diff --git a/bin/cfg b/bin/cfg
index 78e84c5..6f138cf 100755
--- a/bin/cfg
+++ b/bin/cfg
@@ -22,7 +22,62 @@ foreach ($autoloadFiles as $autoloadFile) {
 	}
 }
 
-$version = '0.3';
+// Pre-parse site options so they can be passed after positional args.
+// The input helper library does not handle this case reliably.
+$preparsedSiteOptions = [];
+$rawArgv = $_SERVER['argv'] ?? $GLOBALS['argv'] ?? null;
+if (is_array($rawArgv) && !empty($rawArgv)) {
+	$filteredArgv = [$rawArgv[0]];
+	for ($idx = 1; $idx < count($rawArgv); $idx++) {
+		$arg = $rawArgv[$idx];
+
+		if ($arg === '-s' || $arg === '--site') {
+			$preparsedSiteOptions[] = [
+				'name' => 'site',
+				'value' => $rawArgv[$idx + 1] ?? '',
+			];
+			if (isset($rawArgv[$idx + 1])) $idx++;
+			continue;
+		}
+		if (str_starts_with($arg, '--site=')) {
+			$preparsedSiteOptions[] = [
+				'name' => 'site',
+				'value' => substr($arg, strlen('--site=')),
+			];
+			continue;
+		}
+
+		if ($arg === '--directory-name' || $arg === '--directoryName') {
+			$preparsedSiteOptions[] = [
+				'name' => 'directoryName',
+				'value' => $rawArgv[$idx + 1] ?? '',
+			];
+			if (isset($rawArgv[$idx + 1])) $idx++;
+			continue;
+		}
+		if (str_starts_with($arg, '--directory-name=')) {
+			$preparsedSiteOptions[] = [
+				'name' => 'directoryName',
+				'value' => substr($arg, strlen('--directory-name=')),
+			];
+			continue;
+		}
+		if (str_starts_with($arg, '--directoryName=')) {
+			$preparsedSiteOptions[] = [
+				'name' => 'directoryName',
+				'value' => substr($arg, strlen('--directoryName=')),
+			];
+			continue;
+		}
+
+		$filteredArgv[] = $arg;
+	}
+
+	$_SERVER['argv'] = $filteredArgv;
+	$GLOBALS['argv'] = $filteredArgv;
+}
+
+$version = '0.4';
 
 $actions = [ 'show', 'write', 'help' ];
 $settings = ['key' => '', 'value' => ''];
@@ -60,6 +115,16 @@ $collection = (new Input\InputCollection())
 		->description('Path where the config/ directory of the package conf files is located, defaults to the working dir')
 	) // }}}
 
+	->add( Input\InputTypeFactory::build('LongOption')->name('site')->short('s') // {{{
+		->flags(AbstractInputType::FLAG_OPTIONAL | Input\AbstractInputType::FLAG_VALUE_REQUIRED)
+		->description('Site/instance directory below config/ (target: config/<site>/<prefix>.conf.php), e.g. owner_xyz')
+	) // }}}
+
+	->add( Input\InputTypeFactory::build('LongOption')->name('directoryName') // {{{
+		->flags(AbstractInputType::FLAG_OPTIONAL | Input\AbstractInputType::FLAG_VALUE_REQUIRED)
+		->description('Alias for --site. Accepts owner_xyz or config/owner_xyz (also supports --directory-name)')
+	) // }}}
+
 	->add( Input\InputTypeFactory::build('Argument')->name('action') // {{{
 		->flags(AbstractInputType::FLAG_REQUIRED)
 		->description(
@@ -141,8 +206,18 @@ $usage = Cli\manpage( basename(__FILE__), $version,
 	$collection, Colour::FG_GREEN, Colour::FG_WHITE,
 	[
 		'Examples' =>
-			 'cfg show VeruA db:host'.PHP_EOL.
-			 'cfg write VeruA \'db:host="newHost"\''.PHP_EOL
+			 'cfg write myCEESV \'auth:projectId="218523"\''
+			 .PHP_EOL
+			 .'# writes local config: config/myCEESV.conf.php'
+			 .PHP_EOL
+			 .'cfg write myCEESV \'auth:projectId="218523"\' --directory-name=owner_xyz'
+			 .PHP_EOL
+			 .'# writes instance config: config/owner_xyz/myCEESV.conf.php'
+			 .PHP_EOL
+			 .'cfg show myCEESV auth:projectId --site=owner_xyz'
+			 .PHP_EOL
+			 .'# reads merged config including config/owner_xyz/myCEESV.conf.php'
+			 .PHP_EOL
 	]
 ).PHP_EOL;
 
@@ -181,6 +256,7 @@ echo $argv->find('setting')['value'].PHP_EOL;
 // var_dump($cfg);
 $appPath = $argv->find('appPath');
 if (!$appPath) $appPath = getcwd().'/';
+$appPath = rtrim($appPath, '/').'/';
 
 /* $it = new RecursiveDirectoryIterator($appPath);
 
@@ -194,7 +270,62 @@ foreach(new RecursiveIteratorIterator($it) as $file)
  */
 $mode = ($argv->find('mode') == '') ? null : $argv->find('mode');
 $cfg = (new Settings([], $mode))->appPath($appPath)->prefix($prefix);
-if ($pkgPath = $argv->find('pkgPath')) $cfg->pkgPath($pkgPath);
+// pkgPath points to package defaults (e.g. <prefix>.default.conf.php)
+if ($pkgPath = $argv->find('pkgPath')) $cfg->pkgPath(rtrim($pkgPath, '/').'/');
+
+$site = null;
+$siteFlag = 0x01;
+// Only one site selector may be provided to avoid ambiguous targets.
+$providedSiteOptions = [];
+foreach ($preparsedSiteOptions as $option) {
+	$providedSiteOptions[$option['name']][] = $option['value'] ?? '';
+}
+foreach (['site', 'directoryName'] as $optionName) {
+	$parsedValue = $argv->find($optionName);
+	if ($parsedValue !== null && $parsedValue !== '' && $parsedValue !== false) {
+		$providedSiteOptions[$optionName][] = $parsedValue;
+	}
+}
+
+if (count($providedSiteOptions) > 1) {
+	fwrite(STDERR, 'Please use only one of --site or --directoryName (alias: --directory-name).'.PHP_EOL);
+	exit(1);
+}
+if (!empty($providedSiteOptions)) {
+	$optionName = array_key_first($providedSiteOptions);
+	$siteInput = trim((string)end($providedSiteOptions[$optionName]));
+
+    if ($optionName === 'directoryName') {
+        // Accept both "owner_xyz" and "config/owner_xyz" and normalize to site key.
+        if (str_starts_with($siteInput, $appPath.'config/')) {
+            $siteInput = substr($siteInput, strlen($appPath.'config/'));
+        } elseif (str_starts_with($siteInput, 'config/')) {
+            $siteInput = substr($siteInput, strlen('config/'));
+        }
+    }
+
+    $siteInput = trim($siteInput, '/');
+    if ($siteInput === '') {
+        fwrite(STDERR, "Option --$optionName is empty.".PHP_EOL);
+        exit(1);
+    }
+    // Block directory traversal and hidden-dot segments.
+    if (str_contains($siteInput, '..') || preg_match('~(^|/)\.(?:/|$)~', $siteInput)) {
+        fwrite(STDERR, "Invalid directory in --$optionName: '$siteInput'.".PHP_EOL);
+        exit(1);
+    }
+    // Allow only predictable path segments for instance directories.
+    foreach (explode('/', $siteInput) as $part) {
+        if ($part === '' || !preg_match('/^[A-Za-z0-9._-]+$/', $part)) {
+            fwrite(STDERR, "Invalid directory name segment '$part' in --$optionName.".PHP_EOL);
+            exit(1);
+        }
+    }
+
+    $site = $siteInput;
+    // Reuse util-settings site resolution: config/<site>/<prefix>.conf.php
+    $cfg->site($site);
+}
 
 try {
 	if (is_readable($cfg->buildFileName('default'))) {
@@ -238,27 +369,37 @@ case 'show':
 	
 case 'write':
 	$path = ($settings['key'] !== '') ? explode(':', $settings['key']) : [];
-	var_dump($path);
-	
+
 	$setting2write = $settings['value'];
-	
+
 	while ( ! empty($path))
 	{
 		$setting2write = [array_pop($path) => $setting2write];
 	}
-	if (is_readable($file = $cfg->buildFileName()))
+
+	$writeType = ($site !== null) ? $siteFlag : null;
+	$file = $cfg->buildFileName($writeType);
+	if (is_readable($file))
 	{
 		$setting2write = array_replace_recursive(require($file), $setting2write);
 		copy($file, "$file.bak");
 	}
+
+	$targetDir = dirname($file);
+	if (!is_dir($targetDir) && !mkdir($targetDir, 0775, true) && !is_dir($targetDir)) {
+		fwrite(STDERR, "Can not create directory: $targetDir".PHP_EOL);
+		exit(1);
+	}
+
 	$writeCfg = $cfg->create($setting2write);
 //	var_dump($writeCfg->toArray());
 	try {
-		(new SettingsWriter($writeCfg))->write();
+		(new SettingsWriter($writeCfg, '', $writeType))->write();
 		echo "Written modified settings to: $file".PHP_EOL;
 	}
 	catch (\Exception $e) {
-		echo $e->getMessage().PHP_EOL;
+		fwrite(STDERR, $e->getMessage().PHP_EOL);
+		exit(1);
 	}
 
 	break;
diff --git a/src/SettingsWriter.php b/src/SettingsWriter.php
index d6fc41c..6246a9f 100644
--- a/src/SettingsWriter.php
+++ b/src/SettingsWriter.php
@@ -22,6 +22,8 @@
 declare(strict_types=1);
 namespace rabe\Util;
 
+use ErrorException;
+
 /**
  * Write Settings into a File
  * @author Norbert.e.Wagner dev@norb.me
@@ -45,13 +47,16 @@ class SettingsWriter
 	 *
 	 * @param Settings $settings an object of type settings
 	 * @param String $name The name midfix for the settings File
-	 */
-	public function __construct( Settings $settings, $name='' )
+     * @param String|int|null $type Optional type for Settings::buildFileName(), e.g. site flag
+     */
+    public function __construct( Settings $settings, $name='', $type=null )
 	{
 		$this->settings = $settings;
-		
-		$file = $settings->buildFileName( $name );
-		
+
+        $file = ( $type === null )
+            ? $settings->buildFileName( $name )
+            : $settings->buildFileName( $type );
+
 		if ( ! $this->handle = fopen( $file, 'w' ) )
 		{
 			throw new ErrorException( "Can not open File: $file" );
diff --git a/tests/CfgTest.php b/tests/CfgTest.php
new file mode 100644
index 0000000..508a212
--- /dev/null
+++ b/tests/CfgTest.php
@@ -0,0 +1,307 @@
+<?php
+declare(strict_types=1);
+namespace rabe\Util\tests;
+
+use PHPUnit\Framework\TestCase;
+
+class CfgTest extends TestCase
+{
+	private string $tmpDir;
+
+	protected function setUp(): void
+	{
+		$this->tmpDir = rtrim(sys_get_temp_dir(), '/').'/util-settings-cfg-'.bin2hex(random_bytes(8));
+		mkdir($this->tmpDir, 0775, true);
+		mkdir($this->tmpDir.'/config', 0775, true);
+
+		file_put_contents(
+			$this->tmpDir.'/config/myCEESV.default.conf.php',
+			"<?php return [\n\t'mode' => 'prod',\n\t'auth' => [\n\t\t'projectId' => '',\n\t\t'clientId' => '',\n\t],\n];\n"
+		);
+	}
+
+	protected function tearDown(): void
+	{
+		$this->removeDir($this->tmpDir);
+	}
+
+	public function testWriteWithoutDirectoryNameUsesLocalConfigFile(): void
+	{
+		$result = $this->runCfg([
+			'-a',
+			$this->tmpDir,
+			'write',
+			'myCEESV',
+			'auth:projectId="218523"',
+		]);
+
+		$this->assertSame(0, $result['code'], $result['output']);
+
+		$localFile = $this->tmpDir.'/config/myCEESV.conf.php';
+		$this->assertFileExists($localFile);
+
+		$cfg = require $localFile;
+		$this->assertSame('218523', $cfg['auth']['projectId']);
+	}
+
+	public function testWriteWithDirectoryNameCreatesAndWritesSiteConfig(): void
+	{
+		$result = $this->runCfg([
+			'-a',
+			$this->tmpDir,
+			'write',
+			'myCEESV',
+			'auth:projectId="218523"',
+			'--directory-name',
+			'owner_xyz',
+		]);
+
+		$this->assertSame(0, $result['code'], $result['output']);
+
+		$siteFile = $this->tmpDir.'/config/owner_xyz/myCEESV.conf.php';
+		$this->assertFileExists($siteFile);
+		$this->assertFileDoesNotExist($this->tmpDir.'/config/myCEESV.conf.php');
+
+		$cfg = require $siteFile;
+		$this->assertSame('218523', $cfg['auth']['projectId']);
+	}
+
+	public function testWriteWithDirectoryNameMergesIntoExistingSiteConfig(): void
+	{
+		$firstWrite = $this->runCfg([
+			'-a',
+			$this->tmpDir,
+			'write',
+			'myCEESV',
+			'auth:projectId="218523"',
+			'--directory-name',
+			'owner_xyz',
+		]);
+		$this->assertSame(0, $firstWrite['code'], $firstWrite['output']);
+
+		$secondWrite = $this->runCfg([
+			'-a',
+			$this->tmpDir,
+			'write',
+			'myCEESV',
+			'auth:clientId="service-9999-qual@myceesv.ch"',
+			'--directory-name',
+			'owner_xyz',
+		]);
+		$this->assertSame(0, $secondWrite['code'], $secondWrite['output']);
+
+		$siteFile = $this->tmpDir.'/config/owner_xyz/myCEESV.conf.php';
+		$cfg = require $siteFile;
+
+		$this->assertSame('218523', $cfg['auth']['projectId']);
+		$this->assertSame('service-9999-qual@myceesv.ch', $cfg['auth']['clientId']);
+	}
+
+	public function testWriteWithDirectoryNameUsingConfigPrefixIsNormalized(): void
+	{
+		$result = $this->runCfg([
+			'-a',
+			$this->tmpDir,
+			'write',
+			'myCEESV',
+			'auth:projectId="218523"',
+			'--directory-name=config/owner_xyz',
+		]);
+
+		$this->assertSame(0, $result['code'], $result['output']);
+		$this->assertFileExists($this->tmpDir.'/config/owner_xyz/myCEESV.conf.php');
+		$this->assertFileDoesNotExist($this->tmpDir.'/config/config/owner_xyz/myCEESV.conf.php');
+	}
+
+	public function testWriteWithSiteOptionWritesToSiteConfig(): void
+	{
+		$result = $this->runCfg([
+			'-a',
+			$this->tmpDir,
+			'write',
+			'myCEESV',
+			'auth:projectId="218523"',
+			'--site=owner_xyz',
+		]);
+
+		$this->assertSame(0, $result['code'], $result['output']);
+		$this->assertFileExists($this->tmpDir.'/config/owner_xyz/myCEESV.conf.php');
+	}
+
+	public function testWriteWithDirectoryNameCamelCaseOptionWritesToSiteConfig(): void
+	{
+		$result = $this->runCfg([
+			'-a',
+			$this->tmpDir,
+			'write',
+			'myCEESV',
+			'auth:projectId="218523"',
+			'--directoryName=owner_xyz',
+		]);
+
+		$this->assertSame(0, $result['code'], $result['output']);
+		$this->assertFileExists($this->tmpDir.'/config/owner_xyz/myCEESV.conf.php');
+	}
+
+	public function testWriteToExistingSiteConfigCreatesBackup(): void
+	{
+		$firstWrite = $this->runCfg([
+			'-a',
+			$this->tmpDir,
+			'write',
+			'myCEESV',
+			'auth:projectId="first"',
+			'--directory-name=owner_xyz',
+		]);
+		$this->assertSame(0, $firstWrite['code'], $firstWrite['output']);
+
+		$secondWrite = $this->runCfg([
+			'-a',
+			$this->tmpDir,
+			'write',
+			'myCEESV',
+			'auth:projectId="second"',
+			'--directory-name=owner_xyz',
+		]);
+		$this->assertSame(0, $secondWrite['code'], $secondWrite['output']);
+
+		$siteFile = $this->tmpDir.'/config/owner_xyz/myCEESV.conf.php';
+		$backupFile = $siteFile.'.bak';
+		$this->assertFileExists($backupFile);
+
+		$current = require $siteFile;
+		$backup = require $backupFile;
+		$this->assertSame('second', $current['auth']['projectId']);
+		$this->assertSame('first', $backup['auth']['projectId']);
+	}
+
+	public function testShowWithSiteReturnsSiteSpecificValue(): void
+	{
+		$this->runCfg([
+			'-a',
+			$this->tmpDir,
+			'write',
+			'myCEESV',
+			'auth:projectId="218523"',
+			'--directory-name=owner_xyz',
+		]);
+
+		$show = $this->runCfg([
+			'-a',
+			$this->tmpDir,
+			'show',
+			'myCEESV',
+			'auth:projectId',
+			'--site=owner_xyz',
+		]);
+
+		$this->assertSame(0, $show['code'], $show['output']);
+		$this->assertStringContainsString('218523', $show['output']);
+	}
+
+	public function testDirectoryNameRejectsTraversal(): void
+	{
+		$result = $this->runCfg([
+			'-a',
+			$this->tmpDir,
+			'write',
+			'myCEESV',
+			'auth:projectId="218523"',
+			'--directory-name=../owner_xyz',
+		]);
+
+		$this->assertSame(1, $result['code']);
+		$this->assertStringContainsString('Invalid directory in --directoryName', $result['output']);
+	}
+
+	public function testDirectoryNameRejectsInvalidSegment(): void
+	{
+		$result = $this->runCfg([
+			'-a',
+			$this->tmpDir,
+			'write',
+			'myCEESV',
+			'auth:projectId="218523"',
+			'--directory-name=owner xyz',
+		]);
+
+		$this->assertSame(1, $result['code']);
+		$this->assertStringContainsString("Invalid directory name segment 'owner xyz' in --directoryName.", $result['output']);
+	}
+
+	public function testDirectoryNameRejectsEmptyValue(): void
+	{
+		$result = $this->runCfg([
+			'-a',
+			$this->tmpDir,
+			'write',
+			'myCEESV',
+			'auth:projectId="218523"',
+			'--directory-name=',
+		]);
+
+		$this->assertSame(1, $result['code']);
+		$this->assertStringContainsString('Option --directoryName is empty.', $result['output']);
+	}
+
+	public function testUsingSiteAndDirectoryNameTogetherReturnsError(): void
+	{
+		$result = $this->runCfg([
+			'-a',
+			$this->tmpDir,
+			'write',
+			'myCEESV',
+			'auth:projectId="218523"',
+			'--site',
+			'owner_xyz',
+			'--directory-name',
+			'owner_xyz',
+		]);
+
+		$this->assertSame(1, $result['code']);
+		$this->assertStringContainsString('Please use only one of --site or --directoryName (alias: --directory-name).', $result['output']);
+	}
+
+	private function runCfg(array $args): array
+	{
+		$script = realpath(__DIR__.'/../bin/cfg');
+		$this->assertNotFalse($script);
+
+		$command = escapeshellarg(PHP_BINARY).' '.escapeshellarg($script);
+		foreach ($args as $arg) {
+			$command .= ' '.escapeshellarg($arg);
+		}
+		$command .= ' 2>&1';
+
+		$outputLines = [];
+		$exitCode = 0;
+		exec($command, $outputLines, $exitCode);
+
+		return [
+			'code' => $exitCode,
+			'output' => implode(PHP_EOL, $outputLines),
+		];
+	}
+
+	private function removeDir(string $dir): void
+	{
+		if (!is_dir($dir)) {
+			return;
+		}
+
+		foreach (scandir($dir) ?: [] as $entry) {
+			if ($entry === '.' || $entry === '..') {
+				continue;
+			}
+
+			$path = $dir.'/'.$entry;
+			if (is_dir($path)) {
+				$this->removeDir($path);
+			} else {
+				unlink($path);
+			}
+		}
+
+		rmdir($dir);
+	}
+}
diff --git a/tests/SettingsTest.php b/tests/SettingsTest.php
index 64ed411..c895bba 100644
--- a/tests/SettingsTest.php
+++ b/tests/SettingsTest.php
@@ -211,6 +211,22 @@ class SettingsTest extends TestCase
 							  
 		$this->assertEquals($path.$expected, $cfg->buildFileName($type));
 	}
+
+	public function testBuildFileNameWithPrefixAndSite(): void
+	{
+		$cfg = new Settings();
+		$cfg->appPath('./')->prefix('myCEESV')->site('owner_xyz');
+
+		$this->assertEquals('./config/owner_xyz/myCEESV.conf.php', $cfg->buildFileName(0x01));
+	}
+
+	public function testBuildFileNameWithPrefixAndNestedSite(): void
+	{
+		$cfg = new Settings();
+		$cfg->appPath('./')->prefix('myCEESV')->site('owner_xyz/sub_a');
+
+		$this->assertEquals('./config/owner_xyz/sub_a/myCEESV.conf.php', $cfg->buildFileName(0x01));
+	}
 	
 	public function fileNameData()
 	{