Linux under Variable configuration rules

Tips: In the example below, the Var and str are variables and we want to determine the value of Var against whether Str has a configuration. In general, str: for "STR is not configured or empty string", and Str is only "no this variable".

How variables are configured	STR Not Configured	Str is an empty string	STR is configured not to be an empty string
VAR=${STR-EXPR}	var=expr	Var=	Var= $str
VAR=${STR:-EXPR}	var=expr	var=expr	Var= $str
VAR=${STR+EXPR}	Var=	var=expr	var=expr
VAR=${STR:+EXPR}	Var=	Var=	var=expr
VAR=${STR=EXPR}	str=expr var=expr	STR does not change Var=	STR does not change Var= $str
VAR=${STR:=EXPR}	str=expr var=expr	str=expr var=expr	STR does not change Var= $str
VAR=${STR?EXPR}	Expr output to stderr	Var=	Var= $str
VAR=${STR:?EXPR}	Expr output to stderr	Expr output to stderr	Var= $str

Linux under Variable configuration rules