Proper type casting in a shell script for use with while loop and modulus

Question

I am trying to write a script to get a random, even hex number. I have found the the openssl command has a convenient option for creating random hex numbers. Unfortunately, I need it to be even and my script has a type casting error somewhere. Bash thinks that my newly generated hex number is a string, so when I try to mod it by 2, the script fails. Here is what I have so far:

...
hexVal="$(openssl rand -hex 1)"
while [ `expr $hexVal % 2` -ne 0 ]
do
    hexVal="$(openssl rand -hex 1)"
done
...

I have tried various other combinations as well, to no avail. If someone could tell me what is wrong with my syntax, it would be greatly appreciated.

Answer 1

Using bash

To generate an even random number in hex:

$ printf '%x\n' $((2*$RANDOM))
d056

Or:

$ hexVal=$(printf '%x\n' $((2*$RANDOM)))
$ echo $hexVal
f58a

To limit the output to smaller numbers, use modulo, %:

$ printf '%x\n' $(( 2*$RANDOM % 256 ))
4a

Using openssl

If you really want to use a looping solution with openssl:

while hexVal="$(openssl rand -hex 1)"
do
    ((0x$hexVal % 2 == 0)) && break
done

The 0x signals that the number which follows is hex.

Rules for casting numbers in bash

From man bash:

Constants with a leading 0 are interpreted as octal numbers. A leading 0x or 0X denotes hexadecimal. Otherwise, numbers take the form [base#]n, where the optional base is a decimal number between 2 and 64 representing the arithmetic base, and n is a number in that base. If base# is omitted, then base 10 is used. When specifying n, the digits greater< than 9 are represented by the lowercase letters, the uppercase letters, @, and _, in that order. If base is less than or equal to 36, lowercase and uppercase letters may be used interchangeably to represent numbers between 10 and 35. [Emphasis added]

Answer 2

What about this instead

printf "%0x\n" $(( ($RANDOM*2) & 0xff))

Answer 3

hexVal=1

while (( 16#$hexVal % 2))
do
    hexVal=$(openssl rand -hex 1)
done

printf "%x [%d]\n"  0x$hexVal  0x$hexVal