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UE3 Home > Commandlets > How To Make a Commandlet

How To Make a Commandlet


Commandlets are command line programs that run inside of the Unreal Engine environment. They are most often used to make bulk changes to content, iterate over content to get information about it, or as a unit testing mechanism.

They are basically little applets that have the engine initialized but are mostly just confined to Windows console output. For example, game.exe make executes the make commandlet that compiles UnrealScript classes.

The engine automatically tries to see whether a command you passed on the commandline is a commandlet, and if it matches, executes the associated code.

Check out UContentCommandlets.cpp and UnPackageUtilities for some examples on commandlets.

How To Make a Commandlet in Script

Create a script class for your commandlet as seen below:

class HelloWorldCommandlet extends Commandlet;

event int Main( string Params )
   log( " hello world " );

   return 0;

Or for a native commandlet:

class HelloWorldCommandlet extends Commandlet

INT Main(const FString& CmdLine)
   warnf(TEXT("hello world"));
   return 0;

native event int Main( string Params );

The Main() function is the entry point for your commandlet. The commandlet is passed a string containing all of the parameters for a given run. Note that for

gamename.exe helloworld param1 param2 param3

Your commandlet would see the string param1 param2 param3 as the command line parameters.

How To Make a Commandlet in Native Code

Most commandlets are implemented as intrinsic classes in native code. The procedure for creating a commandlet in this way is quite different. You'll find most commandlet declarations in EditorCommandlets.h.

First, declare your class in a header file like EditorCommandlets.h:

  // declare additional methods here.
The commandlet will have a classname of "UNameCommandlet" and will be placed in the package you specify (like Editor).

Then, in a .cpp file (like UContentCommandlets.cpp), implement your commandlet's Main() function. If necessary, you can also override the virtual CreateCustomEngine() function. See the UCommandlet source for more details.

INT UNameCommandlet::Main(const FString& Params)
    // do yer stuff here...
    return 0;

But you're not done yet! Since this is an intrinsic class, you must inform the script system of its presence. To do this you'll have to manually edit the UnrealEdClasses.h file, specifically the AUTO_INITIALIZE_REGISTRANTS_UNREALED #define. In that list, add a call to the StaticClass() function for your commandlet:

   UNameCommandlet::StaticClass(); \

The next time you rebuild script the compiler will overwrite this file with your Commandlet class in this list in alphabetical order.

Giving Users Some Help

Built into a commandlet is the ability to show help information about it, so that users can discover what it is and what options it supports. To make help information available for your commandlet, you need to add entries into the localized text file that your commandlet resides in. If you package is in Core, then you need to modify Core.int for your help information to show up. Below is a sample setting for the "hello world" commandlet residing in Core.

HelpDescription="This commandlet displays hello world" HelpUsage="gamename.exe helloworld"
HelpParamDescriptions[0]="Ignored since helloworld doesn't need params"

Notice that the section name must include the commandlet as part of the class name. Without this, the commandlet wouldn't be found. To see how this information is used, run the following commands:

game.exe help help
game.exe help list
game.exe help webhelp help

How To Use a Commandlet

To execute a commandlet run your game with the commandlet name that you wish to run. For instance:

examplegame make -full

launches the make commandlet inside of ExampleGame. Note that you do not have to specify the package name for native commandlets. If the make commandlet was a script-only commandlet, then it would be invoked using:

examplegame editor.make -full

since it wouldn't be loaded at startup. Also notice that the commandlet portion of the name isn't needed. The startup code automatically appends _commandlet= when trying to find the specified commandlet.

Common Commandlet Tasks

A common thing to do is iterate over packages and perform some data processing on them. The following code (which can also be found as a comment at the top of UContentCommandlets.cpp) can get you started:

// Below is a template commandlet than can be used when you want to perform an operation on all packages.
INT UPerformAnOperationOnEveryPackage::Main(const FString& Params)
   // Parse command line args.
   TArray<FString> Tokens;
   TArray<FString> Switches;

   const TCHAR* Parms = *Params;
   ParseCommandLine(Parms, Tokens, Switches);

   // Build package file list.
   const TArray<FString> FilesInPath( GPackageFileCache->GetPackageFileList() );
   if( FilesInPath.Num() == 0 )
      warnf( NAME_Warning, TEXT("No packages found") );
      return 1;

   // Iterate over all files doing stuff.
   for( INT FileIndex = 0 ; FileIndex < FilesInPath.Num() ; ++FileIndex )
      const FFilename& Filename = FilesInPath(FileIndex);
      warnf( NAME_Log, TEXT("Loading %s"), *Filename );

      UPackage* Package = UObject::LoadPackage( NULL, *Filename, LOAD_None );
      if( Package == NULL )
         warnf( NAME_Error, TEXT("Error loading %s!"), *Filename );

      // Do your thing here

      TObjectIterator<UStaticMesh> It;...

      UStaticMesh* StaticMesh = *It;
      if( StaticMesh->IsIn( Package )

      UObject::CollectGarbage( RF_Native );

   return 0;

You might also want to look at NormalizePackageNames() in PackageHelperFunctions.h (which appeared in QA_APPROVED_BUILD_JUNE_2007). This function can create a filtered list of packages to iterate over, with a few more niceties beyond just iterating over GPackageFileCache (see the code for details).

Commandlet Listing

Please refer to the Commandlet List page.