# 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. It should not be used to generate an encryption key.

Example 9-26. Generating random numbers

 ``` 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 # ^^ 30 echo "Random number less than \$RANGE --- \$number" 31  32 echo 33  34  35  36 # If you need a random integer greater than a lower bound, 37 #+ then set up a test to discard all numbers below that. 38  39 FLOOR=200 40  41 number=0 #initialize 42 while [ "\$number" -le \$FLOOR ] 43 do 44  number=\$RANDOM 45 done 46 echo "Random number greater than \$FLOOR --- \$number" 47 echo 48  49  # Let's examine a simple alternative to the above loop, namely 50  # let "number = \$RANDOM + \$FLOOR" 51  # That would eliminate the while-loop and run faster. 52  # But, there might be a problem with that. What is it? 53  54  55  56 # Combine above two techniques to retrieve random number between two limits. 57 number=0 #initialize 58 while [ "\$number" -le \$FLOOR ] 59 do 60  number=\$RANDOM 61  let "number %= \$RANGE" # Scales \$number down within \$RANGE. 62 done 63 echo "Random number between \$FLOOR and \$RANGE --- \$number" 64 echo 65  66  67  68 # Generate binary choice, that is, "true" or "false" value. 69 BINARY=2 70 T=1 71 number=\$RANDOM 72  73 let "number %= \$BINARY" 74 # Note that let "number >>= 14" gives a better random distribution 75 #+ (right shifts out everything except last binary digit). 76 if [ "\$number" -eq \$T ] 77 then 78  echo "TRUE" 79 else 80  echo "FALSE" 81 fi 82  83 echo 84  85  86 # Generate a toss of the dice. 87 SPOTS=6 # Modulo 6 gives range 0 - 5. 88  # Incrementing by 1 gives desired range of 1 - 6. 89  # Thanks, Paulo Marcel Coelho Aragao, for the simplification. 90 die1=0 91 die2=0 92 # Would it be better to just set SPOTS=7 and not add 1? Why or why not? 93  94 # Tosses each die separately, and so gives correct odds. 95  96  let "die1 = \$RANDOM % \$SPOTS +1" # Roll first one. 97  let "die2 = \$RANDOM % \$SPOTS +1" # Roll second one. 98  # Which arithmetic operation, above, has greater precedence -- 99  #+ modulo (%) or addition (+)? 100  101  102 let "throw = \$die1 + \$die2" 103 echo "Throw of the dice = \$throw" 104 echo 105  106  107 exit 0```

Example 9-27. Picking a random card from a deck

 ``` 1 #!/bin/bash 2 # pick-card.sh 3  4 # This is an example of choosing random elements of an array. 5  6  7 # Pick a card, any card. 8  9 Suites="Clubs 10 Diamonds 11 Hearts 12 Spades" 13  14 Denominations="2 15 3 16 4 17 5 18 6 19 7 20 8 21 9 22 10 23 Jack 24 Queen 25 King 26 Ace" 27  28 # Note variables spread over multiple lines. 29  30  31 suite=(\$Suites) # Read into array variable. 32 denomination=(\$Denominations) 33  34 num_suites=\${#suite[*]} # Count how many elements. 35 num_denominations=\${#denomination[*]} 36  37 echo -n "\${denomination[\$((RANDOM%num_denominations))]} of " 38 echo \${suite[\$((RANDOM%num_suites))]} 39  40  41 # \$bozo sh pick-cards.sh 42 # Jack of Clubs 43  44  45 # Thank you, "jipe," for pointing out this use of \$RANDOM. 46 exit 0```

Jipe points out a set of techniques for generating random numbers within a range.
 ``` 1 # Generate random number between 6 and 30. 2  rnumber=\$((RANDOM%25+6)) 3  4 # Generate random number in the same 6 - 30 range, 5 #+ but the number must be evenly divisible by 3. 6  rnumber=\$(((RANDOM%30/3+1)*3)) 7  8 # Note that this will not work all the time. 9 # It fails if \$RANDOM%30 returns 0. 10  11 # Frank Wang suggests the following alternative: 12  rnumber=\$(( RANDOM%27/3*3+6 ))```

Bill Gradwohl came up with an improved formula that works for positive numbers.
 ` 1 rnumber=\$(((RANDOM%(max-min+divisibleBy))/divisibleBy*divisibleBy+min))`

Here Bill presents a versatile function that returns a random number between two specified values.

Example 9-28. Random between values

 ``` 1 #!/bin/bash 2 # random-between.sh 3 # Random number between two specified values. 4 # Script by Bill Gradwohl, with minor modifications by the document author. 5 # Used with permission. 6  7  8 randomBetween() { 9  # Generates a positive or negative random number 10  #+ between \$min and \$max 11  #+ and divisible by \$divisibleBy. 12  # Gives a "reasonably random" distribution of return values. 13  # 14  # Bill Gradwohl - Oct 1, 2003 15  16  syntax() { 17  # Function embedded within function. 18  echo 19  echo "Syntax: randomBetween [min] [max] [multiple]" 20  echo 21  echo -n "Expects up to 3 passed parameters, " 22  echo "but all are completely optional." 23  echo "min is the minimum value" 24  echo "max is the maximum value" 25  echo -n "multiple specifies that the answer must be " 26  echo "a multiple of this value." 27  echo " i.e. answer must be evenly divisible by this number." 28  echo 29  echo "If any value is missing, defaults area supplied as: 0 32767 1" 30  echo -n "Successful completion returns 0, " 31  echo "unsuccessful completion returns" 32  echo "function syntax and 1." 33  echo -n "The answer is returned in the global variable " 34  echo "randomBetweenAnswer" 35  echo -n "Negative values for any passed parameter are " 36  echo "handled correctly." 37  } 38  39  local min=\${1:-0} 40  local max=\${2:-32767} 41  local divisibleBy=\${3:-1} 42  # Default values assigned, in case parameters not passed to function. 43  44  local x 45  local spread 46  47  # Let's make sure the divisibleBy value is positive. 48  [ \${divisibleBy} -lt 0 ] && divisibleBy=\$((0-divisibleBy)) 49  50  # Sanity check. 51  if [ \$# -gt 3 -o \${divisibleBy} -eq 0 -o \${min} -eq \${max} ]; then 52  syntax 53  return 1 54  fi 55  56  # See if the min and max are reversed. 57  if [ \${min} -gt \${max} ]; then 58  # Swap them. 59  x=\${min} 60  min=\${max} 61  max=\${x} 62  fi 63  64  # If min is itself not evenly divisible by \$divisibleBy, 65  #+ then fix the min to be within range. 66  if [ \$((min/divisibleBy*divisibleBy)) -ne \${min} ]; then 67  if [ \${min} -lt 0 ]; then 68  min=\$((min/divisibleBy*divisibleBy)) 69  else 70  min=\$((((min/divisibleBy)+1)*divisibleBy)) 71  fi 72  fi 73  74  # If max is itself not evenly divisible by \$divisibleBy, 75  #+ then fix the max to be within range. 76  if [ \$((max/divisibleBy*divisibleBy)) -ne \${max} ]; then 77  if [ \${max} -lt 0 ]; then 78  max=\$((((max/divisibleBy)-1)*divisibleBy)) 79  else 80  max=\$((max/divisibleBy*divisibleBy)) 81  fi 82  fi 83  84  # --------------------------------------------------------------------- 85  # Now, to do the real work. 86  87  # Note that to get a proper distribution for the end points, 88  #+ the range of random values has to be allowed to go between 89  #+ 0 and abs(max-min)+divisibleBy, not just abs(max-min)+1. 90  91  # The slight increase will produce the proper distribution for the 92  #+ end points. 93  94  # Changing the formula to use abs(max-min)+1 will still produce 95  #+ correct answers, but the randomness of those answers is faulty in 96  #+ that the number of times the end points (\$min and \$max) are returned 97  #+ is considerably lower than when the correct formula is used. 98  # --------------------------------------------------------------------- 99  100  spread=\$((max-min)) 101  # Omair Eshkenazi points out that this test is unnecessary, 102  #+ since max and min have already been switched around. 103  [ \${spread} -lt 0 ] && spread=\$((0-spread)) 104  let spread+=divisibleBy 105  randomBetweenAnswer=\$(((RANDOM%spread)/divisibleBy*divisibleBy+min)) 106  107  return 0 108  109  # However, Paulo Marcel Coelho Aragao points out that 110  #+ when \$max and \$min are not divisible by \$divisibleBy, 111  #+ the formula fails. 112  # 113  # He suggests instead the following formula: 114  # rnumber = \$(((RANDOM%(max-min+1)+min)/divisibleBy*divisibleBy)) 115  116 } 117  118 # Let's test the function. 119 min=-14 120 max=20 121 divisibleBy=3 122  123  124 # Generate an array of expected answers and check to make sure we get 125 #+ at least one of each answer if we loop long enough. 126  127 declare -a answer 128 minimum=\${min} 129 maximum=\${max} 130  if [ \$((minimum/divisibleBy*divisibleBy)) -ne \${minimum} ]; then 131  if [ \${minimum} -lt 0 ]; then 132  minimum=\$((minimum/divisibleBy*divisibleBy)) 133  else 134  minimum=\$((((minimum/divisibleBy)+1)*divisibleBy)) 135  fi 136  fi 137  138  139  # If max is itself not evenly divisible by \$divisibleBy, 140  #+ then fix the max to be within range. 141  142  if [ \$((maximum/divisibleBy*divisibleBy)) -ne \${maximum} ]; then 143  if [ \${maximum} -lt 0 ]; then 144  maximum=\$((((maximum/divisibleBy)-1)*divisibleBy)) 145  else 146  maximum=\$((maximum/divisibleBy*divisibleBy)) 147  fi 148  fi 149  150  151 # We need to generate only positive array subscripts, 152 #+ so we need a displacement that that will guarantee 153 #+ positive results. 154  155 disp=\$((0-minimum)) 156 for ((i=\${minimum}; i<=\${maximum}; i+=divisibleBy)); do 157  answer[i+disp]=0 158 done 159  160  161 # Now loop a large number of times to see what we get. 162 loopIt=1000 # The script author suggests 100000, 163  #+ but that takes a good long while. 164  165 for ((i=0; i<\${loopIt}; ++i)); do 166  167  # Note that we are specifying min and max in reversed order here to 168  #+ make the function correct for this case. 169  170  randomBetween \${max} \${min} \${divisibleBy} 171  172  # Report an error if an answer is unexpected. 173  [ \${randomBetweenAnswer} -lt \${min} -o \${randomBetweenAnswer} -gt \${max} ] \ 174  && echo MIN or MAX error - \${randomBetweenAnswer}! 175  [ \$((randomBetweenAnswer%\${divisibleBy})) -ne 0 ] \ 176  && echo DIVISIBLE BY error - \${randomBetweenAnswer}! 177  178  # Store the answer away statistically. 179  answer[randomBetweenAnswer+disp]=\$((answer[randomBetweenAnswer+disp]+1)) 180 done 181  182  183  184 # Let's check the results 185  186 for ((i=\${minimum}; i<=\${maximum}; i+=divisibleBy)); do 187  [ \${answer[i+displacement]} -eq 0 ] \ 188  && echo "We never got an answer of \$i." \ 189  || echo "\${i} occurred \${answer[i+displacement]} times." 190 done 191  192  193 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 9-29. Rolling a single die with RANDOM

 ``` 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 ones=0 # Must initialize counts to zero, 11 twos=0 #+ since an uninitialized variable is null, not zero. 12 threes=0 13 fours=0 14 fives=0 15 sixes=0 16  17 print_result () 18 { 19 echo 20 echo "ones = \$ones" 21 echo "twos = \$twos" 22 echo "threes = \$threes" 23 echo "fours = \$fours" 24 echo "fives = \$fives" 25 echo "sixes = \$sixes" 26 echo 27 } 28  29 update_count() 30 { 31 case "\$1" in 32  0) let "ones += 1";; # Since die has no "zero", this corresponds to 1. 33  1) let "twos += 1";; # And this to 2, etc. 34  2) let "threes += 1";; 35  3) let "fours += 1";; 36  4) let "fives += 1";; 37  5) let "sixes += 1";; 38 esac 39 } 40  41 echo 42  43  44 while [ "\$throw" -lt "\$MAXTHROWS" ] 45 do 46  let "die1 = RANDOM % \$PIPS" 47  update_count \$die1 48  let "throw += 1" 49 done 50  51 print_result 52  53 exit 0 54  55 # The scores should distribute fairly evenly, assuming RANDOM is fairly random. 56 # With \$MAXTHROWS at 600, all should cluster around 100, plus-or-minus 20 or so. 57 # 58 # Keep in mind that RANDOM is a pseudorandom generator, 59 #+ and not a spectacularly good one at that. 60  61 # Randomness is a deep and complex subject. 62 # Sufficiently long "random" sequences may exhibit 63 #+ chaotic and other "non-random" behavior. 64  65 # Exercise (easy): 66 # --------------- 67 # Rewrite this script to flip a coin 1000 times. 68 # Choices are "HEADS" and "TAILS".```

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 9-30. Reseeding RANDOM

 ``` 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  # When is it useful to duplicate a "random" number series? 28  29 echo; echo 30  31 RANDOM=2 # Trying again, but with a different seed... 32 random_numbers # gives a different number series. 33  34 echo; echo 35  36 # RANDOM=\$\$ seeds RANDOM from process id of script. 37 # It is also possible to seed RANDOM from 'time' or 'date' commands. 38  39 # Getting fancy... 40 SEED=\$(head -1 /dev/urandom | od -N 1 | awk '{ print \$2 }') 41 # Pseudo-random output fetched 42 #+ from /dev/urandom (system pseudo-random device-file), 43 #+ then converted to line of printable (octal) numbers by "od", 44 #+ finally "awk" retrieves just one number for SEED. 45 RANDOM=\$SEED 46 random_numbers 47  48 echo; echo 49  50 exit 0``` The /dev/urandom pseudo-device file provides a method 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, Example 15-13, and Example A-37), or using dd (see Example 15-56), or even piping to md5sum (see Example 33-14).

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

Example 9-31. Pseudorandom numbers, using awk

 ``` 1 #!/bin/bash 2 # random2.sh: Returns a pseudorandom number in the range 0 - 1. 3 # Uses the awk rand() function. 4  5 AWKSCRIPT=' { srand(); print rand() } ' 6 # Command(s) / parameters passed to awk 7 # Note that srand() reseeds awk's random number generator. 8  9  10 echo -n "Random number between 0 and 1 = " 11  12 echo | awk "\$AWKSCRIPT" 13 # What happens if you leave out the 'echo'? 14  15 exit 0 16  17  18 # Exercises: 19 # --------- 20  21 # 1) Using a loop construct, print out 10 different random numbers. 22 # (Hint: you must reseed the "srand()" function with a different seed 23 #+ in each pass through the loop. What happens if you fail to do this?) 24  25 # 2) Using an integer multiplier as a scaling factor, generate random numbers 26 #+ in the range between 10 and 100. 27  28 # 3) Same as exercise #2, above, but generate random integers this time.```

The date command also lends itself to generating pseudorandom integer sequences.

### Notes

  True "randomness," insofar as it exists at all, can only be found in certain incompletely understood natural phenomena such as radioactive decay. Computers can only simulate randomness, and computer-generated sequences of "random" numbers are therefore referred to as pseudorandom.  The seed of a computer-generated pseudorandom number series can be considered an identification label. For example, think of the pseudorandom series with a seed of 23 as series #23.A property of a pseurandom number series is the length of the cycle before it starts repeating itself. A good pseurandom generator will produce series with very long cycles.