Deterministic random number generator in Bash

drandom.bash

# deterministic random number generator
# Usage: [DRANDOM_SEED=<whatever>] drandom [-h|--hex|--help]
# Outputs an integer between 0 and 2^256-1 or an equivalent hex string if -h|--hex is specified
drandom() {
  needs sha256sum || return 1
  # if unset, initialize to hash of nanoseconds since epoch,
  # otherwise, set to hash of previous seed
  if [ -z $DRANDOM_SEED ]; then
    export DRANDOM_SEED="$(date +%s%N | sha256sum | cut -d' ' -f1)"
  else
    export DRANDOM_SEED="$(echo -n $DRANDOM_SEED | sha256sum | cut -d' ' -f1)"
  fi
  case "$1" in
    --hex)
      echo $DRANDOM_SEED
      ;;
    -h|--help)
      echo "drandom is a deterministic random number generator."
      echo "Usage: [DRANDOM_SEED=<whatever>] drandom [--hex]"
      echo "Outputs an integer between 0 and 2^256-1 or an equivalent hex string if --hex is specified"
      echo "If no seed is specified, it will be seeded to the sha256 hash of the nanoseconds since epoch on first run."
      ;;
    *)
      # convert to unsigned int from 256-bit hex
      printf "%u\n" $((16#$DRANDOM_SEED))
      ;;
  esac
}

# well, this should be easy to test...
assert "$(DRANDOM_SEED=0 drandom)" == "10852665827039288777"
assert "$(DRANDOM_SEED=1 drandom --hex)" == "6b86b273ff34fce19d6b804eff5a3f5747ada4eaa22f1d49c01e52ddb7875b4b"