add Shell with ERROR and SUCCESS code

src/Console/Command/GenerateRectorOverviewCommand.php

@@ -5,6 +5,7 @@ namespace Rector\Console\Command;
 use Nette\Loaders\RobotLoader;
 use Nette\Utils\Strings;
 use Rector\Console\ConsoleStyle;
+use Rector\Console\Shell;
 use Rector\ConsoleDiffer\MarkdownDifferAndFormatter;
 use Rector\Contract\Rector\RectorInterface;
 use Rector\Exception\ShouldNotHappenException;
@@ -65,8 +66,7 @@ final class GenerateRectorOverviewCommand extends Command
         $rectorsByGroup = $this->groupRectors($this->getGeneralRectors());
         $this->printRectorsByGroup($rectorsByGroup);
 
-        // success
-        return 0;
+        return Shell::CODE_SUCCESS;
     }
 
     /**

src/Console/Command/LevelsCommand.php

@@ -4,6 +4,7 @@ namespace Rector\Console\Command;
 
 use Nette\Utils\Strings;
 use Rector\Console\ConsoleStyle;
+use Rector\Console\Shell;
 use Symfony\Component\Console\Command\Command;
 use Symfony\Component\Console\Input\InputArgument;
 use Symfony\Component\Console\Input\InputInterface;
@@ -43,7 +44,7 @@ final class LevelsCommand extends Command
         $this->consoleStyle->title(sprintf('%d available levels:', count($levels)));
         $this->consoleStyle->listing($levels);
 
-        return 0;
+        return Shell::CODE_SUCCESS;
     }
 
     /**

src/Console/Command/ProcessCommand.php

@@ -10,6 +10,7 @@ use Rector\Autoloading\AdditionalAutoloader;
 use Rector\Configuration\Option;
 use Rector\Console\ConsoleStyle;
 use Rector\Console\Output\ProcessCommandReporter;
+use Rector\Console\Shell;
 use Rector\ConsoleDiffer\DifferAndFormatter;
 use Rector\FileSystem\FilesFinder;
 use Rector\Guard\RectorGuard;
@@ -165,7 +166,7 @@ final class ProcessCommand extends Command
 
         if ($this->errors) {
             $this->processCommandReporter->reportErrors($this->errors);
-            return 1;
+            return Shell::CODE_ERROR;
         }
 
         if ($input->getOption(Option::OPTION_WITH_STYLE)) {
@@ -179,7 +180,7 @@ final class ProcessCommand extends Command
 
         $this->consoleStyle->success('Rector is done!');
 
-        return 0;
+        return Shell::CODE_SUCCESS;
     }
 
     /**

src/Console/Shell.php

@@ -0,0 +1,19 @@
+<?php declare(strict_types=1);
+
+namespace Rector\Console;
+
+/**
+ * Mimics https://github.com/cakephp/cakephp/blob/ae7ec50e12adbb30d9e7d1187b385bc32b7d31dc/src/Console/Shell.php#L39
+ */
+final class Shell
+{
+    /**
+     * @var int
+     */
+    public const CODE_ERROR = 1;
+
+    /**
+     * @var int
+     */
+    public const CODE_SUCCESS = 0;
+}