9.6. $RANDOM: generate random integer

   1 #!/bin/bash
   2 
   3 # $RANDOM returns a different random integer at each invocation.
   4 # Nominal range: 0 - 32767 (signed 16-bit integer).
   5 
   6 MAXCOUNT=10
   7 count=1
   8 
   9 echo
  10 echo "$MAXCOUNT random numbers:"
  11 echo "-----------------"
  12 while [ "$count" -le $MAXCOUNT ]      # Generate 10 ($MAXCOUNT) random integers.
  13 do
  14   number=$RANDOM
  15   echo $number
  16   let "count += 1"  # Increment count.
  17 done
  18 echo "-----------------"
  19 
  20 # If you need a random int within a certain range, use the 'modulo' operator.
  21 # This returns the remainder of a division operation.
  22 
  23 RANGE=500
  24 
  25 echo
  26 
  27 number=$RANDOM
  28 let "number %= $RANGE"
  29 echo "Random number less than $RANGE  ---  $number"
  30 
  31 echo
  32 
  33 # If you need a random int greater than a lower bound,
  34 # then set up a test to discard all numbers below that.
  35 
  36 FLOOR=200
  37 
  38 number=0   #initialize
  39 while [ "$number" -le $FLOOR ]
  40 do
  41   number=$RANDOM
  42 done
  43 echo "Random number greater than $FLOOR ---  $number"
  44 echo
  45 
  46 
  47 # May combine above two techniques to retrieve random number between two limits.
  48 number=0   #initialize
  49 while [ "$number" -le $FLOOR ]
  50 do
  51   number=$RANDOM
  52   let "number %= $RANGE"  # Scales $number down within $RANGE.
  53 done
  54 echo "Random number between $FLOOR and $RANGE ---  $number"
  55 echo
  56 
  57 
  58 # Generate binary choice, that is, "true" or "false" value.
  59 BINARY=2
  60 number=$RANDOM
  61 T=1
  62 
  63 let "number %= $BINARY"
  64 # let "number >>= 14"    gives a better random distribution
  65 # (right shifts out everything except last binary digit).
  66 if [ "$number" -eq $T ]
  67 then
  68   echo "TRUE"
  69 else
  70   echo "FALSE"
  71 fi  
  72 
  73 echo
  74 
  75 
  76 # May generate toss of the dice.
  77 SPOTS=7   # Modulo 7 gives range 0 - 6.
  78 DICE=2
  79 ZERO=0
  80 die1=0
  81 die2=0
  82 
  83 # Tosses each die separately, and so gives correct odds.
  84 
  85   while [ "$die1" -eq $ZERO ]     # Can't have a zero come up.
  86   do
  87     let "die1 = $RANDOM % $SPOTS" # Roll first one.
  88   done  
  89 
  90   while [ "$die2" -eq $ZERO ]
  91   do
  92     let "die2 = $RANDOM % $SPOTS" # Roll second one.
  93   done  
  94 
  95 let "throw = $die1 + $die2"
  96 echo "Throw of the dice = $throw"
  97 echo
  98 
  99 
 100 exit 0

   1 #!/bin/bash
   2 # How random is RANDOM?
   3 
   4 RANDOM=$$       # Reseed the random number generator using script process ID.
   5 
   6 PIPS=6          # A die has 6 pips.
   7 MAXTHROWS=600   # Increase this, if you have nothing better to do with your time.
   8 throw=0         # Throw count.
   9 
  10 zeroes=0        # Must initialize counts to zero.
  11 ones=0          # since an uninitialized variable is null, not zero.
  12 twos=0
  13 threes=0
  14 fours=0
  15 fives=0
  16 sixes=0
  17 
  18 print_result ()
  19 {
  20 echo
  21 echo "ones =   $ones"
  22 echo "twos =   $twos"
  23 echo "threes = $threes"
  24 echo "fours =  $fours"
  25 echo "fives =  $fives"
  26 echo "sixes =  $sixes"
  27 echo
  28 }
  29 
  30 update_count()
  31 {
  32 case "$1" in
  33   0) let "ones += 1";;   # Since die has no "zero", this corresponds to 1.
  34   1) let "twos += 1";;   # And this to 2, etc.
  35   2) let "threes += 1";;
  36   3) let "fours += 1";;
  37   4) let "fives += 1";;
  38   5) let "sixes += 1";;
  39 esac
  40 }
  41 
  42 echo
  43 
  44 
  45 while [ "$throw" -lt "$MAXTHROWS" ]
  46 do
  47   let "die1 = RANDOM % $PIPS"
  48   update_count $die1
  49   let "throw += 1"
  50 done  
  51 
  52 print_result
  53 
  54 # The scores should distribute fairly evenly, assuming RANDOM is fairly random.
  55 # With $MAXTHROWS at 600, all should cluster around 100, plus-or-minus 20 or so.
  56 #
  57 # Keep in mind that RANDOM is a pseudorandom generator,
  58 # and not a spectacularly good one at that.
  59 
  60 # Exercise for the reader (easy):
  61 # Rewrite this script to flip a coin 1000 times.
  62 # Choices are "HEADS" or "TAILS".
  63 
  64 exit 0

   1 #!/bin/bash
   2 # seeding-random.sh: Seeding the RANDOM variable.
   3 
   4 MAXCOUNT=25       # How many numbers to generate.
   5 
   6 random_numbers ()
   7 {
   8 count=0
   9 while [ "$count" -lt "$MAXCOUNT" ]
  10 do
  11   number=$RANDOM
  12   echo -n "$number "
  13   let "count += 1"
  14 done  
  15 }
  16 
  17 echo; echo
  18 
  19 RANDOM=1          # Setting RANDOM seeds the random number generator.
  20 random_numbers
  21 
  22 echo; echo
  23 
  24 RANDOM=1          # Same seed for RANDOM...
  25 random_numbers    # ...reproduces the exact same number series.
  26 
  27 echo; echo
  28 
  29 RANDOM=2          # Trying again, but with a different seen...
  30 random_numbers    # gives a different number series.
  31 
  32 echo; echo
  33 
  34 # RANDOM=$$  seeds RANDOM from process id of script.
  35 # It is also possible to seed RANDOM from 'time' or 'date'.
  36 
  37 # Getting fancy...
  38 SEED=$(head -1 /dev/urandom | od -N 1 | awk '{ print $2 }')
  39 # Pseudo-random output fetched from /dev/urandom (system pseudo-random "device"),
  40 # then converted to line of printable (octal) numbers by "od",
  41 # finally "awk" retrieves just one number for SEED.
  42 RANDOM=$SEED
  43 random_numbers
  44 
  45 echo; echo
  46 
  47 exit 0