How To Use Dd Command.
In this tutorial, we learn about dd command in Linux with practical examples.
The Linux dd command is one of the most powerful utilities for Unix and Unix-like operating systems. It can be used in a variety of ways.
dd command in Linux
dd command is mainly used for copying and converting data, hence it stands for data duplicator.
Using dd command we can do:
Backing up and restoring an entire hard drive or a partition.
Creating virtual filesystem and backup images of CD or DVDs called ISO files
Copy regions of raw device files like backing up MBR (master boot record).
Converting data formats like ASCII to EBCDIC.
Converting lowercase to uppercase and vice versa.
Only the superuser can execute dd command.
Syntax of dd command
The basic use of the dd utility is rather easy because it takes just two arguments: if= to specify the input file and of= to specify the output file. The arguments for those options can be either files or block devices. I would, however, not recommend using dd to copy files because cp does that in a much simpler way. However, you can use it to clone a hard disk.
dd command Examples
We will learn various options while going through dd command examples.
1. Backing up and restoring an entire disk or a partition
It is possible to save all the data from an entire disk/partition to another disk/partition. Not a simple copy as cp command but a block size copy.
a. Backup entire disk to disk
You can copy all the data (entire disk) from the disk /dev/sda to /dev/sdb. dd doesn’t know anything about the filesystem or partitions; it will just copy everything from /dev/sda to /dev/sdb.
You need to indicate the block size to be copied at time with bs option. So, this will clone the disk with the same data on the same partition.
This works only if the second device is as large as or larger than the first. Otherwise, you get truncated and worthless partitions on the second one.
if stands for input file , of stands for output file
bs stands for the block size (number of bytes to be read/write at a time). Make sure you use block sizes in multiples of 1024 bytes which is equal to 1KB. If you don’t specify block size, dd use a default block size of 512 bytes.
The conv value parameter noerror allows the tool to continue to copy the data even though it encounters any errors. The sync option allows to use synchronized I/O.
b. Creating dd disk image (file image)
You can create an image of a disk or a file image. Backing up a disk to an image will be faster than copying the exact data. Also, disk image makes the restoration much easier.
You can store the output file where you want but you have to give a filename ending with .img an extension. Instead of /tmp/sdadisk.img, you could store it for example at /sdadisk.img if you want.
c. Creating a compressed disk image
Because dd creates the exact content of an entire disk, it means that it takes too much size. You can decide to compress the disk image with the command below
The pipe | operator makes the output on the left command become the input on the right command. The -c option writes output on standard output and keeps original files unchanged.
d. Backup a partition or clone one partition to another
Instead of an entire disk, you can only backup a simple partition. You just need to indicate the partition name in the input file.
This will synchronize the partition /dev/sda1 to /dev/sdb1. You must verify that the size of /dev/sdb1 should be larger than /dev/sda1. Or you can create a partition image as below
e. Restoring a disk or a partition image
Save a disk or a partition helps to restore all the data, if there is any problem with our original drive.
To restore, you need to inverse the input file with the output file indicated during the backup operation:
You will retrieve data that were present before the backup operation and not after the operation
f. Restoring compressed image
You need to first indicate the compressed file and the output file which is the disk compressed before.
The -d option here is to uncompress. Note the output file. You can mount the restored disk to see the content.
2. Creating virtual filesystem/Backup images of CD or DVDs as iso files
You can need to create a virtual filesystem on Linux for some reasons as creating a virtual machine on your Linux host. You can also need to create a backup iso image of a CD or DVD
a. Creating a virtual filesystem
A virtual filesystem is a filesystem that exists in a file, which in turn exists on a physical disk. You can need it to create for example an additional swap or loop device or a virtual machine. We need /dev/zero which is a file used to create a file with no data but with required size (a file with all zero’s). In other words, this will create a data file with all zeros in the file which will give the size to a file.
The option count refers to the number of input blocks to be copied. Combined with block size value, it indicates the total size to copy. For example bs=1024k and count=500 give a size=1024K*500 =524288000 bytes =524MB
Now let’s check the size of our file
You can see that we have our virtual filesystem created with the size indicated. You can now use it to create loop device or a virtual disk or anything else.
b. Modify the first 512 bytes of a file with null data
If during the operation you indicate an existing output file, you will lose its data. For some reason, you can need to replace a block size of the output file.
The notrunc option refers to do not truncate the file, only replace the first 512 bytes, if it exists. Otherwise, you will get a 512-byte file
c. Creating a backup iso image of CD or DVD
You may wonder why not just copy the contents of your CD to a directory. How would you handle the boot sector of a CD? You can’t find that as a file on the device because it’s just the first sector. Because dd copies sector by sector, on the other hand, it will copy that information as well.
You need to know that you have to use the -o loop option, which allows you to mount a file like any normal device. So, to mount /mycd.iso on the /mnt/cd directory, do as below.
d. Create a bootable USB Drive
Let’s assume we have downloaded AlmaLinux iso image here to this directory “~/Downloads/”.
if=~/Downloads/AlmaLinux-9-latest-x86_64-dvd.iso – input file is in the path ‘~/Downloads/AlmaLinux-9-latest-x86_64-dvd.iso’
of=/dev/sdb – output file is in the path ‘/dev/sdb’
bs=1M – Read from ‘~/Downloads/AlmaLinux-9-latest-x86_64-dvd.iso’ and write to ‘/dev/sdb’ 1 Megabytes of data at a time.
3. Backing up and restoring MBR
The GRUB bootloader is most commonly stored in the MBR of the bootable drive. The MBR makes up the first 512 bytes of the disk, allowing up to 466 bytes of storage for the bootloader. The additional space will be used to store the partition table for that drive. If MBR gets corrupted, we will not be able to boot into Linux.
a. Backing up MBR
Because the MBR makes up the first 512 bytes of the disk, we just need to copy that block size
With the count=1 and bs=512, only 512 bytes will be copied which corresponds to the size of our MBR.
You can display the saved MBR with the od command which dump files in octal and other formats as below
-a option selects named characters and -x selects hexadecimal 2-byte units
b. Backing up the boot data of MBR excluding the partition table
The MBR 512 bytes data is located at the first sector of the hard disk. It consists of 446 bytes bootstrap, 64 bytes partition table and 2 bytes signature. It means that we can exclude the partition table and bytes signature while backing up the MBR with conserving only a block size equal to the bootstrap size.
c. Restoring MBR from MBR image
You can restore your MBR as shown on the previous commands with
3. Converting data formats
If an input file uses a character set that is not the native character set of the host computer, the import operator must perform a conversion. For example, if ASCII is the native format for strings on your host computer, but the input data file represents strings using EBCDIC, you must convert EBCDIC to ASCII and vice versa.
a. Convert the data format of a file from EBCDIC to ASCII
If there’s an ebcdic file with you, mostly retrieved from mainframe systems, then, you would like to convert them to ASCII for making modifications using text editors on UNIX servers
The conv value parameter now is ascii because we convert from EBCDIC to ASCII
b. Convert the data format of a file from ASCII to EBCDIC
After modifying the ASCII version and once done, you may convert it back to EBCDIC to be used by your application.
The conv value parameter now is ebcdic because we convert from ASCII to EBCDIC. If you’re just replacing the particular number of bytes with an equivalent number of bytes having different characters, the conversion would be smooth and the application reading the file should not have any issues.
4. Converting case of a file
DD command can be also used for an amazing thing. It can convert all text (alphabets) in a file to upper or lower case and vice versa. For the example below, we will have a file for the tests.
a. Converting a file to uppercase
Because our text file example is on lowercase, we will convert it to uppercase
The command will create the new file indicated. See that now conv option takes ucase value. Let’s check the result
b. Converting a file to lowercase
Now we will do the reverse operation which will convert to lowercase
See that we use lcase of conv option to convert from upper case to lower case.
dd command does not convert the file names, only it’s content.
5. Show Progress
dd utility is bunded with Coreutils. The versions above 8.24 are able to show a progress bar while copying.
You can check the coreutils version by checking dd –version command.
The following time shows a progress bar showing amount of data copied and elapsed time.
By default, dd command won’t show the progress bar need to add the option status=progress.
In this tutorial, we learn in-depth about the dd command in Linux. We have illustrated with dd command examples to understand the options.