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

internal/devbox/envvars.go

@@ -89,14 +89,23 @@ 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 '$', '`', '"', '\\', '\n':
+				//
+				// Note: newline is intentionally NOT escaped. A literal newline
+				// inside double quotes is preserved as-is, but a backslash
+				// followed by a newline is a line continuation that the shell
+				// *removes*, silently joining the two lines. Escaping it would
+				// corrupt any multi-line value (e.g. a PROMPT_COMMAND that spans
+				// several lines), producing broken shell such as
+				// `... 2>&1__bp_interactive_mode` and errors like
+				// "ambiguous redirect". See issue #2814.
+				case '$', '`', '"', '\\':
 					strb.WriteRune('\\')
 				}
-				strb.WriteRune(r)
+				strb.WriteRune(char)
 			}
 			strb.WriteString("\";\n")
 		}

internal/devbox/envvars_test.go

@@ -47,6 +47,28 @@ func TestExportifySkipsInvalidNames(t *testing.T) {
 	}
 }
 
+// TestExportifyPreservesNewlines ensures multi-line env values (e.g. a
+// PROMPT_COMMAND that spans several lines) survive round-tripping through the
+// shell. A newline must be emitted literally, not as a backslash-newline: inside
+// double quotes the latter is a line continuation that the shell removes, which
+// silently joins adjacent lines and corrupts the value. See issue #2814, where a
+// multi-line PROMPT_COMMAND collapsed into `... 2>&1__bp_interactive_mode` and
+// produced a "bash: ...: ambiguous redirect" error at every prompt.
+func TestExportifyPreservesNewlines(t *testing.T) {
+	value := "line1 >/dev/null 2>&1\n__bp_interactive_mode"
+	got := exportify(io.Discard, map[string]string{"PROMPT_COMMAND": value})
+
+	// The value must be emitted with a literal newline, never a
+	// backslash-newline (which bash would strip as a line continuation).
+	if strings.Contains(got, "2>&1\\\n") {
+		t.Errorf("newline was escaped as a line continuation, which corrupts the value:\n%s", got)
+	}
+	want := "export PROMPT_COMMAND=\"line1 >/dev/null 2>&1\n__bp_interactive_mode\";"
+	if got != want {
+		t.Errorf("exportify(...) = %q, want %q", got, want)
+	}
+}
+
 func TestExportifyNushellSkipsInvalidNames(t *testing.T) {
 	got := exportifyNushell(io.Discard, map[string]string{
 		"GOOD": "value",