fix(shellenv): preserve newlines in exported env values (ambiguous redirect)

internal/devbox/envvars.go

@@ -89,14 +89,21 @@ func exportify(w io.Writer, vars map[string]string) string {
 			strb.WriteString("export ")
 			strb.WriteString(key)
 			strb.WriteString(`="`)
-			for _, r := range vars[key] {
-				switch r {
+			for _, char := range vars[key] {
+				switch char {
 				// Special characters inside double quotes:
 				// https://pubs.opengroup.org/onlinepubs/009604499/utilities/xcu_chap02.html#tag_02_02_03
-				case '$', '`', '"', '\\', '\n':
+				//
+				// Note: a literal newline is NOT escaped. Inside double quotes
+				// a bare newline is preserved verbatim, whereas a backslash
+				// immediately followed by a newline is a line continuation that
+				// the shell deletes. Escaping it would join a multi-line value's
+				// adjacent lines and corrupt values like a multi-command
+				// PROMPT_COMMAND (see jetify-com/devbox#2814).
+				case '$', '`', '"', '\\':
 					strb.WriteRune('\\')
 				}
-				strb.WriteRune(r)
+				strb.WriteRune(char)
 			}
 			strb.WriteString("\";\n")
 		}

internal/devbox/envvars_test.go

@@ -47,6 +47,27 @@ func TestExportifySkipsInvalidNames(t *testing.T) {
 	}
 }
 
+// TestExportifyPreservesNewlines ensures that multi-line values (e.g. a
+// PROMPT_COMMAND made of several commands separated by newlines) are emitted so
+// that the shell preserves the newlines. A backslash before a newline would be a
+// line continuation that the shell deletes, joining adjacent lines and corrupting
+// the value (see jetify-com/devbox#2814).
+func TestExportifyPreservesNewlines(t *testing.T) {
+	value := "__bp_precmd_invoke_cmd\ndbus-send ... >/dev/null 2>&1\n__bp_interactive_mode"
+	got := exportify(io.Discard, map[string]string{"PROMPT_COMMAND": value})
+
+	// The emitted value must contain a bare newline, not an escaped one. If the
+	// newline were escaped, "2>&1\" and "__bp_interactive_mode" would collapse
+	// into "2>&1__bp_interactive_mode" when the shell sourced the export.
+	if strings.Contains(got, "\\\n") {
+		t.Errorf("newline should not be backslash-escaped, got:\n%q", got)
+	}
+	want := "export PROMPT_COMMAND=\"" + value + "\";"
+	if !strings.Contains(got, want) {
+		t.Errorf("expected exported value to preserve newlines.\ngot:\n%q\nwant to contain:\n%q", got, want)
+	}
+}
+
 func TestExportifyNushellSkipsInvalidNames(t *testing.T) {
 	got := exportifyNushell(io.Discard, map[string]string{
 		"GOOD": "value",