How would I generate an inclusive random number between 1 to 10 in Bash Shell Script?
Would it be $(RANDOM 1+10)?
6 Answers
$(( ( 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
7 Comments
Spaceghost
I get a syntax error: syntax error near unexpected token `+' on Linux
Brad Koch
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.
Ads20000
@BradKoch How do we know that Bash's RNG makes that error?
Brad Koch
@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.
hmedia1
By far the simplest solution
|
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, 2014 at 19:53
Reinstate Monica Please
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1 Comment
jia103
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.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
4 Comments
likid1412
I found $RANDOM cannot generate big number, so I use
date +%s to get a big number. echo $(date +%s) % 4 | bc
ebi
What does the "| bc" part do?
Reinstate Monica Please
@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
Mirko Ebert
Works well on Mac OS X. +1
You can also use /dev/urandom:
grep -m1 -ao '[0-9]' /dev/urandom | sed s/0/10/ | head -n1
3 Comments
jcarballo
Useful when you don't have $RANDOM available, i.e. busybox
Jannie Theunissen
clever trick
sed s/0/10/ !
CodingNagger
Great trick especially for Github Actions!
To generate in the range: {0,..,9}
r=$(( $RANDOM % 10 )); echo $r
To generate in the range: {40,..,49}
r=$(( $RANDOM % 10 + 40 )); echo $r
Comments
Here is example of pseudo-random generator when neither $RANDOM nor /dev/urandom is available
echo $(date +%S) | grep -o .$ | sed s/0/10/
2 Comments
FrustratedWithFormsDesigner
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.
danielkza
A date is not random at all. It is completely predictable and follows an obvious pattern.
export CUDA_VISIBLE_DEVICES=$((( RANDOM % 8 )))