt special file.
The Backup Overview for System Management
in AIX 5L Version 5.2 System Management Concepts: Operating System and Devices provides information on using backups and using memory
devices.
The Files Overview in AIX 5L Version 5.2 System User's Guide: Operating System and Devices provides
information on working with files.
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