Docker 101: Run a Container Overriding its Default Command

Some container images include more than one executable file, and you may want to run a container using other than the default command and/or pass some arguments to it. In this challenge, you'll practice running an alternative executable in an off-the-shelf container image.

When invoked without extra arguments, the docker run redis command will start a redis-server binary in a container. However, the redis image also includes the redis-cli executable, which can be used to interact with any Redis server instance (not necessarily the one running in the same container). This makes a redis container a handy way to run redis-cli without installing it on your machine. But only if you know how to override its default command 😉

To get started, run redis-cli with the --version flag in a redis container:

Hint 1 💡

Docker designed containers in such a way that they can be run without any additional arguments by default. For instance, docker run redis will start a redis process in a container, because the redis image has a default redis-server command defined in its Dockerfile:

redis' Dockerfile

FROM ...some base image...
RUN ..install redis...

# Run the redis-server command by default
CMD ["redis-server"]

However, oftentimes container images include more than one executable file, and you may want to run a container using other than the default command and/or pass some arguments to it.

Luckily, the docker run command is flexible enough to support this:

docker run [OPTIONS] IMAGE [COMMAND] [ARG...]

Now, let's connect to a remote Redis server using the containerized redis-cli binary. The Redis server is reachable at 172.16.0.3:6379, and you can use redis-cli -h <host> to connect to it.

Run the redis-cli -h 172.16.0.3 ping command in a redis container:

Hint 2 💡

If the container is failing to connect to Redis, make sure you use the -h flag with the right IP address and without the port number. If you really want to specify the port number, you can use the -p flag for that.

Hint 3 💡

While it wouldn't be a strict requirement in a real-world scenario, the challenge expects you to run the ping command after connecting to the Redis server.