Check operating system inside a bash script and change commands

check_os_ostype.sh

# Flush DNS cache
if [[ $OSTYPE == darwin* ]]; then
  # works on macOS
  alias flushdns='sudo dscacheutil -flushcache'
elif [[ $OSTYPE == linux* ]]; then
  # works on Ubuntu 18.04+
  alias flushdns='sudo systemd-resolve --flush-caches'
fi

check_os_uname.sh

UNAME=$(uname -s | tr '[:upper:]' '[:lower:]')

# example if/esle statement 1
if [[ $UNAME = darwin ]]; then
  echo "You are on macOS"
elif [[ $UNAME = linux ]]; then 
  echo "You are on Linux"
fi

# example if/esle statement 2
[[ $UNAME = darwin ]] && echo "You are on macOS" || echo "You are on macOS"

# example if/esle statement 3
if [[ $UNAME = darwin ]]; then echo "You are on macOS"; else echo "You are on macOS"; fi