Generating random number between 1 and 10 in Bash Shell Script

Question

How would I generate an inclusive random number between 1 to 10 in Bash Shell Script?

Would it be $(RANDOM 1+10)?

this works for what I need `export CUDA_VISIBLE_DEVICES=$((( RANDOM % 8 )))` — Charlie Parker, Mar 11 '21 at 20:47

score 380 · Accepted Answer · edited Feb 11 '16 at 20:21

380

$(( ( RANDOM % 10 )  + 1 ))

EDIT. Changed brackets into parenthesis according to the comment. http://web.archive.org/web/20150206070451/http://islandlinux.org/howto/generate-random-numbers-bash-scripting

edited Feb 11 '16 at 20:21

toxefa

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answered Jan 24 '12 at 14:53

OrangeTux

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I get a syntax error: syntax error near unexpected token `+' on Linux – Spaceghost Aug 19 '13 at 17:57
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The `% 10` reduces the result to a set of ranges from 0-9. However, the very top set only has a range of 0-7. Thus, there are two fewer draws for 8 & 9 than 0-7. The + 1 translates this to a bias against 9 & 10. This would be a major flaw in security contexts - an RNG must not display bias, and at scale this is noticeable. Further explanation: [Anatomy of a pseudorandom number generator - visualising Cryptocat's buggy PRNG](http://nakedsecurity.sophos.com/2013/07/09/anatomy-of-a-pseudorandom-number-generator-visualising-cryptocats-buggy-prng/). – Brad Koch Sep 18 '13 at 19:02
@BradKoch How do we know that Bash's RNG makes that error? – Ads20000 Feb 13 '17 at 16:12
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@ads20000 The docs say so, `man bash`, "Each time this parameter is referenced, a random integer between 0 and 32767 is generated." Note that this is not a bash error, it's an error of the implementer. Assuming bash implemented `$RANDOM` correctly, there should be a fair draw from numbers 0 to 32767, but because you can't divide that evenly into groups of 10, there is a very small bias against 9 and 10 in this implementation. Most people shouldn't need to worry about this, because you shouldn't be using `$RANDOM` for security purposes anyway. – Brad Koch Feb 13 '17 at 16:22
By far the simplest solution – hmedia1 Nov 07 '19 at 00:57
this works for what I need `export CUDA_VISIBLE_DEVICES=$((( RANDOM % 8 )))` – Charlie Parker Mar 11 '21 at 20:47

score 115 · Answer 2 · answered Apr 15 '14 at 19:53

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Simplest solution would be to use tool which allows you to directly specify ranges, like gnu shuf

shuf -i1-10 -n1

If you want to use $RANDOM, it would be more precise to throw out the last 8 numbers in 0...32767, and just treat it as 0...32759, since taking 0...32767 mod 10 you get the following distribution

0-8 each: 3277 
8-9 each: 3276

So, slightly slower but more precise would be

while :; do ran=$RANDOM; ((ran < 32760)) && echo $(((ran%10)+1)) && break; done

answered Apr 15 '14 at 19:53

Reinstate Monica Please

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I found this question looking for `/bin/sh` even though the question requests bash. The `shuf` technique appears to work in both bash and sh. – jia103 Jan 14 '22 at 05:01

score 43 · Answer 3 · answered Jan 24 '12 at 14:53

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To generate random numbers with bash use the $RANDOM internal Bash function. Note that $RANDOM should not be used to generate an encryption key. $RANDOM is generated by using your current process ID (PID) and the current time/date as defined by the number of seconds elapsed since 1970.

 echo $RANDOM % 10 + 1 | bc

answered Jan 24 '12 at 14:53

shadyabhi

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I found $RANDOM cannot generate big number, so I use `date +%s` to get a big number. `echo $(date +%s) % 4 | bc` – likid1412 Apr 15 '15 at 06:00
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What does the "| bc" part do? – ebi May 11 '15 at 17:27
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@ebi `echo` just prints the string (variable substitution happens before the command is processed, so `$RANDOM` is already replaced with an integer by that point), `bc` is what actually evaluates it when the `|` redirects the stdout from the `echo` into the stdin of the `bc` – Reinstate Monica Please May 14 '15 at 12:12
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Works well on Mac OS X. +1 – Mirko Ebert Jun 07 '17 at 11:16

score 32 · Answer 4 · answered Jan 24 '12 at 17:06

32

You can also use /dev/urandom:

grep -m1 -ao '[0-9]' /dev/urandom | sed s/0/10/ | head -n1

answered Jan 24 '12 at 17:06

choroba

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Useful when you don't have $RANDOM available, i.e. busybox – jcarballo Feb 11 '14 at 20:43
clever trick `sed s/0/10/` ! – Jannie Theunissen Apr 25 '18 at 13:29

Thomas Bratt · Answer 5 · 2012-11-29T10:24:13.747

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To generate in the range: {0,..,9}

r=$(( $RANDOM % 10 )); echo $r

To generate in the range: {40,..,49}

r=$(( $RANDOM % 10 + 40 )); echo $r

edited Nov 29 '12 at 10:24

answered Nov 29 '12 at 10:17

Thomas Bratt

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score -6 · Answer 6 · answered Jul 11 '14 at 18:39

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Here is example of pseudo-random generator when neither $RANDOM nor /dev/urandom is available

echo $(date +%S) | grep -o .$ | sed s/0/10/

answered Jul 11 '14 at 18:39

l0pan

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It just prints an increasing sequence of numbers every time I run it: "1... 2... 3.... 4..." basically, the least-significant digit of the current second. – FrustratedWithFormsDesigner Sep 11 '15 at 19:23
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A date is not random at all. It is completely predictable and follows an obvious pattern. – danielkza Sep 29 '15 at 21:02

Generating random number between 1 and 10 in Bash Shell Script

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