i'm starting using discord.py and i don't understand why my bot does not respond to my `test` command .the bot is starting correcly and says his line in the channel while starting but do not react to my command

```py
import discord
from discord.ext import commands

bot = commands.Bot(command_prefix='$')

class MyClient(discord.Client):
    async def on_ready(self):
        print('Logged on as {0}!'.format(self.user))
        channel = client.get_channel(123456789)
        await channel.send("online")

@bot.command()
async def test(ctx):
    await ctx.send('test')

```

thank you !