The go-to source for randomness in Linux is /dev/urandom, is it not?

I’m not sure the best way to make use of it as a source of random decimals — there are surely a dozen ways to do so — but this works:

  printf %d 0x`
    head -c 1000 /dev/urandom
    | shasum 
    | head -c 8
  `

As always, my advice for things like this is that if you’re in need of true random numbers in your shell script then you should probably stop writing whatever it is you are writing as a shell script.