Linux Management disk Command: DD command detailed

Linux/unix DD command is a very good management disk command

DD Command function: A block of a specified size copies a file and, at the same time as a copy, a specified conversion, playing the role of an initialization disk

1. Introduction to the Order

Main options for DD:

The place where the number is specified is multiplied by the corresponding number at the end of the following character:

b=512, C=1, k=1024, w=2, Xm=number m

If=file

Enter a filename, which defaults to standard input.

Of=file

The output file name, which defaults to standard output.

Ibs=bytes

Read bytes bytes at a time (that is, a block size of bytes bytes).

Obs=bytes

Write bytes bytes at a time (that is, a block size of bytes bytes).

Bs=bytes

At the same time, the size of the read-write block is bytes, which can replace IBS and OBS.

Cbs=bytes

Converts bytes bytes at a time, that is, the size of the conversion buffer.

Skip=blocks

Skips blocks blocks from the start of the input file before copying.

Seek=blocks

Skips blocks blocks from the start of the output file before copying. (usually only works if the output file is disk or tape.)

Count=blocks

Copies only blocks blocks, and the block size equals the number of bytes specified by IBS.

Conv=conversion[,conversion ...]

Converts a file with the specified parameters.

Conversion parameters:

ASCII conversion EBCDIC to ASCII.

EBCDIC converts ASCII to EBCDIC.

IBM converts ASCII to alternate EBCDIC.

Block converts each line into a record length of CBS, with less than a space filled in.

Unblock the length of each row is CBS, and the less part is filled with spaces.

LCase converts uppercase characters to lowercase characters.

UCase converts lowercase characters to uppercase characters.

Swab exchange input for each pair of bytes.

NoError does not stop when an error occurs.

Notrunc does not truncate the output file. Do not take the entire instruction

Sync fills each input block to IBS bytes, and the insufficient part is padded with empty (NUL) characters.

DD Application Example

1. Backup local/dev/hdb whole disk to/DEV/HDD

DD if=/dev/hdb f=/dev/hdd hdb–> memory-> HDD

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2./dev/hdb The overall data back to the specified path image file

DD if=/dev/hdb f=/root/image hdb–> memory-> image file

3. Restore the backup file to the specified disk

DD if=/root/image f=/dev/hdb image file –> memory-> HDB

4. Backup/DEV/HDB overall data, and use the Gzip tool to compress, save to the specified path

DD If=/dev/hdb | Gzip>/root/image.gz

5. Restore the compressed backup file to the specified disk

gzip-dc/root/image.gz | DD F=/dev/hdb

6.512-byte MBR master boot record information to the specified file at the beginning of the backup disk

DD If=/dev/hda f=/root/image count=1 bs=512 read 512 bytes from hard disk 0 cylinder 0 Track 1 sectors by default

Count=1 refers to copying only one block, and the bs=512 block size is 512 bytes.

Recovery: DD If=/root/image F=/dev/hda

7. Backup floppy disk

DD if=/dev/fd0 f=disk.img count=1 bs=1440k (i.e. block size 1.44M)

8. Copy the contents of the memory to the hard disk

DD If=/dev/mem F=/root/mem.bin bs=1024 (Specify block size of 1k)

9. Copy the contents of the CD to the specified folder and save it as a cd.iso image file

DD If=/dev/cdrom (HDC) Of=/root/cd.iso

10. Increase Swap partition file size

First step: Create a file size of 256M:

DD If=/dev/zero of=/swapfile bs=1024 count=262144

Step Two: Turn this file into swap swap partition file:

Mkswap/swapfile

Step three: Enable this swap partition file:

Swapon/swapfile

Fourth step: Edit the/etc/fstab file so that the swap partition file is automatically loaded at each boot:

/swapfile Swap default 0 0

11. Destroying disk data

DD If=/dev/urandom F=/DEV/HDA1

Note: Using random data to populate a hard disk can be used to destroy data on some necessary occasions.

12. Test hard disk read and write speed

DD If=/dev/zero bs=1024 count=1000000 f=/root/1gb.file Write speed

DD If=/root/1gb.file bs=64k | DD F=/dev/null Read speed

The read and write speed of the hard drive can be computed by the command execution time of the above two commands.

13. Determine the best block blocks size of the hard disk: Initialize the hard drive

DD If=/dev/zero bs=1024 count=1000000 f=/root/1gb.file initialize the hard drive

DD If=/dev/zero bs=2048 count=500000 f=/root/1gb.file initialize the hard drive

DD If=/dev/zero bs=4096 count=250000 f=/root/1gb.file initialize the hard drive

DD If=/dev/zero bs=8192 count=125000 f=/root/1gb.file initialize the hard drive

By comparing the command execution times shown in the above command output, you can determine the optimal block size for the system.

14. Repair hard disk: Self-initialized hard drive

DD IF=/DEV/SDA F=/DEV/SDA SCSI hard drive or DD If=/dev/hda F=/dev/hda IDE

When the hard disk is not used for a long time (more than one year), the magnetic flux point electromagnetic dot phenomenon is generated on the disk, which is difficult when the head is read to these areas and may cause I/O errors. When this situation affects the first sector of the hard disk, the MBR (Master bootstrapper 446, hard disk partition table 64, hard disk Identification 2 bytes), the hard disk may be scrapped. The commands above may bring the data back to the dead. And the process is safe and efficient.