DD in Wikipedia
dd is a command-line utility for Unix and Unix-like operating systems whose primary purpose is to convert and copy files.[1]
On Unix, device drivers for hardware (such as hard disk drives) and special device files (such as /dev/zero and /dev/random) appear in the file system just like normal files; ddcan also read and/or write from/to these files, provided that function is implemented in their respective driver. As a result, dd can be used for tasks such as backing up the boot sector of a hard drive, and obtaining a fixed amount of random data. The dd program can also perform conversions on the data as it is copied, including byte order swapping and conversion to and from the ASCII and EBCDIC text encodings.[2]
The name dd is an allusion to the DD statement found in IBM's Job Control Language (JCL),[3][4] in which the initials stand for "Data Description".[5] The command's syntax resembles the JCL statement more than it does other Unix commands, so the syntax may have been a joke.[3]
Originally intended to convert between ASCII and EBCDIC, dd first appeared in Version 5 Unix.[6] The dd command is specified by IEEE Std 1003.1-2008, which is part of theSingle UNIX Specification.
DD help
[root@db1 ~]# dd --help
Usage: dd [OPERAND]...
or: dd OPTION
Copy a file, converting and formatting according to the operands.
bs=BYTES read and write BYTES bytes at a time (also see ibs=,obs=)
设置读入/输出的块大小为bytes个字节
ibs=BYTES read BYTES bytes at a time (default: 512)
一次读入bytes个字节，即指定一个块大小为bytes个字节
obs=BYTES write BYTES bytes at a time (default: 512)
一次输出bytes个字节，即指定一个块大小为bytes个字节
[root@db1 cdb1]# dd if=big_ohs.dbf ibs=1024 obs=4096 of=big_robin 8+0 records in 2+0 records out 8192 bytes (8.2 kB) copied, 0.000521054 s, 15.7 MB/s [root@db1 cdb1]if=FILE read from FILE instead of stdin
输入文件名，默认是标准输入
[root@db1 cdb1]# dd of=block1 <<abc > abcedfg > quit > hello > abc 0+1 records in 0+1 records out 19 bytes (19 B) copied, 0.000565558 s, 33.6 kB/s [root@db1 cdb1]# ls -l block1 -rw-r--r-- 1 root root 19 Jun 15 11:29 block1 [root@db1 cdb1]# cat block1 abcedfg quit hello [root@db1 cdb1]#of=FILE write to FILE instead of stdout
输出文件名，默认是标准输出
[root@db1 ~]# cat ohsdba.log Hello,this is ohsdba welcome [root@db1 ~]# dd if=ohsdba.log Hello,this is ohsdba welcome 0+1 records in 0+1 records out 29 bytes (29 B) copied, 0.000459775 s, 63.1 kB/s [root@db1 ~]#cbs=BYTES convert BYTES bytes at a time
一次转化bytes个字节，也可以说是指定转换缓冲区大小
count=BLOCKS copy only BLOCKS input blocks
要复制blocks的个块，块大小等于ibs指定的字节数
skip=BLOCKS skip BLOCKS ibs-sized blocks at start of input
从输入文件头部开始跳过blocks个块
seek=BLOCKS skip BLOCKS obs-sized blocks at start of output
从输出文件头部开始跳过blocks个块
iflag=FLAGS read as per the comma separated symbol list
oflag=FLAGS write as per the comma separated symbol list
status=WHICH WHICH info to suppress outputting to stderr;
'noxfer' suppresses transfer stats, 'none' suppresses all
conv=CONVS convert the file as per the comma separated symbol list
BLOCKS and BYTES may be followed by the following multiplicative suffixes:
c =1, w =2, b =512, kB =1000, K =1024, MB =1000*1000, M =1024*1024, xM =M
GB =1000*1000*1000, G =1024*1024*1024, and so on for T, P, E, Z, Y.
Each CONV symbol may be:
ascii from EBCDIC to ASCII
转换EBCDIC为ASCII
ebcdic from ASCII to EBCDIC
转换ASCII为EBCDIC
ibm from ASCII to alternate EBCDIC
转换ASCII为Alternate EBCDIC
block pad newline-terminated records with spaces to cbs-size
把每一行转换为长度为cbs，不足部分用空格填充
unblock replace trailing spaces in cbs-size records with newline
使每一行的长度都为cbs，不足部分用空格填充
lcase change upper case to lower case
把大写字符转换为小写字符
nocreat do not create the output file
不创建输出文件，输出文件必须已存在
[root@db1 cdb1]# dd if=big1.dbf of=abc conv=nocreat dd: opening `abc': No such file or directory [root@db1 cdb1]# touch abc [root@db1 cdb1]# dd if=big1.dbf of=abc conv=nocreat 40976+0 records in 40976+0 records out 20979712 bytes (21 MB) copied, 0.223515 s, 93.9 MB/s [root@db1 cdb1]# ls -l abc -rw-r--r-- 1 root root 20979712 Jun 15 12:43 abc [root@db1 cdb1]#excl fail if the output file already exists
如果输出文件存在停止
[root@db1 cdb1]# dd if=big1.dbf of=block1 conv=excl dd: opening `block1': File exists [root@db1 cdb1]#notrunc do not truncate the output file
不截断输出文件，这个很重要，比如备份oracle数据库头部，恢复时这个一定要加上，要不把整个文件都毁了
ucase change lower case to upper case
把小写字符转换为大写字符
sparse try to seek rather than write the output for NUL input blocks
swab swap every pair of input bytes
交换输入的每对字节
noerror continue after read errors
如果出错不停止
sync pad every input block with NULs to ibs-size; when used
with block or unblock, pad with spaces rather than NULs
将每个输入块填充到ibs个字节，不足部分用空（NUL）字符补齐
fdatasync physically write output file data before finishing
相当于不经过缓存直接写数据
fsync likewise, but also write metadata
和fdatasync类似，包含fdatasync功能再加上元数据
https://www.gnu.org/software/coreutils/manual/html_node/dd-invocation.html
[root@db1 cdb1]# dd if=system01.dbf of=system01.dbf.bak oflag=direct 1638416+0 records in 1638416+0 records out 838868992 bytes (839 MB) copied, 179.386 s, 4.7 MB/s [root@db1 cdb1]# dd if=system01.dbf of=system01.dbf.bak conv=fsync 1638416+0 records in 1638416+0 records out 838868992 bytes (839 MB) copied, 6.07085 s, 138 MB/s [root@db1 cdb1]# dd if=system01.dbf of=system01.dbf.bak conv=fdatasync 1638416+0 records in 1638416+0 records out 838868992 bytes (839 MB) copied, 6.55396 s, 128 MB/s [root@db1 cdb1]#
Each FLAG symbol may be:
append append mode (makes sense only for output; conv=notrunc suggested)
direct use direct I/O for data
directory fail unless a directory
dsync use synchronized I/O for data
sync likewise, but also for metadata
fullblock accumulate full blocks of input (iflag only)
nonblock use non-blocking I/O
noatime do not update access time
noctty do not assign controlling terminal from file
nofollow do not follow symlinks
Sending a USR1 signal to a running `dd' process makes it
print I/O statistics to standard error and then resume copying.
$ dd if=/dev/zero of=/dev/null& pid=$!
$ kill -USR1 $pid; sleep 1; kill $pid
18335302+0 records in
18335302+0 records out
9387674624 bytes (9.4 GB) copied, 34.6279 seconds, 271 MB/s
Options are:
--help display this help and exit
--version output version information and exit
Report dd bugs to bug-coreutils@gnu.org
GNU coreutils home page: <http://www.gnu.org/software/coreutils/>
General help using GNU software: <http://www.gnu.org/gethelp/>
For complete documentation, run: info coreutils 'dd invocation'
[root@db1 ~]#
dd能干啥？
dd可以备份、还原磁盘，复制文件，Linux上扩大交换分区，销毁数据等
备份、还原磁盘数据
dd if=/dev/sda of=/dev/sdb dd if=/dev/sda of=/root/image dd if=/root/image of=/dev/sda dd if=/dev/sda|gzip > image.gz gzip -dc /root/image.gz | dd of=/dev/sda dd if=/dev/sda of=/root/image count=1 bs=512 dd if=/root/image of=/dev/sda dd if=/dev/mem of=/root/mem bs=8192
在Linux上扩大交换分区
dd if=/dev/zero of=/var/swapfile bs=1M count=102400 mkswap /var/swapfile swapon /var/swapfile /var/swapfile swap swap default 0 0销毁磁盘/文件数据
dd if=/dev/zero of=/dev/sda3 bs=4k dd if=/dev/urandom of=/dev/sda3 bs=4k [root@db1 ~]# cat a this is ohsdba [root@db1 ~]# dd if=/dev/urandom of=a count=1 1+0 records in 1+0 records out 512 bytes (512 B) copied, 0.000669252 s, 765 kB/s [root@db1 ~]# [root@db1 ~]# strings -a a K7Mk @Ha\+ [root@db1 ~]#测试硬盘读写速度及最合适的块的大小
dd if=/dev/zero bs=1M count=1000 of=/root/1G dd if=/root/1G bs=8k | dd of=/dev/null dd if=/dev/zero bs=1024 count=1000000 of=/root/1G dd if=/dev/zero bs=2048 count=500000 of=/root/1G dd if=/dev/zero bs=4096 count=250000 of=/root/1G dd if=/dev/zero bs=8192 count=125000 of=/root/1G修复硬盘
dd if=/dev/sda of=/dev/sda bs=4k制作USB启动盘
dd if=rhel6.4.iso of=/dev/sdb bs=1M
Reference
ftp://alpha.gnu.org/gnu/coreutils/
http://forensicswiki.org/wiki/Dcfldd
https://sourceforge.net/projects/dcfldd/
http://www.chrysocome.net/dd
https://en.wikipedia.org/wiki/Dd_%28Unix%29https://www.gnu.org/software/coreutils/manual/html_node/dd-invocation.html
https://sourceforge.net/projects/parallelhash/
https://sourceforge.net/projects/dc3dd/
http://pkgs.repoforge.org/
