Linear congruential generator (POSIX-ish shell)

lcgrng2

#! /bin/bash

# If the special variable isn't available (e.g. dash shell), we fall back to a BSD-style
# Linear congruential generator (LCG) which we use to create our own '$RANDOM' variable
# See: https://rosettacode.org/wiki/Linear_congruential_generator

# If two invocations of RANDOM are the same (or both are blank), 
# then we're working with a shell that does not support $RANDOM.
if [ "${RANDOM}" = "${RANDOM}" ]; then

  # We set the initial seed for the LCG
  # First we check if /dev/urandom is available.
  if [ -c /dev/urandom ] && command -v od >/dev/null 2>&1; then
    # Get a string of bytes from /dev/urandom using od
    rnSeed=$(od -N 4 -A n -t uL /dev/urandom | tr -d " ")
  # Otherwise we can just seed it using the epoch
  else
    rnSeed=$(date +%s)
  fi
 
  # BSD style LCG modulus'ed against 2^31
  rnSeed=$(( (1103515245 * rnSeed + 12345) % 2147483648 ))

  # Print as an unsigned integer, divided by 2^16
  RANDOM="$(printf "%u\n" "$(( rnSeed / 65536 ))")"
fi