Ports and Environment Variables

Publishing ports with -p

By default a container is fully isolated — its ports are not reachable from the host. The -p flag publishes a port, creating a mapping between a host port and a container port:

docker run -d -p 8080:80 nginx:alpine

The format is -p <host-port>:<container-port>. After this command, opening http://localhost:8080 in your browser reaches port 80 inside the container.

You can publish multiple ports:

docker run -d -p 8080:80 -p 8443:443 nginx:alpine

Bind to a specific host IP

# Only accessible from localhost, not the network
docker run -d -p 127.0.0.1:8080:80 nginx:alpine

Passing environment variables with -e

Containers read configuration from environment variables. Pass them with -e:

docker run -d \
  -e MYSQL_ROOT_PASSWORD=secret \
  -e MYSQL_DATABASE=myapp \
  mysql:8

Each -e sets one variable inside the container.

Using an env file

For more than a few variables, keep them in a file and use --env-file:

.env

MYSQL_ROOT_PASSWORD=secret
MYSQL_DATABASE=myapp
MYSQL_USER=app
MYSQL_PASSWORD=apppass

docker run -d --env-file .env mysql:8

This keeps secrets out of your shell history.

Naming containers with --name

Giving a container a predictable name makes every subsequent command easier:

docker run -d \
  --name mysite \
  -p 8080:80 \
  nginx:alpine

Now you can use mysite instead of a random ID in every docker stop, docker logs, and docker exec command.

Putting it together

docker run -d \
  --name webserver \
  -p 8080:80 \
  -e NGINX_HOST=localhost \
  nginx:alpine

# Verify
docker ps
curl http://localhost:8080

# Run nginx with a port mapping and an env var
docker run -d \
  --name webserver \
  -p 8080:80 \
  -e NGINX_HOST=localhost \
  nginx:alpine

# Confirm the mapping
docker ps

# Fetch the default page (should return HTML)
curl -s http://localhost:8080 | head -5

# Inspect the env var inside the container
docker exec webserver env | grep NGINX

# Clean up
docker stop webserver && docker rm webserver

Run nginx with a port and an env var ↗

🐳 -p 8080:80 maps host 8080 to container 80

Without -p, the container port exists only inside Docker’s internal network — your browser and curl on the host cannot reach it. The mapping -p 8080:80 means: traffic arriving at port 8080 on the host is forwarded to port 80 inside the container.

What does '-p 3000:8080' mean?

Host port 8080 maps to container port 3000 Host port 3000 maps to container port 8080 Both ports 3000 and 8080 are exposed on the host The container listens on all ports between 3000 and 8080

Which flag passes a file of KEY=VALUE pairs as environment variables?

-e --env --env-file --config

If you run a container WITHOUT -p, can you reach its port from the host?

Yes — all container ports are always reachable Yes — but only on port 80 No — the port is only accessible inside Docker's internal network Only if the container uses EXPOSE in its Dockerfile

What is the benefit of using --name when running a container?

It speeds up the container startup time It gives the container a predictable name for use in subsequent commands It automatically publishes all ports It prevents the container from being removed

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