Running Your First Container

Now that Docker is installed, let's run your first container. This hands-on lesson introduces the fundamental docker run command and its essential options.

The docker run Command

The docker run command creates and starts a container from an image:

docker run [OPTIONS] IMAGE [COMMAND] [ARGS]

Running Hello World

Let's start with the simplest example:

docker run hello-world

What happens:

1. Docker looks for hello-world image locally
2. If not found, pulls it from Docker Hub
3. Creates a container from the image
4. Runs the container
5. Container prints a message and exits

Running an Interactive Container

Let's run a more useful container - an Ubuntu shell:

docker run -it ubuntu bash

Breaking down the options:

• -i (--interactive): Keep STDIN open
• -t (--tty): Allocate a pseudo-terminal
• ubuntu: The image name
• bash: The command to run

You're now inside an Ubuntu container! Try some commands:

# Check the OS
cat /etc/os-release

# List files
ls -la

# Exit the container
exit

Running a Web Server

Let's run Nginx, a popular web server:

docker run -d -p 8080:80 nginx

Breaking down the options:

• -d (--detach): Run in background
• -p 8080:80: Map port 8080 on host to port 80 in container
• nginx: The image name

Visit http://localhost:8080 in your browser to see Nginx running!

Essential docker run Options

Option	Short	Description
--detach	-d	Run in background
--interactive	-i	Keep STDIN open
--tty	-t	Allocate pseudo-TTY
--publish	-p	Publish port (host:container)
--name		Assign a name to container
--rm		Remove container when it exits
--env	-e	Set environment variables
--volume	-v	Mount a volume

Naming Containers

Give containers meaningful names for easier management:

# Run with a custom name
docker run -d --name my-nginx -p 8080:80 nginx

# Now you can reference it by name
docker stop my-nginx
docker start my-nginx
docker logs my-nginx

Automatic Cleanup

Use --rm to automatically remove the container when it exits:

# Container is removed after the command completes
docker run --rm ubuntu echo "Hello from container"

# Useful for one-off commands
docker run --rm -it python:3.11 python --version

Environment Variables

Pass environment variables to configure containers:

# Single variable
docker run -e MY_VAR=hello ubuntu env

# Multiple variables
docker run -e DB_HOST=localhost -e DB_PORT=5432 myapp

# From a file
docker run --env-file .env myapp

Running in the Background

For long-running services, use detached mode:

# Start in background
docker run -d --name web -p 80:80 nginx

# Check it's running
docker ps

# View logs
docker logs web

# Follow logs in real-time
docker logs -f web

Working with Container Output

View output from containers:

# View all logs
docker logs container_name

# Follow logs (like tail -f)
docker logs -f container_name

# Last 100 lines
docker logs --tail 100 container_name

# Logs since a timestamp
docker logs --since 2023-01-01 container_name

Executing Commands in Running Containers

Use docker exec to run commands in an existing container:

# Start a container
docker run -d --name mycontainer nginx

# Execute a command
docker exec mycontainer ls -la

# Open an interactive shell
docker exec -it mycontainer bash

# Run as a different user
docker exec -u root mycontainer whoami

Stopping and Removing Containers

# Stop a running container (graceful)
docker stop container_name

# Stop immediately (force)
docker kill container_name

# Remove a stopped container
docker rm container_name

# Stop and remove in one command
docker rm -f container_name

# Remove all stopped containers
docker container prune

Practical Examples

Run a Database

docker run -d \
  --name postgres \
  -e POSTGRES_PASSWORD=mysecretpassword \
  -p 5432:5432 \
  postgres:15

Run Redis

docker run -d \
  --name redis \
  -p 6379:6379 \
  redis:alpine

Run a One-off Command

# Run Node.js script without installing Node
docker run --rm -v $(pwd):/app -w /app node:18 node script.js

# Run Python script
docker run --rm -v $(pwd):/app -w /app python:3.11 python script.py

Common Mistakes

Forgetting to expose ports

# Wrong - can't access the web server
docker run -d nginx

# Correct - accessible on localhost:8080
docker run -d -p 8080:80 nginx

Running interactive containers in detached mode

# Container exits immediately
docker run -d ubuntu

# Correct - use -it for interactive shells
docker run -it ubuntu bash

Key Takeaways

• docker run creates and starts containers from images
• Use -d for background services, -it for interactive sessions
• Port mapping (-p host:container) exposes container services
• Name containers with --name for easier management
• Use --rm for one-off commands to avoid container buildup
• docker exec runs commands in existing containers
• Always expose necessary ports when running services