dd — Copy data block by block: make disk images, write ISO files to USB, create files of a fixed size, wipe or test disks.

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Wat het doet

dd if=INPUT of=OUTPUT bs=SIZE count=N reads blocks from the input and writes them to the output. It works on files and on raw devices (whole disks), so it can copy or destroy anything — people call it "disk destroyer" for a reason. Always check of=.

Options are written as name=value, not with dashes.

Zo schrijf je het

dd [OPERAND]...

Opties 15

Meest gebruikt

if=FILE: Read from this file or device (default stdin).
of=FILE: Write to this file or device (default stdout).
bs=BYTES: Block size for read and write (like 1M, 4M).
count=N: Copy only N blocks.
ibs=BYTES: Read block size.
obs=BYTES: Write block size.
cbs=BYTES: Conversion block size.
skip=N, iseek=N: Skip N input blocks.
seek=N, oseek=N: Skip N output blocks.
conv=LIST: Conversions: notrunc, noerror, sync, sparse, fsync, fdatasync, excl, nocreat, ucase, lcase …
iflag=LIST: Input flags: fullblock, direct, nocache, skip_bytes, count_bytes …
oflag=LIST: Output flags: direct, sync, dsync, append, nocache, seek_bytes …
status=LEVEL: none, noxfer or progress (show progress).
--help: Show help.
--version: Show the version.

Voorbeelden 11

$ als gewone gebruiker# als root (of met sudo)

1Create a file of 1 GiB with zeros
```
#dd if=/dev/zero of=/swapfile bs=1M count=1024
```
Good for swap files (no holes).
2Write an ISO to a USB stick
```
#dd if=rhel-10.0-x86_64-dvd.iso of=/dev/sdc bs=4M status=progress oflag=sync
```
/dev/sdc is the whole stick (no number). Check with lsblk first!
3Back up a partition to an image
```
#dd if=/dev/vdb1 of=/backup/vdb1.img bs=4M status=progress
```
Unmount it first for a consistent copy.
4Back up the MBR (first 512 bytes)
```
#dd if=/dev/vda of=/root/mbr.bin bs=512 count=1
```
Boot code and MBR table.
5Wipe the start of a disk
```
#dd if=/dev/zero of=/dev/vdb bs=1M count=10
```
Removes partition table and signatures at the start (wipefs is cleaner).

6Test write speed

$dd if=/dev/zero of=/var/tmp/test bs=1M count=1024 oflag=direct status=progress

direct skips the cache; delete the file after.

7Test read speed

$dd if=/var/tmp/test of=/dev/null bs=1M iflag=direct

Real disk reads.

8A busy CPU process for labs
```
$dd if=/dev/zero of=/dev/null &
```
Practice top, nice and kill.
9Random data file
```
$dd if=/dev/urandom of=random.bin bs=1M count=10
```
Test files that do not compress.
10Copy a failing disk and go on after errors
```
#dd if=/dev/sdb of=/backup/sdb.img bs=64K conv=noerror,sync status=progress
```
Bad blocks become zeros (ddrescue is better for this).
11Restore only the boot code
```
#dd if=/root/mbr.bin of=/dev/vda bs=446 count=1
```
Restores only the boot code, not the partition table (446 bytes).

Let op

of= on the wrong disk destroys it at once. There is no question and no undo.
Without status=progress dd prints nothing until it is done; send kill -USR1 PID for a status.

Exitcodes

0 :: Done. 1 :: Error.

`dd`

Wat het doet

Zo schrijf je het

Opties 15

Voorbeelden 11

1Create a file of 1 GiB with zeros

2Write an ISO to a USB stick

3Back up a partition to an image

4Back up the MBR (first 512 bytes)

5Wipe the start of a disk

6Test write speed

7Test read speed

8A busy CPU process for labs

9Random data file

10Copy a failing disk and go on after errors

11Restore only the boot code

Let op

Exitcodes

Volledige handleiding

dd

Wat het doet

Zo schrijf je het

Opties 15

Voorbeelden 11

1Create a file of 1 GiB with zeros

2Write an ISO to a USB stick

3Back up a partition to an image

4Back up the MBR (first 512 bytes)

5Wipe the start of a disk

6Test write speed

7Test read speed

8A busy CPU process for labs

9Random data file

10Copy a failing disk and go on after errors

11Restore only the boot code

Let op

Exitcodes

Verwante commando's

Gebruikt in deze lessen

Gebruikt in deze labs

Volledige handleiding

`dd`