Purpose
Converts and copies a file.
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.
Note: - 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
occurrence 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
| Item |
Description |
|---|
| 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:
| Item |
Description |
|---|
| 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
| Item |
Description |
|---|
| /usr/bin/dd |
Contains the dd command. |