Add detection of "special parameters" for substitution

Detect Special parameters as defined in
http://pubs.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html#tag_02_05_02

Treat these as parameters that are not set, instead of
producing an error that a modifier is missing.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

builder/dockerfile/shell/envVarTest

@@ -174,3 +174,38 @@ A|$9                        |
 A|${9}                      |
 A|${9:+bbb}                 |
 A|${9:-bbb}                 |     bbb
+
+# Special parameters won't be set in the Dockerfile:
+# http://pubs.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html#tag_02_05_02
+A|$@                        |
+A|${@}                      |
+A|${@:+bbb}                 |
+A|${@:-bbb}                 |     bbb
+A|$*                        |
+A|${*}                      |
+A|${*:+bbb}                 |
+A|${*:-bbb}                 |     bbb
+A|$#                        |
+A|${#}                      |
+A|${#:+bbb}                 |
+A|${#:-bbb}                 |     bbb
+A|$?                        |
+A|${?}                      |
+A|${?:+bbb}                 |
+A|${?:-bbb}                 |     bbb
+A|$-                        |
+A|${-}                      |
+A|${-:+bbb}                 |
+A|${-:-bbb}                 |     bbb
+A|$$                        |
+A|${$}                      |
+A|${$:+bbb}                 |
+A|${$:-bbb}                 |     bbb
+A|$!                        |
+A|${!}                      |
+A|${!:+bbb}                 |
+A|${!:-bbb}                 |     bbb
+A|$0                        |
+A|${0}                      |
+A|${0:+bbb}                 |
+A|${0:-bbb}                 |     bbb

builder/dockerfile/shell/lex.go

@@ -318,7 +318,7 @@ func (sw *shellWord) processName() string {
 
 	for sw.scanner.Peek() != scanner.EOF {
 		ch := sw.scanner.Peek()
-		if name.Len() == 0 && unicode.IsDigit(ch) {
+		if name.Len() == 0 && (unicode.IsDigit(ch) || isSpecialParam(ch)) {
 			ch = sw.scanner.Next()
 			return string(ch)
 		}
@@ -332,6 +332,18 @@ func (sw *shellWord) processName() string {
 	return name.String()
 }
 
+// isSpecialParam checks if the provided character is a special parameters,
+// as defined in http://pubs.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html#tag_02_05_02
+func isSpecialParam(char rune) bool {
+	switch char {
+	case '@', '*', '#', '?', '-', '$', '!', '0':
+		// Special parameters
+		// http://pubs.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html#tag_02_05_02
+		return true
+	}
+	return false
+}
+
 func (sw *shellWord) getEnv(name string) string {
 	for _, env := range sw.envs {
 		i := strings.Index(env, "=")

builder/dockerfile/shell/lex_test.go

@@ -26,13 +26,11 @@ func TestShellParser4EnvVars(t *testing.T) {
 		line := scanner.Text()
 		lineCount++
 
-		// Trim comments and blank lines
-		i := strings.Index(line, "#")
-		if i >= 0 {
-			line = line[:i]
+		// Skip comments and blank lines
+		if strings.HasPrefix(line, "#") {
+			continue
 		}
 		line = strings.TrimSpace(line)
-
 		if line == "" {
 			continue
 		}