Advanced BashScripting Guide: A complete guide to shell scripting  

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$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 920. Generating random numbers
#!/bin/bash # $RANDOM returns a different random integer at each invocation. # Nominal range: 0  32767 (signed 16bit integer). MAXCOUNT=10 count=1 echo echo "$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 921. 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, plusorminus 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 922. Reseeding RANDOM
#!/bin/bash # seedingrandom.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 $2 }') # Pseudorandom output fetched from /dev/urandom (system pseudorandom "device"), # then converted to line of printable (octal) numbers by "od", # finally "awk" retrieves just one number for SEED. RANDOM=$SEED random_numbers echo; echo exit 0 
The /dev/urandom devicefile 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 wellscattered 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 1234). There are also other means of generating pseudorandom numbers in a script. Awk provides a convenient means of doing this. Example 923. Pseudorandom numbers, using awk
