# Docker - Use docker-compose to override CMD

I've been dabbling with Docker to run services on my new  [Raspberry Pi](https://blog.manoj.ch/setting-up-ubuntu-server-on-raspberry-pi4-with-boot-from-usb-ssd)  and being a beginner at Docker myself, it was hard sometimes to debug why a docker container was failing as soon as it was started. Since a container "stays alive" only until its `CMD` process is running, there is no way to do a quick look-see inside the container if it has already Exited.

To change the `Dockerfile` to perform trial and error means that you will have to rebuild it again - which is time consuming.

One way to keep a container running without having to do this, is to override the `Dockerfile` command from the `docker-compose.yml` file.

For example:

```yaml
version: "3"
services:
  redis:
    image: redis
  web:
    image: web:v001
    build:
      context: .
      dockerfile: Dockerfile
    ports:
      - "3000:3000"
    volumes:
      - .:/opt/www
    depends_on:
      - redis
```

And in the `Dockerfile` you have a command:

```Dockerfile
FROM node:14-alpine

ENV APP_PATH /opt/www
RUN mkdir -p $APP_PATH

WORKDIR $APP_PATH

COPY package.json ./
COPY yarn.lock ./

RUN yarn

COPY . .

RUN yarn build

CMD yarn start

EXPOSE 3000

```
Anytime `CMD yarn start` fails, the container exits and is no more accessible for debugging.

Modify `docker-compose.yml` to include `command` override.

```yaml
version: "3"
services:
  redis:
    image: redis
  web:
    image: web:v001
    build:
      context: .
      dockerfile: Dockerfile
    command: "tail -f /dev/null"     #   <--- <--- <---
    ports:
      - "3000:3000"
    volumes:
      - .:/opt/www
    depends_on:
      - redis
```

Now, when you start the container using `docker-compose up -d`, the container stays running.

Next, use `docker ps -a` to list all containers and obtain the hash for the container id.

Then, run `sh` within the container by: `docker exec -it <sha> sh`.

If everything goes well, you should see the shell prompt within the container:

```sh
/opt/www #
```

Now, you may debug for missing files, env vars, or even try to run `yarn start` and see why it is failing.