diff --git a/phpcs.xml.dist b/phpcs.xml.dist
index 081e4b7..66ac36d 100644
--- a/phpcs.xml.dist
+++ b/phpcs.xml.dist
@@ -61,9 +61,25 @@
 
 	<!-- Exclude test classes from naming conventions. -->
 	<rule ref="WordPress.Files.FileName.InvalidClassFileName">
-		<exclude-pattern>/tests/phpunit/tests/*</exclude-pattern>
+		<exclude-pattern>/tests/phpunit/*</exclude-pattern>
 	</rule>
 	<rule ref="WordPress.Files.FileName.NotHyphenatedLowercase">
+		<exclude-pattern>/tests/phpunit/*</exclude-pattern>
+	</rule>
+
+	<!-- Exclude test files from overriding globals sniff. -->
+	<rule ref="WordPress.WP.GlobalVariablesOverride.Prohibited">
 		<exclude-pattern>/tests/phpunit/tests/*</exclude-pattern>
 	</rule>
+
+	<!-- Exclude test files from some filesystem sniffs. -->
+	<rule ref="WordPress.WP.AlternativeFunctions.file_get_contents_file_get_contents">
+		<exclude-pattern>/tests/phpunit/*</exclude-pattern>
+	</rule>
+	<rule ref="WordPress.WP.AlternativeFunctions.file_system_operations_file_put_contents">
+		<exclude-pattern>/tests/phpunit/*</exclude-pattern>
+	</rule>
+	<rule ref="WordPress.WP.AlternativeFunctions.unlink_unlink">
+		<exclude-pattern>/tests/phpunit/*</exclude-pattern>
+	</rule>
 </ruleset>
diff --git a/tests/phpunit/bootstrap.php b/tests/phpunit/bootstrap.php
index 3899953..186c80a 100644
--- a/tests/phpunit/bootstrap.php
+++ b/tests/phpunit/bootstrap.php
@@ -40,5 +40,9 @@ function _manually_load_plugin() {
 // Start up the WP testing environment.
 require "{$_tests_dir}/includes/bootstrap.php";
 
+// Load unit test mocks and fakes.
+require __DIR__ . '/includes/class-ap-fake-filesystem.php';
+
 // Load unit test abstract classes.
 require __DIR__ . '/includes/AdminSettings_UnitTestCase.php';
+require __DIR__ . '/includes/Debug_UnitTestCase.php';
diff --git a/tests/phpunit/includes/Debug_UnitTestCase.php b/tests/phpunit/includes/Debug_UnitTestCase.php
new file mode 100644
index 0000000..0058fd0
--- /dev/null
+++ b/tests/phpunit/includes/Debug_UnitTestCase.php
@@ -0,0 +1,116 @@
+<?php
+/**
+ * Abstract base test class for \AspireUpdate\Debug.
+ *
+ * All \AspireUpdate\Debug unit tests should inherit from this class.
+ */
+abstract class Debug_UnitTestCase extends WP_UnitTestCase {
+	/**
+	 * The path to the log file.
+	 *
+	 * @var string
+	 */
+	protected static $log_file;
+
+	/**
+	 * Previously created filesystems.
+	 *
+	 * @var array
+	 */
+	protected static $filesystems = [];
+
+	/**
+	 * The original value of $wp_filesystem before any tests run.
+	 *
+	 * @var WP_Filesystem_Base|null|false False if not already set.
+	 */
+	protected static $default_filesystem;
+
+	/**
+	 * Gets the log file's path, and deletes if it exists before any tests run.
+	 * Backs up the default filesystem.
+	 *
+	 * @return void
+	 */
+	public static function set_up_before_class() {
+		parent::set_up_before_class();
+
+		$get_file_path = new ReflectionMethod(
+			'AspireUpdate\Debug',
+			'get_file_path'
+		);
+
+		$get_file_path->setAccessible( true );
+		self::$log_file = $get_file_path->invoke( null );
+		$get_file_path->setAccessible( false );
+
+		if ( file_exists( self::$log_file ) ) {
+			unlink( self::$log_file );
+		}
+
+		if ( isset( $GLOBALS['wp_filesystem'] ) ) {
+			self::$default_filesystem = $GLOBALS['wp_filesystem'];
+		} else {
+			self::$default_filesystem = false;
+		}
+	}
+
+	/**
+	 * Filters the filesystem method before each test runs.
+	 *
+	 * Filters are removed in the tear_down() parent method.
+	 *
+	 * @return void
+	 */
+	public function set_up() {
+		parent::set_up();
+
+		add_filter(
+			'filesystem_method',
+			static function () {
+				return 'direct';
+			}
+		);
+	}
+
+	/**
+	 * Delete the log file and restores the filesystem after each test runs.
+	 *
+	 * @return void
+	 */
+	public function tear_down() {
+		if ( file_exists( self::$log_file ) ) {
+			unlink( self::$log_file );
+		}
+
+		if ( false === self::$default_filesystem ) {
+			unset( $GLOBALS['wp_filesystem'] );
+		} else {
+			$GLOBALS['wp_filesystem'] = self::$default_filesystem;
+		}
+
+		parent::tear_down();
+	}
+
+	/**
+	 * Creates a fake filesystem.
+	 *
+	 * @param bool|null $exists      Whether paths should exist.
+	 *                               Default null uses default implemenation.
+	 * @param bool|null $is_readable Whether paths should be readable.
+	 *                               Default null uses default implemenation.
+	 * @param bool|null $is_writable Whether paths should be writable.
+	 *                               Default null uses default implemenation.
+	 */
+	public function get_fake_filesystem( $exists = null, $is_readable = null, $is_writable = null ) {
+		$hash = ( null === $exists ? '-1' : (int) $exists ) . ',' .
+				( null === $is_readable ? '-1' : (int) $is_readable ) . ',' .
+				( null === $is_writable ? '-1' : (int) $is_writable );
+
+		if ( ! isset( self::$filesystems[ $hash ] ) ) {
+			self::$filesystems[ $hash ] = new AP_FakeFilesystem( $exists, $is_readable, $is_writable );
+		}
+
+		return self::$filesystems[ $hash ];
+	}
+}
diff --git a/tests/phpunit/includes/class-ap-fake-filesystem.php b/tests/phpunit/includes/class-ap-fake-filesystem.php
new file mode 100644
index 0000000..d2de759
--- /dev/null
+++ b/tests/phpunit/includes/class-ap-fake-filesystem.php
@@ -0,0 +1,82 @@
+<?php
+
+require_once ABSPATH . 'wp-admin/includes/class-wp-filesystem-base.php';
+require_once ABSPATH . 'wp-admin/includes/class-wp-filesystem-direct.php';
+
+class AP_FakeFilesystem extends WP_Filesystem_Direct {
+	/**
+	 * Whether paths should exist.
+	 *
+	 * @var bool|null
+	 */
+	private $exists;
+
+	/**
+	 * Whether paths should be readable.
+	 *
+	 * @var bool|null
+	 */
+	private $is_readable;
+
+	/**
+	 * Whether paths should be writable.
+	 *
+	 * @var bool|null
+	 */
+	private $is_writable;
+
+	/**
+	 * Contructor.
+	 *
+	 * @param bool|null $exists      Whether paths should exist.
+	 *                               Default null to use parent implementation.
+	 * @param bool|null $is_readable Whether paths should be readable.
+	 *                               Default null to use parent implementation.
+	 * @param bool|null $is_writable Whether paths should be writable.
+	 *                               Default null to use parent implementation.
+	 */
+	public function __construct( $exists = null, $is_readable = null, $is_writable = null ) {
+		$this->exists      = $exists;
+		$this->is_readable = $is_readable;
+		$this->is_writable = $is_writable;
+	}
+
+	/**
+	 * Checks whether a path exists.
+	 *
+	 * @param string $path The path to check.
+	 * @return bool Whether the path exists.
+	 */
+	public function exists( $path ) {
+		if ( null === $this->exists ) {
+			return parent::exists( $path );
+		}
+		return $this->exists;
+	}
+
+	/**
+	 * Checks whether a path is readable.
+	 *
+	 * @param string $path The path to check.
+	 * @return bool Whether the path is readable.
+	 */
+	public function is_readable( $path ) {
+		if ( null === $this->is_readable ) {
+			return parent::is_readable( $path );
+		}
+		return $this->is_readable;
+	}
+
+	/**
+	 * Checks whether a path is writable.
+	 *
+	 * @param string $path The path to check.
+	 * @return bool Whether the path is writable.
+	 */
+	public function is_writable( $path ) {
+		if ( null === $this->is_writable ) {
+			return parent::is_writable( $path );
+		}
+		return $this->is_writable;
+	}
+}