Add support for more advanced ${xxx:...} syntax

Just ${xxx:+...} and ${xxx:-...} for now

Signed-off-by: Doug Davis <dug@us.ibm.com>

builder/shell_parser.go

@@ -157,7 +157,40 @@ func (sw *shellWord) processDollar() (string, error) {
 			sw.next()
 			return sw.getEnv(name), nil
 		}
-		return "", fmt.Errorf("Unsupported ${} substitution: %s", sw.word)
+		if ch == ':' {
+			// Special ${xx:...} format processing
+			// Yes it allows for recursive $'s in the ... spot
+
+			sw.next() // skip over :
+			modifier := sw.next()
+
+			word, err := sw.processStopOn('}')
+			if err != nil {
+				return "", err
+			}
+
+			// Grab the current value of the variable in question so we
+			// can use to to determine what to do based on the modifier
+			newValue := sw.getEnv(name)
+
+			switch modifier {
+			case '+':
+				if newValue != "" {
+					newValue = word
+				}
+				return newValue, nil
+
+			case '-':
+				if newValue == "" {
+					newValue = word
+				}
+				return newValue, nil
+
+			default:
+				return "", fmt.Errorf("Unsupported modifier (%c) in substitution: %s", modifier, sw.word)
+			}
+		}
+		return "", fmt.Errorf("Missing ':' in substitution: %s", sw.word)
 	}
 	// $xxx case
 	name := sw.processName()