I want to create a large file ~10G filled with zeros and random values. I have tried using:

dd if=/dev/urandom of=10Gfile bs=5G count=10

it creates a file of about 2Gb and exits with a exit status 0. I fail to understand why?

I also tried creating file using:

head -c 10G </dev/urandom >myfile

but it takes about 28-30 mins to create it. But i want it created faster. Anyone has a solution?

Also i wish to create multiple files with same (pseudo) random pattern for comparison. Does anyone know a way to do that? Thanks

  • If it's important that the files contain random numbers, that should be part of the title! What means "filled with zeros and random values."? – Volker Siegel Aug 4 '14 at 22:20

I've seen a pretty neat trick at commandlinefu: use /dev/urandom as a source of randomness (it is a good source), and then using that as a password to an AES stream cipher.

I can't tell you with 100% sure, but I do believe that if you change the parameters (i.e. use way more than just 128 bytes from /dev/urandom), it is at least close enough to a cryptographically secure PRNG, for all practical purposes:

This command generates a pseudo-random data stream using aes-256-ctr with a seed set by /dev/urandom. Redirect to a block device for secure data scrambling.

openssl enc -aes-256-ctr -pass pass:"$(dd if=/dev/urandom bs=128 count=1 2>/dev/null | base64)" -nosalt < /dev/zero > randomfile.bin

How does this work?

openssl enc -aes-256-ctr will use openssl to encrypt zeroes with AES-256 in CTR mode.

  • What will it encrypt?


  • What is the password it will use to encrypt it?

    dd if=/dev/urandom bs=128 count=1 | base64

    That is one block of 128 bytes of /dev/urandom encoded in base64 (the redirect to /dev/null is to ignore errors).

  • I'm actually not sure why -nosalt is being used, since OpenSSL's man page states the following:

        use a salt in the key derivation routines. This is the default.
        don't use a salt in the key derivation routines. This option SHOULD NOT be used except for test purposes or compatibility with ancient versions of OpenSSL and SSLeay.

    Perhaps the point is to make this run as fast as possible, and the use of salts would be unjustified, but I'm not sure whether this would leave any kind of pattern in the ciphertext. The folks at the Cryptography Stack Exchange may be able to give us a more thorough explanation on that.

  • The input is /dev/zero. This is because it really doesn't matter what is being encrypted - the output will be something resembling random data. Zeros are fast to get, and you can get (and encrypt) as much as you want without running out of them.

  • The output is randomfile.bin. It could also be /dev/sdz and you would randomize a full block device.

But I want to create a file with a fixed size! How do I do that?


dd if=<(openssl enc -aes-256-ctr -pass pass:"$(dd if=/dev/urandom bs=128 count=1 2>/dev/null | base64)" -nosalt < /dev/zero) of=filename bs=1M count=100 iflag=fullblock

Just dd that command with a fixed blocksize (which is 1 MB here) and count. The file size will be blocksize * count = 1M * 100 = 100M.

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  • I was able to generate file quickly but it doesn't stop w/o Ctrl+C.Is there any way of giving out a file size? Also i didn't understand the "-nosalt < /dev/zero >" part.Frm wht i found ol it gives a initialization vector. So in this case is the IV /dev/zero? Also if i want to generate another file with same contents is that possible? – egeek Aug 6 '14 at 18:47
  • It should stop by itself with a "disk full" warning. I'm updating the post to explain how it works. – Valmiky Arquissandas Aug 6 '14 at 18:52
  • I appreciate it.Able to understand clearly now, Thank you! The only issue is generating a output file of a particular size.I need a bunch of them to transfer and check timings n stuff.Can't have it running till the "disk full" warning – egeek Aug 6 '14 at 19:33
  • In that case, you can also use dd. The following line will create a 100 MB file with random data (count * blocksize = 100 * 1M): dd if=<(openssl enc -aes-256-ctr -pass pass:"$(dd if=/dev/urandom bs=128 count=1 2>/dev/null | base64)" -nosalt < /dev/zero) of=filename bs=1M count=100 – Valmiky Arquissandas Aug 6 '14 at 22:36
  • Sorry, the above line doesn't work because dd can't handle the input file being a stream. To make it accumulate the input blocks, you need to add the option iflag=fullblock to the outer dd, like this: dd if=<(openssl enc -aes-256-ctr -pass pass:"$(dd if=/dev/urandom bs=128 count=1 2>/dev/null | base64)" -nosalt < /dev/zero) of=filename bs=1M count=100 iflag=fullblock. I am adding this to the answer. – Valmiky Arquissandas Aug 6 '14 at 22:43

I'm getting good speeds using the shred utility.

  • 2G with dd in=/dev/urandom - 250sec
  • 2G with openssl rand - 81sec
  • 2G with shred - 39sec

So I expect about 3-4 minutes for 10G with shred.

Create an empty file and shred it by passing the desired file size.

touch file
shred -n 1 -s 10G file

I'm not sure how cryptographically secure the generated data is, but it looks random. Here's some info on that.

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  • 6
    +1 for introducing me to shred <3. So useful. I used to loop dd. – aggregate1166877 Apr 17 '18 at 21:50
  • so that would still be, uh... 300-400 minutes per TB – Michael Feb 18 at 17:49
  • @Michael Exactly! – lyuboslav kanev Feb 23 at 10:27

There is a random number generator program sharand, it writes random bytes to a file. (The program was originally called sharnd, with one letter a less ( see http://mattmahoney.net/dc/)

It takes roughly one third of the time compared to reading /dev/urandom

It's a secure RNG - there are faster, but not secure RNG, but that's not what's needed normally.
To be really fast, look for the collection of RNG algorithms for perl: libstring-random-perl.

Let's give it a try (apt-get install sharand):

$ time sharand a 1000000000                      
sharand a 1000000000  21.72s user 0.34s system 99% cpu 22.087 total

$ time head -c 1000000000 /dev/urandom > urand.out
head -c 1000000000 /dev/urandom > urand.out  0.13s user 61.22s system 99% cpu 1:01.41 total

And the result files - (they do look more random from the inside):

$ ls -l
-rw-rw-r-- 1 siegel siegel 1000000000 Aug  5 03:02 sharand.out
-rw-rw-r-- 1 siegel siegel 1000000000 Aug  5 03:11 urand.out

Comparing the 'total' time values, sharand took only a third of the time needed by the urandom method to create a little less than a GB random bytes:

sharand: 22s total
urandom: 61s total

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  • I am using CentOS 6.5 and sharand is not available. I tried installing using: yum install sharand. It gives me "No package sharand available." Also if i run "time sharand a 1000000000", it says : command not found – egeek Aug 5 '14 at 20:36
  • Oh, I just found that the program originally was not called sharand, as in ubuntu, but sharnd whith one "a" less. So that may be just a different package name. Looking at the home page of the software, it seems like he does not provide any packages, except the source; But most tools are just an algorithm in a single .c file, and very simple to build. If you can not find a package with the original name either, we'll find another way. (Sources and math papers here: mattmahoney.net/dc) – Volker Siegel Aug 6 '14 at 1:16
  • Unfortunately,i am unable to figure out how to work with sharnd also. One method bewlo with the openssl is working fine for now but only issue is how to specify the output file size. – egeek Aug 6 '14 at 19:36

You want a special file in Linux, /dev/random serves as a random number generator on a Linux system. /dev/random will eventually block unless your system has a lot of activity, /dev/urandom in non-blocking. We don't want blocking when we're creating our files so we use /dev/urandom.

try this command:

dd if=/dev/urandom bs=1024 count=1000000 of=file_1GB conv=notrunc

This will create a file with bs*count random bytes, in our case 1024*1000000 = 1GB. The file will not contain anything readable, but there will be some newlines in it.

xKon@xK0n-ubuntu-vm:~/tmp$ dd if=/dev/urandom of=file.txt bs=1048576 count=100 conv=notrunc
100+0 records in
100+0 records out
104857600 bytes (105 MB) copied, 13.4593 s, 7.8 MB/s
xKon@xK0n-ubuntu-vm:~/tmp$ wc -l file.txt
410102 file.txt

You can use the option seek with dd to speed up the process a little more:

$ dd if=/dev/zero of=1g.img bs=1 count=0 seek=1G
1+0 records in
1+0 records out
1073741824 bytes (1.1 GB) copied, 8.12307 s, 132 MB/s
$ ls -lh t
-rw-rw-r-- 1 xK0n  xK0n  1.1G 2014-08-05 11:43 t

The disadvantages here are the fact that the file does not contain anything readable and the fact that it is quite a bit slower than the /dev/zero method (around 10 seconds for 100Mb).

You may also like fallocate command that Preallocates space to a file.

fallocate -l 1G test.img


-rw-r--r--. 1 xK0n xK0n 1.0G Aug 05 11:43 test.img

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