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diff --git a/src/test/java/org/apache/commons/io/FileSystemTest.java b/src/test/java/org/apache/commons/io/FileSystemTest.java
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+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.commons.io;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.io.IOException;
+
+import org.apache.commons.lang3.SystemUtils;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.condition.EnabledOnOs;
+import org.junit.jupiter.api.condition.OS;
+
+/**
+ * Tests {@link FileSystem}.
+ */
+public class FileSystemTest {
+
+
+    @Test
+    public void testGetCurrent() {
+        if (SystemUtils.IS_OS_WINDOWS) {
+            assertEquals(FileSystem.WINDOWS, FileSystem.getCurrent());
+        }
+        if (SystemUtils.IS_OS_LINUX) {
+            assertEquals(FileSystem.LINUX, FileSystem.getCurrent());
+        }
+        if (SystemUtils.IS_OS_MAC_OSX) {
+            assertEquals(FileSystem.MAC_OSX, FileSystem.getCurrent());
+        }
+    }
+
+    @Test
+    public void testIsLegalName() {
+        for (final FileSystem fs : FileSystem.values()) {
+            assertFalse(fs.isLegalFileName(""), fs.name()); // Empty is always illegal
+            assertFalse(fs.isLegalFileName(null), fs.name()); // null is always illegal
+            assertFalse(fs.isLegalFileName("\0"), fs.name()); // Assume NUL is always illegal
+            assertTrue(fs.isLegalFileName("0"), fs.name()); // Assume simple name always legal
+            for (final String candidate : fs.getReservedFileNames()) {
+                // Reserved file names are not legal
+                assertFalse(fs.isLegalFileName(candidate));
+            }
+        }
+    }
+
+    @Test
+    public void testIsReservedFileName() {
+        for (final FileSystem fs : FileSystem.values()) {
+            for (final String candidate : fs.getReservedFileNames()) {
+                assertTrue(fs.isReservedFileName(candidate));
+            }
+        }
+    }
+
+    @Test
+    @EnabledOnOs(OS.WINDOWS)
+    public void testIsReservedFileNameOnWindows() throws IOException {
+        final FileSystem fs = FileSystem.WINDOWS;
+        for (final String candidate : fs.getReservedFileNames()) {
+            // System.out.printf("Reserved %s exists: %s%n", candidate, Files.exists(Paths.get(candidate)));
+            assertTrue(fs.isReservedFileName(candidate));
+            assertTrue(fs.isReservedFileName(candidate + ".txt"), candidate);
+        }
+
+// This can hang when trying to create files for some reserved names, but it is interesting to keep
+//
+//        for (final String candidate : fs.getReservedFileNames()) {
+//            System.out.printf("Testing %s%n", candidate);
+//            assertTrue(fs.isReservedFileName(candidate));
+//            final Path path = Paths.get(candidate);
+//            final boolean exists = Files.exists(path);
+//            try {
+//                PathUtils.writeString(path, "Hello World!", StandardCharsets.UTF_8);
+//            } catch (IOException ignored) {
+//                // Asking to create a reserved file either:
+//                // - Throws an exception, for example "AUX"
+//                // - Is a NOOP, for example "COM3"
+//            }
+//            assertEquals(exists, Files.exists(path), path.toString());
+//        }
+    }
+
+    @Test
+    public void testReplacementWithNUL() {
+        for (final FileSystem fs : FileSystem.values()) {
+            try {
+                fs.toLegalFileName("Test", '\0'); // Assume NUL is always illegal
+            } catch (final IllegalArgumentException iae) {
+                assertTrue(iae.getMessage().startsWith("The replacement character '\\0'"), iae.getMessage());
+            }
+        }
+    }
+
+    @Test
+    public void testSorted() {
+        for (final FileSystem fs : FileSystem.values()) {
+            final char[] chars = fs.getIllegalFileNameChars();
+            for (int i = 0; i < chars.length - 1; i++) {
+                assertTrue(chars[i] < chars[i + 1], fs.name());
+            }
+        }
+    }
+
+    @Test
+    public void testSupportsDriveLetter() {
+        assertTrue(FileSystem.WINDOWS.supportsDriveLetter());
+        assertFalse(FileSystem.GENERIC.supportsDriveLetter());
+        assertFalse(FileSystem.LINUX.supportsDriveLetter());
+        assertFalse(FileSystem.MAC_OSX.supportsDriveLetter());
+    }
+
+    @Test
+    public void testToLegalFileNameWindows() {
+        final FileSystem fs = FileSystem.WINDOWS;
+        final char replacement = '-';
+        for (char i = 0; i < 32; i++) {
+            assertEquals(replacement, fs.toLegalFileName(String.valueOf(i), replacement).charAt(0));
+        }
+        final char[] illegal = { '<', '>', ':', '"', '/', '\\', '|', '?', '*' };
+        for (char i = 0; i < illegal.length; i++) {
+            assertEquals(replacement, fs.toLegalFileName(String.valueOf(i), replacement).charAt(0));
+        }
+        for (char i = 'a'; i < 'z'; i++) {
+            assertEquals(i, fs.toLegalFileName(String.valueOf(i), replacement).charAt(0));
+        }
+        for (char i = 'A'; i < 'Z'; i++) {
+            assertEquals(i, fs.toLegalFileName(String.valueOf(i), replacement).charAt(0));
+        }
+        for (char i = '0'; i < '9'; i++) {
+            assertEquals(i, fs.toLegalFileName(String.valueOf(i), replacement).charAt(0));
+        }
+    }
+}