os: keep the $ if a variable is not detected

If the character after $ cannot be detected as a valid
variable declaration, do not gobble the $.

Fixes #24345

Change-Id: Iec47be1f2e4f8147b8ceb64c30778eae8045b58f
Reviewed-on: https://go-review.googlesource.com/103055
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/os/env.go

@@ -21,7 +21,12 @@ func Expand(s string, mapping func(string) string) string {
 		if s[j] == '$' && j+1 < len(s) {
 			buf = append(buf, s[i:j]...)
 			name, w := getShellName(s[j+1:])
+			// If the name is empty, keep the $.
+			if name == "" {
+				buf = append(buf, s[j])
+			} else {
 				buf = append(buf, mapping(name)...)
+			}
 			j += w
 			i = j + 1
 		}

src/os/env_test.go

@@ -49,6 +49,8 @@ var expandTests = []struct {
 	{"${HOME}", "/usr/gopher"},
 	{"${H}OME", "(Value of H)OME"},
 	{"A$$$#$1$H$home_1*B", "APIDNARGSARGUMENT1(Value of H)/usr/foo*B"},
+	{"start$+middle$^end$", "start$+middle$^end$"},
+	{"mixed$|bag$$$", "mixed$|bagPID$"},
 }
 
 func TestExpand(t *testing.T) {