How to generate a random number within a range in Bash?

25 Answers

Use $RANDOM. It's often useful in combination with simple shell arithmetic. For instance, to generate a random number between 1 and 10 (inclusive):

$ echo $((1 + $RANDOM % 10))
3

The actual generator is in variables.c, the function brand(). Older versions were a simple linear generator. Version 4.0 of bash uses a generator with a citation to a 1985 paper, which presumably means it's a decent source of pseudorandom numbers. I wouldn't use it for a simulation (and certainly not for crypto), but it's probably adequate for basic scripting tasks.

If you're doing something that requires serious random numbers you can use /dev/random or /dev/urandom if they're available:

$ dd if=/dev/urandom count=4 bs=1 | od -t d

Please see $RANDOM:

$RANDOM is an internal Bash function (not a constant) that returns a pseudorandom integer in the range 0 - 32767. It should not be used to generate an encryption key.

You can also use shuf (available in coreutils).

shuf -i 1-100000 -n 1

Try this from your shell:

$ od -A n -t d -N 1 /dev/urandom

Here, -t d specifies that the output format should be signed decimal; -N 1 says to read one byte from /dev/urandom.

you can also get random number from awk

awk 'BEGIN { # seed srand() for (i=1;i<=1000;i++){ print int(1 + rand() * 100) }
}'

I like this trick:

echo ${RANDOM:0:1} # random number between 1 and 9
echo ${RANDOM:0:2} # random number between 1 and 99

...

There is $RANDOM. I don't know exactly how it works. But it works. For testing, you can do :

echo $RANDOM

bash 5.1 introduces a new variable, SRANDOM, which gets its random data from the system's entropy engine and so is not linear and cannot be reseeded to get an identical random sequence. This variable can be used as a substitute for RANDOM for generating more random numbers.

$ echo $((1 + SRANDOM % 10))
4

Random number between 0 and 9 inclusive.

echo $((RANDOM%10))

If you are using a linux system you can get a random number out of /dev/random or /dev/urandom. Be carefull /dev/random will block if there are not enough random numbers available. If you need speed over randomness use /dev/urandom.

These "files" will be filled with random numbers generated by the operating system. It depends on the implementation of /dev/random on your system if you get true or pseudo random numbers. True random numbers are generated with help form noise gathered from device drivers like mouse, hard drive, network.

You can get random numbers from the file with dd

Reading from /dev/random or /dev/urandom character special files is the way to go.

These devices return truly random numbers when read and are designed to help application software choose secure keys for encryption. Such random numbers are extracted from an entropy pool that is contributed by various random events. {LDD3, Jonathan Corbet, Alessandro Rubini, and Greg Kroah-Hartman]

These two files are interface to kernel randomization, in particular

void get_random_bytes_arch(void* buf, int nbytes)

which draws truly random bytes from hardware if such function is by hardware implemented (usually is), or it draws from entropy pool (comprised of timings between events like mouse and keyboard interrupts and other interrupts that are registered with SA_SAMPLE_RANDOM).

dd if=/dev/urandom count=4 bs=1 | od -t d

This works, but writes unneeded output from dd to stdout. The command below gives just the integer I need. I can even get specified number of random bits as I need by adjustment of the bitmask given to arithmetic expansion:

me@mymachine:~/$ x=$(head -c 1 /dev/urandom > tmp && hexdump -d tmp | head -n 1 | cut -c13-15) && echo $(( 10#$x & 127 ))

I wrote several articles on this.

$ RANDOM=$(date +%s%N | cut -b10-19)
$ echo $(( $RANDOM % 113 + 13 ))

The above will give a number between 13 and 125 (113-1+13), with reasonable random entropy.

I have taken a few of these ideas and made a function that should perform quickly if lots of random numbers are required.

calling od is expensive if you need lots of random numbers. Instead I call it once and store 1024 random numbers from /dev/urandom. When rand is called, the last random number is returned and scaled. It is then removed from cache. When cache is empty, another 1024 random numbers is read.

Example:

rand 10; echo $RET

Returns a random number in RET between 0 and 9 inclusive.

declare -ia RANDCACHE
declare -i RET RAWRAND=$(( (1<<32)-1 ))
function rand(){ # pick a random number from 0 to N-1. Max N is 2^32 local -i N=$1 [[ ${#RANDCACHE[*]} -eq 0 ]] && { RANDCACHE=( $(od -An -tu4 -N1024 /dev/urandom) ); } # refill cache RET=$(( (RANDCACHE[-1]*N+1)/RAWRAND )) # pull last random number and scale unset RANDCACHE[${#RANDCACHE[*]}-1] # pop read random number
};
# test by generating a lot of random numbers, then effectively place them in bins and count how many are in each bin.
declare -i c; declare -ia BIN
for (( c=0; c<100000; c++ )); do rand 10 BIN[RET]+=1 # add to bin to check distribution
done
for (( c=0; c<10; c++ )); do printf "%d %d\n" $c ${BIN[c]}
done

UPDATE: That does not work so well for all N. It also wastes random bits if used with small N. Noting that (in this case) a 32 bit random number has enough entropy for 9 random numbers between 0 and 9 (10*9=1,000,000,000 <= 2*32) we can extract multiple random numbers from each 32 random source value.

#!/bin/bash
declare -ia RCACHE
declare -i RET # return value
declare -i ENT=2 # keep track of unused entropy as 2^(entropy)
declare -i RND=RANDOM%ENT # a store for unused entropy - start with 1 bit
declare -i BYTES=4 # size of unsigned random bytes returned by od
declare -i BITS=8*BYTES # size of random data returned by od in bits
declare -i CACHE=16 # number of random numbers to cache
declare -i MAX=2**BITS # quantum of entropy per cached random number
declare -i c
function rand(){ # pick a random number from 0 to 2^BITS-1 [[ ${#RCACHE[*]} -eq 0 ]] && { RCACHE=( $(od -An -tu$BYTES -N$CACHE /dev/urandom) ); } # refill cache - could use /dev/random if CACHE is small RET=${RCACHE[-1]} # pull last random number and scale unset RCACHE[${#RCACHE[*]}-1] # pop read random number
};
function randBetween(){ local -i N=$1 [[ ENT -lt N ]] && { # not enough entropy to supply ln(N)/ln(2) bits rand; RND=RET # get more random bits ENT=MAX # reset entropy } RET=RND%N # random number to return RND=RND/N # remaining randomness ENT=ENT/N # remaining entropy
};
declare -ia BIN
for (( c=0; c<100000; c++ )); do randBetween 10 BIN[RET]+=1
done
for c in ${BIN[*]}; do echo $c
done

Generate random number in the range of 0 to n (signed 16-bit integer). Result set in $RAND variable. For example:

#!/bin/bash
random()
{ local range=${1:-1} RAND=`od -t uI -N 4 /dev/urandom | awk '{print $2}'` let "RAND=$RAND%($range+1)"
}
n=10
while [ $(( n -=1 )) -ge "0" ]; do random 500 echo "$RAND"
done

What about:

perl -e 'print int rand 10, "\n"; '

Maybe I am a bit too late, but what about using jot to generate a random number within a range in Bash?

jot -r -p 3 1 0 1

This generates a random (-r) number with 3 decimal places precision (-p). In this particular case, you'll get one number between 0 and 1 (1 0 1). You can also print sequential data. The source of the random number, according to the manual, is:

Random numbers are obtained through arc4random(3) when no seed is specified, and through random(3) when a seed is given.

Based on the great answers of @Nelson, @Barun and @Robert, here is a Bash script that generates random numbers.

Can generate how many digits you want.
each digit is separately generated by /dev/urandom which is much better than Bash's built-in $RANDOM

#!/usr/bin/env bash
digits=10
rand=$(od -A n -t d -N 2 /dev/urandom |tr -d ' ')
num=$((rand % 10))
while [ ${#num} -lt $digits ]; do rand=$(od -A n -t d -N 1 /dev/urandom |tr -d ' ') num="${num}$((rand % 10))"
done
echo $num

Random branching of a program or yes/no; 1/0; true/false output:

if [ $RANDOM -gt 16383 ]; then # 16383 = 32767/2 echo var=true/1/yes/go_hither
else echo var=false/0/no/go_thither
fi

of if you lazy to remember 16383:

if (( RANDOM % 2 )); then echo "yes"
else echo "no"
fi

Generate random 3-digit number

This is great for creating sample data. Example: put all testing data in a directory called "test-create-volume-123", then after your test is done, zap the entire directory. By generating exactly three digits, you don't have weird sorting issues.

printf '%02d\n' $((1 + RANDOM % 100))

This scales down, e.g. to one digit:

printf '%01d\n' $((1 + RANDOM % 10))

It scales up, but only to four digits. See above as to why :)

Wanted to use /dev/urandom without dd and od

function roll() { local modulus=${1:-6}; echo $(( 1 + 0x$(env LC_CTYPE=C tr -dc '0-9a-fA-F' < /dev/urandom | head -c5 ) % $modulus )); }

Testing

$ roll
5
$ roll 12
12

Just how random is it?

$ (echo "count roll percentage"; i=0; while [ $i -lt 10000 ]; do roll; i=$((i+1)); done | sort | uniq -c | awk '{print $0,($1/10000*100)"%"}') | column -t
count roll percentage
1625 1 16.25%
1665 2 16.65%
1646 3 16.46%
1720 4 17.2%
1694 5 16.94%
1650 6 16.5%

A bash function that uses perl to generate a random number of n digits. Specify either the number of digits or a template of n 0s.

rand() { perl -E '$ARGV[0]||=""; $ARGV[0]=int($ARGV[0])||length($ARGV[0]); say join "", int(rand(9)+1)*($ARGV[0]?1:0), map { int(rand(10)) } (0..($ARGV[0]||0)-2)' $1
}

Usage:

$ rand 3
381
$ rand 000
728

Demonstration of calling rand n, for n between 0 and 15:

$ for n in {0..15}; do printf "%02d: %s\n" $n $(rand $n); done
00: 0
01: 3
02: 98
03: 139
04: 1712
05: 49296
06: 426697
07: 2431421
08: 82727795
09: 445682186
10: 6368501779
11: 51029574113
12: 602518591108
13: 5839716875073
14: 87572173490132
15: 546889624135868

Demonstration of calling rand n, for n a template of 0s between length 0 and 15

$ for n in {0..15}; do printf "%15s :%02d: %s\n" $(printf "%0${n}d" 0) $n $(rand $(printf "%0${n}d" 0)); done 0 :00: 0 0 :01: 0 00 :02: 70 000 :03: 201 0000 :04: 9751 00000 :05: 62237 000000 :06: 262860 0000000 :07: 1365194 00000000 :08: 83953419 000000000 :09: 838521776 0000000000 :10: 2355011586 00000000000 :11: 95040136057 000000000000 :12: 511889225898 0000000000000 :13: 7441263049018 00000000000000 :14: 11895209107156
000000000000000 :15: 863219624761093

You can use a seed:

RANDOM=$(date +%s%N | cut -b10-19)
echo $(( $RANDOM % 100 + 1 ))

Here is a function I wrote which will output a random number in a desired range>

Description:

random <min> <max>
Generate a random number from min to max, inclusive. Both min and max can be positive OR negative numbers, and the generated random number can be negative too, so long as the range (max - min + 1) is less than or equal to 32767. Max must be >= min.

The core of it is this:

random() { min="$1" max="$2" range=$((max - min + 1)) rand=$((min + (RANDOM % range))) echo "$rand"
}

Usage:

# general form: obtain a random number between min and max, inclusive
random <min> <max>
# Example: obtain a random number from -10 to 10, inclusive
random -10 10

This works from the bash built-in variable RANDOM, which probably just uses C rand() under the hood, since they both have a max value of 32767--see:

For the bash documentation, see man bash:

RANDOM
Each time this parameter is referenced, a random integer between 0 and 32767 is generated. The sequence of random numbers may be initialized by assigning a value to RANDOM. If RANDOM is unset, it loses its special properties, even if it is subsequently reset.

Robust, runnable, `source`able version of the script

Here is a much more robust version of my random function above. It includes full error checking, bounds checking, a help menu via random --help or random -h, and a special run_check feature which allows you to source OR run this script so that you can source it to import the random function into any other script--just like you can do in Python!

random.sh <-- click this link to always get the latest version from my eRCaGuy_dotfiles repo.

RETURN_CODE_SUCCESS=0
RETURN_CODE_ERROR=1
HELP_STR="\
Generate a random integer number according to the usage styles below.
USAGE STYLES: 'random' Generate a random number from 0 to 32767, inclusive (same as bash variable 'RANDOM'). 'random <max>' Generate a random number from 0 to 'max', inclusive. 'random <min> <max>' Generate a random number from 'min' to 'max', inclusive. Both 'min' and 'max' can be positive OR negative numbers, and the generated random number can be negative too, so long as the range (max - min + 1) is less than or equal to 32767. Max must be >= min.
This file is part of eRCaGuy_dotfiles:
"
print_help() { echo "$HELP_STR" | less -RFX
}
# Get a random number according to the usage styles above.
# See also `utils_rand()` in utilities.c:
#
random() { # PARSE ARGUMENTS # help menu if [ "$1" = "-h" ] || [ "$1" = "--help" ]; then print_help exit $RETURN_CODE_SUCCESS fi # 'random' if [ $# -eq 0 ]; then min=0 max="none" # 'random max' elif [ $# -eq 1 ]; then min=0 max="$1" # 'random min max' elif [ $# -eq 2 ]; then min="$1" max="$2" else echo "ERROR: too many arguments." exit "$RETURN_CODE_ERROR" fi # CHECK FOR ERRORS if [ "$max" = "none" ]; then rand="$RANDOM" echo "$rand" exit "$RETURN_CODE_SUCCESS" fi if [ "$max" -lt "$min" ]; then echo "ERROR: max ($max) < min ($min). Max must be >= min." exit "$RETURN_CODE_ERROR" fi # CALCULATE THE RANDOM NUMBER # See `man bash` and search for `RANDOM`. This is a limitation of that value. RAND_MAX=32767 range=$((max - min + 1)) if [ "$range" -gt "$RAND_MAX" ]; then echo "ERROR: the range (max - min + 1) is too large. Max allowed = $RAND_MAX, but actual" \ "range = ($max - $min + 1) = $range." exit "$RETURN_CODE_ERROR" fi # NB: `RANDOM` is a bash built-in variable. See `man bash`, and also here: # rand=$((min + (RANDOM % range))) echo "$rand"
}
# Set the global variable `run` to "true" if the script is being **executed** (not sourced) and
# `main` should run, and set `run` to "false" otherwise. One might source this script but intend
# NOT to run it if they wanted to import functions from the script.
# See:
# 1. *****
# 1. my answer:
# 1.
run_check() { # This is akin to `if __name__ == "__main__":` in Python. if [ "${FUNCNAME[-1]}" == "main" ]; then # This script is being EXECUTED, not sourced run="true" fi
}
# ----------------------------------------------------------------------------------------------------------------------
# Main program entry point
# ----------------------------------------------------------------------------------------------------------------------
# Only run main function if this file is being executed, NOT sourced.
run="false"
run_check
if [ "$run" == "true" ]; then random "$@"
fi

$(((RANDOM % $((upperBound - lowerBound))) + lowerBound))

the random number range is [lowerBound,upperBound)

Pure Bash random number without moduloing

lowerRange=10 # inclusive
upperRange=20 # exclusive
randomNumber=$(( RANDOM * ( upperRange - lowerRange) / 32767 + lowerRange ))

Pop Glow

How to generate random number in Bash?

25 Answers

Generate random 3-digit number

Robust, runnable, `source`able version of the script

Your Answer

Sign up or log in

Post as a guest

You Might Also Like

List of equipment you can parry with in Dark Souls 3?

How do I get the golden treasure in the Yoshi Sphinx?

What exactly does the shield protect you from?

What is the name of the view in games like Diablo 3, Starcraft, and Warcraft?

How to generate random number in Bash?

25 Answers

Generate random 3-digit number

Robust, runnable, sourceable version of the script

Your Answer

Sign up or log in

Post as a guest

You Might Also Like

Robust, runnable, `source`able version of the script