# 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 9-23. 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. 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```

Example 9-24. Picking a random card from a deck

 ```#!/bin/bash # pick-card.sh # This is an example of choosing a random element of an array. # Pick a card, any card. Suites="Clubs Diamonds Hearts Spades" Denominations="2 3 4 5 6 7 8 9 10 Jack Queen King Ace" suite=(\$Suites) # Read into array variable. denomination=(\$Denominations) num_suites=\${#suite[*]} # Count how many elements. num_denominations=\${#denomination[*]} echo -n "\${denomination[\$((RANDOM%num_denominations))]} of " echo \${suite[\$((RANDOM%num_suites))]} # \$bozo sh pick-cards.sh # Jack of Clubs # Thank you, "jipe," for pointing out this use of \$RANDOM. exit 0``` Jipe points out another set of techniques for generating random numbers within a range.

 ```# Generate random number between 6 and 30. rnumber=\$((RANDOM%25+6)) # Generate random number in the same 6 - 30 range, #+ but the number must be evenly divisible by 3. rnumber=\$(((RANDOM%30/3+1)*3)) # Exercise: Try to figure out the pattern here.```

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 9-25. 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 (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 9-26. 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. # # When is it useful to duplicate a "random" number series? echo; echo RANDOM=2 # Trying again, but with a different seed... 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' commands. # Getting fancy... SEED=\$(head -1 /dev/urandom | od -N 1 | awk '{ print \$2 }') # Pseudo-random output fetched #+ from /dev/urandom (system pseudo-random device-file), #+ 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``` 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 12-42).

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

Example 9-27. 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: # --------- # 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.```