fix(shellenv): preserve newlines in exported env values (#2814)

internal/devbox/envvars.go

@@ -89,11 +89,19 @@ func exportify(w io.Writer, vars map[string]string) string {
 			strb.WriteString("export ")
 			strb.WriteString(key)
 			strb.WriteString(`="`)
+			// Escape the characters that are special inside double quotes:
+			// https://pubs.opengroup.org/onlinepubs/009604499/utilities/xcu_chap02.html#tag_02_02_03
+			//
+			// A newline is intentionally NOT escaped. Inside double quotes a
+			// literal newline is preserved as-is, but a backslash-newline is a
+			// line continuation that the shell removes entirely. Escaping it
+			// would silently join adjacent lines of a multiline value and
+			// corrupt it (see #2814, where a multiline PROMPT_COMMAND turned
+			// `2>&1\<newline>foo` into `2>&1foo` and produced an "ambiguous
+			// redirect" error).
 			for _, r := range vars[key] {
 				switch r {
-				// Special characters inside double quotes:
-				// https://pubs.opengroup.org/onlinepubs/009604499/utilities/xcu_chap02.html#tag_02_02_03
-				case '$', '`', '"', '\\', '\n':
+				case '$', '`', '"', '\\':
 					strb.WriteRune('\\')
 				}
 				strb.WriteRune(r)

internal/devbox/envvars_test.go

@@ -47,6 +47,28 @@ func TestExportifySkipsInvalidNames(t *testing.T) {
 	}
 }
 
+// TestExportifyMultilineValue ensures that env vars whose values contain
+// newlines are emitted with literal newlines inside the double quotes, not a
+// backslash-newline. A backslash-newline is a shell line continuation that gets
+// removed on eval, which would join adjacent lines and corrupt the value (see
+// issue #2814: a multiline PROMPT_COMMAND produced an "ambiguous redirect").
+func TestExportifyMultilineValue(t *testing.T) {
+	got := exportify(io.Discard, map[string]string{
+		"MULTI": "line1\nline2",
+	})
+
+	want := "export MULTI=\"line1\nline2\";"
+	if got != want {
+		t.Errorf("exportify multiline value = %q, want %q", got, want)
+	}
+
+	// A backslash-newline (line continuation) must not appear; that is the
+	// exact corruption we are guarding against.
+	if strings.Contains(got, "\\\n") {
+		t.Errorf("exportify escaped a newline as a line continuation, got:\n%s", got)
+	}
+}
+
 func TestExportifyNushellSkipsInvalidNames(t *testing.T) {
 	got := exportifyNushell(io.Discard, map[string]string{
 		"GOOD": "value",