1 files changed, 21 insertions, 3 deletions
diff --git a/libgo/go/os/env.go b/libgo/go/os/env.go
index 4e0171f..330297b 100644
--- a/libgo/go/os/env.go
+++ b/libgo/go/os/env.go
@@ -14,18 +14,33 @@ import (
 // Expand replaces ${var} or $var in the string based on the mapping function.
 // For example, os.ExpandEnv(s) is equivalent to os.Expand(s, os.Getenv).
 func Expand(s string, mapping func(string) string) string {
-	buf := make([]byte, 0, 2*len(s))
+	var buf []byte
 	// ${} is all ASCII, so bytes are fine for this operation.
 	i := 0
 	for j := 0; j < len(s); j++ {
 		if s[j] == '$' && j+1 < len(s) {
+			if buf == nil {
+				buf = make([]byte, 0, 2*len(s))
+			}
 			buf = append(buf, s[i:j]...)
 			name, w := getShellName(s[j+1:])
-			buf = append(buf, mapping(name)...)
+			if name == "" && w > 0 {
+				// Encountered invalid syntax; eat the
+				// characters.
+			} else if name == "" {
+				// Valid syntax, but $ was not followed by a
+				// name. Leave the dollar character untouched.
+				buf = append(buf, s[j])
+			} else {
+				buf = append(buf, mapping(name)...)
+			}
 			j += w
 			i = j + 1
 		}
 	}
+	if buf == nil {
+		return s
+	}
 	return string(buf) + s[i:]
 }
 
@@ -63,10 +78,13 @@ func getShellName(s string) (string, int) {
 		// Scan to closing brace
 		for i := 1; i < len(s); i++ {
 			if s[i] == '}' {
+				if i == 1 {
+					return "", 2 // Bad syntax; eat "${}"
+				}
 				return s[1:i], i + 1
 			}
 		}
-		return "", 1 // Bad syntax; just eat the brace.
+		return "", 1 // Bad syntax; eat "${"
 	case isShellSpecialVar(s[0]):
 		return s[0:1], 1
 	}