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.

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

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

I wrote several articles on this.

• https://linuxconfig.org/generating-random-numbers-in-bash-with-examples
• https://linuxconfig.org/random-entropy-in-bash
• https://www.cloudsavvyit.com/7572/how-to-generate-better-random-numbers-at-the-bash-command-line/

$ 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.

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

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%

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 :)

You can use a seed, see documentation:

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

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

Pure Bash random number without moduloing

lowerRange=10   # inclusive
upperRange=20   # exclusive

randomNumber=$(( RANDOM * ( upperRange - lowerRange) / 32767 + lowerRange ))

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:

1. https://en.cppreference.com/w/c/numeric/random/rand
2. https://en.cppreference.com/w/c/numeric/random/RAND_MAX

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, sourceable 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: https://github.com/ElectricRCAircraftGuy/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:
# https://github.com/ElectricRCAircraftGuy/eRCaGuy_hello_world/blob/master/c/utilities.c#L176
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:
    # https://stackoverflow.com/a/1195035/4561887
    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. *****https://github.com/ElectricRCAircraftGuy/eRCaGuy_hello_world/blob/master/bash/argument_parsing__3_advanced__gen_prog_template.sh
# 1. my answer: https://stackoverflow.com/a/70662049/4561887
# 1. https://github.com/ElectricRCAircraftGuy/eRCaGuy_hello_world/blob/master/bash/check_if_sourced_or_executed.sh
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)