dd command
dd command to copy the contents of the file and the original file and the format conversion process. dd command function is very powerful, for some of the more underlying problem, use the dd command can often get unexpected results. Or more with the back up dd nude equipment. But not recommended, if you need to backup oracle raw device, you can use rman backup, or use third-party backup software, use dd, then it is not good to engage in management.
When in need can use dd to physical disk operation, if it is, then the file system or the use of tar, backup, easy cpio commands. Further, when using the disk dd operation, preferably the block device file.
grammar
dd (option)
Options
bs = <number of bytes> of: IBS (input) and OBS (output) set to the number of bytes specified;
CBS = <number of bytes>: conversion, a time conversion specified number of bytes;
CONV = < keywords>: file conversion specified manner;
COUNT = <block number>: only a specified number of read blocks;
IBS = <number of bytes>: read each time the number of bytes;
OBS = <bytes >: number of bytes per output;
of = <file>: output to a file;
Seek = <block number>: output the outset, to skip a specified number of blocks;
skip = <block number>: a read at the beginning, skipping the specified number of blocks;
- help: help;
--version: show the version information.
Examples
[root@localhost text]# dd if=/dev/zero of=sun.txt bs=1M count=1
1+0 records in
1+0 records out
1048576 bytes (1.0 MB) copied, 0.006107 seconds, 172 MB/s
[root@localhost text]# du -sh sun.txt
1.1M sun.txt
This command creates a file sun.txt 1M size, which explain the parameters:
- if the representative of the input file. If you do not specify if, by default it will read input from stdin.
- of the representative of the output file. If you do not specify of, the default will be the default stdout output.
- bs representative of block size in units of bytes.
- count represents the number of blocks to be copied.
- / dev / zero is a character device, it will continue to return a zero byte (\ 0).
Unit table block size may be used
| Unit size | Code |
| Byte (1B) | c |
| Byte (2B) | w |
| Block (512B) | b |
| Kilobytes (1024B) | k |
| Megabyte (1024KB) | M |
| Gigabytes (1024MB) | G |
As can be seen above command dd command to test the operation speed of the memory:
1048576 bytes (1.0 MB) copied, 0.006107 seconds, 172 MB/s