test for the OS with the shell environment variable $OSTYPE

OSTYPE test

#!/bin/bash -x
#
# From: http://stackoverflow.com/questions/394230/detect-the-os-from-a-bash-script

  PLATFORM=$(uname)
  case "$PLATFORM" in
  "Darwin")
    # macOS
    ;;
  "Linux")
    # Linux distro, 
    ;;
  esac


case "$OSTYPE" in
  solaris*) echo "SOLARIS" ;;
  darwin*)  echo "OSX" ;; 
  linux*)   echo "LINUX" ;;
  bsd*)     echo "BSD" ;;
  *)        echo "unknown: $OSTYPE" ;;
esac


if [[ "$OSTYPE" == "linux-gnu" ]]; then
  # ...
  echo "linux-gnu"
elif [[ "$OSTYPE" == "darwin"* ]]; then
  # ...
  echo "darwin"
elif [[ "$OSTYPE" == "cygwin" ]]; then
  # ...
  echo "cygwin"
elif [[ "$OSTYPE" == "win32" ]]; then
  # ...
  echo "win32" 
elif [[ "$OSTYPE" == "freebsd" ]]; then
  # ...
  echo "freebsd"
else
  # Unknown.
  echo "Wow, that's a weird one.\n"
fi