BinaryProbe

// Assuming module is called "myproto"
import qbs.File
import qbs.Probes

Module {
    // search for a protoc executable
    Probes.BinaryProbe {
        id: protocProbe
        names: "protoc"
    }
    property string executableFilePath: protocProbe.filePath

    validate: {
        if (!File.exists(executableFilePath))
            throw "The executable '" + executableFilePath + "' does not exist.";
    }

    // use the found executable
    Rule {
        // rule input/outputs here...

        // run executable for the module called "myproto":
        prepare: {
            var args = // initialize arguments...
            var cmd = new Command(input.myproto.executableFilePath, args);
            cmd.highlight = "codegen";
            cmd.description = "generating protobuf files for " + input.fileName;
            return [cmd];
        }
    }
}