expand: support more format escape sequences

Character escape codes like \x and \u remain to be done.

Updates mvdan#338.

expand/expand.go

@@ -199,14 +199,24 @@ func Format(cfg *Config, format string, args []string) (string, int, error) {
 		case esc:
 			esc = false
 			switch c {
-			case 'n':
+			case 'a': // bell
+				buf.WriteRune('\a')
+			case 'b': // backspace
+				buf.WriteRune('\b')
+			case 'e', 'E': // escape
+				buf.WriteRune('\x1b')
+			case 'f': // form feed
+				buf.WriteRune('\f')
+			case 'n': // new line
 				buf.WriteRune('\n')
-			case 'r':
+			case 'r': // carriage return
 				buf.WriteRune('\r')
-			case 't':
+			case 't': // horizontal tab
 				buf.WriteRune('\t')
-			case '\\':
-				buf.WriteRune('\\')
+			case 'v': // vertical tab
+				buf.WriteRune('\v')
+			case '\\', '\'', '"', '?': // just the character
+				buf.WriteRune(c)
 			default:
 				buf.WriteRune('\\')
 				buf.WriteRune(c)

interp/interp_test.go

@@ -231,6 +231,8 @@ var fileCases = []struct {
 	{`echo $'%s'`, "%s\n"},
 	{`a=$'\r\t\\'; echo "$a"`, "\r\t\\\n"},
 	{`a=$"foo\nbar"; echo "$a"`, "foo\\nbar\n"},
+	{`echo $'\a\b\e\E\f\v'`, "\a\b\x1b\x1b\f\v\n"},
+	{`echo $'\\\'\"\?'`, "\\'\"?\n"},
 
 	// escaped chars
 	{"echo a\\b", "ab\n"},