Demo Docker Container Operations Continued

In this lesson, we’ll explore advanced Docker flags and best practices for managing containers. You’ll learn how to:

Automatically clean up ephemeral containers
Customize container hostnames
Control container restarts with various policies
Inspect Docker events for troubleshooting
Copy files between host and containers
Expose ports using random and static mappings

Automatically Remove Ephemeral Containers with `--rm`

When running short-lived or CI/CD containers, you can use --rm to automatically delete them upon exit. This helps keep your environment clean and free of stale containers.

docker container run -itd --name=kodekloud --rm ubuntu
docker container ls -l   # Lists only the most recent container

Stop and watch it disappear:

docker container stop kodekloud

Note

Using --rm is ideal for one-off or CI tasks. Remember that you won’t be able to inspect logs or the filesystem after the container exits.

Setting Container Name vs. Hostname

Docker allows you to assign both a unique container name and an internal hostname:

docker container run -itd \
  --name=yogesh \
  --hostname=kodekloud \
  --rm ubuntu

docker container ls -l
docker exec -it yogesh hostname

Container names must be unique per host, but you can reuse the same internal hostname across multiple containers.

Restart Policies

Docker’s --restart flag offers fine-grained control over container uptime. Below is a summary of each policy:

Policy	Description
`no`	(default) Container will not restart after exit.
`on-failure[:N]`	Restart only on non-zero exit status. Optionally limit retries to `N`.
`always`	Always restart the container regardless of exit code or Docker daemon restarts.
`unless-stopped`	Like `always`, but the container won’t restart if you manually stopped it.

1. `--restart=no` (Default)

docker container run -itd --name=caseone --restart=no ubuntu
docker container stop caseone
docker container ls -l

The container will remain stopped until you manually start it again.

2. `--restart=on-failure`

docker container run -itd --name=casetwo --restart=on-failure ubuntu
docker container kill casetwo   # Simulate a failure
sleep 2
docker container ls -l

The container automatically restarts if it exits with a non-zero status.

3. `--restart=always`

docker container run -itd --name=casethree --restart=always ubuntu
docker container stop casethree
docker container ls -l

Regardless of exit reason or daemon restart, the container will be restarted.

4. `--restart=unless-stopped`

docker container run -itd --name=casefour --restart=unless-stopped ubuntu
docker container stop casefour
docker container ls -l

Even if Docker restarts, casefour stays down after a manual stop.

Warning

Use always or unless-stopped for critical services. Excessive restart loops on failure can degrade performance—consider on-failure with a retry limit.

Inspecting Docker Events

To diagnose restart loops or networking events, view real-time Docker system events:

docker system events --since 60m

Sample output:

2020-05-04T08:38:43.747Z network connect cf10… (container=2daf…, name=bridge) 2020-05-04T08:38:43.976Z container start … (image=ubuntu, name=casethree) 2020-05-04T08:39:43.633Z network connect cf10… (container=74b0…, name=bridge) 2020-05-04T08:39:43.890Z container start … (image=ubuntu, name=casefour)

Copying Files Between Host and Container (`docker cp`)

You don’t need an interactive shell to move files; docker cp handles it directly.

Host → Container

mkdir -p /var/temp
echo "hello for KodeKloud" > /var/temp/yogishtest

docker container run -itd --name=copytest --rm ubuntu
docker container exec copytest mkdir -p /root
docker container cp /var/temp/yogishtest copytest:/root/
docker container exec -it copytest cat /root/yogishtest

Container → Host

docker container exec copytest bash -c "mkdir -p /var/temp && echo 'container to host' > /var/temp/docker-host"
docker container cp copytest:/var/temp/docker-host /var/temp/
ls -l /var/temp/docker-host
cat /var/temp/docker-host

Publishing Ports

By default, containers don’t expose ports to the host. You can use random or static port mappings:

Option	Description	Access URL
none	No mapping; container ports are isolated	Unreachable from host
`-P`	Random high host port mapped to container’s port	`http://<host-ip>:<random>`
`-p`	Static host-to-container port mapping (e.g., 82:80)	`http://<host-ip>:82`

No Port Mapping

docker container run -itd --name=case1 httpd
docker container ls -l

Accessing via host IP will fail.

Random Port Mapping (`-P`)

docker container run -itd -P --name=case2 httpd
docker container ls -l

Docker picks a random port (e.g., 32768) for container port 80.

Static Port Mapping (`-p`)

docker container run -itd --name=case3 -p 82:80 httpd
docker container ls -l

Now host port 82 forwards to container port 80.

Port Mapping on Restart

docker container restart case2
docker container restart case3
docker container ls -l

With -P, Docker may assign a different random port after restart.
With -p, the mapping remains consistent.

Best Practice

For production services, always use -p for predictable port assignments and easier firewall configuration.

References

We hope this lesson helps you master advanced Docker container operations!

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Automatically Remove Ephemeral Containers with --rm