9.6. $RANDOM: generate random integer

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

Example 9-20. Generating random numbers

#!/bin/bash

# $RANDOM returns a different random integer at each invocation.
# Nominal range: 0 - 32767 (signed 16-bit integer).

MAXCOUNT=10
count=1

echo
echo "$MAXCOUNT random numbers:"
echo "-----------------"
while [ "$count" -le $MAXCOUNT ]      # Generate 10 ($MAXCOUNT) random integers.
do
  number=$RANDOM
  echo $number
  let "count += 1"  # Increment count.
done
echo "-----------------"

# If you need a random int within a certain range, use the 'modulo' operator.
# This returns the remainder of a division operation.

RANGE=500

echo

number=$RANDOM
let "number %= $RANGE"
echo "Random number less than $RANGE  ---  $number"

echo

# If you need a random int greater than a lower bound,
# then set up a test to discard all numbers below that.

FLOOR=200

number=0   #initialize
while [ "$number" -le $FLOOR ]
do
  number=$RANDOM
done
echo "Random number greater than $FLOOR ---  $number"
echo


# May combine above two techniques to retrieve random number between two limits.
number=0   #initialize
while [ "$number" -le $FLOOR ]
do
  number=$RANDOM
  let "number %= $RANGE"  # Scales $number down within $RANGE.
done
echo "Random number between $FLOOR and $RANGE ---  $number"
echo


# Generate binary choice, that is, "true" or "false" value.
BINARY=2
number=$RANDOM
T=1

let "number %= $BINARY"
# let "number >>= 14"    gives a better random distribution
# (right shifts out everything except last binary digit).
if [ "$number" -eq $T ]
then
  echo "TRUE"
else
  echo "FALSE"
fi  

echo


# May generate toss of the dice.
SPOTS=7   # Modulo 7 gives range 0 - 6.
DICE=2
ZERO=0
die1=0
die2=0

# Tosses each die separately, and so gives correct odds.

  while [ "$die1" -eq $ZERO ]     # Can't have a zero come up.
  do
    let "die1 = $RANDOM % $SPOTS" # Roll first one.
  done  

  while [ "$die2" -eq $ZERO ]
  do
    let "die2 = $RANDOM % $SPOTS" # Roll second one.
  done  

let "throw = $die1 + $die2"
echo "Throw of the dice = $throw"
echo


exit 0

Just how random is RANDOM? The best way to test this is to write a script that tracks the distribution of "random" numbers generated by RANDOM. Let's roll a RANDOM die a few times...

Example 9-21. Rolling the die with RANDOM

#!/bin/bash
# How random is RANDOM?

RANDOM=$$       # Reseed the random number generator using script process ID.

PIPS=6          # A die has 6 pips.
MAXTHROWS=600   # Increase this, if you have nothing better to do with your time.
throw=0         # Throw count.

zeroes=0        # Must initialize counts to zero.
ones=0          # since an uninitialized variable is null, not zero.
twos=0
threes=0
fours=0
fives=0
sixes=0

print_result ()
{
echo
echo "ones =   $ones"
echo "twos =   $twos"
echo "threes = $threes"
echo "fours =  $fours"
echo "fives =  $fives"
echo "sixes =  $sixes"
echo
}

update_count()
{
case "$1" in
  0) let "ones += 1";;   # Since die has no "zero", this corresponds to 1.
  1) let "twos += 1";;   # And this to 2, etc.
  2) let "threes += 1";;
  3) let "fours += 1";;
  4) let "fives += 1";;
  5) let "sixes += 1";;
esac
}

echo


while [ "$throw" -lt "$MAXTHROWS" ]
do
  let "die1 = RANDOM % $PIPS"
  update_count $die1
  let "throw += 1"
done  

print_result

# The scores should distribute fairly evenly, assuming RANDOM is fairly random.
# With $MAXTHROWS at 600, all should cluster around 100, plus-or-minus 20 or so.
#
# Keep in mind that RANDOM is a pseudorandom generator,
# and not a spectacularly good one at that.

# Exercise for the reader (easy):
# Rewrite this script to flip a coin 1000 times.
# Choices are "HEADS" or "TAILS".

exit 0

As we have seen in the last example, it is best to "reseed" the RANDOM generator each time it is invoked. Using the same seed for RANDOM repeats the same series of numbers. (This mirrors the behavior of the random() function in C.)

Example 9-22. Reseeding RANDOM

#!/bin/bash
# seeding-random.sh: Seeding the RANDOM variable.

MAXCOUNT=25       # How many numbers to generate.

random_numbers ()
{
count=0
while [ "$count" -lt "$MAXCOUNT" ]
do
  number=$RANDOM
  echo -n "$number "
  let "count += 1"
done  
}

echo; echo

RANDOM=1          # Setting RANDOM seeds the random number generator.
random_numbers

echo; echo

RANDOM=1          # Same seed for RANDOM...
random_numbers    # ...reproduces the exact same number series.

echo; echo

RANDOM=2          # Trying again, but with a different seen...
random_numbers    # gives a different number series.

echo; echo

# RANDOM=$$  seeds RANDOM from process id of script.
# It is also possible to seed RANDOM from 'time' or 'date'.

# Getting fancy...
SEED=$(head -1 /dev/urandom | od -N 1 | awk '{ print $2 }')
# Pseudo-random output fetched from /dev/urandom (system pseudo-random "device"),
# then converted to line of printable (octal) numbers by "od",
# finally "awk" retrieves just one number for SEED.
RANDOM=$SEED
random_numbers

echo; echo

exit 0

Note

The /dev/urandom device-file provides a means of generating much more "random" pseudorandom numbers than the $RANDOM variable. dd if=/dev/urandom of=targetfile bs=1 count=XX creates a file of well-scattered pseudorandom numbers. However, assigning these numbers to a variable in a script requires a workaround, such as filtering through od (as in above example) or using dd (see Example 12-34).

There are also other means of generating pseudorandom numbers in a script. Awk provides a convenient means of doing this.

Example 9-23. Pseudorandom numbers, using awk

#!/bin/bash
# random2.sh: Returns a pseudorandom number in the range 0 - 1.
# Uses the awk rand() function.

AWKSCRIPT=' { srand(); print rand() } '
# Command(s) / parameters passed to awk
# Note that srand() reseeds awk's random number generator.

echo -n "Random number between 0 and 1 = "
echo | awk "$AWKSCRIPT"

exit 0


# Exercises for the reader:
# -------------------------

# 1] Using a loop construct, print out 10 different random numbers.
#      (Hint: you must reseed the "srand()" function with a different seed
#      in each pass through the loop. What happens if you fail to do this?)

# 2] Using an integer multiplier as a scaling factor, generate random numbers 
#    in the range between 10 and 100.

# 3] Same as exercise #2, above, but generate random integers this time.