envsubst: support a negative substring length instead of panicking

envsubst/funcs.go

@@ -121,6 +121,18 @@ func toSubstr(s string, args ...string) string {
 		return s
 	}
 
+	if length < 0 {
+		// a negative length counts back from the end of the string, so
+		// ${var:1:-1} drops the first and the last character.
+		end := len(s) + length
+		if end < pos {
+			// bash rejects this as a negative substring expression.
+			return ""
+		}
+
+		return s[pos:end]
+	}
+
 	if pos+length >= len(s) {
 		if pos < len(s) {
 			// if the position exceeds the length of the

envsubst/funcs_test.go

@@ -121,4 +121,24 @@ func Test_substr(t *testing.T) {
 	if got != want {
 		t.Errorf("Expect substr function to cut entire string if pos is itself out of bound")
 	}
+
+	got, want = toSubstr("hello world", "2", "-1"), "llo worl"
+	if got != want {
+		t.Errorf("Expect substr function to count a negative length back from the end, got %q want %q", got, want)
+	}
+
+	got, want = toSubstr("hello world", "0", "-3"), "hello wo"
+	if got != want {
+		t.Errorf("Expect substr function to count a negative length back from the end at offset 0, got %q want %q", got, want)
+	}
+
+	got, want = toSubstr("hello world", "8", "-8"), ""
+	if got != want {
+		t.Errorf("Expect substr function to return empty when a negative length ends before the offset, got %q want %q", got, want)
+	}
+
+	got, want = toSubstr("hello world", "-4", "-1"), "orl"
+	if got != want {
+		t.Errorf("Expect substr function to combine a negative offset with a negative length, got %q want %q", got, want)
+	}
 }