Evaluating shell variables

The following set of rules govern the evaluation of all shell variables.

Definition            Description

$var                  signifies the value of var or nothing,
                      if var is undefined.

${var}                same as above except the braces enclose
                      the name of the variable to be substituted.

${var-thing}          value of var if var is defined; otherwise thing.
                      $var is not set to thing.

${var=thing}          value of var if var is defined; otherwise thing.
                      If undefined $var is set to thing.

${var?message}        If defined, $var; otherwise print message
                      and exit the shell.  If the message is
                      empty, print a standard message.

${var+thing}          thing if $var is defined, otherwise nothing.