${parameter:-word}
     If parameter is set and  is  non-null,  substitute  its
     value; otherwise substitute word.

${parameter:=word}
     If parameter is not set or is null set it to word;  the
     value  of  the  parameter  is  substituted.  Positional
     parameters may not be assigned in this way.

${parameter:?word}
     If parameter is set and  is  non-null,  substitute  its
     value;  otherwise,  print word and exit from the shell.
     If word is omitted, the message "parameter null or  not
     set" is printed.

${parameter:+word}
     If parameter is set and is non-null,  substitute  word;
     otherwise substitute nothing.

If the colon (:)  is omitted from the above expressions, the
shell only checks whether parameter is set or not.