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11.2 dd: Convert and copy a file

dd copies a file (from standard input to standard output, by default) with a changeable I/O block size, while optionally performing conversions on it. Synopses:

dd [operand]…
dd option

The only options are --help and --version. See Common options. dd accepts the following operands, whose syntax was inspired by the DD (data definition) statement of OS/360 JCL.

if=file

Read from file instead of standard input.

of=file

Write to file instead of standard output. Unless ‘conv=notrunc’ is given, dd truncates file to zero bytes (or the size specified with ‘seek=’).

ibs=bytes

Set the input block size to bytes. This makes dd read bytes per block. The default is 512 bytes.

obs=bytes

Set the output block size to bytes. This makes dd write bytes per block. The default is 512 bytes.

bs=bytes

Set both input and output block sizes to bytes. This makes dd read and write bytes per block, overriding any ‘ibs’ and ‘obs’ settings. In addition, if no data-transforming conv option is specified, input is copied to the output as soon as it’s read, even if it is smaller than the block size.

cbs=bytes

Set the conversion block size to bytes. When converting variable-length records to fixed-length ones (conv=block) or the reverse (conv=unblock), use bytes as the fixed record length.

skip=n

Skip nibs’-byte blocks in the input file before copying. If ‘iflag=skip_bytes’ is specified, n is interpreted as a byte count rather than a block count.

seek=n

Skip nobs’-byte blocks in the output file before copying. if ‘oflag=seek_bytes’ is specified, n is interpreted as a byte count rather than a block count.

count=n

Copy nibs’-byte blocks from the input file, instead of everything until the end of the file. if ‘iflag=count_bytes’ is specified, n is interpreted as a byte count rather than a block count. Note if the input may return short reads as could be the case when reading from a pipe for example, ‘iflag=fullblock’ will ensure that ‘count=’ corresponds to complete input blocks rather than the traditional POSIX specified behavior of counting input read operations.

status=level

Transfer information is normally output to stderr upon receipt of the ‘INFO’ signal or when dd exits. Specifying level will adjust the amount of information printed, with the last level specified taking precedence.

none

Do not print any informational or warning messages to stderr. Error messages are output as normal.

noxfer

Do not print the final transfer rate and volume statistics that normally make up the last status line.

progress

Print the transfer rate and volume statistics on stderr, when processing each input block. Statistics are output on a single line at most once every second, but updates can be delayed when waiting on I/O.

conv=conversion[,conversion]…

Convert the file as specified by the conversion argument(s). (No spaces around any comma(s).)

Conversions:

ascii

Convert EBCDIC to ASCII, using the conversion table specified by POSIX. This provides a 1:1 translation for all 256 bytes. This option implies ‘conv=unblock’; input is converted to ASCII before trailing spaces are deleted.

ebcdic

Convert ASCII to EBCDIC. This is the inverse of the ‘ascii’ conversion. This option implies ‘conv=block’; trailing spaces are added before being converted to EBCDIC.

ibm

This acts like ‘conv=ebcdic’, except it uses the alternate conversion table specified by POSIX. This is not a 1:1 translation, but reflects common historical practice for ‘~’, ‘[’, and ‘]’.

The ‘ascii’, ‘ebcdic’, and ‘ibm’ conversions are mutually exclusive. If you use any of these options, you should also use the ‘cbs=’ option.

block

For each line in the input, output ‘cbs’ bytes, replacing the input newline with a space and padding with spaces as necessary.

unblock

Remove any trailing spaces in each ‘cbs’-sized input block, and append a newline.

The ‘block’ and ‘unblock’ conversions are mutually exclusive.

lcase

Change uppercase letters to lowercase.

ucase

Change lowercase letters to uppercase.

The ‘lcase’ and ‘ucase’ conversions are mutually exclusive.

sparse

Try to seek rather than write NUL output blocks. On a file system that supports sparse files, this will create sparse output when extending the output file. Be careful when using this option in conjunction with ‘conv=notrunc’ or ‘oflag=append’. With ‘conv=notrunc’, existing data in the output file corresponding to NUL blocks from the input, will be untouched. With ‘oflag=append’ the seeks performed will be ineffective. Similarly, when the output is a device rather than a file, NUL input blocks are not copied, and therefore this option is most useful with virtual or pre zeroed devices.

swab

Swap every pair of input bytes. GNU dd, unlike others, works when an odd number of bytes are read—the last byte is simply copied (since there is nothing to swap it with).

sync

Pad every input block to size of ‘ibs’ with trailing zero bytes. When used with ‘block’ or ‘unblock’, pad with spaces instead of zero bytes.

The following “conversions” are really file flags and don’t affect internal processing:

excl

Fail if the output file already exists; dd must create the output file itself.

nocreat

Do not create the output file; the output file must already exist.

The ‘excl’ and ‘nocreat’ conversions are mutually exclusive.

notrunc

Do not truncate the output file.

noerror

Continue after read errors.

fdatasync

Synchronize output data just before finishing. This forces a physical write of output data.

fsync

Synchronize output data and metadata just before finishing. This forces a physical write of output data and metadata.

iflag=flag[,flag]…

Access the input file using the flags specified by the flag argument(s). (No spaces around any comma(s).)

oflag=flag[,flag]…

Access the output file using the flags specified by the flag argument(s). (No spaces around any comma(s).)

Here are the flags. Not every flag is supported on every operating system.

append

Write in append mode, so that even if some other process is writing to this file, every dd write will append to the current contents of the file. This flag makes sense only for output. If you combine this flag with the ‘of=file’ operand, you should also specify ‘conv=notrunc’ unless you want the output file to be truncated before being appended to.

cio

Use concurrent I/O mode for data. This mode performs direct I/O and drops the POSIX requirement to serialize all I/O to the same file. A file cannot be opened in CIO mode and with a standard open at the same time.

direct

Use direct I/O for data, avoiding the buffer cache. Note that the kernel may impose restrictions on read or write buffer sizes. For example, with an ext4 destination file system and a Linux-based kernel, using ‘oflag=direct’ will cause writes to fail with EINVAL if the output buffer size is not a multiple of 512.

directory

Fail unless the file is a directory. Most operating systems do not allow I/O to a directory, so this flag has limited utility.

dsync

Use synchronized I/O for data. For the output file, this forces a physical write of output data on each write. For the input file, this flag can matter when reading from a remote file that has been written to synchronously by some other process. Metadata (e.g., last-access and last-modified time) is not necessarily synchronized.

sync

Use synchronized I/O for both data and metadata.

nocache

Request to discard the system data cache for a file. When count=0 all cached data for the file is specified, otherwise the cache is dropped for the processed portion of the file. Also when count=0, failure to discard the cache is diagnosed and reflected in the exit status.

Note data that is not already persisted to storage will not be discarded from cache, so note the use of the “sync” options in the examples below, which are used to maximize the effectiveness of the ‘nocache’ flag.

Here are some usage examples:

# Advise to drop cache for whole file
dd if=ifile iflag=nocache count=0

# Ensure drop cache for the whole file
dd of=ofile oflag=nocache conv=notrunc,fdatasync count=0

# Drop cache for part of file
dd if=ifile iflag=nocache skip=10 count=10 of=/dev/null

# Stream data using just the read-ahead cache.
# See also the ‘direct’ flag.
dd if=ifile of=ofile iflag=nocache oflag=nocache,sync
nonblock

Use non-blocking I/O.

noatime

Do not update the file’s access time. Some older file systems silently ignore this flag, so it is a good idea to test it on your files before relying on it.

noctty

Do not assign the file to be a controlling terminal for dd. This has no effect when the file is not a terminal. On many hosts (e.g., GNU/Linux hosts), this option has no effect at all.

nofollow

Do not follow symbolic links.

nolinks

Fail if the file has multiple hard links.

binary

Use binary I/O. This option has an effect only on nonstandard platforms that distinguish binary from text I/O.

text

Use text I/O. Like ‘binary’, this option has no effect on standard platforms.

fullblock

Accumulate full blocks from input. The read system call may return early if a full block is not available. When that happens, continue calling read to fill the remainder of the block. This flag can be used only with iflag. This flag is useful with pipes for example as they may return short reads. In that case, this flag is needed to ensure that a ‘count=’ argument is interpreted as a block count rather than a count of read operations.

count_bytes

Interpret the ‘count=’ operand as a byte count, rather than a block count, which allows specifying a length that is not a multiple of the I/O block size. This flag can be used only with iflag.

skip_bytes

Interpret the ‘skip=’ operand as a byte count, rather than a block count, which allows specifying an offset that is not a multiple of the I/O block size. This flag can be used only with iflag.

seek_bytes

Interpret the ‘seek=’ operand as a byte count, rather than a block count, which allows specifying an offset that is not a multiple of the I/O block size. This flag can be used only with oflag.

These flags are not supported on all systems, and ‘dd’ rejects attempts to use them when they are not supported. When reading from standard input or writing to standard output, the ‘nofollow’ and ‘noctty’ flags should not be specified, and the other flags (e.g., ‘nonblock’) can affect how other processes behave with the affected file descriptors, even after dd exits.

The numeric-valued strings above (n and bytes) can be followed by a multiplier: ‘b’=512, ‘c’=1, ‘w’=2, ‘xm’=m, or any of the standard block size suffixes like ‘k’=1024 (see Block size).

Any block size you specify via ‘bs=’, ‘ibs=’, ‘obs=’, ‘cbs=’ should not be too large—values larger than a few megabytes are generally wasteful or (as in the gigabyte..exabyte case) downright counterproductive or error-inducing.

To process data that is at an offset or size that is not a multiple of the I/O block size, you can use the ‘skip_bytes’, ‘seek_bytes’ and ‘count_bytes’ flags. Alternatively the traditional method of separate dd invocations can be used. For example, the following shell commands copy data in 512 KiB blocks between a disk and a tape, but do not save or restore a 4 KiB label at the start of the disk:

disk=/dev/rdsk/c0t1d0s2
tape=/dev/rmt/0

# Copy all but the label from disk to tape.
(dd bs=4k skip=1 count=0 && dd bs=512k) <$disk >$tape

# Copy from tape back to disk, but leave the disk label alone.
(dd bs=4k seek=1 count=0 && dd bs=512k) <$tape >$disk

For failing disks, other tools come with a great variety of extra functionality to ease the saving of as much data as possible before the disk finally dies, e.g. GNU ddrescue. However, in some cases such a tool is not available or the administrator feels more comfortable with the handling of dd. As a simple rescue method, call dd as shown in the following example: the options ‘conv=noerror,sync’ are used to continue after read errors and to pad out bad reads with NULs, while ‘iflag=fullblock’ caters for short reads (which traditionally never occur on disk based devices):

# Rescue data from an (unmounted!) partition of a failing disk.
dd conv=noerror,sync iflag=fullblock </dev/sda1 > /mnt/rescue.img

Sending an ‘INFO’ signal (or ‘USR1’ signal where that is unavailable) to a running dd process makes it print I/O statistics to standard error and then resume copying. In the example below, dd is run in the background to copy 5GB of data. The kill command makes it output intermediate I/O statistics, and when dd completes normally or is killed by the SIGINT signal, it outputs the final statistics.

# Ignore the signal so we never inadvertently terminate the dd child.
# Note this is not needed when SIGINFO is available.
trap '' USR1

# Run dd with the fullblock iflag to avoid short reads
# which can be triggered by reception of signals.
dd iflag=fullblock if=/dev/zero of=/dev/null count=5000000 bs=1000 & pid=$!

# Output stats every second.
while kill -s USR1 $pid 2>/dev/null; do sleep 1; done

The above script will output in the following format:

3441325+0 records in
3441325+0 records out
3441325000 bytes (3.4 GB, 3.2 GiB) copied, 1.00036 s, 3.4 GB/s
5000000+0 records in
5000000+0 records out
5000000000 bytes (5.0 GB, 4.7 GiB) copied, 1.44433 s, 3.5 GB/s

The ‘status=progress’ option periodically updates the last line of the transfer statistics above.

On systems lacking the ‘INFO’ signal dd responds to the ‘USR1’ signal instead, unless the POSIXLY_CORRECT environment variable is set.

An exit status of zero indicates success, and a nonzero value indicates failure.


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