test: Add support for the -s option

Tests/Utilities/CMakeLists.txt

@@ -1,6 +1,7 @@
 set(TEST_SOURCES
-    TestSed.cpp
     TestPatch.cpp
+    TestSed.cpp
+    TestTest.cpp
     TestUniq.cpp
 )
 

Tests/Utilities/TestTest.cpp

@@ -0,0 +1,34 @@
+/*
+ * Copyright (c) 2026, Lucas Chollet <lucas.chollet@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/StringView.h>
+#include <LibCore/File.h>
+#include <LibCore/Process.h>
+#include <LibFileSystem/TempFile.h>
+#include <LibTest/TestCase.h>
+
+static void run_test(Vector<ByteString>&& arguments, int expected_exit_status)
+{
+    auto test = TRY_OR_FAIL(Core::Process::spawn(
+        Core::ProcessSpawnOptions { .executable = "test"sv,
+            .search_for_executable_in_path = true,
+            .arguments = arguments }));
+    auto exited_with_code_0 = TRY_OR_FAIL(test.wait_for_termination());
+    EXPECT_EQ(expected_exit_status, exited_with_code_0 ? 0 : 1);
+}
+
+TEST_CASE(option_s)
+{
+    run_test({ "-s", "file_that_do_not_exist" }, 1);
+
+    // empty file
+    auto temp_file = TRY_OR_FAIL(FileSystem::TempFile::create_temp_file());
+    run_test({ "-s", MUST(ByteString::from_utf8(temp_file->path())) }, 1);
+
+    auto file = TRY_OR_FAIL(Core::File::open(temp_file->path(), Core::File::OpenMode::Write));
+    TRY_OR_FAIL(file->write_until_depleted("file content\n"sv));
+    run_test({ "-s", MUST(ByteString::from_utf8(temp_file->path())) }, 0);
+}

Userland/Utilities/test.cpp

@@ -265,6 +265,28 @@ class FileIsOwnedBy : public Condition {
     Owner m_kind { EffectiveGID };
 };
 
+class FileExists : public Condition {
+public:
+    FileExists(StringView path)
+        : m_path(path)
+    {
+    }
+
+private:
+    virtual bool check() const override
+    {
+        // "True if pathname resolves to an existing directory entry for a file that
+        // has a size greater than zero. False if pathname cannot be resolved, or if
+        // pathname resolves to an existing directory entry for a file that does not
+        // have a size greater than zero."
+
+        auto result = Core::System::stat(m_path);
+        return !result.is_error() && result.value().st_size > 0;
+    }
+
+    ByteString m_path;
+};
+
 class StringCompare : public Condition {
 public:
     enum Mode {
@@ -478,8 +500,9 @@ static OwnPtr<Condition> parse_simple_expression(char* argv[])
             return make<FileIsOwnedBy>(value, FileIsOwnedBy::EffectiveGID);
         case 'O':
             return make<FileIsOwnedBy>(value, FileIsOwnedBy::EffectiveUID);
-        case 'N':
         case 's':
+            return make<FileExists>(value);
+        case 'N':
             // 'optind' has been incremented to refer to the argument after the
             // operator, while we want to print the operator itself.
             fatal_error("Unsupported operator \033[1m%s", argv[optind - 1]);