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