Docker Containers

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Images vs Containers

Docker's default image registry is called Docker Hub

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Docker container commands

Start a new container

👉 To start a new container from an image, use docker container run.

Let's have a look at how to start a container from an image. Here we run an nginx container as follows:

# 80:80 = <host-IP>:<container-IP>
# port 80 on host IP routes traffic to container IP on port 80
docker container run --publish 80:80 nginx

This is what the above command does:

1. Looks for an nginx image locally in image cache
2. Did it find the image?

• No?
  • looks for the same in remote image repository (defaults to Docker Hub)
  • downloads the latest version (nginx:latest)
  • stores it in the image cache and continues.
• Yes?
  • Continues to next step

3. Creates a new container based on that image and starts
4. Assigns it a virtual IP on a private network inside the docker engine
5. Opens port 80 on host IP and routes traffic to container IP on port 80

❗ This runs the process in foreground.

🛑 To send a stop signal to a process running in the foreground, press Ctrl + C.

👉 To run the same command in background, run the above command with the detach option as follows:

# 80:80 = <host-IP>:<container-IP>
# port 80 on host IP routes traffic to container IP on port 80
docker container run --publish 80:80 --detach nginx

This will return a unique container ID.

Use a specific version of the image

docker container run --publish 80:80 --d nginx:1.11

💡 d is short for detach

Start an existing container

# docker container start e6a
docker container start <first-few-digits-of-container-id>

List all running containers

docker container ls
# Alternate command
docker ps

This shows all running containers in the following format:

CONTAINER ID   IMAGE     COMMAND                  CREATED              STATUS              PORTS                NAMES
e6ac9bd67074   nginx     "/docker-entrypoint.…"   About a minute ago   Up About a minute   0.0.0.0:80->80/tcp   heuristic_joliot

Stop a running container

# docker container stop e6a
docker container stop <first-few-digits-of-container-id>

Name a container

To name a container, use name option as follows:

docker container run --publish 80:80 --detach --name webhost nginx

Get logs for a specific container

# docker container logs webhost
# docker container logs e6a
docker container logs <first-few-digits-of-container-id | container-name>

Remove container(s)

docker container rm d34 e6a fb0 9c1

❗ Be sure to stop running containers before trying to remove them.

💡 There is always the option to force remove running containers with f option.

docker container rm -f d34 e6a fb0 9c1

View container processes

To list running processes in a specific container:

docker container top <first-few-digits-of-container-id | container-name>

Inspect containers

To see metadata about the container (startup, config, volumes, networking, etc.)

docker container inspect <first-few-digits-of-container-id | container-name>

View container usage statistics

Display a live stream of container(s) resource usage statistics

docker container stats <first-few-digits-of-container-id | container-name>

Running commands on a container

# docker container run -it --name ubuntu ubuntu
# docker container run -it --name proxy mariadb bash

# -t simulates a real terminal, like what SSH does
# -i Keeps session open so that we can run more commands
docker container run -it <first-few-digits-of-container-id | container-name> <command>

This will open a shell inside the container as follows:

# root means root on the container
# the number after `root@` is the container ID
root@3eeb605c1258:/# 

👉 To start an existing container interactively:

# -a attach session and forward signals to the container
docker container start -ai <first-few-digits-of-container-id | container-name>

👉 To run additional processes in a running container without affecting the running container:

# docker container exec -it mariadb bash
docker container exec -it <first-few-digits-of-container-id | container-name> <command>

💡 Exiting the command in the above case won't stop the container.

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Containers vs Virtual machines

A container is just a process running on top of an image, i.e., they have access to limited resources and a process can be stopped.

A container is ❌ not exactly a virtual machine.

💡 To monitor all processes running on your system, use ps aux.

💡 To view all processes matching a string, use ps aux | grep <search-string>.