It is not blackmagic, it is bashmagic

Generating a simple* random number in bash is really easy.

Let say you need a random number between 10 and 30.
Set the variables $MIN and $MAX
MAX=30
MIN=10

Generate the random number in variable $T.
T=$((MIN+$RANDOM%$((MAX-MIN))))

Notice the $RANDOM that is build into bash.

* Simple as in don't use this for security applications.