Bash: Random numbers for fun and profit

This post appeared originally in our sysadvent series and has been moved here following the discontinuation of the sysadvent microsite

bash has many things that just works automagically. Did you know it has a built-in pseudo-random number generator? Let’s play games!

Before continuing, note that this is pseudo-randomization with a small footprint. DO NOT use it for security, scrambling, passwords, or anything even scarcely security related.

The random function in bash is called by the magic variable RANDOM. It gives you a more or less random integer between 0 and 32767.

$ echo $RANDOM
720
$ echo $RANDOM
29582

This seems optimized for silly games:

#!/bin/bash
echo -n "Let's play a game: Pick a number between 0 and 32767: "
read a
if (( RANDOM == a )); then
  echo AMAZING, your answer is correct
else
  echo Sorry, that was wrong
fi

Note that “unsetting” the variable removes the magic for the rest of the shell session.

$ unset RANDOM
$ RANDOM=42
$ echo $RANDOM
42
$ echo $RANDOM
42

You may format the output by using the modulo operator % and the built-in integer arithmetics in bash, called by ((expression)):

# A number 0-9
$ echo $(( RANDOM % 10 ))
4

# A number 0-99
$ echo $(( RANDOM % 100 ))
67

# A number 0-999
$ echo $(( RANDOM % 1000 ))
783

So we can make our game a bit more interesting:

#!/bin/bash
prs=("paper" "rock" "scissors")
echo -n "Get ready Player 1: Choose ${prs[@]}: "
read a

case $a in
  "paper") a=0;; "rock")  a=1;; "scissors") a=2;;
  *) echo No cheating; exit 1 ;;
esac

echo "You chose ${prs[$a]}"

b=$(( RANDOM % 3 ))
echo "The computer choose ${prs[$b]}"

if (( a == b )); then echo "A draw!"; exit 0; fi

case "$a$b" in
  01) winner="You";;
  02) winner="The computer";;
  10) winner="The computer";;
  12) winner="You";;
  20) winner="You";;
  21) winner="The computer";;
esac

echo "Winner: $winner"

One might think that, since RANDOM is a number between 0 and 32k, it would be skewed away from the decimal system, so let’s check that: Picking 10000 numbers between 0 and 999, the mean should be about 499,5.

for n in 1 2 3 4 5; do for i in $(seq 1 10000); do
  echo $((RANDOM%1000));
done | awk '{SUM+=$1} END{print SUM/10000}'; done

498.253
501.122
495.839
503.898
498.537

So, it seems this is good enough for most purposes.

For a testing purpose, I need a tree of directories with variable depth sub-directories and some diverse data:

cd "$(mktemp -d)"
for n in $(seq 1 100); do
  depth=$((RANDOM%10))
  for i in $(seq 1 $depth); do
    dir=dir$((RANDOM%10))
    mkdir -p $dir;
    pushd $dir;
  done;
  echo $((RANDOM%100)) > file$((RANDOM%100));
  for i in $(seq 1 $depth); do popd; done;
done;
find; echo; ls

Some times I need to test a script or a function with just true or false values. RANDOM can be used for this as well:

for i in $(seq 1 10); do
  if (( RANDOM %2 )); then
    echo true
  else
    echo false
  fi
done

false
true
true
false
false
false
true
false
true
false

To sum up: Bash has a handy random number generator, called by the magic variable RANDOM. It should NOT be used anything related to security. Also, make sure you do not use the variable name RANDOM for other purposes, as the behaviour will be, well, random.