Using Parameter Expansion in POSIX Shell

In POSIX shell scripting, we can use parameter expansion to reference a variable

message="hello"
echo ${message}

The usefulness of this syntax is in its variations that handle variables when null/not null:

Form	Meaning
`${var:-word}`	use word if var unset/null
`${var:=word}`	use word and set var to word if var unset/null
`${var:+word}`	use word if var set and not null
`${var:?word}`	exit with error and print word if var unset/null