dd is a common Unix program whose primary purpose is the low-level copying and conversion of raw data. You can backup whole hard drives, create a large file filled with only zeros, create and modify image files at specific points, and even do conversions to upper case.
To display dd‘s help simply enter:
| erik@debian:~$dd –help |
Alright, lets get to the juicy stuff.
1. Make an ISO of a your favourite CD just for backing up purposes with dd:
| dd if=/dev/cdrom of=/home/erik/myCD.iso bs=2048 conv=sync |
Breaking down the commands:
- if is “input file”, so in this case our cdrom drive at /dev/cdrom
- of is “output file”, in this case myCD.iso
- bs is “block size”, in this case 2048 bytes per block
- conv is for conversion, in this case we are using “sync” which tells DD to execute synchronized input and output, this is needed for the CD-ROM as we want to read a whole block to ensure no data loss occurs.
2. Duplicate one hard disk partition to another hard disk with dd:
| dd if=/dev/sda1 of=/dev/sdb1 bs=4096 conv=noerror |
In this case everything is the same as example 1 but our conversion methods states that noerror should be executed, this tells DD to continue after read errors.
3. Fill a file with 1MB of random bytes with dd:
| erik@debian:~$dd if=/dev/urandom bs=1024 count=1000 of=fun.bin 1000+0 records in 1000+0 records out 1024000 bytes (1.0 MB) copied, 0.198349 s, 5.2 MB/s |
This time I stated that our block size is 1024 bytes, and we are going to make 1000 of them sequentially. I also used the built-in kernel device urandom which provides random bytes.
4. Skip first 128K of input file then write remaining with dd:
| dd if=/home/erik/fun.bin skip=128k bs=1 of=/home/erik/fun2.bin |
The skip command tells DD to move passed the (in this case) 128k of data in fun.bin then write the rest to fun2.bin. This can be handy if you have a large file that needs to be written across more than one partition. For instance, if you had 3 partitions each 128k. You wouldn’t want to write the same 128k to each partition, you would want to write the first 128k to partition 1, then from 128k-256k of the file to partition 2 and so on.
5. Using dd to convert a file to uppercase:
| dd if=erik.txt of=erik_up.txt conv=ucase |
Finally, we use conv again to do a conversion. In this case we convert with the specifier of ucase.
What is your favourite use of dd?
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