diff --git a/core/command/app/listapps.php b/core/command/app/listapps.php
index e30baddb74..e483037d45 100644
--- a/core/command/app/listapps.php
+++ b/core/command/app/listapps.php
@@ -73,7 +73,7 @@ class ListApps extends Base {
 	 */
 	protected function writeAppList(InputInterface $input, OutputInterface $output, $items) {
 		switch ($input->getOption('output')) {
-			case 'plain':
+			case self::OUTPUT_FORMAT_PLAIN:
 				$output->writeln('Enabled:');
 				parent::writeArrayInOutputFormat($input, $output, $items['enabled']);
 
diff --git a/core/command/base.php b/core/command/base.php
index 7936e683dc..0a4b002c1c 100644
--- a/core/command/base.php
+++ b/core/command/base.php
@@ -27,7 +27,11 @@ use Symfony\Component\Console\Input\InputOption;
 use Symfony\Component\Console\Output\OutputInterface;
 
 class Base extends Command {
-	protected $defaultOutputFormat = 'plain';
+	const OUTPUT_FORMAT_PLAIN = 'plain';
+	const OUTPUT_FORMAT_JSON = 'json';
+	const OUTPUT_FORMAT_JSON_PRETTY = 'json_pretty';
+
+	protected $defaultOutputFormat = self::OUTPUT_FORMAT_PLAIN;
 
 	protected function configure() {
 		$this
@@ -49,10 +53,10 @@ class Base extends Command {
 	 */
 	protected function writeArrayInOutputFormat(InputInterface $input, OutputInterface $output, $items, $prefix = '  - ') {
 		switch ($input->getOption('output')) {
-			case 'json':
+			case self::OUTPUT_FORMAT_JSON:
 				$output->writeln(json_encode($items));
 				break;
-			case 'json_pretty':
+			case self::OUTPUT_FORMAT_JSON_PRETTY:
 				$output->writeln(json_encode($items, JSON_PRETTY_PRINT));
 				break;
 			default:
@@ -89,10 +93,10 @@ class Base extends Command {
 		}
 
 		switch ($input->getOption('output')) {
-			case 'json':
+			case self::OUTPUT_FORMAT_JSON:
 				$output->writeln(json_encode($item));
 				break;
-			case 'json_pretty':
+			case self::OUTPUT_FORMAT_JSON_PRETTY:
 				$output->writeln(json_encode($item, JSON_PRETTY_PRINT));
 				break;
 			default:
diff --git a/core/command/config/listconfigs.php b/core/command/config/listconfigs.php
index d80bd2b98c..fed6a8e9db 100644
--- a/core/command/config/listconfigs.php
+++ b/core/command/config/listconfigs.php
@@ -30,7 +30,7 @@ use Symfony\Component\Console\Input\InputOption;
 use Symfony\Component\Console\Output\OutputInterface;
 
 class ListConfigs extends Base {
-	protected $defaultOutputFormat = 'json_pretty';
+	protected $defaultOutputFormat = self::OUTPUT_FORMAT_JSON_PRETTY;
 
 	/** @var array */
 	protected $sensitiveValues = [
diff --git a/core/command/encryption/listmodules.php b/core/command/encryption/listmodules.php
index d55480def8..cc436ea562 100644
--- a/core/command/encryption/listmodules.php
+++ b/core/command/encryption/listmodules.php
@@ -65,7 +65,7 @@ class ListModules extends Base {
 	 * @param array $items
 	 */
 	protected function writeModuleList(InputInterface $input, OutputInterface $output, $items) {
-		if ($input->getOption('output') === 'plain') {
+		if ($input->getOption('output') === self::OUTPUT_FORMAT_PLAIN) {
 			array_walk($items, function(&$item) {
 				if (!$item['default']) {
 					$item = $item['displayName'];