Advanced Bash-Scripting Guide: A complete guide to shell scripting, using Bash | ||
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$RANDOM is an internal Bash function (not a constant) that returns a |

**Example 9-18. 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 $T ] then echo "TRUE" else echo "FALSE" fi echo # May generate toss of the dice. SPOTS=7 # Modulo 7 gives range 0 - 6. DICE=2 ZERO=0 die1=0 die2=0 # Tosses each die separately, and so gives correct odds. while [ "$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 9-19. 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 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 9-20. 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. 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 $2 }') # Pseudo-random output fetched from /dev/urandom (system pseudo-random "device"), # then converted to line of printable (octal) numbers by "od", # finally "awk" retrieves just one number for SEED. RANDOM=$SEED random_numbers echo; echo exit 0 |

The |