Variables

${VAR+T} True if VAR is set, even if it is empty

${VAR:+T} True if VAR is set and non-null

${VAR:-default_value} use the value of VAR if VAR is set and non-null,
   otherwise use "default_value"

${VAR:+value} return "value" if VAR is set and non-null,
   otherwise return VAR