How to read in N random characters from /dev/urandom?

Question

read /dev/urandom 3

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

score 70 · Answer 1 · edited Aug 12 '15 at 14:16

70

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:

head -c 500 /dev/urandom: Obtain the first 500 characters (bytes) from /dev/urandom.
tr -dc 'a-zA-Z0-9~!@#$%^&*_-': Remove all but the characters specified in 'a-zA-Z0-9~!@#$%^&*_-' from the output of the first command.
fold -w 3: Format the output of the second command such that it has 3 characters per line.
head -n 1: Display the first line of the result of the third command on stdout.

edited Aug 12 '15 at 14:16

Slothworks

105

answered Feb 21 '13 at 16:41

ADM

1,393

1

He's looking for 3 bytes... – Chris S Feb 21 '13 at 16:56
1

3 plus exact pattern – ADM Feb 21 '13 at 17:05
4

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
6

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
2

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

Flimzy · Accepted Answer · 2022-04-14T08:58:57.120

65

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)

edited Apr 14 '22 at 08:58

answered Jun 23 '11 at 05:44

Flimzy

2,472

1

@Flimzy,how does $() work? – linux Jun 23 '11 at 06:07
2

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
1

@Flimzy,It treats the output of a command like a variable.,you actually mean as string,right? – linux Jun 23 '11 at 06:14
2

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
1

$() 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. (Granted, /dev/random should not be used anyway.) 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
Be careful with that: Bash strings cannot contain null bytes, so they are dropped from the string which may thus not be of the requested length. – Gabriel Devillers Mar 28 '24 at 17:01

score 44 · Answer 3 · answered Sep 01 '15 at 07:18

44

Please check man od.

You can use, for example

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

to generate unsigned decimal 4 bytes random numbers.

answered Sep 01 '15 at 07:18

Ekrem Aksoy

441

2

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

score 27 · Answer 4 · answered Jan 04 '15 at 06:34

27

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

answered Jan 04 '15 at 06:34

Umur Kontacı

378

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
2

@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
1

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
1

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
4

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

score 15 · Answer 5 · edited Sep 01 '15 at 08:03

15

The easiest solution would be as simple as:

$ head -cN /dev/urandom

edited Sep 01 '15 at 08:03

MadHatter

80,590

answered Sep 01 '15 at 08:00

Spack

1,624

2

What does this add to the answers already given? – MadHatter Sep 01 '15 at 08:03
10

simpler invocation – m.kocikowski Jun 27 '16 at 19:08

score 11 · Answer 6 · answered Jun 23 '11 at 05:43

11

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.

answered Jun 23 '11 at 05:43

Eduardo Ivanec

15,071
2
37
43

Can the command also be uses in order to read 3 random bytes of binary data as well? – Dimitrios Desyllas Apr 26 '18 at 21:32

score 1 · Answer 7 · answered Nov 03 '22 at 07:00

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.

How to read in N random characters from /dev/urandom?

7 Answers7