--------------------- Table 5.3 ---------------------

$var            value of var; nothing if var is undefined
${var}          same; useful if alphanumerics follows variable name
${var-thing}    value of var if defined; otherwise thing.
                    $var unchanged.
${var=thing}    value of var if defined; otherwise thing.
                    If undefined, $var set to thing
${var?message}  if defined, $var. Otherwise print message
                    and exit shell. if Message empty, print:
                        var: parameter not set
${var+thing}    thing if $var defined, otherwise nothing