Linux Commands Examples

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convert and copy a file


dd [OPERAND]...

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dd if=file1 of=file2 bs=2M
example added by an anonymous user

Why does 'dd' not work for creating bootable USB?

Have you made sure that your motherboard is set to boot from the USB device before it tries booting from your HDD? I would guess that may be your only issue - there's not much to using dd as you can see.


What happens if I dd zeros to the drive where dd resides?

Yes. Of course, it'll also cost you most of your filesystem, but presumably you already know that...


How can I mount dd image of a partition?

You might first have to use losetup to create a device from your file, and then mount that device. Here's what I do to mount a backup file with partition image inside:

losetup /dev/loop1 /home/backup-file
mount /dev/loop1 /mnt/backup 

My partition then appears under /mnt/backup, and the original file is /home/backup-file. Maybe you can do this all with "mount -o loop" but I haven't been successful with that, so I'm using losetup separately.

After I'm finished, I umount the partition and then delete the loop with "losetup -d /dev/loop1", just in case.

Also, you can use losetup to find out what loop device is currently free in your system, with losetup -f

Let me know if this works.


How to pad a file with "FF" using dd?

i found a solution on comandlinefu, it involves tr to change the zeros to ones.

Good Luck


What exactly does the dd command do?

dd does a byte-by-byte copy from the source to the destination, with an optional conversion specified by the conv argument. It performs reads and writes as specified by the *bs and *flag options, with the range defined by the count, skip, and seek options.

what happens if the specified output file is too small to be turned into the specified input file?

If of is too small to contain if then the data is truncated to fit. Note that if of is a regular file then it is overwritten.


How do I create a 1GB random file in Linux?

Try this script.

$random=(openssl rand -base64 1000)
dd if=$random of="sample.txt bs=1G count=1"

This script might work as long as you don't mind using /dev/random.

dd if=/dev/random of="sample.txt bs=1G count=1"


How to create VHD disk image from a Linux live system?

One approach is to use a couple of handy technologies: VirtualBox, and the ntfsprogs package.

Recent versions of VirtualBox allow you to create VHD hard disk files, while ntfsprogs provides the ntfsclone utility. As its name suggests, ntfsclone clones NTFS filesystems, and I believe that it does it at the filesystem level, skipping over unused disk blocks.

So, to begin, create a new VM in VirtualBox, and provision a new, empty VHD-file drive for it. The VHD drive need only be as large as the size of data in use on the physical drive you want to clone (well actually, make it a little bit larger, to allow for some wiggle room).

Next, find a Linux live CD that contains the ntfsprogs package, as well as openssh-server. I like System Rescue CD for this, but pretty much any Debian- or Ubuntu-based live CD should work as well.

Boot the VirtualBox VM with the Linux live CD, and start sshd within the VM so that you will be able execute commands on it remotely. Partition the empty VHD drive approriately, using whatever partitioning tool you prefer (I like plain old fdisk, but I'm somewhat old school).

With another copy of the Linux live CD, boot the machine containing the physical disk you want to clone. I assume that the VirtualBox VM and this machine are accessible to each other over the network. On this machine, execute the following command (all on one line):

ntfsclone --save-image -o - /dev/sdXX |
    ssh root@VirtualBox-VM 'ntfsclone --restore-image --overwrite /dev/sdYY -'


  • /dev/sdXX is the device name (on the local machine) of the physical drive you want to clone, and
  • /dev/sdYY is the device name (in the VM) of the VHD destination drive.

Explanation: The first ntfsclone command in the pipeline extracts an image of the source NTFS filesystem and sends it out through the ssh tunnel, while the second ntfsclone command receives the image and restores it to the VHD drive.

Once the operation completes, the VHD file should contain a file-for-file exact clone of the original physical disk (barring any hardware errors, like bad sectors, that might cause the process to abort prematurely).

One last thing you may want to do is to run a Windows chkdsk on the VHD drive, just to ensure the cloning didn't introduce any problems (it shouldn't have, but hey, I'm a bit paranoid about these things).


dd_rescue vs dcfldd vs dd

The three are different, and the two varients are derived for the needs of specific communities. dd is a general purpose software for imaging, dd-rescue is designed to rebuild damaged files from multiple passes and sources, and forensic dd varients are designed to make verifiable, legally sound copies

dd is the baseline version - its the generic product, so to speak. DD is designed to make a bit perfect copy. Its what you use when you want to make a disk image, with no fancy addons.dd does one thing well, and absolutely nothing else. While there's distinct gnu and bsd versions, their functionality and commands are identical to both the unix dd, and a previous software made for the IBM JCL

gnu ddrescue is optimised for data recovery - it will note down where bad sectors are, and will attempt to fill in those areas with data from subsequent runs.As a result, the aim is to get files that are readable, as opposed to bit perfect. You will want to use it to recover data from a drive you suspect is damaged

From the DD Rescue Webpage

Ddrescue does not write zeros to the output when it finds bad sectors in the input, and does not truncate the output file if not asked to. So, every time you run it on the same output file, it tries to fill in the gaps without wiping out the data already rescued.

Automatic merging of backups: If you have two or more damaged copies of a file, cdrom, etc, and run ddrescue on all of them, one at a time, with the same output file, you will probably obtain a complete and error-free file. This is so because the probability of having damaged areas at the same places on different input files is very low. Using the logfile, only the needed blocks are read from the second and successive copies.

dcfldd and other forensic dd varients are designed to make forensic copies. These need to be bit perfect AND verifiable. Use this when you absolutely need to know that a copy and subsequent copies are identical to the original - forensic dd varients add additional features such as hashing

From the website, additional features of dcfldd are

Hashing on-the-fly - dcfldd can hash the input data as it is being transferred, helping to ensure data integrity.

Status output - dcfldd can update the user of its progress in terms of the amount of data transferred and how much longer operation will take. Flexible disk wipes - dcfldd can be used to wipe disks quickly and with a known pattern if desired.

Image/wipe Verify - dcfldd can verify that a target drive is a bit-for-bit match of the specified input file or pattern. Multiple outputs - dcfldd can output to multiple files or disks at the same time.

Split output - dcfldd can split output to multiple files with more configurability than the split command. Piped output and logs - dcfldd can send all its log data and output to commands as well as files natively.


creating a bootable USB from command line on linux

This is a common issue with SanDisk USB sticks, or sticks not formatted in FAT32.

If not either of those it is most certainly an issue with your stick partition order or the syslinux.cfg file.


Compressing a file in place - does "gzip -c file | dd of=file" really work?

Experiment shows that this does not work.

I created a 2-megabyte file from /dev/urandom, then tried the above command on it. Here are the results:

% ls -l
total 41008
-rw-r--r-- 1 kst kst 20971520 2012-01-18 03:47 file
-rw-r--r-- 1 kst kst 20971520 2012-01-18 02:48 orig
% gzip -c file | dd of=file
0+1 records in
0+1 records out
25 bytes (25 B) copied, 0.000118005 s, 212 kB/s
% ls -l
total 20508
-rw-r--r-- 1 kst kst       25 2012-01-18 03:47 file
-rw-r--r-- 1 kst kst 20971520 2012-01-18 02:48 orig

Obviously a 2-megabyte random file won't compress to 25 bytes, and in fact running gunzip on the compressed file yields an empty file.

I got similar results for a much smaller random file (100 bytes).

So what happened?

In this case, the dd command truncated file to zero bytes before starting to write to it; gzip started reading from the newly empty file and produced 25 bytes of output, which dd then appended to the empty file. (An empty file "compresses" to a non-zero size; it's theoretically impossible for any compressor to make all input smaller).

Other results may be possible, depending on the timing of the gzip, dd, and shell processes, all of which are running in parallel.

There's a race condition because one process, gzip, reads from file, while another parallel process, the shell, writes to it.

It should be possible to implement an in-place file compressor that reads and writes to the same file, using whatever internal buffering is necessary to avoid clobbering data. But I've never heard of anyone actually implementing that, probably because it usually isn't necessary and because if the compressor fails partway through, the file will be permanently corrupted.


How can I mount a disk image?

Found this:

which seems exactly what I was looking for.

Here's the key part:

mount -o loop,ro,offset=32256 hda.img /mnt/rabbit

where the value of offset is in bytes. The suggested way to get the offset is to point parted at the image, then unit B for bytes and take the start value from the print output. As an alternative, assuming you have the disk space, do the obvious: once you have the offset and size, just use dd to extract each partition to a separate file.


Mac OSX - Why is /dev/rdisk 20 times faster than /dev/disk

From man hdiutil:

/dev/rdisk nodes are character-special devices, but are "raw" in the BSD sense and force block-aligned I/O. They are closer to the physical disk than the buffer cache. /dev/disk nodes, on the other hand, are buffered block-special devices and are used primarily by the kernel's filesystem code.

In layman's terms /dev/rdisk goes almost directly to disk and /dev/disk goes via a longer more expensive route


dd performance on Mac OS X vs. Linux

For OS X, use /dev/rdisk3.

For some reason rdisk is faster than disk. I believe it has to do with buffers.

Also in general using the bs flag with dd helps with speed.

dd if=/path/to/image.iso of=/dev/sdc bs=1M

The bytesize is 1M which transfers faster. On OS X you have to use 1m (lowercase) instead of 1M.


How Do I Find The Hardware Block Read Size for My Hard Drive?

Linux exposes the physical sector size in files /sys/block/sdX/queue/physical_block_size. Although, to get the best performance you should probably do a little testing with different sizes and meassure. I could not find a clear answer in that using exactly the physical block size would get the optimal result (although I assume it cannot be a bad choice).


Copy a file, converting and formatting according to the operands.

read and write up to BYTES bytes at a time


convert BYTES bytes at a time


convert the file as per the comma separated symbol list


copy only N input blocks


read up to BYTES bytes at a time (default: 512)


read from FILE instead of stdin


read as per the comma separated symbol list


write BYTES bytes at a time (default: 512)


write to FILE instead of stdout


write as per the comma separated symbol list


skip N obs-sized blocks at start of output


skip N ibs-sized blocks at start of input


WHICH info to suppress outputting to stderr; ’noxfer’ suppresses transfer stats, ’none’ suppresses all

N and BYTES may be followed by the following multiplicative suffixes: c =1, w =2, b =512, kB =1000, K =1024, MB =1000*1000, M =1024*1024, xM =M GB =1000*1000*1000, G =1024*1024*1024, and so on for T, P, E, Z, Y.

Each CONV symbol may be:






from ASCII to alternate EBCDIC


pad newline-terminated records with spaces to cbs-size


replace trailing spaces in cbs-size records with newline


change upper case to lower case


change lower case to upper case


try to seek rather than write the output for NUL input blocks


swap every pair of input bytes


pad every input block with NULs to ibs-size; when used with block or unblock, pad with spaces rather than NULs


fail if the output file already exists


do not create the output file


do not truncate the output file


continue after read errors


physically write output file data before finishing


likewise, but also write metadata

Each FLAG symbol may be:


append mode (makes sense only for output; conv=notrunc suggested)


use direct I/O for data


fail unless a directory


use synchronized I/O for data


likewise, but also for metadata


accumulate full blocks of input (iflag only)


use non-blocking I/O


do not update access time


discard cached data


do not assign controlling terminal from file


do not follow symlinks


treat ’count=N’ as a byte count (iflag only)


treat ’skip=N’ as a byte count (iflag only)


treat ’seek=N’ as a byte count (oflag only)

Sending a USR1 signal to a running ’dd’ process makes it print I/O statistics to standard error and then resume copying.

$ dd if=/dev/zero of=/dev/null& pid=$!
$ kill -USR1 $pid; sleep 1; kill $pid

18335302+0 records in 18335302+0 records out 9387674624 bytes (9.4 GB) copied, 34.6279 seconds, 271 MB/s

Options are:


display this help and exit


output version information and exit


Copyright © 2012 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later <>.
This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law.

reporting bugs

Report dd bugs to bug-coreutils[:at:]gnu[:dot:]org
GNU coreutils home page: <>
General help using GNU software: <>
Report dd translation bugs to <>

see also

The full documentation for dd is maintained as a Texinfo manual. If the info and dd programs are properly installed at your site, the command

info coreutils 'dd invocation'

should give you access to the complete manual.


Written by Paul Rubin, David MacKenzie, and Stuart Kemp.

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