Some useful bash functions to check if something doesn't exist in Linux OS.

doesnt_exist.sh

# check if user doesn't exist
function user_doesnt_exist {
  if id -u "$1" >/dev/null 2>&1; then
    return 1
  else
    return 0
  fi
}

# usage. do something if the user abc doesn't exist.
if user_doesnt_exist abc; then
  echo "abc user doesn't exist."
fi


# check if program doesn't exist
function program_doesnt_exist {
  # set to 1 initially
  local return_=1
  # set to 0 if not found
  type $1 >/dev/null 2>&1 || { local return_=0; }
  # return value
  return $return_
}

# usage. install node.js if it doesn't exist.
if program_doesnt_exist node; then
  curl -sL https://deb.nodesource.com/setup_11.x | sudo -E bash -
  sudo apt install -y nodejs
fi