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read /dev/urandom 3

The above is not working..How can I read random bytes from /dev/urandom in bash?

Slothworks
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linux
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7 Answers7

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head -c 500 /dev/urandom | tr -dc 'a-zA-Z0-9~!@#$%^&*_-' | fold -w 3 | head -n 1

(If you want literal dash characters, the dash character must go at the end of the string as done above, as opposed to *-_).

And to explain what gets done due to the above set of commands:

  1. head -c 500 /dev/urandom: Obtain the first 500 characters (bytes) from /dev/urandom.
  2. tr -dc 'a-zA-Z0-9~!@#$%^&*_-': Remove all but the characters specified in 'a-zA-Z0-9~!@#$%^&*_-' from the output of the first command.
  3. fold -w 3: Format the output of the second command such that it has 3 characters per line.
  4. head -n 1: Display the first line of the result of the third command on stdout.
Slothworks
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ADM
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    He's looking for 3 bytes... – Chris S Feb 21 '13 at 16:56
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    3 plus exact pattern – ADM Feb 21 '13 at 17:05
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    Great! There are also the char class names ([:graph|print|etc:]) available for the tr func, and it could be further simplified by replacing the `fold -w 3 | head -n 1` by a `head -c 3` if the new line char is not needed at the end. – Zimmi Dec 14 '14 at 22:39
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    If the system's default encoding is multibyte (e.g. UTF-8) then tr will fail with an `Illegal byte sequence` error. To fix that, use the "C" encoding; change the command to: `head -c 500 /dev/urandom | LC_ALL=C tr -dc 'a-zA-Z0-9~!@#$%^&*_-' | fold -w 3 | head -n 1` – devstuff Nov 10 '15 at 17:06
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    What is the purpose of grabbing 500 bytes at the beginning? Don't you obtain the same result with: `cat /dev/urandom | tr -dc 'a-zA-Z0-9~!@#$%^&*_-' | fold -w 3 | head -n 1` – philraj Aug 09 '19 at 15:49
  • Why not: `tr -dc 'a-zA-Z0-9~!@#$%^&*_-' < /dev/urandom | head -c 3` – Vlad Didenko Jun 14 '20 at 02:48
  • I'm not sure but won't the stripping of characters lower the entropy? – Jupiter Dec 02 '21 at 14:02
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random="$(dd if=/dev/urandom bs=3 count=1)"

if specifies the input file, bs the block size (3 bytes), and count the number of blocks (1 * 3 = 3 total bytes)

Flimzy
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    @Flimzy,how does `$()` work? – linux Jun 23 '11 at 06:07
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    It treats the output of a command like a variable. Note: That's a bashism. If you're not using bash, you may need to use `` instead. `` is more universal, but I think $() is easier to read. – Flimzy Jun 23 '11 at 06:08
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    @Flimzy,`It treats the output of a command like a variable.`,you actually mean as string,right? – linux Jun 23 '11 at 06:14
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    A variable _can_ be a string... or a number. It treats it as a variable... then depending on context, it's treated like a string, or a number. – Flimzy Jun 23 '11 at 06:27
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    `$()` is pretty universal, not bash-specific. BTW, be careful you don't run into limitations on what characters the shell can store in variables -- for example, the version of bash I tested with leaves out nulls (\x00) from the string. – Gordon Davisson Jun 23 '11 at 13:13
  • Putting "" marks around the $() may solve that problem (untested). It will certainly help solve problems of other odd characters, though. I'm just not sure if it solves the case of \x00. – Flimzy Jun 23 '11 at 20:49
  • @Flimzy: Worth a try, but it didn't help with nulls, at least with bash 3.2.48(1) (under OS X 10.6.6). – Gordon Davisson Jun 24 '11 at 03:25
  • Note that [while reading 3 bytes is guaranteed with `/dev/urandom`, it would not be for `/dev/random`](https://unix.stackexchange.com/questions/32988/why-does-dd-from-dev-random-give-different-file-sizes/33009#33009). (Granted, [`/dev/random` should not be used anyway](http://security.stackexchange.com/questions/3936/is-a-rand-from-dev-urandom-secure-for-a-login-key).) But some of these three bytes can be null bytes, and most shells will ignore them and possibly the following bytes as well. There's no way to store a null byte into a variable in any of the common shells except zsh. – Gilles 'SO- stop being evil' Aug 12 '15 at 09:35
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Please check man od.

You can use, for example

od -vAn -N4 -tu4 < /dev/urandom

to generate unsigned decimal 4 bytes random numbers.

Ekrem Aksoy
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    Ekrem, welcome to SF. I don't normally much favour new answers to long-accepted questions, but this is genuinely different from all previous answers, and has some real advantages - nice! +1 from me, and I hope you stay around SF and contribute like this for some time to come. – MadHatter Sep 01 '15 at 08:00
  • I really like this way of generating random strings, it's much more efficient. – Jaakko Apr 20 '20 at 11:06
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Here's one that creates base64 strings, note that even though they are limited to base64 strings, the padding is removed from them, so you can't decode them, you probably won't need it anyway.

 cat /dev/urandom | base64 | head -c 5

Replace 5 with the number of chars you'd like.

If you however need to decode them for some reason, move base64 to the end of the pipe. It will then collect 5 chars from urandom and base64 encode it with the right padding, but the final string may be longer than what you wanted due to padding.

cat /dev/urandom | head -c 5 | base64
Umur Kontacı
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  • This is not exactly what the question was asking for. Your answer might still be useful, but the question doesn't have enough context to say for sure. – kasperd Jan 04 '15 at 10:23
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    @kasperd The question asks (in the title) how to read N random characters from /dev/urandom but in the body it asks for reading bytes. I agree there is an inconsistency. I came to this question from Google for reading characters and I think that it may help the future visitors as an option. – Umur Kontacı Jan 04 '15 at 14:31
  • I think your answer is the simplest yet easiest to understand. Thanks! – Robert Sep 23 '16 at 08:52
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    Using `cat` will read far more than a given amount `N` chars, and can deplete the entropy. Not useful answer. – DrBeco Mar 20 '17 at 18:52
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    While Dr Beco is correct that this is an abuse of `cat` since both `base64` and `head` can take a filename as an argument, in this case it works and shouldn't deplete the entropy. See http://stackoverflow.com/questions/10031344/why-is-this-pipe-terminated – J.Money Mar 20 '17 at 22:34
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    Hey! its yet another useless use of cat. ```$ head -c 5 /dev/urandom | base64``` produces exactly the same output. – Joshua Clayton Oct 02 '17 at 23:42
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The easiest solution would be as simple as:

$ head -cN /dev/urandom
MadHatter
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Spack
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Try this: dd if=/dev/urandom bs=1 count=3

If you want to put the result in $variable:

variable=`dd if=/dev/urandom bs=1 count=3`

Do note that it'll probably not be printable.

Eduardo Ivanec
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1

The following command generates random characters

</dev/urandom tr -dc 'A-Za-z0-9!"#$%&\(\)*+,./: ;<=>?@\[\]^_`{|}~-' | head -c 15  ; echo

from all the ASCII Printable characters and only takes 15 head characters (or any number that you specifies) from it.

Adeel Raza Azeemi
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