Generating random number with bounds produces unexpected output in Bash-CodePudding

I am trying to learn bash so I was working on a "guess the number" type game. I am at the point of trying to generate random numbers based on the user's input for lower and upper bounds. But some test cases seem to create numbers that don't make sense; some being outside my bounds and others not being what I would expect.

Here is my program

echo "Enter the lower bound: "
echo -n "> "
read lowerBound

while [ "$lowerBound" -lt 0 ]
do
    echo "Lower bound must be >= 0.  Please try again: "
    echo -n "> "
    read lowerBound
done

echo
echo "Enter the upper bound: "
echo -n "> "
read upperBound

lowerLimit=$(($lowerBound   1))

while [ "$upperBound" -lt "$lowerLimit" ]
do
    echo "Upper bound must be larger than lower bound. Please try again: "
    echo -n "> "
    read upperBound
done

echo
randNum=$(( $RANDOM % ( ($upperBound - $lowerBound)   1 ) ))
echo "RANDOM=$RANDOM randNum=$randNum"

Here is an output that produced unexpected results. I would expect 6: (9237 % (10-1 1) = 7 but output is 5)

Enter the lower bound:
> 1

Enter the upper bound:
> 10

RANDOM=9237 randNum=5

Finally here is some output where the generated number lies outside my bounds. I understand this from doing the math myself, but thought this equation was supposed to generate random numbers in a range. Here 26921 % (126-123 1) = 9237. Most likely just a coincidence that 9237 appeared in subsequent runs.

Enter the lower bound:
> 123

Enter the upper bound:
> 126

RANDOM=26921 randNum=0

I'm not sure if my equation is wrong or if it is something I am doing wrong in Bash. Any pointers would be appreciated.

Note: I am not new to programming, just new to Bash.

CodePudding user response：

Each reference of $RANDOM generates a new number.

You can see this with something as simple as:

$ echo "$RANDOM : $RANDOM : $RANDOM"
297 : 20330 : 14461

In your code you get one $RANDOM value when you calculate randNum, and a second (and likely different) $RANDOM value in the echo; net result is randNum=5 was not generated based on $RANDOM=9237.

If you want to reference the same $RANDOM value more than once you should first store it in a variable and then (re)use said variable as needed, eg:

ranx=$RANDOM
randNum=$(( $ranx % ( ($upperBound - $lowerBound)   1 ) ))
echo "RANDOM=$ranx randNum=$randNum"