Bash script to convert milliseconds since epoch to human readable iso-8601 local time

ms-to-local

#!/usr/bin/env bash

millis=${1-}

if [ -z "$millis" ]; then
  script="${0##*/}"
  echo "usage: $script milliseconds-since-epoch"
  echo "example: $script 1620418406902"
  exit 1
fi

float=$(echo "scale=3; $millis / 1000" | bc)

command="date"
# add for MacOS portability, requires coreutils to be installed via homebrew
if [ ! -z "$(which gdate)" ]; then
  command="gdate"
fi
$command -d "@$float" --iso-8601='seconds'