$signature = 'command:name {your_command?*}' のように?*を追加するだけカンタン

app/Console/Commands/shell_exec.php

<?php

namespace App\Console\Commands;

use Illuminate\Console\Command;

class shell_exec extends Command
{
    /**
     * The name and signature of the console command.
     *
     * @var string
     */
    protected $signature = 'command:name {your_command?*}';

    /**
     * The console command description.
     *
     * @var string
     */
    protected $description = 'Command description';

    /**
     * Create a new command instance.
     *
     * @return void
     */
    public function __construct()
    {
        parent::__construct();
    }

    /**
     * Execute the console command.
     *
     * @return int
     */
    public function handle()
    {
        $command = join(' ', $this->argument('your_command'));
        if ($command) {
            $output = shell_exec($command);
            if ($output) $this->line($output);
        }
        return 0;
    }
}